MOZAMBIQUE UPSCALING NATURE-BASED FLOOD PROTECTION IN MOZAMBIQUE’S CITIES Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane January 2020 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane i Project Client: World Bank (WB) Project: Consultancy Services for Upscaling Nature-Based Flood Protection in Mozambique’s Cities (Selection No. 1254774) Document Title: Task 3 – Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane Cover photo by: IL/CES Handling and document control Prepared by CES Consulting Engineers Salzgitter GmbH and Inros Lackner SE (Team Leader: Matthias Fritz, CES) Quality control and review by World Bank Task Team: Bontje Marie Zangerling (Task Team Lead), Brenden Jongman, Michel Matera, Lorenzo Carrera, Xavier Agostinho Chavana, Steven Alberto Carrion, Amelia Midgley, Alvina Elisabeth Erman, Boris Ton Van Zanten, Mathijs Van Ledden Peer Reviewers: Lizmara Kirchner, João Moura Estevão Marques da Fonseca, Zuzana Stanton-Geddes, Julie Rozenberg Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane i LIST OF CONTENT 1 Introduction 11 Objectives of the consultancy 11 Nature-Based Solutions 12 Background 13 Climate 14 Vegetation and fauna 15 Soils, geology and topography 18 Historical Situation and Specific Issues in Various Areas of Nacala 20 2 Nacala Report 24 Field Visits, Data Collection and Assessment of Current Situation 24 Desktop Review and Preparatory Works 24 Field Visit Nacala 24 Data Collection 25 Community Based Mapping Campaign 29 Summary and Assessment of Current Situation – Nacala 31 Risk Assessment – City Risk Profile Nacala 34 General Considerations regarding Risks in Nacala 34 Urban Expansion Scenario 35 Risk Assessment – Parameters and Results 37 Identification of Project Measures - Nacala 45 Sustainable Urban Drainage Systems (SUDS) 47 Large Scale Runoff Reduction 47 Protection of V-shaped Gullies 53 Combined Measures 55 Preventive Erosion Protection Measures to face further Urbanization / Meso- Scale Retention Ponds 62 Alternative and Accompanying Measures 63 Do Nothing Alternative 65 Technical Analysis of nature-based Flood and Erosion Protection Measures 67 Basics 67 Hydrology Nacala 67 Climate Change (Hydrological Impact) 82 Required Protection Goal and Return Period for Modelling 85 Hydraulic Modelling Nacala 86 Additional Remarks for Technical Analysis 98 Aquatic Ecosystem Mapping and Assessment of Proposed Solutions - Nacala 99 Ecosystem mapping 99 Description of the aquatic ecosystems of Nacala 100 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 1 Ecosystem services provided by aquatic ecosystems 101 Ecological Evaluation of Proposed Measures 102 3 Quelimane Report 104 Field Visits, Data Collection and Assessment of Current Situation 104 Desktop Review and Preparatory Works 104 Field Visit to Quelimane 104 Data Collection 145 Community Based Mapping Campaign 155 Summary and Assessment of Current Situation - Quelimane 156 Identification of Possible Measures - Quelimane 159 Overview of Possible Measures to be Applied in Quelimane 159 Allocation of Possible Measures to Sites 159 Risk Assessment - City Risk Profile Quelimane 171 Introduction 171 Approach 171 Assessment of Probabilities of Risk Events 172 Assessment of Socio-Economic Impacts of Risk Events 186 Estimation of Risks 190 Evaluation of Risk Reduction Due to Implementation of Proposed Measures 190 Ecosystem Mapping and Assessment of Proposed Solutions - Quelimane 192 Mapping of Ecological Infrastructure 192 Description of the aquatic ecosystems of Quelimane 193 Ecosystem services provided by aquatic ecosystems 199 Ecological Evaluation of Proposed Measures 200 4 Community-Based Measures 202 Awareness Raising Campaigns 202 Tree Planting 204 Community Works 205 5 Cost Benefit Assessment 207 Cost Estimation for project measures in Quelimane 207 Cost - Benefit Assessment Nacala 209 Cost - Benefit Assessment Quelimane 209 6 List of References 210 7 Annex I – Field Visit Report Nacala 214 8 Annex II - Community Based Mapping Campaign – Collected Data 239 9 Annex III - Ecological Evaluation of Proposed Measures 249 10 Annex IV – Results of Hydraulic Model Nacala 256 Results of Hydraulic Model - Existing Situation 256 Results of Hydraulic Model - Including Detention Ponds 263 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 2 Results of Hydraulic Model - Final Situation / Check dams 270 11 Annex V – Proposed Project Measures for Quelimane 277 12 Annex VI – Site Development of project sites in Quelimane 278 FIGURES Figure 1-1 Green, Grey and Hybrid Solutions (source: www.hollandwaterchallenge.com) 13 Figure 1-2 Overview Map Mozambique / Project Locations 13 Figure 1-3 Annual rainfall (mm) for Nampula from 1969 – 2016 15 Figure 1-4 Climate Graph for Quelimane (from https://en.climate-data.org, March 2019) 15 Figure 1-5 Lone baobab tree in a residential area of Nacala 16 Figure 1-6 Mesquite (Prosopis sp.) growing in one of the erosion gullies in Nacala. 16 Figure 1-7 Vetiver grass (Chrysopogon zizanioides). 16 Figure 1-8 Elepchant grass (Pennisetum purpureum). 17 Figure 1-9 Moringa trees (Moringa olifeira) 17 Figure 1-10 Grass and sedges typical of the freshwater wetlands around the City of Quelimane 17 Figure 1-11 Sandstone in the foreground exposed within an erosion gully 18 Figure 1-12 Sandstone cliffs near the sea, where the short west-facing watercourses meet the coastline 18 Figure 1-13 Geological Map of Mozambique [Grantham et al., 2008] 19 Figure 1-14 Erosion due to storm water runoff in Nacala City (www.voaportugues.com) 20 Figure 1-15 Cassava field in northern part of Nacala 21 Figure 1-16 Gully caused by redirected rainwater from the airport 21 Figure 1-17 deposited material at the sea in 2019 22 Figure 1-18 collapsed house adjacent to a gully 23 Figure 2-1 Selected photos of the field visit in Nacala 25 Figure 2-2 World Bank's R5 Population Count Data for 2010 and 2050 (source: The World Bank) 27 Figure 2-3 Population Growth 2010 to 2050 (1 = No Growth; 0.5 = Doubled Population) 27 Figure 2-4 Kobo Field Data Collection App (Sample Screens) 29 Figure 2-5 Questionnaire for Community Mapping (codes for results table in brackets) 30 Figure 2-6 Nacala, Community Mapping Results for Questions 4, 5 31 Figure 2-7 Erosion due to storm water runoff in Nacala City (August 2018) 33 Figure 2-8 Location of Erosion Gullies (Red) and Deposits in the Sea (Yellow) 35 Figure 2-9 Urban Expansion Nacala – 2006 to 2018 36 Figure 2-10 Identification of 13 prioritized catchment areas in Nacala 41 Figure 2-11 Overview map with indication of catchment areas and urban districts Nacala 42 Figure 2-12 Risk Assessment – Total Risk Score Map for Nacala 44 Figure 2-13 Proposed Main Measures per Catchment 46 Figure 2-14 soil bunds - schematic sketch (source: knowledge.unccd.int) 51 Figure 2-15 Soil bunds to reduce runoff and erosion risks (Source CES, Somaliland) 52 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 3 Figure 2-16 Ecologs as an alternative to soil bunds for areas with sandy soils 52 Figure 2-17 V-shaped gully in northern part of Nacala, depth of more than 8 meters 53 Figure 2-18 Rock Bag/Filter Units (source: Sumitomo) 53 Figure 2-19 Installation of Rock Bags / Filter Units 54 Figure 2-20 Schematic Cross-Section Detention Pond 56 Figure 2-21 Cross-section used for preliminary hydraulic and static assessment 56 Figure 2-22 SUDS - Recreational Areas as Meso-Scale Retention Basins (source: www.lizlake.com) 63 Figure 2-23 Instream River Training - Meandering Ramps (source www.engineering- group.ch) 64 Figure 2-24 Limits of Nature-Based Solutions 65 Figure 2-25 Study Area - Catchment ID 09 68 Figure 2-26 Station Overview and distance of stations in proximity of Nacala. 69 Figure 2-27 Average annual precipitation for observation points in proximity of Nacala (Maputo shown for comparison) 69 Figure 2-28 Daily summaries, precipitation Lumbo station 70 Figure 2-29 Double-Mass-Plot, Lumbo (ordinate) tested against Nampula (abscissa) 70 Figure 2-30 Annual maximum daily precipitation Lumbo 71 Figure 2-31 Return Period for Maximum Annual Precipitation, Log Pearson III distribution and 90% confidence and prediction intervals 71 Figure 2-32 Return Period for Maximum Annual Precipitation, Log Pearson III distribution and 90% confidence and prediction intervals for Reference Stations 73 Figure 2-33 Annual Maximum Precipitation Return Periods for Lumbo and Comparison Stations 74 Figure 2-34 Average Annual Precipitation for Lumbo and Reference Stations 74 Figure 2-35 Annual maximum daily precipitation Maputo 75 Figure 2-36 Rainfall regions of Mozambique – Map of K coefficient 76 Figure 2-37 Intensity Duration Frequency curve for Hydrological Zone D 77 Figure 2-38 Hyetographs for Different Return Periods (Alternating Block Method) 77 Figure 2-39 Catchment Area and Land use Classification 80 Figure 2-40 Discharge Values for logPearson III Distribution and 90% Prediction Interval 81 Figure 2-41 Discharge Values for CN-Value Variation 82 Figure 2-42 Projections for Precipitation Sum (top left), Heavy Rain Events (top right) and Precipitation Seasonality (bottom left). 83 Figure 2-43 Return Period for Maximum Annual Precipitation, Log Pearson III distribution and 90% confidence and prediction intervals (reduced time series 1971-2000) 84 Figure 2-44 Typical Cross-Section (scale exaggerated) 87 Figure 2-45 Long Profile - Existing Situation Q2 (P=0.2) = 15.1 m³/s 89 Figure 2-46 Long Profile with Indication of left and right embankment for hydraulic model with detention ponds; Q2 = 15.1 m³/s 91 Figure 2-47 Long Profile with Indication of left and right embankment for hydraulic model with check dams; Q2 = 15.1 m³/s 93 Figure 2-48 stage discharge relation (average values) 94 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 4 Figure 2-49 flow velocity vs. discharge 94 Figure 2-50 Bed Shear Stress vs. Q 97 Figure 2-51 Map of the main watercourses, wetlands and sub basins in Nacala. 99 Figure 2-52 The main wetland systems of Mozambique 100 Figure 2-53 Vegetable gardens in a wetland to the east of the City 101 Figure 3-1 Overview of the site locations 105 Figure 3-2 City district east of airport 106 Figure 3-3 Location of Site 2 (Google Earth) 107 Figure 3-4 Shore line with mangroves in front of bank wall at Site 2 107 Figure 3-5 Cut mangroves west of the container terminal 108 Figure 3-6 Water outlets without flap gates at Site 2 108 Figure 3-7 Erosion-induced settlements behind bank wall at the SE end of Site 2 109 Figure 3-8 Erosion protection measures at the south-eastern end of the bank wall 109 Figure 3-9 Location of Site 3 (Google Earth) 110 Figure 3-10 Disconnected or blocked pipe in urban area 110 Figure 3-11 Location of Site 4 (Google Earth) 111 Figure 3-12 High ground water level in urban areas 111 Figure 3-13 Map of drainage channels and outlets in the city centre 112 Figure 3-14 Drainage inlets and manholes in the city centre 112 Figure 3-15 Uncleaned channels 113 Figure 3-16 Uncleaned and unprotected inlet 113 Figure 3-17 Lack of maintenance of drainage channels 114 Figure 3-18 Drainage channel inlet 114 Figure 3-19 Location of Site 5 – Inhangome (Google Earth) 115 Figure 3-20 Wetlands and shore line without mangroves at Site 5 115 Figure 3-21 Wetlands southwest of the airport at Site 5 116 Figure 3-22 Wetlands at Inhangome 116 Figure 3-23 Condition of the temporary bridge crossing the tributary at Inhangome 117 Figure 3-24 Wooden bridge crossing the tributary at Inhangome / Insufficient erosion protection with thin concrete layer 117 Figure 3-25 Cut mangroves at Inhangome 117 Figure 3-26 Wetlands at Inhangome 118 Figure 3-27 Re-growing trees at Incidua. Trees are establishing well 118 Figure 3-28 Location of Site 6 (Google Earth) 119 Figure 3-29 Drainage channel downstream of water outlet at Site 6 (SE of the airport) 119 Figure 3-30 Erosion-prone embankment downstream of water outlet at Site 6 (southwest of the airport) 120 Figure 3-31 Flooded settlement during high tide and elevated pathways to avoid flooding at Site 6 120 Figure 3-32 Flooded settlement during high tide at Site 6 120 Figure 3-33 Location of Site 7 (Google Earth) 121 Figure 3-34 Commercial area on elevated ground and flooded wetlands / settlements during spring tide at Site 7 121 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 5 Figure 3-35 Drainage channel / tidal stream along the settlement boundary at Site 7 122 Figure 3-36 Flooded plots and settlements during spring tide at Site 7 122 Figure 3-37 (Informal) harbor at tidal stream at Site 7 and 122 Figure 3-38 Playground upstream of drainage channel at Site 7 122 Figure 3-39 Location of Site 8 (Google Earth) 123 Figure 3-40 Wetlands without mangroves at Site 8 123 Figure 3-41 Emergency bridge with ongoing erosion in the west of Site 8 124 Figure 3-42 Existing bearings of emergency bridge (with risk of scouring) 124 Figure 3-43 Insufficient erosion protection measures with geotextiles at Site 8 125 Figure 3-44 Collapsed bridge in the east of at Site 8 125 Figure 3-45 Aerial views of collapsed bridge at Site 8 125 Figure 3-46 Location of Site 9 (Google Earth) 126 Figure 3-47 Settlement at the edge of the wetlands at Site 9 126 Figure 3-48 Emergency bridge with ongoing erosion at Site 9 127 Figure 3-49 Location of Site 10 (Google Earth) 128 Figure 3-50 Existing groins (constructed by USAID) at Site 10 128 Figure 3-51 Aerial picture of existing groins (constructed by USAID) at Site 10 129 Figure 3-52 Erosion upstream and downstream of existing groins at Site 10 129 Figure 3-53 Location of Site 11 (Google Earth) 130 Figure 3-54 Wetlands at Site 11 northeast of Quelimane 130 Figure 3-55 Location of Site 12 (Google Earth) 131 Figure 3-56 Informal settlements in wetlands at Site 12 131 Figure 3-57 Wetlands / riverbed at Site 12 132 Figure 3-58 Wetlands and drainage channel at Site 12 132 Figure 3-59 Extract of nautical charts for Quelimane 146 Figure 3-60 Bathymetric Measurements 2011 [INAHINA, 2011] 147 Figure 3-61 Bathymetric Measurements 2011 [INAHINA, 2011] 147 Figure 3-62 Predicted tide curves for Quelimane for August 2018 [Wxtide, 2018] 148 Figure 3-63 Tide curves for Quelimane for the year 2018 [Wxtide, 2018] 148 Figure 3-64 Tide tables for Quelimane [INAHINA, 2018] 149 Figure 3-65 Tide values for Quelimane [INAHINA, 2018] 149 Figure 3-66 Radar gauge (defective) and water level gauge at pier of fishing port 150 Figure 3-67 Tidal levels for Quelimane according to sea chart 16402 150 Figure 3-68 Existing benchmark at collapsed bridge 150 Figure 3-69 Normal spring tide situation in Quelimane (13th of August 2018) 151 Figure 3-70 Flooded areas / Delineation map (Storm Desmond, 01/2019) [https://emergency.copernicus.eu] 152 Figure 3-71 Flooded areas / Delineation map (Storm IDAI, 03/2019) [https://emergency.copernicus.eu] 152 Figure 3-72 Vulnerability (A), Exposure (B), Sensitivity (C) and Adaption Capacity (D) for Natural Disaster at Quelimane Municipality with indexes ranging from very low (green) to very high (red) [USAID, 2015] 153 Figure 3-73 Quelimane, Community Mapping Results for Questions 4, 5 155 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 6 Figure 3-74 Industrial development (within the red outline) in a wetland. 157 Figure 3-75 Proposed flood and erosion measures at Site 1 159 Figure 3-76 Proposed flood and erosion measures at Site 2 160 Figure 3-77 Proposed flood and erosion measures at Site 3 and 4 160 Figure 3-78 Proposed flood and erosion measures at Sites 5 and 6 161 Figure 3-79 Proposed flood and erosion measures at Site 7 162 Figure 3-80 Proposed flood and erosion measures at Site 8 163 Figure 3-81 Proposed flood and erosion measures at Site 9 164 Figure 3-82 Proposed flood and erosion measures at Site 10 164 Figure 3-83 Proposed flood and erosion measures at Site 12 165 Figure 3-84 Potential Inundation Areas for Selected Sea States (mCD) 177 Figure 3-85 Drainage Channels in the City 177 Figure 3-86 Monthly Rainfall in Quelimane (Source: Instituto Nacional de Estatísta) 179 Figure 3-87 Neap and Spring Tidal Curves 181 Figure 3-88 Global Mean Sea Level Rise Scenarios relative to 1990 Levels (Source: World Bank, 2010) 182 Figure 3-89 Monthly Wind Data for Quelimane (Source: www.meteoblue.com) 184 Figure 3-90 Wind Rose for Quelimane (Source: www.meteoblue.com) 184 Figure 3-91: Map of the main tidal rivers, wetlands and mangroves in and around Quelimane 192 Figure 3-92 The main wetland systems of Mozambique (source: Chabwela, 1991 and Saket, 1994) 194 Figure 4-1 Beach cleaning by ‘Cooperativa Repensar’ 202 Figure 4-2 Community Recycling workshop, Beira 202 Figure 4-3 Community awareness session, Beira 203 Figure 4-4 Chiveve colouring book 203 Figure 4-5 Mangrove reforestation through the NGO ADEL in Beira 204 Figure 4-6 Community works for Goto drainage ditches in Beira 205 Figure 4-7 Technical staff and farmers coordinating works in Somaliland 206 TABLES Table 2-1 Received data and information 26 Table 2-2 List of Selected Parameters, Type of Parameter, Score Classification and Data Source 40 Table 2-3 Summarized Input Data for All Catchments Areas Nacala 43 Table 2-4 Risk Assessment – Score Table and Results Discussion of Risk Assessment Results and Recommendations of Priority Areas 43 Table 2-5 Summary of Proposed Main and Accompanying Measures per Catchment 47 Table 2-6 Vetiver Grass 48 Table 2-7 Elephant Grass 49 Table 2-8 Moringa Oleifera / Drumstick Tree 50 Table 2-9 Alternative Measures for Storm Water detention and drainage measures 57 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 7 Table 2-10 Feasible Erosion Protection Measures - Embankment 59 Table 2-11 Additional Measures for Erosion Prevention (gullies and drains) 61 Table 2-12 Design Precipitation Values and Return Periods of Extreme Events from Statistical Analysis (Log Pearson III) – Lumbo 72 Table 2-13 90% Prediction Intervals for Daily Precipitation Summary Lumbo 72 Table 2-14 Design Precipitation Values and Return Periods for Extreme Events from Statistical Analysis (Log Pearson III) – Regional Stations 74 Table 2-15 IDF-Curve Values for Hydrological Zone D 76 Table 2-16 Comparison Cumulative Precipitation from IDF-Curves and Values Derived from Statistical Analysis 78 Table 2-17 Utilized Design Values for Peak Precipitation 78 Table 2-18 Land use Classification of a 200m x 200m sample area and related CN values 79 Table 2-19 Calculated discharges for the catchment area considering return periods of 2, 5, 10, 20, 50 years (standard values) 81 Table 2-20 Precipitation Values and Return Periods from Statistical Analysis of Extreme Events for Climate Fact Sheet Reference Time Period (Log Pearson III) – Lumbo 83 Table 2-21 Projected Precipitation Values and Return based on Climate Fact Sheet 84 Table 2-22 Project Design Values for Precipitation Sum and Upper Bounds of Projections from Climate Fact Sheet 85 Table 2-23 Discharges in m³/s for Hydraulic Model 87 Table 2-24 Results Hydraulic Model - Existing Situation 88 Table 2-25 Results Hydraulic Model – Detention Ponds 90 Table 2-26 Results Hydraulic Model – Final Situation / Check Dams 91 Table 2-27 Bed Shear Stress Analysis – 5yr Return Period 96 Table 2-28 Evaluation Criteria for Environmental Consequences of Measures 102 Table 2-29 Ecological Evaluation of Proposed Measures 103 Table 3-1 Overview of representative sites and their categorization 133 Table 3-2 Summary table of biophysical characteristics recorded at the field sites in Quelimane 135 Table 3-3 Received data and information 145 Table 3-4 Estimated population per site (based on surface area and counted no. of houses) 158 Table 3-5 Description of measures possibly applied in Quelimane 166 Table 3-6 Overview of problems / risks observed in Quelimane 173 Table 3.7 IDF-Curve Values for Quelimane 175 Table 3.8 Critical Precipitation Intensity 176 Table 3.9 Critical Sea Level per Site (indicative only) 178 Table 3.10 Monthly Rainfall in Quelimane (mm) (Source: Instituto Nacional de Estatísta) 179 Table 3.11 Monthly Wind Data for Quelimane (Source: www.meteoblue.com) 183 Table 3.12 Distribution of Wind Speed and Direction for Quelimane (Source: www.meteoblue.com) 183 Table 3.13 Estimated Yearly Occurrences for Storm Surge levels 185 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 8 Table 3.14 Overview of Occurrence of Flooding Events due to High Sea Level 185 Table 3.15 Summary of Probabilities per Year of Risk (Rainfall and High Sea Level) Events 186 Table 3.16 Summary of Estimated Potential Socio-Economic Impacts 189 Table 3.17 Risk Estimates for Existing Conditions 190 Table 3.18 Risk Estimates after Implementation of Proposed Measures 191 Table 3-19 Species list for planting in Quelimane 195 Table 3-20 Vetiver grass 197 Table 3-21 Elephant grass 198 Table 3-22 Evaluation Criteria for Environmental Consequences of Measures 200 Table 3-23 Ecological evaluation of the proposed measures for Quelimane. 201 Table 5-1 Cost estimation for project measures in Quelimane 208 Table 7-1 Summary table of characteristics and challenges recorded at the field sites in Nacala. 214 Table 8-1 Collected Data – Community Mapping Campaign Quelimane and Nacala 239 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 9 ABREVIATIONS CAD Computer Aided Design CES CES Consulting Engineers Salzgitter GmbH EACC Economics of Adaptation to Climate Change ESIA Environmental and Social Impact Assessment EU European Union GI Green Infrastructure HEC US Army Corps of Engineers – Hydraulic Engineering Center HMS Hydrological Modelling System MICOA Ministério da Coordenação Ambiental NAPA National Adaptation Program of Action SRTM Shuttle Radar Topo Mission ToR Terms of Reference Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 10 1 INTRODUCTION OBJECTIVES OF THE CONSULTANCY The objective of the Consultancy Services is to provide technical and analytical support to contribute in the upscaling of nature-based solutions for urban flood risk management, particularly in coastal cities. The activity will also leverage the lessons learnt in Mozambique to support the future application of similar solutions in the wider Africa region. Mozambique is one of the countries most exposed to coastal and river flooding in Africa. The World Bank has been active in providing emergency recovery after flooding in Mozambique and is increas- ingly supporting the Government in preventive disaster risk management operations on the city and regional levels (e.g. the environment resilience program). While traditional infrastructure-based inter- ventions still make up the majority of global financing to improve disaster risk management, the ap- plication of nature-based solutions is gaining momentum. One of the first nature-based urban flood management projects supported by the World Bank is in the coastal city of Beira in Mozambique. To maximize results from such projects, it is important to clarify the benefits for urban flood risk management and how such approaches can be best adjusted and scaled up to other cities in Mozambique and other countries in Africa. More generally, nature-based urban flood risk management projects struggle to provide a structured and comprehensive assess- ment of grey and green infrastructure solutions and produce and communicate evidence on the cost effectiveness of such solutions compared to other priorities. The Program for Forests (PROFOR) aims to enhance and upscale the green infrastructure pilot for urban flood risk management to other cities in Mozambique by building on lessons learnt in Beira and using guidelines produced under PROFOR’s ongoing “Harnessing Forests for Nature-Based Solu- tions to Disaster Risk Management” funded under the DRM and Forestry Global Knowledge Manage- ment program. The two pilot cities that were selected for this assignment are Quelimane in Zambezi Province and Nacala in Nampula Province. Wrong practices caused by the ignorance of the importance of symbiosis between mangroves and man led to horrendous degradation of the environment in Quelimane. The uncontrolled cutting down of mangrove trees for fuelwood, timber and coffins led to extensive, irre- versible salinization of coastal waters and soils are disrupt from natural protection against flooding. The Mayor of the northern Mozambican port of Nacala has warned that erosion in the city is becoming alarming and may even threaten the future of Nacala as a deep-water port. There is a danger that one day these deep waters may cease to exist because of the sediments accumulating in the access channel and in the port itself. Specific objectives of the consultancy services are: (1) to identify the lessons learnt of the green infrastructure pilot project in Beira, as well as legal, regulatory and institutional constraints and opportunities to integrate nature-based risk management solutions in the cities of Mozambique; and (2) to identify different options for nature-based and hybrid solutions to manage urban flood risks in two pilot cities and assess their effectiveness, costs and benefits. Based on the Terms of Reference (ToR), the Consultancy comprises five main tasks: — Task 1: Identification and analysis of Lessons Learnt from Beira — Task 2: Assessment of the Enabling Environment — Task 3: Assessment of urban flood and erosion risks and nature-based or hybrid solutions Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 11 — Task 4: Cost-Benefit Analysis — Task 5: Knowledge sharing and dissemination The following activities have been covered within this Task 3 for the elaboration of the “Quelimane and Nacala Report”: — Field visits to Quelimane and Nacala, data collection, interviews with stakeholders — Assessment of the city risk profile for both cities under different climate change scenarios, return periods, different urban expansion scenarios, incl. different land-use typologies — Identification and analysis of possible innovative nature-based and/or hybrid flood & erosion pro- tection solutions for Quelimane and Nacala — Conduction of a Workshop in Quelimane and Nacala to discuss and select potential solutions — Ecosystem Mapping and assessment of proposed solutions — Flood modelling and elaboration of flood and erosion risk maps for different climate change and urban growth scenarios and potential solutions NATURE-BASED SOLUTIONS The majority of flood protection investments, not only in Mozambique, are still made in the rehabilita- tion and construction of grey infrastructure, such as drainage canals, retention basins, protection walls and their appurtenant infrastructures. While there are several reasons to consider for and against grey infrastructure, incl. degree of urbanization, existing infrastructure, local capacities (construction and operation), etc., nature-based solutions are becoming a preferred option by international financing institutions, national agencies as well as local stakeholders. Especially when looking at small-scale interventions, nature-based solutions can be a more cost-effective option and may also be imple- mented and operated/ maintained by local agents, including communities and NGOs (e.g. afforesta- tion measures). While the concept of nature-based solutions is firmly rooted in the climate change context, it is currently understood to be able to address a range of policy objectives, ranging from climate change to disaster risk management, stimulating green economies, and addressing poverty and disease (Pauleit et al., 2017). The concept of “nature” is also wide-ranging, including the stock of all-natural capital. Furthermore, the concept aims to foster an integrative and action-oriented ap- proach. Nevertheless, it needs to be pointed out that nature-based and hybrid flood and erosion protection measures may also be very complex in their planning, especially when looking at their impacts. Eco- systems and their service provision are a condition for the interventions’ success, meaning that many aspects of their functioning need to be considered. Often, a network of ecosystems can be found, which are linked to each other, so that influencing one will also affect the others. Accordingly, the publication ‘Implementing nature-based flood protection’ (World Bank, 2017) concludes that there is no ‘one-size fits all’ solution. Based on a specific hazard and risk assessment, a variety of natural as well as social aspects need to be assessed for a well-designed project. In context of urban areas most times pure nature-based solutions can not applied and a mix between green and grey infrastructure will be required. In between these two approaches hybrid solutions might be a preferred choice due to the special conditions in urban areas. Different green, grey and hybrid solutions are shown within the following figure. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 12 Figure 1-1 Green, Grey and Hybrid Solutions (source: www.hollandwaterchallenge.com) BACKGROUND The cities of Quelimane and Nacala are both located along the Mozambican coast at riverine estuar- ies, with a similar size of population of 350.000 (2017) and 250.000 respectively. However, when looking at climate risks, particularities in terms of exposure, hazards and vulnerabilities apply for each city. Figure 1-2 Overview Map Mozambique / Project Locations Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 13 The city of Nacala is located on a coastal inlet on the Mozambican coast. The city boasts a deep- water harbour and a busy port, and a population 250 000. The city centre, in the Bairro of Cidade Baixa, is located on the steep slopes that characterise the west-facing parts of the city. The city’s economy is based primarily on the activities around the railroads and the port, and the trades and services associated with these. The city has seen a significant increase in built structures since Inde- pendence, as a result of immigration of rural Mozambicans into the city. Houses have been built to high density in parts of the city, even in areas where construction is prohibited. Quelimane is the largest city and capital of Zambezia Province and situated north of the extensive Zam- bezi delta. It is located at the bank of Rio Dos Bons Sinais, also known as Rio Quelimane or Rio Quá- qua, approx. 25 km inland from the Indian Ocean and Mozambique Channel, respectively. The area is rich in floodplain wetlands, wetland depressions and mangroves. The vegetation type is that character- istic of formations on alluvium. Soils are sandy in the drier parts of the City, tending towards organic silt in the wetter areas and wetlands. Due to its geographic position, the city of Quelimane is located close to the medium sea level, so inundations after intense rainfall and storm surges together with coastal erosion and saltwater intrusion are major challenges for the city’s resilience. Parts of the City are inun- dated during spring tide each day. Rio Dos Bons Sinais is strongly influenced by tidal periods and forms the single access to Quelimanes Port. The population currently amounts to approx. 350,000 (Census 2017) with a tendency to further increase in the future due to population growth and increased influx into urban areas. Informal settlements, which still develop in flood-prone parts of the municipality are particularly exposed and show a high vulnera- bility due to their socio-economic conditions. Ongoing mangrove deforestation and degradation in and around Quelimane increase the exposure and result in exacerbating storm impacts with flooding and erosion. Subsistence economy prevails in the entire region with sources of income being artisanal fishery, agri- culture as well as informal businesses and trade. Except for the fishing port / container terminal / landing facilities, noteworthy industry or formal business are largely absent. Climate Nacala is located in Nampula Province, which has one of the highest average seasonal rainfall in the country (> 1500mm per year), in addition to a high number and frequency of heavy rainfall days (World Food Programme, 2018). However, the Nacala area is described as semi-arid sub-tropical. Rainfall data for nearby Nampula (which is wetter than Nacala) for the period 1969 – 2016 gives an average of 1100 mm / year and a standard deviation of 200 mm. Average annual rainfall for Nacala is considerably lower at around 780 mm / year, but similarly variable. The wettest months are December to February and the driest August – September. Cyclones and tropical storms regularly make landfall along the Mozambican coast every few years. High intensity or prolonged precipitation events have been recorded in nearby Nampula in 1981/1982, 1984 (Tropical Storm Domoina), 1990/1991, 1994 (Cyclone Nadia – 127 mm of rain fell in Nampula in one day in March 1994), 1996 (Cyclone Bonita), 2003 (Tropical Storm Delfina - in Nampula the rainfall over January 2003 exceeded 800 mm), 2008 (Cyclone Jokwe), 2012 (Cyclone Funso), 2015 (Cyclone Chedza – 134 mm of rain fell in Nampula in one day in January 2015). These cyclones and storms bring rainfall that falls at high intensity and volume over a very short period. Average temperatures range between 24 and 27°C, with January being the hottest month, and July the coolest (Tecnica, 1994). Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 14 Figure 1-3 Annual rainfall (mm) for Nampula from 1969 – 2016 The City of Quelimane is characterized by a tropical savannah climate with wet and dry season with the wet season lasting from November until April. Average temperature is 25.3°C and an average of 1,346 mm precipitation per annum has been recorded (source: en.climate-data.org). The warmest month of the year is January, with an average temperature of 28.1 °C, while in July the average tem- perature slightly decreases to 21.0 °C, being the lowest average temperature of the whole year. Figure 1-4 Climate Graph for Quelimane (from https://en.climate-data.org, March 2019) The driest month is September, with 16 mm of rainfall. Most precipitation falls in January, with an aver- age of 251 mm. The difference in precipitation between the driest month and the wettest month is 235 mm. The average temperatures vary during the year by 7.1 °C. Vegetation and fauna Broadly speaking, the vegetation in and around Nacala is dry savannah. This vegetation type is char- acterised by low grassland and few trees. The South African National Biodiversity Institute mapped woody plant species zones across East Africa in 2000, and mapped three different species zones in the Nacala area (see Map) – Guibourtia schliebenii (a flowering tree common to Mozambique), Adan- sonia sp. (baobab), and a third zone characterised by an unknown species. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 15 Figure 1-5 Lone baobab tree in a residential area of Nacala There is little indigenous vegetation within the urbanised area, with more natural areas of vegetation persisting near the coast, and to the east of the city. Isolated individual mangrove trees (Avicennia marina) are scattered along the coastline south and north of the port, but more healthy mangrove forests are located in the south of the coastal inlet and along the coast to the east of the city (map of mangroves). A few baobab trees are scattered throughout the urban area (Figure 1-5), with more trees occurring to the south of the city. Several species of exotic trees were noted during the field visit. Many of these are planted for food, privacy, wind and erosion protection, marking the boundaries of properties, and for aesthetics. Examples include the invasive mesquite tree (Prosopis spp. (possibly P. glandulosa) – from Central and South America), vetiver grass (Chrysopogon zizanioides – from India), moringa (Moringa oleifera – from India), mango (Mangifera indica – from India), cashew (Anacardium occi- dentale - from Brazil, and a number of shrubs and grasses. Elephant grass (Pennisetum purpureum), which is indigenous to Africa, also occurs throughout the city. While most of these exotic species are useful species, the mesquite is problematic. This species transpires significant quantities of water, often tapping into precious groundwater resources. It also spreads very easily, and has been known to invade river channels as a result of seeds dispersing downstream. It also appears that the longer flowering period of the species can lead to a longer viability of the mosquito adults that feed on the sugar-rich flowers (Muller et al., 2017). This may enhance the capacity of the Anopheles mosquito to host and transmit the malaria parasite. Figure 1-6 Mesquite (Prosopis sp.) Figure 1-7 Vetiver grass (Chrysopogon growing in one of the erosion gullies in zizanioides). Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 16 Nacala. Figure 1-8 Elepchant grass (Pennisetum Figure 1-9 Moringa trees (Moringa oli- purpureum). feira) Moringa trees planted along the boundary of a property. The leaves are eaten like spinach. The tree has many other medicinal properties. The vegetation type in the Quelimane area is that characteristic of formations on alluvium. Much of the vegetation currently observed in and around the City comprises exotic species, such as the mango (Mangifera indica – from India) and cashew (Anacardium occidentale - from Brazil), and the naturalised coconut palm (Cocos nucifera). The floodplain wetlands around the City are dominated by mangroves (predominantly the white mangrove, Avicennia marina, but also the black mangrove, Lumnitzera race- mosa, and the stilt mangrove, Rhizophera mucronata), where these have not been cut down for use as building materials or for charcoal manufacture. The freshwater wetlands further inland are inhabited by a range of grasses and sedges, with some floating macrophytes (including the exotic invasive species, water lettuce (Pistia stratiotes). Figure 1-10 Grass and sedges typical of the freshwater wetlands around the City of Quelimane The mangroves are associated with a number of faunal species, but the dominants are the mudskippers (Periophthalmus kalolo), mud crabs (Scylla serrata), and mangrove whelks (Terebralia pallustris). The wetlands support a variety of bird species – during the field visit the following were observed: Grey and Striated Herons (Ardea cinerea and Butorides striata), little egrets (Egretta garzetta) and the Black Kite (Milvus migrans). The Painted Reed Frog (Hyperolius marmoratus) was recorded in the freshwater Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 17 around Micajune. This record is the eastern-most location of this species recorded thus far. The man- groves and freshwater wetlands appear to be in good condition around the City, despite the pressures of population expansion and water quality deterioration. Soils, geology and topography The soils in Nacala are predominantly sandy soils on sandstone. The upper sandy layer of soil ranges from 0 – 8 m thick, and is extremely vulnerable to erosion due to the lack of cohesion between the very fine particles of sediment (Tecnica, 1994). This is exacerbated by the lack of vegetation cover, which would stabilise the sandy soils to some extent. The underlying sandstone is exposed in places, such as in the erosion gullies (Figure 1-11) and on the cliffs close to the sea (Figure 1-12). Figure 1-11 Sandstone in the foreground Figure 1-12 Sandstone cliffs near the sea, exposed within an erosion gully where the short west-facing watercourses meet the coastline Nacala has its city centre located on elevated ground, only with a narrow strip of the port exposed to the sea. The urban area has many steep slopes that run towards the port area. Intense rains, partic- ularly those deriving from tropical cyclones cause excessive runoff. Such excessive storm water in combination with fragile soils (without vegetation or supporting structures) lead to significant erosion problems, threatening both public and private assets. Major flooding and erosion risks in Nacala are thus mainly associated to its steep terrain and storm water runoff. As it can be derived from the geological map (see Figure 1-13), the prevailing soils in the coastal area of Quelimane are quaternary sedimentary deposits as: • alluvium, sand, silt, gravel • colluvium • coastal sand dune and beach sand • alluvial mud of fluvial-marine origin The origin of the alluvial materials are: • gneissic rocks and granitites of the Precambrian formation found further inland • sandy deposits from coastal dunes Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 18 • organic debris Key: Qa- Alluvium, sand, silt, gravel. Qc- Colluvium. Qd- Coastal sand dune and beach sand. Qst- Alluvial mud of fluvial-marine origin. Figure 1-13 Geological Map of Mozambique [Grantham et al., 2008] Due to climatic conditions and the geographic location, the soils within the area of Quilemane are wet throughout the year. Sandy soils tend to be well drained with low capacity of retaining water and nutri- tions. The clayey and silty soils and the mud, however, are poorly drained and poorly consolidated. Furthermore, all present superficial soils are prone to erosion. One of the main problems for the local agriculture is the salinisation of soils caused by the deforestation of mangrove belts along the shoreline. According to the Preliminary City Risk Profiles from the CityCORE Africa Project provided with the ToR, the following priorities were identified for Nacala, amongst others: • Reshape Primary Channels to a stable cross-section (space permitting) and protected with heavy armouring (gabion baskets are the preferred method) • Construct service roads to run parallel to the channels • Protect channels with drought-resistant vegetation above the 1:10 year flood elevation (Veti- ver grass recommended) • Storm sewer construction According to the Preliminary City Risk Profiles from the CityCORE Africa Project provided with the ToR and after evaluation of the visited sites, following strategic objectives were identified for Quelimane: • Expand and improve the city's drainage system • Improve the city's water capture and supply system • Introduce Coastal Protection Measures taking into account sea level rise, marine intrusion and storms Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 19 • Introduce erosion protection measures Figure 1-14 Erosion due to storm water runoff in Nacala City (www.voaportugues.com) Historical Situation and Specific Issues in Various Areas of Nacala As described in the previous chapters the situation in Nacala (Nacala-o-Porto) and its history is quite different in various locations of the City. Within this section site specific changes shall be addressed in more detail. Looking at the peninsula of Nacala (which includes the relatively young Nacala-o-Porto) and the sur- roundings on a larger scale the natural historical and thus pre-urbanized situation can be described as follows: — Area-wide vegetation cover — No erosion gullies — Minor erosion takes place as a natural process The development of Nacala-o-Porto as a relatively young and fast-growing city had huge implications on the natural landscape resulting in various changes: — Removal of natural vegetation cover due to densely populated areas / settlements — Increased runoff in case of heavy rainfall due to higher share of sealed surfaces and open land — No major drainage system which is capable of diverting runoff existent — Surroundings of settlements have been utilized for agricultural purposes causing higher runoff rates (after harvest the land remains uncovered) As a result of the urbanization the landscape changed drastically while general frame conditions for possible flood and erosion risks remained unchanged (such as slope, rainfall, in-situ soil). This com- bination of urbanization without construction of proper drainage systems leads to higher runoffs in general and a creation of erosion gullies. Due to the steep character of the landscape the higher runoff does not result in major flooding as most of the water is drained towards the sea quickly. Some minor areas are prone to flooding. Still, most of these areas are wetlands and thus have a natural tendency to retain water as more detailed described in chapter 2.5.1. Site Specific Issues in Nacala Within the following section different specific sites in Nacala are described with a focus on the current drainage situation which is – mainly – affected by the most recent urbanisation (past 50 years) com- pared to a natural, all vegetation covered state. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 20 SITUATION IN NORTHERN PART OF NACALA The northern part of Nacala is less densely populated. Still, most of the area is used for agricultural purposes which caused most of its natural vegetation cover being removed. During the period of planting and growing crops the area is covered with vegetation until (rainy season). After harvesting the crops, the land remains open and vulnerable to erosion before new planting of crops takes place. In addition, most of the cultivated land (mainly cassava and corn) have a higher runoff coefficient than the natural vegetation cover which leads to a higher runoff in general (as can be seen in the following figure). Figure 1-15 Cassava field in northern part of Nacala A second major impact on the disposal of rainwater in the northern part of Nacala is due to the exten- sion of Nacala airport which was completed in 2014. In general, the sealed surface has been in- creased. The rainwater at the airport is collected at a central retention pond which seems to be un- dersized based on a brief visual assessment. All water is drained towards the east without major drainage infrastructure downstream of the retention pond. This causes erosion on its way to the sea. Figure 1-16 Gully caused by redirected rainwater from the airport Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 21 Due to the collection and redirection of rainwater lots of material is disposed at the shore line. Within recent years an area of about 2 ha was disposed as can be seen in the following figure: Figure 1-17 deposited material at the sea in 2019 SITUATION IN DENSELY POPULATED AREAS IN CENTRAL PARTS OF THE CITY The situation within the central parts can be characterized by a generally high building density result- ing in a high share of sealed surface. In general, with every new building constructed the runoff in case of rainfall will increase. Rainwater collection is rarely used, most of the rainwater falling on sealed surfaces will drain with almost no retention. Situation is most critical in informal settlements which are located directly next to the gullies. Existing erosion gullies tend to grow with further urbanization and re-densification of central parts. Occasionally houses collapse into the gully as can be seen in the picture below. Although resettlement is not a preferred option, a buffer zone restricting construction of buildings of up to 25m along the gullies could help to protect the population and their properties. There is no general town development plan and no general drainage master plan for Nacala. Discus- sions with the municipality and community members in Nacala showed that those plans will be re- quired to address the problems surrounding drainage and town development. The ideas and methods proposed within this project can be a part of the solution. Still, management of the situation and as- sertiveness of protection zones adjacent to the gullies from the side of the municipality can be de- scribed as little. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 22 Figure 1-18 collapsed house adjacent to a gully SITUATION TOWARDS THE EASTERN EDGE OF TOWN The catchments draining towards the eastern parts are less steep compared to the catchments drain- ing to the west. In addition, building and population density is far less and some of the natural vege- tation cover is still in place. As a result, no major erosion took place in these parts. With further ur- banization towards the east indication of the same issues of the central and northern parts of Nacala are shown already. In some areas smaller erosion gullies have formed already, mainly in the more densely populated areas within the upper parts of the catchments. Still, due to the vegetation cover closer to the shore these gullies disappear and no disposed material at the shoreline can be found. CONCLUSION REGARDING HISTORICAL AND CURRENT SITUATION: In general, due to the removal of the natural vegetation cover without construction of adequate drain- age systems erosion gullies appear in several locations of the city as a direct result of the progressive urbanisation in Nacala and its surroundings. Due to the steep character flooding is not a major issue. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 23 2 NACALA REPORT FIELD VISITS, DATA COLLECTION AND ASSESSMENT OF CURRENT SITUATION Desktop Review and Preparatory Works After the consulting team has familiarized itself with the data, the field visit took place (see chapter 2.1.1). Based on the compiled data, a desktop mapping of the urban catchments within the project area was elaborated, showing the location and rough extent (not ground-truthed) of wetlands and rivers. Besides the existing vector-data, the mapping included the analysis of the satellite imagery and remote-sensed data grids (such as SRTM data) to identify potential additional features. Further- more, urban expansion was reviewed so that future developments in the area are also included in the measures (see chapter 2.2.2). All data was compiled in GIS system and verified on the ground during field surveys, with participation of stakeholder institutions’ technicians. The community mapping cam- paign was planned before the IC’s first field visit to Nacala and Quelimane and conducted during this visit. Results are summarized in chapter 2.1.4. Field Visit Nacala The field visit to Nacala was conducted from the 17th – 24th August 2018. Nine areas were visited, and these are described and illustrated in Annex I. Based on the consultant’s first assessment of the situation in Nacala, the IC held a workshop with stakeholders on August 20, 2018 to discuss the collected information and identify and select a range of potential options. Observed problems: — Insufficient capacity in the retention ponds that — Location of fuel tanks and truck parking area in were built for the airport. the watercourse means that drainage will al- — Sand blocking drainage channels. ways be a problem. — Removal of vegetation, leading to instability of — Erosion gullies start between the houses. Water soils and erosion of gullies across landscape. flows along the streets, and gathers momentum — Trees are being removed in the area, making and sediment, causing massive erosion gullies. soils unstable. — People place obstructions in older gullies, which — Roads without any stormwater, drainage sys- causes the water to merely find another route. tems turn into watercourses during heavy rains, — Walls, houses, parts of school property, and in- as water just flows along them. frastructure (FIPAG pipeline) have washed — Houses very close to steep banks of gullies; away in places. some have collapsed into the gully. — Eroding roads threaten infrastructure such as — Children play on the steep banks of the gully. pipeline carrying water from FIPAG boreholes. — Poor stormwater management in the upper — Short, steep erosion gullies causing damage to parts of the catchment leads to water flowing buildings and infrastructure. along pathways and tracks, causing erosion. — Road crossing over Watercourse#1 becomes very dangerous during high discharge. Four people drowned at this crossing in 2017/2018. — Sand deposited in the port area. — Litter from upstream is deposited on the beach. — Silting up of stormwater channels and pipes. — Truck parking area next to fuel depot was built on top of the drainage line, diverting water to- wards the fuel tanks. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 24 For a complete documentation of the field visit including indication of locations and photos please refer to Annex I. Deeply incised erosion gullies going down to- Wide erosion gully in Mocone. This gully ends wards the coast. up at the hotel development site. Wall washed away by an erosion gully, Watercourse #1 where it flows along the road, Watercourse #1. causing erosion around FIPAG pipeline. Figure 2-1 Selected photos of the field visit in Nacala Data Collection Summary of Information and Requirements compiled Based on the meetings and contacts hold with the client, the Storm Water Drainage team, the Munic- ipality and other stakeholders, the consultant could compile the following data and information: Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 25 Table 2-1 Received data and information Information Source Comments Basemap / Vector OpenStreetMap (OSM) data Vector data DIVA-GIS Vector data WFP Mapping Elevation Grids SRTM, ASTER Orthophotos ESRI Imagery, Google Imagery Hydrological Data INAM Precipitation, Temperature R5 Hazard and Expo- World Bank sure Data Bathymetry GEBCO Landcover CCI Climate Data WorldClim Gridded Soil, Land- Africa Soil Information Service / Derived from remote sensing data cover, Vegetation Africagrids Data Based on this information, the consultant defined the key requirements for the study and development of feasible options in terms of physical and recreational infrastructure within the project site and the overall management of those structures. A look at the World Bank’s R5 exposure data for Mozambique shows that the overall population ex- posure shows an increase within the timeframe between 2010 and 2050 as shown in the following figures. Looking at the population growth rates for 2010 and 2050 the average value of 0.54 for all of Mozam- bique (this indicates almost doubled population from 2010 to 2050). For Nacala-o-Porto a value slightly above average of 0.56 can be determined which implies population growth of ~79% within the indicated time frame. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 26 Figure 2-2 World Bank's R5 Population Count Data for 2010 and 2050 (source: The World Bank) Figure 2-3 Population Growth 2010 to 2050 (1 = No Growth; 0.5 = Doubled Population) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 27 Additional Data Collection The already collected data was further supplemented with data from various sources as outlined be- low. GEOGRAPHICAL INFORMATION Nautical charts for the project area and as-built drawings of existing coastal protection structures in Nacala and Quelimane were reviewed, as available. USAGE OF UAV The use of an Unmanned Aerial Vehicle (UAV) collected high-resolution aerial imagery of hotspots. Depending on the identified hotspots and proposed measures, the UAV was used to collect imagery for limited areas to supplement the dataset. LOCAL INFORMATION Through interviews and workshops in Quelimane and Nacala, the IC gathered information from the communities on existing erosion, drainage and flooding problems to consider in the assessments. SOIL CONDITIONS To assess the soil conditions in the project area, the IC acquired geological maps of the area, as well as existing geotechnical reports/assessments. Furthermore, existing data from sediment samples along the shoreline of the cities could be used for the assessments. METEOROLOGICAL / HYDROLOGICAL / TIDAL DATA Hydrological data from nearby gauging stations was collected. This also includes gauging data from ports and rivers and tidal calendars, if available. LITERATURE REVIEW In addition, a literature research was conducted to identify and evaluate the recent internationally published literature related to “Building with Nature” or “Engineering with Nature”. Known references are amongst others: — Engineering with Nature (USACE, USA): https://ewn.el.erdc.dren.mil/ — Working with Nature (PIANC): http://www.pianc.org/workingwithnature.php — Building with Nature (Ecoshape, NL): https://www.ecoshape.org/en/ — Nature based Solutions (IUCN & EU): https://www.iucn.org/commissions/commission-ecosystem- management/our-work/nature-based-solutions — IPCC, 2012: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. A Special Report of Working Groups I and II of the Intergovernmental Panel on Cli- mate Change. Cambridge University Press, Cambridge, UK, and New York, NY, USA, 582 pp., 2012 — Renaud, Fabrice G. et al., 2016: Ecosystem-Based Disaster Risk Reduction and Adaptation in Practice, Springer, Switzerland — The World Bank, 2017: Implementing nature-based flood protection, Principles and implementa- tion guidance — The World Bank, 2010: Economics of Adaptation to Climate Change, Mozambique, EACC Publi- cations and Reports, World Bank, Washington, 2010 However, internationally recognized standards and guidelines for the design, evaluation, and con- struction of nature-based structures are not yet available. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 28 FIELD DATA VERIFICATION Based on the outcome of the mapping activities, the IC's team identified flooding/erosion zones on the ground and possibly collect additionally required data, such as dimensions of culvert and bridge structures or conduct basic soil classifications in erosion gullies. Community Based Mapping Campaign Methodology The community mapping aimed to gather information from residents in the city regarding the investi- gated problems. On the one hand it provides valuable insights regarding the issues in the city, on the other hand, some public awareness for the problems as well as the project is raised. The mapping was based on mobile data collectors (Android smartphones) which ran the data collec- tion interface “Kobo Collect”. The application is part of the “Kobo Toolbox”, a free and open source data collection framework. A customized questionnaire was pre-loaded in the application (see 6.2). The information has been collected georeferenced and includes photographic documentation. Figure 2-4 Kobo Field Data Collection App (Sample Screens) The different locations have been visited during a preparatory field visit in August 2018. For data collection, the consultant’s team identified stakeholders in the city. In Nacala, the mapping was con- ducted with technicians from the city. At the different locations, the community leaders and/or resi- dents were also briefly interviewed to get additional insight at the sites. To ensure proper usage of the application, a member from the consultant’s team accompanied the technicians during the field works. During the campaign, information was collected at 74 different points. The mapping was monitored by the consultant’s experts remotely. The outputs were exported from the Kobo Framework and analysed in Excel and QGIS. Questionnaire / Survey Form A simple questionnaire has been developed in Portuguese to collect locations of known issues as well as photos and some first information to screen and classify the collected points. The questionnaire (English translation) is shown in the figure below. The first question aims to achieve a first classifica- tion of the recorded issue in the different categories relevant for the project. By selecting the type of endangered infrastructures, it can be possible to further classify the points. The third section (location and photo) collects the most valuable information for the project. By recording the coordinates and taking photos of the site, the result can be mapped and also further assessed remotely by the con- sulting team. Furthermore, this section collects mostly objective information, whereas the results from Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 29 the other sections rely on the judgement of the person collecting the data and/or bystanders. Espe- cially section 4 (issue rating) is very subjective. Section 5 aims to get an approximate idea whether the issue is long-standing or is a result of more recent developments. Before closing the survey form, the data collector can also enter some additional comments regarding specific issues at the site. Figure 2-5 Questionnaire for Community Mapping (codes for results table in brackets) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 30 Results Summary The main goal of the community mapping campaign was to identify sites and gather first information – especially photos and locations. Furthermore, the free-text comments (Section 6) were important for a first assessment. The gathered information was used to identify sites to visit during the consult- ant’s team field visit. Questions 4 and 5 were collecting mostly subjective information. Most issues recorded were considered to have a high severity of impact and also a high importance of mitigation. The mapping also shows that the answers regarding the affected population and timing of the issue are rather inconsistent. Therefore, the information gathered in these steps is considered to have a low significance and has been verified on site during the field visit. Figure 2-6 Nacala, Community Mapping Results for Questions 4, 5 In conclusion, the Community Based Mapping Campaign provided the following inputs: — Community Mapping Campaign helped to identify sites and gather lots of information — The results of the campaign gave a positive input for the further advancement of the project — Preparation of the questionnaire and selection of applicable answers have a high influence on the significance of the results and must be prepared carefully A complete documentation of the results of the Community Mapping Campaign is given in Annex II - Community Based Mapping Campaign – Collected Data. Summary and Assessment of Current Situation – Nacala The management and maintenance of the stormwater drainage system within Nacala City pose some real challenges. The City has seen rapid urbanisation and densification of houses since the 1980s (see chapter 2.2.2). There has been repeated encroachment of legal and illegal housing into high risk (in terms of erosion and flooding) areas within the City, which has been followed, in some areas, by Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 31 the removal of families to safer areas, and then the consequent return of families back to the same risky areas. Industrial complexes have also been constructed across drainage lines, leading to the diversion of surface flow into channels and pipelines that cannot always accommodate the volumes of water received. This does not allow for the systematic and effective design of stormwater management systems within the City, which would be able to cope with the volume and velocity of runoff experienced during the high rainfall months. Erosion is the product of rainfall, soil, slope, cover and land-use and cultivation practices. Nacala receives high rainfall volumes falling in intense rainfall events over short periods. The rainfall lands on soils that are mostly sandy, on steep slopes with little vegetative cover. The proliferation of buildings, industries and roads in the City has led to hardening of the catchments, through soil sealing and this has the direct result of changing the hydrology (i.e. the volume, velocity and frequency) of surface flow throughout the City. There are several parts of the City where the slopes are steep, which further increases the velocity of surface runoff. The removal of indigenous and even exotic tree species throughout the City for use as building materials and for charcoal pro- duction, causes instability of the soils by removing stabilising root matter from the soil, and changing the manner in which water falls on the soil – rain falling on bare soil has a higher erosive capacity than the same rain falling on leaves before it hits the soil. The rain falls with greater intensity on bare soil, washing the finer soil particles away and leading to greater soil instability. Unstable soils have a greater potential for erosion, and sedimentation along the coastline where these soils are deposited. Trees, shrubs and grasses also get rid of water through transpiration, thereby reducing the total vol- ume of water exiting the catchment. Furthermore, formalised stormwater infrastructure in the City appears to be blocked or damaged in several areas. Sand, rubble and litter blocks drainage channels and pipelines, and accumulates in detention and retention basins. These blockages and damaged infrastructure (for instance, the theft of stones and wire from gabions) lead to the switching of flow pathways, from the desired route to alternative routes in residential or industrial areas. Scouring and incision of these alternative routes then follows, and an erosion gully is formed, and more sediment deposited downstream. The steep slopes and unstable soils merely exacerbate the problem. In some cases, the banks of these erosion gullies are dangerously unstable, leading to loss of lives where banks have subsided. In other cases, roads and houses are washed away and parts of the City flooded and/or isolated. In summary, the stormwater infrastructure is not keeping pace with the rapid urbanisation and densi- fication of houses in Nacala City and does not cater for the predicted increases in the frequency and intensity of storms due to climate change across the African continent. Based on the findings of the field visit in August 2018 the main priorities for Nacala are – amongst others: — Reshaping Primary Channels to a stable cross-section (space permitting) and slope protection — Construct service roads to run parallel to the channels — Reduce erosion risks wherever possible — Protect channels with drought-resistant vegetation above the 1:10 year flood elevation (e.g. Vetiver grass or Elephant grass) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 32 Figure 2-7 Erosion due to storm water runoff in Nacala City (August 2018) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 33 RISK ASSESSMENT – CITY RISK PROFILE NACALA General Considerations regarding Risks in Nacala In general, a risk is defined as the likelihood of an event of a certain severity combined with the estimated damage. To determine this risk, the risk elements in the project area must be identified and defined. Risk elements are regularly or temporarily occurring hazards, together with the exposure and vulnerability of “real world objects” such as infrastructure, cultural assets, conservation areas and population. Damage is generally defined as the opposite of a benefit, i.e. the deterioration of a risk element. Damages may be expressible financially (e.g. property damage) or not financially quantifia- ble (e.g. damage to conservation areas). Main risks elements for Nacala have been previously identified as follows: a) Flood Risks b) Erosion Risks The situation regarding flood risks in Nacala is quite different from the one in Quelimane. The flooding hazard is mainly existent as an urban flood risk in case of heavy rainfall. The terrain profile of Nacala is quite steep and there is no general risk of flooding of entire districts in Nacala. The risk for flooding has been determined and assessed within previous steps and the field visits. As a result of this as- sessment flood risks and possible damages due to flooding of large areas can be neglected for Nacala. Although heavy rainfalls do not lead to large-area flooding, rainfall induces very concentrated runoff forming multiple gullies which dispose the rainfall towards the sea. Due to the high flow velocity and high bed shear stress the gullies tend to deepen every year. This leads to collapsing side slopes and the disposal of this material which results in growing – relating to their width – gullies. Therefore, the main risk in Nacala is further erosion along existing gullies or forming of new gullies if runoff paths are changed or disturbed. Further erosion along the gullies is endangering people, houses and infrastructure located adjacent to these existing gullies. As an outcome of the collected and assessed data (as described in the previous chapter) the focus for the municipality of Nacala is on erosion mitigation strategies along the existing drainage canals. The risk assessment and the risk profile for Nacala is covering the erosion risks induced by heavy rainfall. Although the severity of the rainfall events is varying, erosion will continue unless measures are taken to prevent it. Based on the collected rainfall data it can be summarized that the risk of erosion is permanent in Nacala as with every above average rainfall event – although in its extent most time minor – erosion happens along the existing gullies Urban erosion risks and possible areas at risk can be localized based on the mapping of the existing gullies as presented in the figure below. Prior to the erosion risk assessment within this chapter different urban growth and climate change scenarios are investigated and described in more detailed. Based on the outcome of these scenarios various parameters have been selected and an erosion risk profile for the city of Nacala was elabo- rated indicating areas with major risks. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 34 Figure 2-8 Location of Erosion Gullies (Red) and Deposits in the Sea (Yellow) Urban Expansion Scenario Regarding the assessment of potential erosion risks in Nacala it needs to be analysed if there is urban expansion in Nacala and if, what might be the potential impacts regarding erosion if the city is growing. For this assessment urban expansion is defined as an increased area with high density of buildings and population as these areas tend to show the highest risk for creation of new erosion gullies. Generally, it can be expected that in case of urbanisation of an area the vegetation cover will be reduced, and the percentage of sealed surface will increase. Both effects lead to higher runoff rates in case of rainfall which leads to potential of erosion and creation of gullies. Hence, assessing the erosion risks in Nacala is also dependent on further urbanization and the implication in what areas urbanization might happen. Within the map presented below the city limits of Nacala for the years 2006 to 2018 are presented in time steps of 4 years. The aim of the map is to identify if the city is growing in its urbanized area in general and if so, where are the areas that have been urbanized in the past 12 years. In addition, it needs to be clarified, in what direction future urbanization might head. The shown city limits do not present administrative boundaries or the boundary of generally populated areas in Nacala. Due to the loosely scattered buildings and small settlements within the boundaries of Nacala it would be difficult to assess urbanization if all populated areas would have been included. A different method was chosen to assess potential urbanization in Nacala. The presented lines indi- cate the border between high density areas (urbanized areas) and areas with low density / loose settlements. This way for classification urban expansion has been chosen as densely populated areas tend to show much more characteristics leading to erosion in case of heavy rainfall which met the Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 35 requirements for the assessment best. The data base for the large-scale assessment have been different satellite images (BING, GE) which have been captured during the dry seasons in the years 2006, 2010, 2014 and 2018. The assessment of urbanized areas was generated by identification of high-density building areas within QGIS. Results are the four coloured boundary lines for the four selected time steps. Figure 2-9 Urban Expansion Nacala – 2006 to 2018 Results of the urban expansion assessment for the years 2006 - 2018 are as follows: — There is clear evidence of urbanization in Nacala in the considered time — Main direction indicating urban expansion in 2006 to 2018 was the eastern part of Nacala Based on this assessment the following can be concluded: — Further urban expansion needs to be considered — Due to availability of free land the main direction of further urbanization will be towards east — Although currently not as much affected by erosion as the areas draining towards the west the two catchment areas draining to the east should be included in the risk assessment — Preventive erosion protection measures should be foreseen in areas where further urban expan- sion is expected (details see chapter 2.3.5) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 36 To face the drainage related problems of further urbanization a general town development plan and a drainage master plan should help to identify at what locations nature-based solutions can be se- lected. There is no general town development plan and no general drainage master plan for Nacala according to information from the municipality. Discussions with the municipality and community mem- bers in Nacala showed that those plans will be required to address the problems surrounding drainage and town development. The ideas and methods proposed within this project can be a part of this solution. Still, keeping in mind that nature-based solutions are only one piece of the puzzle additional projects and studies are required to solve the drainage problems in Nacala. Risk Assessment – Parameters and Results As a principle for the assessment of the risks in Nacala it needs to be defined at what scale/unit the assessment needs to be conducted. Possible units for the assessment can be: — The whole city of Nacala — Every gully separately — Per catchment area A general assessment for the entire city wouldn’t help to identify locations that need to be prioritized within future projects. An assessment per gully would help to specify micro-scale conditions and po- tential risks. Still, the assessment would be divided into lots of small sections without clear identifica- tion at what size a gully needs to be considered. Some parts of Nacala might be excluded for the assessment. An assessment per catchment area was chosen to be most suitable due to the following reasons: — All prioritized areas in Nacala will be assessed without any gaps — The characteristics of a catchment usually apply for all gullies within that area and erosion risks will be similar within this catchment — Erosion is caused by heavy rainfall which usually will be disposed within the existing catchment. Assessment per catchment allows to correct problems within the area where they are created. The risk assessment will be based on various parameters chosen for the catchments in Nacala. Within this chapter the parameters to be considered for the risk assessment will be explained in more detail. General aim is to select relevant parameters that have a high impact regarding existing and future erosion risks. As described in previous chapters the main risk identified during the field visits and desktop studies is progressive erosion along the existing gullies in case of heavy rainfall events. Ero- sion endangers adjacent buildings, infrastructure and population. As there is no existing drainage system which covers all parts of Nacala the run-off rainwater accu- mulates at the lowest elevation and runs on a direct path towards the sea. Low runoff coefficients (roads, roofs, unvegetated land) tend to increase concentrated runoff resulting in higher flow velocities and bed shear stresses which lead to higher erosion rates along the existing gullies. For the risk assessment all selected parameters will help to describe erosion relevant specifics of the prioritized catchment areas. Finally, the collected and calculated results for all 13 prioritized catchment areas in Nacala will be summarised and every catchment gets a separate score per parameter. The indicated score will be either: — “1” – low risk / outcome for the parameter reduces erosion risk — “2” – medium risk — “3” – high risk / outcome for the parameter increases erosion risk An example for this classification method can be the average longitudinal slope within an existing gully. Generally, the slope can be either steep or shallow. Flow velocities tend to be higher if the slope Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 37 is steep which leads to increased bed shear stress and to increased erosion within a gully. In this case, a steep slope increases the risk of erosion and a score of “3” would be justified. Vice versa, a shallow gully slope leads to lower erosion risk and a score of “1”. The details and classification for all selected parameters will be described in more detail below. In principal the selected parameters can be divided into parameters that are related to potential dam- ages that occur in case of further erosion (e.g. total number of buildings adjacent to existing gullies) or rather general catchment specific parameters that amplify or weaken erosion in general (e.g. lon- gitudinal slope). Within this risk assessment both types of parameters will be considered in conjunc- tion in order to identify maximum risk locations which should be prioritized. Selected parameters in relation to potential damages in case of further erosion are as follows: — Number of Affected Houses (25m Buffer Zone along gullies) — Number of Affected Main Infrastructure — Formal/Informal Residential Areas and Population Density — Commercial/Industrial Areas Parameters that describe catchment area specifics in relation to general erosion tendency within a catchment are as follows: — Longitudinal Slope — Vegetation Cover — Sealed Surface (due to its high runoff coefficient) — Expected Urban Expansion Description of selected Parameters Within the following section all considered parameters are described in more detail and an indication is given why the parameter is relevant in terms of the erosion risk assessment for Nacala. In addition, a summarizing list with indication of the parameter type, the chosen score classification and the data basis is given. 1) Population Density A high number of people living in a catchment area indicates a higher potential for damages within this specific area due to a higher building density and possibly higher number of affected residents in case of heavy rainfall. In addition, it reduces the overall possibilities for large-area measures to reduce runoff and erosion risks. 2) Share of Sealed Surface A high share of sealed surfaces within a catchment area increases the runoff coefficient for the area significantly which leads to higher runoff in case of heavy rainfall which results in increased flow ve- locities and higher erosion rates within the gullies. As an example, sealed surfaces can be roofing or roads. 3) Vegetation Cover Generally, vegetated areas usually have a lower runoff coefficient which implies less runoff etc. Indi- cation of a high vegetation cover within the separate catchments is helpful to reduce runoff which leads to reduced erosion risks. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 38 4) Slope The longitudinal slope of an erosion gully has a major impact on the overall flow velocity and subse- quently bed shear stresses at the gully river bed. In contrast to the vegetation cover described previ- ously there is no possibility to change the longitudinal slope within a gully on a large scale. Still, for identification of erosion risk the slope needs to be regarded. 5) Share of Informal Residential Areas In comparison to formal residential areas, which are usually in accordance to general city master plans, informal residential areas induce problems related to road planning, drainage and general in- stitutionalization. Houses have been built in areas where construction is prohibited as described in the field visit chapter. Evidence of informal residential areas increase potential risk as described in (e.g. some buildings constructed adjacent to gullies have collapsed already). 6) Commercial Areas Commercial areas have been included as a parameter for the risk assessment as the potential dam- age in case of further erosion can lead to reduced economic output and reduced attractiveness for investors. 7) Industrial Areas In a similar way to the previously mentioned commercial areas industrial areas have a high potential for damages if further erosion and the management of heavy rainfall events will not be addressed and is therefore included in the assessment. 8) Affected Population For this parameter a small-scale analysis of the number of inhabitants living directly adjacent to ex- isting gullies has been identified. The affected population in case of heavy rainfall was located by counting all residential buildings a 25m buffer zone adjacent to all identified existing gullies. 9) Affected Major Infrastructure Like the affected population the affected major infrastructure was included as well due to a high po- tential for damages to main infrastructure (main roads) in case of further erosion and a high overall significance for the city’s operational capability. 10) Urban Expansion Impact As described in chapter 2.2.2 different urban expansion scenarios have been discussed. Due to fur- ther urbanization of currently uninhabited land or low-density areas it is expected that areas impacted by urbanisation will result in a higher runoff coefficient (less vegetation, higher percentage of sealed surface) leading to higher erosion risks. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 39 Table 2-2 List of Selected Parameters, Type of Parameter, Score Classification and Data Source Type of Score Classifica- Score Classifica- # Parameter Data Source Parameter tion* tion Boundaries Catchment Area Boundaries: 1 - Low Population Watershed Generated in GIS Potential 1: < 20 Pers. / ha Population Density based on SRTM Data 1 for Dam- 2: 20 – 60 Pers./ ha Density 3 - High Population Population Figures: Based on age 3: > 60 Pers. / ha Density latest census and population forecast for 2019 OSM Data and Data collected 1 - Low % of Sealed Erosion 1: < 7% Sealed during Field Visit (drones), Sat- Sealed Sur- Surface 2 Rate Re- 2: 7-15% Sealed ellite Images. Data Compiled face 3 - High % of Sealed lated 3: > 15% Sealed and generated within GIS and Surface classification. 1 - High Vegetation 1: > 30% Vegetation Erosion Based on most recent Google Vegetation Cover Rate Cover (VC) 3 Rate Re- Earth Satellite Images, Gener- Cover 3 - Low Vegetation 2: 20-30% VC lated ated within GIS Cover Rate 3: <20% VC Erosion 1: < 5% 1 - Shallow SRTM Data + Field Visits, Gen- 4 Slope Rate Re- 2: 5-10% 3 - Steep erated within GIS lated 3: >10% 1 - No Informal Res. 1: No Informal Resi- Areas Informal Potential dential Areas (IRA) 2 - Sign of Informal Based on Field Visit Results 5 Residential for Dam- 2: Minor Evidence of Res. Areas. and Satellite Images Areas age IRA 3 - Informal Resi- 3: IRA Identified dential Areas 1 - No Commercial 1: No Commercial Potential Commercial Areas Activities (CA) Based on Field Visit, OSM Data 6 for Dam- Areas 3 - High Share of 2: Minor CA and Satellite Images age Comm. Areas 3: Major CA 1 - No Industrial Ar- 1: No Industrial Acti- Potential Industrial Ar- eas vities (IA) Based on Field Visit, OSM Data 7 for Dam- eas 3 - High Share of 2: Minor IA and Satellite Images age Ind. Areas 3: Major IA 1 - No/Minor Af- Potential 1: < 10 People Based on OSM Data, Drone Affected fected Pop. 8 for Dam- 2: 10 – 200 People Photos (Field Visit) and Satel- Population 3 - High No. of Af- age 3: > 200 People lite Images fected Pop. 1 - No Affected In- Affected In- Potential 1: < 1,000m frastructure 9 frastructure for Dam- 2: 1-5,000m OSM Data 3 - Infrastructure (Roads) age 3: >5,000m heavily affected 1 - Decreasing Ur- 1: Positive (Decreas- banisation Urban Ex- Erosion ing Urbanization) Based on Field Visit, OSM 2 - No Major Change 10 pansion Im- Rate Re- 2: Unchanged Data, Satellite Images and Dis- expected pact lated 3: Negative (Increas- cussion with Client 3 - Further Urbani- ing U.) sation expected *) Generally, a score of "2" can be defined as a medium risk/potential in between "1" and "3" Risk Assessment – Results per Catchment Area The catchments to be assessed regarding their erosion risks have been selected based on the results of the field visit and the collected data. The selected catchments cover major parts of Nacala and should be prioritized if implementation of the proposed measures will be realized. The subsequent risk assessment will help to identify areas with the highest need for action and should be included in any following projects with top priority. The numbering of the 13 catchment areas can be found in the figure below. Catchments with ID 01 to Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 40 11 are draining towards the west. Catchments ID 12 and 13 are draining towards the east. As some parts of these two catchments are outside the urban area, they show clearly different characteristics which leads to a slightly different assessment within the following tables: — Major parts of the two catchments 12 and 13 show high vegetation cover which results in little or no erosion gullies, especially in the lower elevated part close to the sea — The higher elevated parts of these two catchments show a higher grade of urbanisation with the same problems as the 11 west-draining catchments (ID 01 – 11), e.g. high runoff coefficients and creation of erosion gullies — As the most realistic urban expansion scenario indicates further urbanisation towards the east these two catchments need to be included in the assessment — Still, their current risk regarding erosion is not as critical as in the catchments draining towards the west. Figure 2-10 Identification of 13 prioritized catchment areas in Nacala For better understanding of the location of the 13 selected catchments in Nacala the following map shows the catchments with an additional indication of the approximate borders of the most relevant districts (barrios). Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 41 Figure 2-11 Overview map with indication of catchment areas and urban districts Nacala The summarized input data for the 13 catchment areas in Nacala is presented in the following table. Based on this data and the score classification boundaries presented in table 2-2 the overall risk assessment for these catchments is determined Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 42 Table 2-3 Summarized Input Data for All Catchments Areas Nacala # Catchment Nr. Unit 1 2 3 4 5 6 7 8 9 10 11 12* 13* Population Population/ 1 11 5 1 4 48 33 100 15 103 63 39 8 7 Density ha Percentage of 2 % 6 4 2 7 9 11 17 23 18 16 11 n/a n/a sealed surface % of Area 3 Covered with % 13.1 36.1 34.7 24.5 17.3 22.9 30.2 25.6 16.6 13.2 13.5 n/a n/a Vegetation 4a Slope Category - shallow - steep steep medium medium medium - shallow medium medium shallow shallow Avg. Longitudinal 4b Slope of Existing % 4.3 0.0 10.7 10.2 6.6 7.0 5.8 0.0 2.8 6.1 5.2 2.7 3.2 Gullies Indication of 5 Informal - No No No No Minor Minor Yes No Yes Yes Yes Yes Yes Residential Areas Indication of Major 6 Commercial - Minor No No No Minor Minor Minor Minor Yes Minor No Minor Minor Activities Indication of Major 7 Industrial - Minor No No No No Minor No Yes Yes Yes Yes No Minor Activities Affected 8 - 0 0 0 30 143 15 1,088 0 2,063 606 10 0 192 Population Affected Main Infrastructure 9 m 2,059 0 0 0 1,117 0 1,311 0 8,741 1,218 2,301 8,359 8,793 (Main Roads within Catchment) Impact of urban Un- Un- Un- Un- Un- Un- 10 expansion Negative Negative Negative Negative Negative changed changed changed changed changed canged Negative Negative scenario Within the following table all data presented above is merged into one table and the total score is calculated (score for all 10 parameters added up for each catchment area). A maximum total score of 30 points represents the highest possible erosion risk within a catchment. Vice-versa, a minimal pos- sible score of 10 points indicates lowest possible risks regarding erosion. The results of the assess- ment are discussed in the following chapter. Table 2-4 Risk Assessment – Score Table and Results Discussion of Risk Assessment Results and Recommendations of Priority Areas Catchment Nr. 1 2 3 4 5 6 7 8 9 10 11 12* 13* Risk Assessment - Score per Category 1 - Population Density 1 1 1 1 2 2 3 1 3 3 2 1 1 2 - Sealed Surface 1 1 1 2 2 2 3 3 3 3 2 1 1 3 - Vegetation Cover 3 1 1 2 3 2 1 2 3 3 3 1 1 4 - Slope 1 1 3 3 2 2 2 1 1 2 2 1 1 5 - Informal Residential Areas (IRA) 1 1 1 1 2 2 3 1 3 3 3 3 3 6 - Commercial Areas 2 1 1 1 2 2 2 2 3 2 1 2 2 7 - Industrial Areas 2 1 1 1 1 2 1 3 3 3 3 1 2 8 - Affected Population 1 1 1 2 2 2 3 1 3 3 1 1 2 9 - Affected Infrastructure (Roads) 2 1 1 1 2 1 2 1 3 2 2 3 3 10 - Urban Expansion Impact 3 3 3 3 3 2 2 2 2 2 2 3 3 Summary Total Risk Score 17 12 14 17 21 19 22 17 27 26 21 17 19 Risk Classification LOW LOW LOW LOW MEDIUM MEDIUM MEDIUM LOW HIGH HIGH MEDIUM LOW MEDIUM As previously specified the results of the risk assessment shall help to identify areas in Nacala which have a high potential for further erosion of existing gullies or risk for creation of new gullies. Hence, the results help to identify areas which should be prioritized if implementation is foreseen. The risk assessment does not include potential risks which are caused by artificially redirection of runoff to adjacent catchment areas (which applies for example for catchment 08 and catchment 09). Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 43 Looking at the below map with the total risk score above the following can be concluded: — The highest risk regarding further erosion (with a total risk score of 27/26 points) is given in the densely populated areas in the inner city of Nacala (catchments ID 9 and 10). These areas should be prioritized for implementation of proposed measures in any case. — Low risk areas (with a total risk score below 18 points) are the less densely populated areas to the north and north east. — Catchment ID 8 itself has a low total risk score and no immediate measures are required in this area. Still, due to its adjacent location to the high-risk zone ID 9 and the potential redirection of runoff from ID 9 to ID 8 there is a risk that damages might occur in this area (which is not reflected in the way the risk assessment was conducted). Improving the situation in ID 9 or avoiding artifi- cial redirection of runoff from catchment 09 will help to prevent flood-related damages in ID 8. — The two eastern catchments (ID 12 and 13) show low or medium risk regarding erosion. Still, due to the expected urbanization towards this direction measures to avoid initialization of gullies and preventive erosion protection should be considered to avoid the rather difficult situations as in ID 9 or 10 Figure 2-12 Risk Assessment – Total Risk Score Map for Nacala Although erosion gullies exist in most of the investigated catchment areas the risk assessment iden- tified areas which are much more vulnerable to further erosion. Within the following chapter 2.3 dif- ferent measures are proposed to address the erosion problems in Nacala with nature-based solutions. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 44 IDENTIFICATION OF PROJECT MEASURES - NACALA As discussed in the previous chapter drainage management and erosion protection is the main iden- tified risk for Nacala. The proposed measures to be presented in this chapter are selected for the different conditions in the catchment areas. The proposed measures can be divided into different categories in principle: — Less densely populated areas and if unused land available (mainly northern catchments): Large- scale runoff reduction using revegetation measures and soil bunds — Stabilization of existing V-shaped gullies: filter unit rock bags (mainly northern catchments) — Small-scale combined measures in densely populated areas (inner city) — Preventive measures in areas prone to further urbanisation (eastern catchments) Principle recommendation to minimize erosion risks in Nacala is reduce the runoff in case of heavy rainfall events as much as possible. The proposed measures aim to reduce erosion and allow dam- age-free disposal of the rainwater towards the sea. The situation in catchment areas that indicate lots of bare soils due to removal of the natural vegetation cover and which are rather loosely populated is improved if the vegetation cover will be increased and soil bunds are constructed. These measures are described in more detail in chapter 2.3.2. Densely populated areas require a combination of sev- eral measures to improve the situation. These combined measures are described in chapter 2.3.4. If there is still open land available and further major urbanisation can be expected the construction of recreational areas / parks which serve as (meso scale) retention basins in case of heavy rainfall can be foreseen. This measure mainly applies for the two big eastern catchment areas (12 and 13) and are described in chapter 2.3.5. The proposed measures can be combined if found suitable. Still, all measures will be presented sep- arately in more detail within the following chapters. Besides these proposed measures a set of alter- native and accompanying measures will be included to allow additional methods for nature-based flood and erosion protection in Nacala. Based on the characteristics of the different catchment areas the following main measures are pro- posed, visualised in Figure 2-13. The indication of the selected main measures for a specific catchment does not imply that additional measures can’t be added to improve the situation in this catchment. E.g. the gullies in catchment ID 13 can be protected with measures described in the chapter “combined measures” besides the main proposed activity – creation of recreational areas that serve as meso or large-scale retention ponds. General objective for Nacala should be a sustainable urban drainage system (see section 2.3.1) which can be supported by NBS. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 45 Figure 2-13 Proposed Main Measures per Catchment Legend for proposed main measures: a) Green Catchments ID 1 -5: Large Scale / Area Wide Runoff Reduction (Revegetation, Soil Bunds), Protection of V-shaped Gullies b) Blue Catchments ID 6 -11: Combined Measures Approach c) Yellow Catchments ID 12-13: Preventive Measures / Meso-Scale Retention Summary of proposed main and accompanying nature-based measures per catchment area are as follows: Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 46 Table 2-5 Summary of Proposed Main and Accompanying Measures per Catchment Accompanying Catchment ID Main Measure Remarks Measure 1 2 Revegetation and Rock Bags for V- Less densely popu- 3 Shaped Gullies lated areas in the north Soil Bunds 4 5 6 7 Combined Densely populated ar- 8 Measures Ap- - eas in the centre of 9 Nacala proach 10 11 12 Combined Measures Eastern catchments Preventive Approach, Rock Bags (prone to further 13 Measures (if applicable) urbanization) Sustainable Urban Drainage Systems (SUDS) The challenges faced by the City of Nacala revolve primarily around drainage (see above) and ero- sion, and the goal of proposed nature-based solutions (NbS) must be to alleviate flood and erosion risks. Specifically, the aim should be to attenuate peak flows, and to control runoff volumes. It is most likely, therefore, that the principles and concepts associated with Sustainable Urban Drainage Sys- tems (SUDS) are most suitable for the City. The principle of SUDS is to slow down and reduce the quantity of water flowing throughout the urban area. In addition, this allows for in situ partial treatment of water through filtering and uptake of pollu- tants (Rose and Lamond 2013; Woods Ballard et al. 2015; Zhou 2014). SUDS achieves these aims by utilising a mix of natural processes and conventional engineered (“grey” infrastructure) components in order to harvest, infiltrate, slow, store, convey and treat runoff onsite. SUDS can be implemented from the start of stormwater system design or can be integrated into ex- isting grey stormwater infrastructure (Davis and Naumann, 2017). If implemented properly (i.e. with the most efficient use of space and taking into account local environmental and socio-economic con- ditions), SUDS is generally a more cost-effective approach than conventional grey infrastructure. The most efficient approach is often a mix of SUDS and conventional approaches (Davis and Naumann, 2017). The proposed measures presented in the following chapters can be seen as different elements of an SUDS and will help to allow enhancement in Nacala with the help of nature-based solutions Large Scale Runoff Reduction Within the less densely populated areas (mainly within the northern catchments ID 1 – 5) large scale and area-wide measures can be foreseen. Main objective is to reduce the direct runoff towards the gullies to a minimum in order to reduce further erosion. Main measures can be revegetation for rein- statement of a more natural vegetation cover within the areas and construction of soil bunds. Both measures are described within the following chapters. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 47 Revegetation Revegetation is one of the preferable used measures for the less populated areas ID 1 – 5 in the northern part of Nacala. Besides utilizing revegetation as a large-scale, area-wide measure the method can be used as a general approach for the combined measures approach on a smaller scale which includes planting of different plants in order to stabilise the existing gullies, toe of slope protec- tion or to revegetate unused land for runoff reduction. It can be applied wherever there is unused or open land. Within the subsequent chapters different plant species which are suitable for the described measures. In addition to simple revegetation to reduce runoff the currently unused areas can be used for urban gardening purposes as well for the benefit of the population. VETIVER GRASS – CHRYSOPOGON ZIZANOIDES The following table provides a summary of the features and advantages of vetiver grass. Table 2-6 Vetiver Grass Feature Advantage Dense tufted clump grass Slows down runoff Increased density of leaves means higher transpiration Intercepts rainfall and reduces its erosive capacity Relatively deep roots Stabilise soils The roots improve the shear strength of soil by between 30 and 40% Allows infiltration of water to depth, and increased soil moisture locally encourages the growth of other plants Strong roots The roots break up compacted soils, allowing water to infiltrate along the roots Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 48 Feature Advantage Stiff overlapping leaf bases, Slows down runoff by increasing roughness (i.e. increasing fric- strong erect stems tion) Encourages deposition of sediment Can withstand high flow velocities and water depths Can retain water behind a line of vetiver (as long as height of water does not exceed ~30 cm behind the vetiver) Blocks the passage of soils and debris, ensuring that this is not washed away and gradually builds up a soil terrace If planted in a dense hedge, this functions as an effective sedi- ment fence Propagates vegetatively Easy to cultivate, grows easily in most soils (as long as there is little frost in winter) Grows easily and establishes well from runners Invasive potential is low New shoots grow up from the base, even if this is buried in sed- iment, so gradually builds up over time (in Fiji – built up 2m ver- tically, over 30 years on a 20% slope) Does not invade Will not outcompete local plants Ecological climax species, Unlikely to invade and outcompete local plants not a pioneer Tolerates high levels of pes- Will grow well even in stormwater drains ticides, herbicides, toxins and heavy metals (i.e. urban stormwater runoff) Drought and flood resistant Withstands fluctuations in conditions Withstands high flows Will grow in erosion gullies, stormwater drains etc Medicinal properties Reduces stomach parasites in livestock In principal, vetiver grass has the potential to be planted in all 13 selected and assessed catchments areas in all different locations as vetiver is undemanding in general and requires little to no mainte- nance. ELEPHANT GRASS – PENNISETUM PURPUREUM The following table provides a summary of the features and advantages of elephant grass. Table 2-7 Elephant Grass Feature Advantages Tall clumped perennial with Stems can be used for fencing, building materials and fodder tough stems (and for silage) Shallow spreading runners, Stabilise soils, but may invade into adjacent areas but also deep roots Easily cultivated from cuttings or runners Grows quickly Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 49 Feature Advantages Stems not very dense, so cannot be planted in a single line Fire tolerant Inhabits a wide ecological Easy to establish in different soil conditions throughout the catch- range, in different soils, alti- ment tudes etc Well adapted to drought con- Will survive dry periods ditions Medicinal qualities Plant extracts used as a diuretic in Africa (www.cabi.org) In comparison to vetiver grass elephant grass is a little bit more demanding. Therefore, it needs to be checked in more detail if elephant grass can be planted everywhere (as it is currently proposed for vetiver). Still, elephant grass has a low water and nutrient demand and therefore can make use of otherwise uncultivated land. MORINGA OLEIFERA / DRUMSTICK TREE The following table provides a summary of the features and advantages of Moringa oleifera. Table 2-8 Moringa Oleifera / Drumstick Tree Feature Advantages Water purifying qualities Seeds can be used to purify water - dry seed suspension is a natural coagulant, also reduces bacterial concentrations Can be used to reduce water hardness Reduces turbidity (Nile River) Medicinal qualities Antioxidant, antibiotic properties, antimicrobial Used to treat malnutrition in Senegal and Haiti (leaf powder) Nutritional qualities Good source of a number of minerals, vitamins, protein, calcium, iron, copper, sulphur Moringa trees themselves will not be favorable for reduction of large-scale and area-wide runoff. In combination with the previously described vetiver and elephant grass moringa tress will help to stabi- lize the soil and thus avoid erosion. In addition, the planting of moringa tress will reduce the probability of complete removal of the newly created vegetation cover (which can happen if grass would be planted solely) and thus will allow long-term improvement of the situation. Soil Bunds Main parts of the northern catchments are used for agricultural use. As lots of Nacala’s population depend on growing crops, revegetation of all areas will be unrealistic. In order to achieve runoff re- duction in these catchments soil bunds can be constructed as an alternative or accompanying meas- ure to the previously proposed revegetation. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 50 A soil bund is a structural measure with an embankment of soil, or soil and stones, constructed along the contour lines and stabilized with vegetative measures, such as grass and fodder trees. The height of the bunds depends on the slope of the landscape. Bunds reduce the velocity of runoff and soil erosion, retain water behind the bund and support water infiltration. It further helps in ground water recharging and increases soil moisture which can results in better yields as a benefit for local farmers. A simplified sketch is shown in the following figure to clarify the principal of soil bunds: Figure 2-14 soil bunds - schematic sketch (source: knowledge.unccd.int) Height of the soil bunds depend on the slope of the landscape, as a general recommendation a height of 0.5m to 1m seems to be suitable for the locations in Nacala. In addition, soil bunds can be – with guidance of trained staff – constructed and maintained by the local population which creates owner- ship and reduces costs for overall implementation of this nature-based solution. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 51 Figure 2-15 Soil bunds to reduce runoff and erosion risks (Source CES, Somaliland) Some locations in Nacala show open, unvegetated land which is covered with sand only. As the soil bunds presented above require either slightly cohesive material (with vegetation cover) or sufficient weight of the bunds (stones) pure sand as can be seen in some locations cannot be used for creation of soil bunds. As an alternative Ecologs as shown in the following table can be installed in these specific locations. Ecologs can be filled with the sand available at place. After first rainfalls eroded material will sediment at the Ecolog which allows vegetation to grow as can be seen in the following figure. Figure 2-16 Ecologs as an alternative to soil bunds for areas with sandy soils Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 52 Protection of V-shaped Gullies Most of the erosion gullies within the densely populated areas of Nacala are quite wide with total widths of more than 30 meters. In opposite to this, within the areas which are foreseen for large-scale and area-wide measures (northern catchment areas) most of the gullies can be described as v-shaped with small bottom widths of 1-3 meters only and steep side slopes with total heights of more than 8 meters in some locations as can be seen in the following figure. Due to possible collapse of these steep side slopes works at the bottom of these gullies are dangerous and cannot be conducted without additional protection measures. Therefore, an alternative for stabilisation of the bed of the gully will be required as construction of gabion walls (as proposed in the combined measures approach) will be not be possible in these steep gullies. Figure 2-17 V-shaped gully in northern part of Nacala, depth of more than 8 meters In order to avoid further erosion and as an accompanying measure to the large-scale and area-wide measures presented in the previous chapter (revegetation, soil bunds) rock bags can be deposited in the gullies for protection and stabilization of the gully bed. Figure 2-18 Rock Bag/Filter Units (source: Sumitomo) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 53 Due to the difficult access the consultant proposes the use filter units / rock bags which can be de- posited at the bottom of the gully by use of a small mobile crane or a dredger. The rock bags shall be installed to block the entire width of the gully bed at its location of installation and thus will serve as small check dams. Due to the small width of these steep gullies at its bottom it is expected that 2-3 rock bags at one location will be enough to serve as a check dam. Installation of several bundles of rock bags with 15-20meters distance in-between (depends on slope of the gully) will lead to significant reduction of the longitudinal gully slope and thus reduce bed shear stresses and erosion. Due to the flexible character of these bags they are perfectly fit to improve the situation in the catchment areas with steep gullies (catchment with ID 1-5 / northern parts and some sections of catchments 12+13 / eastern parts). For wider gullies – which are located in the more central catchment areas (e.g. catchment 9) – the use of rock bags is not proposed as different alternatives (gabion walls) to improve the situation will be more beneficial (for details see chapter 2.3.4.1). The rock bags proposed for the use in Nacala and as shown in the figure above are called filter units and are manufactured by Sumitomo. In general, the filter units have a simple design. The bags are constructed of recycled synthetic ma- terial (PET bottles) and filled with stones available at the location. Total size of the rock bag can be either 2,4 or 8 metric tons. After filling of the bag is completed the bag can be lifted by a mobile crane and deposited in the gully. More than one bag can be installed at one location if necessary. Some advantages of the rock bags are – amongst others – as follows: — Simple installation (single lifting point) — Can be filled with standard stones (50mm – 200mm) — Easy stone collection as community work — No failure due to single stone movement — Bag is made of recycled polyester — Long lifetime — Adaptable and flexible structure — Installation with minimal workforce in little time possible — (comparatively low investment costs) Figure 2-19 Installation of Rock Bags / Filter Units Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 54 Combined Measures For the more populated areas in the inner city of Nacala – catchment areas ID 6 to 11 – a set of different measures is proposed to address the rainwater drainage and erosion problems. The measures include: • Rehabilitation of natural drains / streams • Revegetation with appropriate indigenous plant species / variation in tree species on a smaller scale • Green embankments along the existing gullies • Detention ponds • Rehabilitation and improvement of existing drainage infrastructure Detention Ponds / Check Dams The main purpose of the detention ponds is to reduce the flow velocities within the existing gullies. As the erosion rate is directly connected to the flow velocity this will be the most beneficial measure. For clarity the following naming convention will be used within this report: Detention Ponds: Holds water for a short period of time. Small-scale structures to be used within the combined measures approach. It is expected that most of the detention ponds will be sedimented 1-2 years after construction and thus will serve as check dams further on. Retention Ponds: Can maintain a pool of water throughout the year and holds stormwater runoff following storms. Retention ponds as proposed in this report will be used on a large or meso-scale and will be described in more detail in chapter 2.3.5. Retention ponds are not foreseen to silt up which implies more maintenance in case of sediment input (needs to be removed occasionally). For the hydraulic assessment of the detention ponds as a favourable nature-based solution please refer to chapter 2.4. In this chapter the hydraulics of the main gully in catchment ID 9 is investigated in more detail with indication of critical flow velocities and bed shear stresses. In general, it was proved that the installation of several small ponds reduces the risk of further erosion significantly. For this specific gully, ponds will be required in a distance of 100 to 150 meters to avoid super-critical flow conditions. For other gullies this distance might be slightly lower. The design of the detention ponds based can be as follows: • Earthen dike for the entire width of a gully (up to 35 meters) • Crest height of up to 2 meters to give sufficient detention volume • Core of gabion walls to strengthen in case of extreme events / prevention of failure • Geotextile protection of the soil covering the gabions to avoid scouring • Planting of vegetation on top of the dike with vetiver or elephant grass for strengthening of the top soil layer Besides geotextiles and stones for the gabion walls the soil which are in place can be used for con- struction of the ponds. Borrow areas should be upstream from the dike to increase detention volume. Minimum maintenance requirements for this kind of ponds will be as follows: • Regular check if condition is ok • Remove waste regularly • Check stability after heavy rainfall events • Repair dikes in case of damages Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 55 Most of the erosion can be avoided or significantly reduced after construction of the detention ponds. Still, due to the general entry of new sediments from the catchment into the gully without removal of soil these detention ponds will silt up quickly and the remaining structure will serve as a check dam. As it is expected that detention ponds will turn into several check dams a cascade system will evolve. These cascades are still beneficiary as the longitudinal slope will remain lower and thus the overall erosion rate will remain lower compared to the existing situation. In addition, the time until the deten- tion ponds are silted up the embankments of the gully should be stabilized and vegetated completely which will reduce further growth of the gully in its width. As an example, a schematic cross-section for a typical detention pond is shown in the following figure. As described previously, it is expected that the retention volume will silt up after completion of the pond structure. Figure 2-20 Schematic Cross-Section Detention Pond For assessment of the hydraulic and static calculations of the gabion wall cross-section a preliminary calculation was conducted to determine if stability and hydraulic capability is suitable for the conditions in the gully using the cross section shown in the figure below. Calculations have been conducted using Maccaferri’s Macra2 for gabion weirs. The results showed that the chosen dimension and as- sumptions indicate sufficient safety against sliding, overturning and seepage. Still, as all detention ponds will have slightly different conditions a more detailed assessment will be required in case of implementation. Figure 2-21 Cross-section used for preliminary hydraulic and static assessment Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 56 Alternative measures to the above-mentioned detention pond design can be as follows: Table 2-9 Alternative Measures for Storm Water detention and drainage measures Measure Description Example Image Storm water Unused land can be desig- detention nated for detention ponds ponds forming a decentralized stormwater detention sys- tem. Ponds can feature added indigenous plant spe- cies for additional ecological value. Areas can also be set-up as mixed-use ponds, serving as recreational area during dry periods and detention or in- filtration basins during runoff periods. Gabion bank Especially in gullies with lim- protection ited space for ecological pro- tection, gabion walls are an efficient and flexible struc- ture for bank protection. Vegetation can be initiated as well. Embankment Erosion Protection Measures As an accompanying measure the embankments need to be protected against further erosion. As most of the gullies are not completely filled during rainfall events the main focus should be to protect the toe of the embankments as most of the erosion rates comes from collapsing side slopes into the gully because of previously eroded material at the toe of these slopes. There are several nature-based solutions to prevent erosion. Due to its characteristic’s vetiver grass seems to fit perfect for the requirements. Still, additional measures are presented in this chapter to show possible alternatives. In general, the protection of the slopes will be required for the entire length of the gullies to allow sufficient efficiency. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 57 Feasible technical solutions for erosion protection measures along the embankments of existing ero- sion gullies are provided in the overview below. Still, if the use of vetiver grass is found suitable it Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 58 would be the solution of choice due to its characteristics. Table 2-10 Feasible Erosion Protection Measures - Embankment Stabilisation of the chan- nel can occur through planting of vetiver or ele- Vetiver phant grass “weirs” that Grass will slow down flow and trap sediment, resulting in terrace formation over time. Vetiver Grass (source: uni.marburg.de) Crib walls are one of the oldest gravity wall sys- tems, comprised of a se- ries of stacked members Crib Retain- creating hollow cells filled ing Wall with soil or rock. Cells can be vegetated. Cribs can be made of wood or con- crete structures Vegetated Wooden Crib Retaining Wall (source: www4.lubw.baden-wuerttemberg.de) As an alternative slope protection method various Tire Retain- material are suitable to ing Wall stabilize slopes – e.g. tires. Tire Retaining Wall (source: www.permies.com) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 59 Concrete retaining wall Concrete blocks can be prefabri- Retaining cated. Quick onsite instal- Wall Blocks lation, cells will be filled with soil and vegetated. Concrete Retaining Wall Blocks (source: kuert.com) Possible Sequence of Works For the implementation of the combined measures approach the following sequence of works could be applied after selection of gullies to be protected and preparation of all relevant design documents is completed. Possible sequence of works for implementation of the combined measures approach: 1) Removal of waste / cleaning of gully 2) Large-scale / area-wide runoff reduction measures (revegetation/soil bunds), if applicable 3) Improvement and cleaning of existing drainage system, if applicable 4) Installation of several detention ponds for initial stabilization of the gully 5) Stabilization of side slopes (e.g. vetiver grass, see previous chapter) after completion of con- struction of the detention ponds 6) Fostering of planted slope protection (if required) 7) Silting up of detention ponds will progress, regular checking and maintenance works required Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 60 Alternative measures for embankment protection Table 2-11 Additional Measures for Erosion Prevention (gullies and drains) Measure Description Example Image Geofabric Geofabric allows vegeta- tion to grow through the fabric, which ultimately breaks down. This ap- proach is effective when the bed and banks of rivers and wetlands are reshaped after engineering works Geocells Geocells can also be filled with soil and planted. Geocell Geocell chute designed to Chutes stabilize head-cut and gully erosion. This approach oc- cupies a much smaller footprint than a traditional gabion weir. However, there is risk of failure in high energy systems (e.g. rivers with high discharge). Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 61 Ecologs Ecologs can be used to trap sediment and allow the natural vegetation to establish itself. Ecologs are effective in areas with low gradient, where a natural source of sediment is pre- sent. Geotubes Geotubes enable several applications in marine work: Protecting the cliff along the shore from erosion from wave and wind dam- age Constructing breakwaters Constructing artificial reefs / islands Preventive Erosion Protection Measures to face further Urbanization / Meso-Scale Retention Ponds Preventive measures can be used besides the above described measures. These measures are mainly foreseen for the two big catchments draining towards the east (ID 12 and 13). As it is expected that further urbanization is heading towards the eastern districts creation of recreational areas that serve as possible retention basins (as indicated in the picture below) could help to mitigate flood and erosion risks in these areas. Recreational areas (parks) that can be flooded as a large-scale retention basis should be prioritized prior to construction of new (informal) residential building in these areas. The following points need to be considered in case of creation of these measures: — Should be constructed prior to urbanization process — Requires maintenance In addition to the construction of recreational areas the following can be considered for the areas which are prone to further urbanization: • Low sealed surface • High vegetation cover, • General town planning that includes erosion risks and prevention of initial erosion. In contrast to the densely populated areas in central parts of Nacala there is more space and open land available towards the eastern end of Nacala which can be used to create mesoscale retention ponds besides the basic idea of recreational areas as described above. Opposite to the detention ponds proposed within the combined measures approach these meso-scale retention ponds shall not Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 62 silt up and thus provide retention volume in case of heavy rainfall for the long run. If found suitable main task in order to construct these retention ponds will be to find locations with a beneficial retention volume. Figure 2-22 SUDS - Recreational Areas as Meso-Scale Retention Basins (source: www.lizlake.com) Alternative and Accompanying Measures The following measures illustrate alternatives to the measures presented above or accompanying measures to increase erosion protection in Nacala. These alternatives are not seen as suitable for the conditions in Nacala as the proposed measures. Still, as a basis for further discussion these ideas are included in this report. Instream River Training Instream river training is a nature-based river works development with the goal to reduce river bed and slope erosion by inducing secondary flows within the river. Most measures aim to install over- flowed instream constructions (usually rocks) which change a uniform flow in the river. The principals have been elaborated by Viktor Schauberger almost 100 years ago. Due to the recent valuation of alternative ways for river protection and restoration of heavily modified waterbodies these nature- based principals have been scientifically investigated in more depth and applied in several projects; located mainly in central Europe. Although primarily focusing on bedload management, river bed erosion protection and restoration for running waters some of the basic principles could be applied to avoid heavy river installations, espe- cially if there is enough space along the river banks (which applies for the gullies in catchment areas ID 1 – 5) to allow for meandering which reduces flow velocity and bed shear stress. As an example, the installation of meandering ramps (as presented in the following figure below) or micro groins could replace continuous slope protection measures and installation of retention ponds. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 63 Legend: A – High Point of Line B – Low Point of Line C – Stilling Basin D – Arched Support Figure 2-23 Instream River Training - Meandering Ramps (source www.engineering- group.ch) Most instream river training measures are applied for perennial rivers in opposition to the gullies in Nacala which are just disposing water in case of (heavy) rainfall events. Still, the principal of instream river training for induction secondary flows and reduction of erosion could be a viable option if there is enough space alongside the gully and induction of meandering is imaginable to the municipality in principal. Rainwater Collection As an alternative micro-scale solution rainwater collection could be implemented within the densely populated areas in the inner city of Nacala. Based on the Consultants risk assessment between 15- 20% within catchments ID 9 – 10 can be considered as sealed surface with the majority being roof tops. Collection and storage of rainwater in tanks and small cisterns could reduce the runoff to a certain degree. Rainwater collection as an only-measure will not be suitable as the reduced runoff will be far too little to avoid further erosion. Still, rainwater collection as an accompanying measure and part of possible urban gardening programs could be considered. Potential adverse implications of rainwater collection can be: — Rainwater collection tanks can be breeding ground to mosquitos — High participation required to impact situation significantly Grey Infrastructure Measures The nature-based proposed measures presented in the previous chapters will help to reduce the ero- sion risks along the existing gullies and will prevent creation of new gullies (in case of large-area revegetation). Still, it needs to be pointed out that erosion protection in urbanized areas depending solely on nature-based solutions will not be feasible. Nature-based solutions will help to prevent or at least mitigate erosion risk in Nacala. Still, as urbanized areas have a high share of sealed surfaces the damage-free disposal of the runoff depends on standard (grey) draining structures as well. As an example, the following gully in the inner city of Nacala (catchment ID 09) can be examined: Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 64 Figure 2-24 Limits of Nature-Based Solutions The gully (indicated in red) can be stabilized and protected against further erosion using nature-based solutions – in this case green detention ponds and bank protection with suitable plants. As soon as the runoff leaves the gully various buildings and a major road are in the natural path of the water towards the sea. For crossing of the road, a culvert needs to be constructed which – at least in its standard layout – will be a (grey) reinforced concrete structure. To improve the overall drainage situation in Nacala a hybrid solution needs to be implemented with usage of both methods: Nature-based / green solutions wherever possible and suitable: o Unused Land – Revegetation, Urban gardening o Along existing gullies - Small-scale detention ponds and green embankments o Expansion Areas towards the east - vegetated recreational areas (parks) as a pre- ventive measure Grey infrastructure o Crossing of infrastructure - Culverts o Protection of port area – Sea Outlet o Closely spaced building areas – Channels, Piped Disposal of Rainwater Do Nothing Alternative In opposite to all measures proposed in the previous chapters the implications if doing nothing can be considered as well. As the basic risk a continuous growth of the gullies in its width and depth was identified previously. Main issues will have to be discussed if nothing would be done: a) Public awareness: The community in Nacala is aware of the erosion and the urbanization it caused. Based on information received at the latest workshop in Nacala several children died in a gully during a rainfall event. If doing nothing will be presented as the results of the assess- ment, public opinion will most likely not agree. b) As a minimum, people living adjacent to the gullies would have to be resettled to safe loca- tions. In addition, the municipality will have to control that no new buildings will be constructed Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 65 in these locations. With the recent history in Nacala in mind, both seems to be very unlikely. In addition, costs for resettlement will have to be considered which wouldn’t leave a “do-nothing” alternative anymore. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 66 TECHNICAL ANALYSIS OF NATURE-BASED FLOOD AND EROSION PROTECTION MEASURES Basics Used Software for modelling For all data preparation and/or GIS-related tasks the free and open source GIS software “QGIS” has been selected. It can be used to collect, compile, manage and process the different datasets required for elaboration of the tasks described below. In addition, analyses of previous erosion processes, post-processing of models and creation of maps was done within QGIS. The hydrological and sedimentological analyses was conducted in a hybrid approach between QGIS and the hydrological modelling system HEC-HMS. Details regarding the approach can be found in the following chapter. Based on the chosen and proposed protection measures a 1D hydrodynamic model was established for hydraulic modelling and assessment of the current and the future scenarios. The program utilized for the hydraulic modelling is HEC RAS 5. More details regarding the setup of the model can be found in chapter 2.4.5 Hydrology Nacala The Consultant utilized a simplified rainfall-runoff modelling in order to estimate potential maximum discharges in the catchment area. The individual steps are being described in the next subchapters. Study Area The study area is located in the city of Nacala, which is part of the Nampula Province. The rainfall- runoff model has been implemented for a sub catchment within catchment ID 09 in Nacala with an area of 2.34 km². A geographical representation of the study area as well as 3D visualization of the sub catchment is shown in the following figure. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 67 Figure 2-25 Study Area - Catchment ID 09 Rainfall-Runoff Modelling Rainfall-runoff modelling is the process of simulating the river flow by making use of mathematical models with observed or assumed rainfall and a respective catchment area. Precipitation falling on the ground breaks down into various components at the interface between atmosphere, biosphere and pedosphere. These processes can be divided into the general groups of runoff formation, runoff concentration and the flow process. A description of the processes and selected methods will be described in the next sections. PRECIPITATION DATA Precipitation is the most important input parameter for the rainfall-runoff model. The accuracy and availability of this input parameter strongly influences the performance of the model. Overall, data availability is very limited. Hydro-Meteorological observation data has been collected for several stations in Mozambique as shown in the figure below. A meteorological observation point is present at the recently built airport in Nacala. However, no data could be obtained from this station. Therefore, the assessment must be based on available stations in proximity of the study area. The closest stations are Lumbo, Nampula and Pemba. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 68 Figure 2-26 Station Overview and distance of stations in proximity of Nacala. Daily precipitation summary data is available for these three stations from 1971 to 2016 with relatively small gaps. The average annual precipitation varies between 869 mm and 1092 mm. Figure 2-27 Average annual precipitation for observation points in proximity of Nacala (Maputo shown for comparison) Due to its proximity to the study area and the similar topographical location, Lumbo station has been utilized for the following assessment. Both locations are influenced by similar weather patterns, with convectional rain as dominant type. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 69 Lumbo_PRCP_dly 200 190 180 170 160 150 140 130 120 110 mm 100 90 80 70 60 50 40 30 20 10 0 1971-12-13 00:00 1979-12-11 00:00 1987-12-09 00:00 1995-12-07 00:00 2003-12-05 00:00 2011-12-03 00:00 Figure 2-28 Daily summaries, precipitation Lumbo station To check the homogeneity of the dataset from Lumbo, it has been tested against the station in Nam- pula using a double-mass plot (see figure below). Nampula station is located at the international air- port, so the data from this station is seen as the most reliable / homogenous source of precipitation data in the region. The double mass plot shows a determination factor of 0.999, showing that the dataset from Lumbo can be considered homogenous. Homogeneous line, slope l=.879 Determination factor=.999 1969.01 45,000 40,000 35,000 30,000 25,000 20,000 15,000 10,000 5,000 2017.04 0 0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 Figure 2-29 Double-Mass-Plot, Lumbo (ordinate) tested against Nampula (abscissa) DESIGN VALUES / RETURN PERIODS To determine the design flows for the hydraulic analysis, the precipitation values of different return periods have been determined based on the annual maximum precipitation (see figure below) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 70 Lumbo_PRCP_annualMax 200 190 180 170 160 150 140 130 120 110 mm 100 90 80 70 60 50 40 30 20 10 0 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 2013 2015 Figure 2-30 Annual maximum daily precipitation Lumbo After aggregation, there are 39 maximum values, which are included in the regression as shown be- low. Weibull LogPearsonIII Prediction interval limits 90% Confidence interval limits 90% Return period (T) f or Maximum v alues in y ears - scale: Normal distribution 1000 2000 1.01 1.01 1.02 1.05 1.11 1.25 1.43 1.67 3.33 100 200 500 2.5 10 20 50 1 1 1 2 5 260 240 220 200 180 160 140 mm 120 100 80 60 40 20 0 -3 -2 -1 0 1 2 3 Figure 2-31 Return Period for Maximum Annual Precipitation, Log Pearson III distribution and 90% confidence and prediction intervals The results of the statistical analysis show the following values for the maximum daily rainfall as fol- lows: Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 71 Table 2-12 Design Precipitation Values and Return Periods of Extreme Events from Sta- tistical Analysis (Log Pearson III) – Lumbo Return occurrence precipitation (log Pear- Period Probability son III) [mm] [1:x yr] 2 0.5 87.5 5 0.2 121.2 10 0.1 140.3 20 0.05 165.1 50 0.02 201.2 100 0.01 231.6 Statistical extrapolations of a dataset bear uncertainties. The 90% confidence and prediction intervals are given in the statistical distribution plots. To evaluate the likely range of the resulting precipitation values, the prediction interval of the statistical analysis is evaluated. Especially for the lower occur- rence probabilities, there is a wider likely range of values due to the limited amount and range of available data. Table 2-13 90% Prediction Intervals for Daily Precipitation Summary Lumbo Return occurrence precipitation [mm] Period Probability LOWER LOG PEARSON UPPER [1:x yr] III PREDICTION (90%) PREDICTION (90%) 2 0.5 80.5 (-8%) 87.5 96.6 (+10%) 5 0.2 104.4 (-11%) 121.2 133.0 (+13%) 10 0.1 120.1 (-15%) 140.3 164.5 (+17%) 20 0.05 134.6 (-18%) 165.1 202.4 (+23%) 50 0.02 153.2 (-24%) 201.2 269.1 (+34%) 100 0.01 166.9 (-28%) 231.6 331.1 (+43%) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 72 The return periods for the annual maximum precipitation for the utilized station (Lumbo), nearby sta- tions (Nampula, Pemba) and a reference station (Maputo) are shown in the table and figures below. Figure 2-32 Return Period for Maximum Annual Precipitation, Log Pearson III distribution and 90% confidence and prediction intervals for Reference Stations Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 73 Table 2-14 Design Precipitation Values and Return Periods for Extreme Events from Sta- tistical Analysis (Log Pearson III) – Regional Stations precipitation [mm] Return Period occurrence precipitation [1:x yr] Probability [mm] NAM- MA- LUMBO PEMBA PULA PUTO 2 0.5 87.5 87.5 82.4 90.4 92.5 5 0.2 121.2 121.2 115.3 126.8 148.8 10 0.1 140.3 140.3 140.2 153.3 190.7 20 0.05 165.1 165.1 166.6 180.6 234.0 50 0.02 201.2 201.2 204.7 218.8 294.7 100 0.01 231.6 231.6 236.5 249.8 343.7 Figure 2-33 Annual Maximum Precipitation Return Periods for Lumbo and Comparison Stations The average annual precipitation in Mozambique has a general north-south gradient, with a tendency for an annual precipitation sum of about 1000 mm in the north and approx. 500 mm in the south (locally modified by certain topographic features), as also shown in the figure below. Figure 2-34 Average Annual Precipitation for Lumbo and Reference Stations Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 74 The analysis shows that the results of the extreme evaluation for precipitation of the stations in prox- imity to Nacala (Lumbo, Nampula, Pemba) result in similar precipitation values for the analysed return periods. The resulting values for the reference station “Maputo” are significantly higher for the larger return periods, while the average annual precipitation is lower than for the other observation points. The higher results in Maputo are driven by two extreme events in 1999 and 2008, where the annual maximum daily precipitation record is significantly larger than for the other years on record. Maputo_PRCP_dly_Annual_maximum 350 300 250 200 mm 150 100 50 0 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 2013 2015 Figure 2-35 Annual maximum daily precipitation Maputo PRECIPITATION INTENSITY No intensity data is recorded at the stations; therefore, the daily precipitation summaries must be broken down to a shorter time interval to calculate the expected peak flow in the drainage gullies. The document “Regulamento dos sistemas publicos de Distibuicao de Agua e Drenagem de Agua Resi- duais” (2003) divides Mozambique into 5 climatic zones (Maputo/Matola, Zone A, B, C, D). For each zone a coefficient K is defined. Based on the IDF-Curve for Maputo, IDF-curves for the different hy- drological regions are derived by applying the multiplicator K. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 75 Figure 2-36 Rainfall regions of Mozambique – Map of K coefficient (based on “Regulamento dos sistemas publicos de Distibuicao de Agua e Drenagem de Agua Residu- ais” (2003)) Lumbo and Nacala fall into Zone D. Based on the methodology described above, the following IDF- Curve is derived for Zone D: Table 2-15 IDF-Curve Values for Hydrological Zone D PRECIPITATION INTENSITY [mm/hr] Rainfall duration [min] 20 60 120 160 200 260 T = 2 yr 129.8 66.6 43.7 36.7 32.0 27.3 T = 5 yr 175.9 91.6 60.7 51.2 44.8 38.3 T = 10 yr 206.2 108.2 72.0 60.8 53.4 45.8 T = 20 yr 235.2 124.1 82.9 70.1 61.6 52.9 T = 25 yr 244.8 129.3 86.4 73.1 64.2 55.1 T = 50 yr 273.0 144.8 97.0 82.2 72.2 62.1 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 76 Figure 2-37 Intensity Duration Frequency curve for Hydrological Zone D Under the assumption of the rainfall event length of 120 min, the following hyetographs have been derived using the alternating block method. Figure 2-38 Hyetographs for Different Return Periods (Alternating Block Method) The cumulative depth of the generated rainfall events matches the values derived using the statistical analysis as shown in the table below. This supports the assumption of a 120 min rainfall event. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 77 Table 2-16 Comparison Cumulative Precipitation from IDF-Curves and Values Derived from Statistical Analysis Return Period [1:x Precipitation Value statistical Cumulative precipitation from idf- yr] analysis [mm] curve [mm] 2 87.5 87.4 5 121.2 121.4 10 140.3 144.1 20 165.1 165.8 50 201.2 194.0 The uncertainties derived from the statistical analysis must be also accounted for in this analysis. Especially the possible upper bound of the prediction interval has to be considered, as the larger precipitation (and resulting runoff) values can reduce the designed mitigation level to a larger occur- rence probability / increase the risk of erosion. While the design values derived using the log Pear- son III distribution show in conjunction with the IDF curve a clear indication for a rainfall event length of 120 min, the values for the upper bound of the prediction interval result in significantly longer rainfall periods to achieve a similar cumulative depth using the IDF-Curve (up to 420 min = 7 h for 50 yr. return period). This suggests possible prediction uncertainties in the empirically derived IDF-curve, as common rainfall patterns in the region usually result in shorter rainfall events. However, other IDF- curves could not be obtained for the study location. Therefore, it is assumed that the rainfall event length of 120 min is also applicable for the upper boundary of the prediction interval. Under this as- sumption, the design values are derived using the percentual change as shown in the table below: Table 2-17 Utilized Design Values for Peak Precipitation Return Period [1:x PEAK Precipitation Value sta- Check Value (Upper Boundary of yr] tistical analysis [mm] Prediction Interval) [mm] 2 33.0 36.3 5 44.2 50.0 10 51.6 60.4 20 58.6 72.1 50 67.9 91.0 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 78 RUNOFF FORMATION Runoff formation describes the process of separating the precipitation into effective precipitation. Ef- fective Precipitation is the part of the rain, which is discharged through catchment area causing flood- water. There are several methods available calculating the effective precipitation. In general, there is a distinction between deterministic and stochastic model. Further differentiation includes categories of black-box, conceptual and process-orientated models. The selection of the correct model depends on the needed temporal and spatial scales, data availability and need for accuracy, amongst other. For this study a model should be used which is able to cope with a lack in catchment related infor- mation, such as measured discharges or detailed catchment related parameters. In addition, it should be able to simulate the flow in small catchments over a short time period. This can be done by using the SCS method of the US Department of Agriculture’s Soil Conservation Service. The SCS is a deterministic model which uses only precipitation and a few area-specific parameters. The SCS method calculates the effective precipitation using following formula: where: Peff is the effective precipitation [mm], P the fallen precipitation [mm], CN the curve number [-] and Vi the initial loss in [%] With reference to above equation the fallen precipitation P for different return periods has been de- termined in the chapter above. The initial loss Vi [%] is calculated by dividing the potential maximum storage capacity (with some exceptions) by 5. The potential maximum storage capacity is calculated using following formula: The curve number CN is depending on the land use, general soil properties and soil moisture class. CN values are based on experience, for which common values can be found in tables. In order to estimate the distribution of land use classes in the catchment area the Consultant per- formed a GIS analysis. At first the catchment area was delineation using a SRTM 30*30m elevation model. Based on a recent satellite image it became apparent that the land use distribution among the whole catchment area is rather uniform. Hence a sample area of 200*200m was chosen at a repre- sentative location and classified according common land use classes, i.e. sealed surface, vegetation cover and “open land”. A graphical representation of this task is shown in Figure 3 5. After the classi- fication, CN values were applied to each land use class by considering the soil type and moisture class. A single CN value is then derived by weighting the individual CN values for each class by the percentage of their coverage. A summary is given in the table below. Table 2-18 Land use Classification of a 200m x 200m sample area and related CN values CN-Value Cn Value Classification Area [m²] Percentage [%] per Class Overall Sealed Surface (A) 7,010 18% 90 Vegetation Cover (B) 6,073 15% 50 73.8 Open Land (C) 26,917 67% 75 Considering all input parameters, the effective precipitation was calculated for each rain event of a return period of 2, 5, 10, 20 and 50 years and used to calculate the runoff. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 79 Figure 2-39 Catchment Area and Land use Classification RUNOFF CONCENTRATION The term runoff concentration denotes the transformation of the effective precipitation into direct run- off. In this sense, once the effective precipitation is determined, as has been described in the previous section, the volume of runoff for the catchment area is also known. The transformation from the ef- fective precipitation to the runoff curve via a respective “wave” is described as “concentration” and is generally calculated via a mathematical transfer function (Unit hydrograph, Linear storage cascade etc.). Based on the Kirpich Equation, the time of concentration for the different sub-catchments varies between 10-20 min. Due to the small catchment size and short time of concentration it is assumed that the total volume of runoff from the effective precipitation (SCS Method) is reaching the outlet of the catchment area without alteration to its wave, equally distributed over the timestep. Therefore, the cumulative precipitation around the peak timestep at a length equalling the specific time of concen- tration for the sub-catchment is used for the calculation of the resulting runoff values. For precipitation return periods of 2, 5, 10, 20 and 50 years discharges were calculated and presented in the following table. Runoff coefficients range from Ψ=0.12 to 0.34, which can be considered rea- sonable under such conditions. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 80 Table 2-19 Calculated discharges for the catchment area considering return periods of 2, 5, 10, 20, 50 years (standard values) Sub- Time of con- Catchment Discharge [m³/s] Catch- centration size [km²] 1 IN 5 1 IN 10 ment [min] 1 IN 2 YR 1 IN 20 YR 1 IN 50 YR YR YR P=0.5 P=0.05 P=0.02 P=0.2 P=0.1 1 20 1.08 3.8 8.2 11.6 15.3 20.4 2 18 0.85 2.6 5.8 8.3 11.0 14.8 3 10 0.30 0.4 1.1 1.7 2.3 3.1 TOTAL 2.23 6.8 15.1 21.6 28.6 38.3 RUNOFF UNCERTAINTIES In addition to the values based on the log Pearson III distribution curve, the discharges for the upper and lower bounds of the 90% prediction interval have been modelled as shown in the figure below. Figure 2-40 Discharge Values for logPearson III Distribution and 90% Prediction Interval The other main source of uncertainties in the modelling is the selection of the appropriate CN value for the SCS model. Therefore, the resulting runoff has also been modelled for a variation of the CN- value of ±15%. With the changing CN value, also the soil storage / initial abstraction is changing. The results are shown in the following diagram: Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 81 . Figure 2-41 Discharge Values for CN-Value Variation The range of values for the CN variation of ±5% falls within the modelled prediction interval, for almost the complete range of modelled return periods. For the larger return periods, also a CN-value variation of ±10% falls within the 90% prediction interval. Climate Change (Hydrological Impact) The impact of climate change on the hydrological conditions in Nacala has been analyzed based on the projections presented in the climate fact sheet for Mozambique, published by the Climate Service Center Germany. The fact sheet summarizes the projections from the models AIPCC AR4 and AR5 and shows the expected ranges and uncertainties for the anticipated change of different hydro-climatic parameters. The expected climate change effects for Mozambique compared to the reference period of 1971-2000 are summarized as follows: “Precipitation The likely range of projected change in annual total precipitation is from -10 to +2% by 2085. The very likely range is from -16 to +5%, with most projections showing a decrease. The projected change in precipitation shows for the majority of model simulations a tendency towards an increase in future precipitation amounts during the main rainy season (likely range from -40 to +15%), whereas for the dry season a tendency towards drier condi- tions is projected (likely range from -53 to +9%). Confidence in these figures is medium. The change in annual total precipitation can be considered to be weak.” “Heavy Rains: The likely range of projected change in the intensity of heavy rain events is from +2 to +15% by 2085 and the very likely range is from -3 to +29%, with only a few pro- jections showing a decrease. The likely range of projected change in the frequency of heavy rain events is from -4 to +27% by 2085. Confidence in these figures is medium. The change in the intensity of heavy rain events can be considered to be weak.” “Water balance: The likely range of projected change in the annual mean climatic water balance is from -45 to +84 mm/yr by 2085 and the very likely range is from -92 to +261 mm/yr, with some projections showing an increase and some a decrease. Confidence in these figures is medium. The change in the annual mean climatic water balance can be considered to be medium-strong.” (Climate Service Center Germany / GERICS: Climate-Fact-Sheet Mozambique, 2015) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 82 Figure 2-42 Projections for Precipitation Sum (top left), Heavy Rain Events (top right) and Precipitation Seasonality (bottom left). The projected change in frequency of heavy rainfall events results in a likely increase of the occur- rence probabilities of the rainfall events modelled for Nacala. The hydrological modelling for the pro- ject is based on the available time series from 1969 – 2016. As the climate change forecasts are based on a shorter period (1971-2000), the projected relative changes cannot be directly applied to that time series. Therefore, the extreme analysis has been carried out for the reference time period utilized in the climate fact sheet. The results are given in the table below (together with the analysis results for the complete time series). Table 2-20 Precipitation Values and Return Periods from Statistical Analysis of Extreme Events for Climate Fact Sheet Reference Time Period (Log Pearson III) – Lumbo RETURN PE- OCCURRENCE PRECIPITATION (LOG PEARSON III) [mm] RIOD [1:x yr] PROBABILITY TIME SERIES 1971-2000 TIME SERIES 1969-2016 2 0.5 83.9 87.5 5 0.2 103.6 121.2 10 0.1 115.6 140.3 20 0.05 126.6 165.1 50 0.02 140.2 201.2 100 0.01 150.1 231.6 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 83 Weibull LogPearsonIII Prediction interval limits 90% Confidence interval limits 90% Return period (T) f or Maximum v alues in y ears - scale: Normal distribution 1000 2000 1.01 1.01 1.02 1.05 1.11 1.25 1.43 1.67 3.33 100 200 500 2.5 10 20 50 1 1 1 2 5 170 160 150 140 130 120 110 100 90 mm 80 70 60 50 40 30 20 10 0 -3 -2 -1 0 1 2 3 Figure 2-43 Return Period for Maximum Annual Precipitation, Log Pearson III distribution and 90% confidence and prediction intervals (reduced time series 1971-2000) As the shorter time series does not contain the extreme events that occurred in 2007 and 2015, the resulting values are significantly lower for the smaller occurrence probabilities while also showing a larger 90% prediction interval. The projected increase of intensity of the events can result either in shorter event durations or an increase of the amount of precipitation during these events. If the amount of precipitation increases, and the rainfall duration stays similar, the following values can be derived for the extreme events: Table 2-21 Projected Precipitation Values and Return based on Climate Fact Sheet RETURN PRECIPITATION (LOG PEARSON III) [mm] OCCURRENCE PERIOD 2000 2030 2050 2085 PROBABILITY [1:x yr] (reference) (-3 to +16%) (-4 to +20%) (-4 to +27%) 2 0.5 83.9 81.4 to 97.3 80.5 to 100.7 80.5 to 106.6 5 0.2 103.6 100.5 to 120.2 99.5 to 124.3 99.5 to 131.6 10 0.1 115.6 112.1 to 134.1 111 to 138.7 111 to 146.8 20 0.05 126.6 122.8 to 146.9 121.5 to 151.9 121.5 to 160.8 50 0.02 140.2 136 to 162.6 134.6 to 168.2 134.6 to 178.1 100 0.01 150.1 145.6 to 174.1 144.1 to 180.1 144.1 to 190.6 While the reductions are non-substantial for the different projects and fall within the prediction interval of the analysis for the reference time period, the increases are substantial. The table below compares the upper boundary of the climate change projections to the design values utilized in the project: Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 84 Table 2-22 Project Design Values for Precipitation Sum and Upper Bounds of Projections from Climate Fact Sheet RETURN PRECIPITATION (LOG PEARSON III) [mm] OCCURRENCE PERIOD 1969-2016 PROBABILITY 2030 2050 2085 [1:x yr] (design base) 2 0.5 87.5 97.3 100.7 106.6 5 0.2 121.2 120.2 124.3 131.6 10 0.1 140.3 134.1 138.7 146.8 20 0.05 165.1 146.9 151.9 160.8 50 0.02 201.2 162.6 168.2 178.1 The comparison shows that the design values are mostly larger than the projected values for most return periods. This highlights the uncertainties of statistical extrapolation approaches. As the reduc- tion of the time series to the reference time period of the climate fact sheet eliminates the two largest recorded precipitation event records at the station, the reference values for the projection are also lowered. This leads to lower base results (with larger uncertainties) and therefore rather low projec- tions. However, it has to be taken into consideration that the climate change projections are derived from a global model, therefore not taking into account any specific conditions at local precipitation gauges. Therefore, the results of this analysis only highlight the uncertainties of statistical approaches and extrapolations – both in climate change projections and extreme value analyses – as well as the impact of altering time series for statistical analyses and does not produce useful design values. Even though the statistical analysis and comparison regarding climate change does not result in us- able design values, the tendency towards a larger precipitation intensity during extreme events can still be assumed for future projections. Therefore, the overall conclusion from the climate fact sheet is that the occurrence probability associated with the different precipitation/runoff values utilized in the project will increase in the future, therefore effectively lower the level of protection achieved with the project measures. Required Protection Goal and Return Period for Modelling Prior to hydraulic modelling of the detention ponds the general required protection goal needs to be defined as a basis for the chosen return period. Higher return periods result in higher discharges as can be seen in the previous chapter. Higher discharges usually require higher efforts to avoid or reduce erosion in Nacala. IDENTIFICATION OF REQUIRED PROTECTION GOALS: As described in the previous chapter erosion was identified as main risk in case of heavy rainfall. Due to the erosion the gullies tend to grow year by year endangering population, buildings and infrastruc- ture. Thus, avoidance or reduction of erosion will be one of the main protection goals. As stated pre- viously the natural state with a high vegetation cover (prior to urbanization) showed no or minor ero- sion only. As this natural state was disturbed due to the progressive urbanization in Nacala and its surroundings erosion gullies developed. Still, as erosion is a natural progress complete avoidance of any erosion will be unrealistic on a nature-based approach. Protection goal shall be reduction of further erosion to a minimum, stabilisation of the gullies and avoidance of gully growth in its width and depth as observed within past years. Detention ponds can help to address these issues. In addition, the location of ongoing erosion after construction of the ponds needs to be identified as well. E.g. if there is further erosion in the middle of a gully by trans- porting newly entered sediment along the longitudinal slope of the gully without erosion of the gully’s Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 85 embankments protection goals are already met. RETURN PERIODS FOR HYDRAULIC MODELLING: Based on the protection goals different return periods can be selected and investigated. As higher return periods result in higher discharges and thus higher efforts for stabilization of the gully a balance between potential benefits and potential costs for these measures must be found. For the assessment within this report a 5-year return period was chosen to identify if the proposed measures are suitable to achieve the mentioned goals. Assessment of 2, 10, 20 or 100 years return periods will lead to different requirements with the tendency that higher return periods will required more and higher de- tention ponds in order to reduce erosion. At a certain point, nature-based solutions will not be suitable to address drainage problems in Nacala on its own and accompanying (grey) drainage infrastructure measures will be required anyway. As discussed, and agreed upon with the municipality of Nacala and interested community members during the workshop in Nacala (end of March 2019) nature-based solutions can be one piece of a puzzle to improve the drainage related problems in Nacala. Still, the need for a general drainage master plan and guidance for implementation of drainage infrastructure is required in any way. Based on this, a 5-year return period was considered suitable to identify if this selected and single nature-based measure is fit to achieve stabilisation of the gully and show the general technical feasi- bility of the proposed measure. Still, the results for higher discharges / higher return periods was included and indicated as well to show the impact of the detention ponds for a broad variety of dis- charges. Hydraulic Modelling Nacala Model Setup and Data Input The hydraulic model was set up for three different scenarios: a) Existing situation b) Construction of several detention ponds within the gully (includes minor retention volume) c) Final situation after silting up of detention ponds (will serve as check dams) All geometric data was compiled and prepared in the general GIS model. Cross-sections and the alignment of the reach was imported to HEC-RAS using an open source plugin within QGIS. The total length of the modelled gully is approximately 1,200 m. Starting elevation of the gully is about 87.8 masl and the bottom elevation at the end 51.9 masl. This gives an average longitudinal slope of ~3%. Compared to other gullies in Nacala this is an average longitudinal slope. The profiles have been generated each 50m within the GIS environment and have been interpolated to 5m in HEC-RAS. A typical cross-section is shown in the following figure: Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 86 Figure 2-44 Typical Cross-Section (scale exaggerated) The value of Manning’s n is highly variable and depends on a number of factors including surface roughness, vegetation, channel irregularities and alignment, seasonal changes, suspended materials and some more. As calibration of the values was not possible for this project, the following values have been used based on Chow’s “Open-Channel Hydraulics”: • 0.025 s/m1/3 for the gully (channel - earth, winding and sluggish, no vegetation) • 0.2 s/m1/3 for the left and right overbank (to account for vegetation and obstructions) (As the calculated runoff usually stays within the gully the roughness for left and right overbank is irrelevant for the calculation) As described in the hydrology section the discharges for the catchment have been divided into three sub-catchments (SC): • SC 1: Area upstream of the gully and first section of the gully • SC 2: “Side-Valley” which discharges into the modelled gully at station 491.12 • SC 3: Area downstream of SC 2 discharge point The entire discharge of SC 1 was allocated to the first cross-section of the reach (Station 1191.28). SC 2 was allocated at station 491.12. Due to the limited distance until the end the gully (only minor contribution to total discharge) SC 3 was allocated at this station as well. Table 2-23 Discharges in m³/s for Hydraulic Model Q1 Q2 Q3 Q4 Q5 SUB-CATCH- STATION (P=0.5) (P=0.2) (P=0.1) (P=0.05) (P=0.02) MENTS Section 1 SC 1 1191.28 3.8 8.2 11.6 15.3 20.4 Section 2 SC 1 - 3 491.12 6.8 15.1 21.6 28.6 38.3 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 87 The simulation was conducted as a steady flow simulation. Different discharges (Q1 – Q5) have been added to assess different flow conditions within the gully. For assessment of the hydraulic capacity of the proposed measures (detention ponds) a return interval of 5 years (P = 0.2) was chosen. Hence, the design discharge for the gully is 8.2 m³/s in the upper section (section 1) and 15.1 m³/s for the lower section (section 2). The selected protection level of 5 years will be further detailed and verified during (possible) later stages of this project (e.g. feasibility or detailed design level) including the implications for mainte- nance and rehabilitation measures based on the chosen protection level. After simulating and assessing the existing situation a second model was elaborated based on the proposed measure to construct detention ponds to reduce flow velocities in the gully. The detention ponds have been included into the model as separate inline structures (weir/embankment) with the following characteristics: • Broad crested weir with a crest width of 1m • Maximum height of 2m • Upstream and downstream slope of 1:2 • Width of the embankment is identical to the width of the closest cross-section (distance: 5m) Based on the findings in Nacala it is expected that these detention ponds will silt up quickly after completion and will serve as check dams further on. Therefore, a third model was set up to investigate what happens in this case. As an assumption the detention pond crest level was chosen as new minimum channel bottom elevation for all upstream profiles with lower minimum elevation compared to the crest level. As a result, the retention volume will be lost and a cascade system will remain. Results Hydraulic Model – Existing Situation The results of the hydraulic model for the existing situation are presented in the table below. Table 2-24 Results Hydraulic Model - Existing Situation River Sta- Channel Depth of Energy Q Total Velocity Froude # tion Bottom El. Water Grade (m3/s) (m) (m) (m/m) (m/s) 1191.28 8.2 87.77 1.06 0.014 3.24 1.01 1141.28 8.2 85.14 0.47 0.057 4.96 2.3 1091.28 8.2 83.02 0.73 0.036 4.51 1.69 1041.28 8.2 79.51 0.2 0.079 3.71 2.67 991.29 8.2 79.17 0.46 0.009 2.12 1 941.28 8.2 77.72 0.33 0.028 3.07 1.69 891.28 8.2 76.38 0.18 0.030 2.23 1.66 841.28 8.2 75.8 0.47 0.009 2.17 1.01 791.28 8.2 70.96 0.17 0.101 3.78 2.96 741.15 8.2 69.32 1.73 0.000 0.68 0.16 691.28 8.2 70.55 0.32 0.010 1.78 1.01 641.27 8.2 69.42 0.28 0.021 2.4 1.44 591.28 8.2 68.87 0.46 0.009 2.12 1 541.28 8.2 64.59 0.19 0.088 3.87 2.82 491.12 15.1 63.27 0.66 0.009 2.55 1.01 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 88 River Sta- Channel Depth of Energy Q Total Velocity Froude # tion Bottom El. Water Grade (m3/s) (m) (m) (m/m) (m/s) 441.28 15.1 62.32 0.28 0.024 2.58 1.56 391.28 15.1 61.5 0.66 0.009 2.55 1.01 341.28 15.1 59.7 0.23 0.041 2.99 1.99 291.28 15.1 58.23 1.63 0.000 0.92 0.23 241.28 15.1 59.22 0.41 0.009 2.03 1.01 191.28 15.1 58.27 0.38 0.017 2.63 1.36 141.28 15.1 58.26 0.31 0.009 1.75 1.01 91.28 15.1 55.92 0.24 0.042 3.14 2.04 41.28 15.1 51.96 0.15 0.082 3.27 2.66 Within the table above the results are shown for selected cross-section (stations) each 50m only although the distance for modelling was set to 5m. As the general conclusions for the modelling of the existing situation will be identical the indication of the results for all ~240 stations was not added to this table. Please refer to Annex IV for detailed results table. A long profile for modelling of the existing situation is given in the following figure. Figure 2-45 Long Profile - Existing Situation Q2 (P=0.2) = 15.1 m³/s Discussion of the results for the existing situation: • Most of the sections of the gully show supercritical flow conditions with flow velocities of 3 m/s at average • Some natural ponds exist (e.g. station 240 and 680). In these sections subcritical flow conditions can be observed. Erosion rate is reduced in these areas. • Besides natural ponds some sections show slower flow velocities than the average of around 3 m/s due to reduced slope and widening of the gully • With the inflow of SC2 and SC3 at station 491.12 the water levels are higher in the last section of Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 89 the gully Results Hydraulic Model – Construction of Detention Ponds As described in chapter 2.4.5.1 for the model including detention ponds the previously elaborated model for the existing situation has been expanded. Several detention ponds (embankments in the model) have been added and the results checked in an iterative process. In total, 18 detention ponds have been included in the model to reduce the sections with supercritical flow conditions as much as possible. On average, for the total length of the gully a pond will be required every 70m. The results of the hydraulic model with detention ponds are presented in the table below. Table 2-25 Results Hydraulic Model – Detention Ponds River Sta- Channel Water Sur- Energy Q Total Velocity Froude # tion Bottom El. face E.v Grade (m3/s) (m) (m) (m/m) (m/s) 1191 8.2 87.77 90.48 0.0013 1.26 0.24 1141 8.2 85.14 87.15 0.0009 1.17 0.26 1091 8.2 83.02 87.15 0.0004 0.79 0.12 1041 8.2 79.51 82.28 0.0000 0.26 0.05 991 8.2 79.17 80.73 0.0002 0.62 0.16 941 8.2 77.72 80.73 0.0000 0.34 0.06 891 8.2 76.38 78.93 0.0000 0.16 0.03 841 8.2 75.8 77.77 0.0001 0.52 0.12 791 8.2 70.96 72.19 0.0002 0.51 0.15 741 8.2 69.32 72.19 0.0001 0.41 0.08 691 8.2 70.55 72.19 0.0001 0.34 0.09 641 8.2 69.42 72.19 0.0000 0.25 0.05 591 8.2 68.87 69.56 0.0025 1.4 0.54 541 8.2 64.59 66.86 0.0000 0.33 0.07 491 15.1 63.27 65.09 0.0004 0.92 0.22 441 15.1 62.32 65.12 0.0000 0.26 0.05 391 15.1 61.5 63.56 0.0003 0.81 0.18 341 15.1 59.7 60.93 0.0002 0.56 0.16 291 15.1 58.23 60.92 0.0001 0.56 0.11 241 15.1 59.22 60.92 0.0001 0.49 0.12 191 15.1 58.27 60.92 0.0000 0.38 0.07 141 15.1 58.26 60.92 0.0000 0.2 0.04 91 15.1 55.92 58.75 0.0000 0.27 0.05 41 15.1 51.96 52.12 0.0819 3.27 2.66 As for the model for the existing situation the results for stations each 50m have been shown only. Please refer to Annex IV for detailed results table. A long profile for modelling with detention ponds is given in the following figure. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 90 Figure 2-46 Long Profile with Indication of left and right embankment for hydraulic model with detention ponds; Q2 = 15.1 m³/s Discussion of the results for the hydraulic model with detention ponds: • Supercritical flow conditions can be avoided in almost all sections of the gully • This leads to significantly reduced bed shear stresses and thus reduction of the overall erosion in the gully • The detention pond model proved that construction of several detention ponds can stabilize the bed within the gully, hence further erosion will be reduced. Accompanying measures (e.g. em- bankment protection) will be required to reduce erosion into the gully to a minimum. Results Hydraulic Model – Check Dams / Final Situation As previously described, it is expected that the detention ponds will silt up quickly and there will be no retention volume anymore. For setup of this final state model the crest elevation of the detention ponds was set as the new channel bottom elevation for the upstream profiles until intersection of crest level and original ground level. In some cases, the crest elevation will be identical or even higher than the bottom elevation of the closest detention pond upstream. In this situation a cascade system will be created. Due to silting up of the detention ponds a system of various check dams will be created. The results of the hydraulic model with check dams are presented in the table below. Table 2-26 Results Hydraulic Model – Final Situation / Check Dams River Sta- Channel Water Sur- Energy Q Total Velocity Froude # tion Bottom El. face E.v Grade (m3/s) (m) (m) (m/m) (m/s) 1191.28 8.2 88.99 90.38 0.0067 2.45 0.66 1141.28 8.2 85.14 87.18 0.0009 1.15 0.26 1091.28 8.2 85.02 87.05 0.0023 1.62 0.36 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 91 River Sta- Channel Water Sur- Energy Q Total Velocity Froude # tion Bottom El. face E.v Grade (m3/s) (m) (m) (m/m) (m/s) 1041.28 8.2 81.44 82.25 0.0008 0.91 0.32 991.29 8.2 79.72 80.73 0.0007 0.95 0.3 941.28 8.2 79.72 80.68 0.0010 1.07 0.35 891.28 8.2 78.38 78.9 0.0010 0.79 0.35 841.28 8.2 76.84 77.73 0.0013 1.15 0.39 791.28 8.2 71.42 72.34 0.0004 0.68 0.23 741.15 8.2 71.42 72.19 0.0027 1.52 0.55 691.28 8.2 71.42 72.21 0.0005 0.72 0.26 641.27 8.2 71.42 72.15 0.0010 0.93 0.35 591.28 8.2 68.87 69.57 0.0024 1.38 0.53 541.28 8.2 65.99 66.84 0.0007 0.88 0.3 491.12 15.1 64.32 65.02 0.0069 2.38 0.91 441.28 15.1 64.32 65.07 0.0009 0.95 0.35 391.28 15.1 62.6 63.48 0.0034 1.91 0.65 341.28 15.1 60.26 61.29 0.0003 0.67 0.21 291.28 15.1 60.26 61.08 0.0035 1.86 0.66 241.28 15.1 60.26 61.07 0.0010 1.05 0.37 191.28 15.1 60.26 60.91 0.0030 1.55 0.61 141.28 15.1 60.26 60.89 0.0009 0.86 0.35 91.28 15.1 57.92 58.7 0.0009 0.97 0.35 41.28 15.1 51.96 52.12 0.0819 3.27 2.66 As for the previous sections the results for stations each 50m have been shown only. Please refer to Annex IV for detailed results table. A long profile for modelling with check dams (which are sedimented detention ponds) is given in the following figure. As can be seen in the profile, the top level of the detention pond was used for the upstream stations as the new channel bottom elevation due to sedimentation. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 92 Figure 2-47 Long Profile with Indication of left and right embankment for hydraulic model with check dams; Q2 = 15.1 m³/s Discussion of the results for the hydraulic model with check dams: — Except for the sections which are located downstream of a check dam there is no super-critical flow. Occurrence of hydraulic jumps will addressed by stilling basins (Gabion mattresses) — Due to the lost retention volume because of silting up the flow area is less compared to the previ- ous model with fully functioning detention ponds which is results in slightly higher flow velocities — Still, final situation with check dams allows to stabilize the gullies and shows a significant improve- ment compared to the existing situation Comparison of Results for Different Discharges Within the following diagram the water depth and flow velocities in relation to different discharges (return periods: 2, 5, 10, 20 and 50 years) for all three models are presented (existing situation, con- struction of detention ponds, silting up of these ponds with check dams to remain). The figures are based on average values for the gullies. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 93 3 2.5 h (m) 2 1.5 Existing Situation Detention Ponds 1 Check Dams 0.5 0 HQ2 HQ5 HQ10 HQ20 HQ50 Q total (m³/s) Figure 2-48 stage discharge relation (average values) Looking at the stage discharge relation the results of the model show that the existing situation has the lowest water depth in the gully. Due to the construction of detention ponds the water depth rises due to reduction in flow velocity and creation of retention volume as could be expected. After silting up of the detention ponds the check dam model shows reduced water depth. Still, water depth remains significantly higher compared to the existing situation which indicates lower flow velocities. 4 3.5 3 Vel Chnl (m/s) 2.5 2 Existing Situation 1.5 Detention Ponds Check Dams 1 0.5 0 HQ2 HQ5 HQ10 HQ20 HQ50 Q total (m³/s) Figure 2-49 flow velocity vs. discharge A comparison between the flow velocities for the three models shows the following: — Existing situation shows highest flow velocities — Significant reduction due to construction of detention ponds — Slight increase after silting up of detention ponds (check dams model) but still significantly lower compared to the current state Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 94 A comparison of the models showed that for different discharges the detention model performed best when considering reduction of flow velocity as a major goal. The results for the check dam model are not as good but still indicate significant improvement compared to the existing situation. Bed Shear Stress and Erosion Rate In general, for a fluid to begin transporting sediment that is currently at rest, the bed shear stress τ0 exerted by the fluid must exceed the critical shear stress τcr for the initiation of motion of sediment at the bed: !" = !$ The investigated gully in catchment ID 09 has an average width of 15 to 20 meters and an average water depth (for the existing situation) of less than one meter. The impact of the side slopes can therefore be neglected, and the maximum shear stress can be expected at the bed of the gully. With these conditions the following formula for calculation of the bed shear stress can be applied: max !0 ("+,) = . × 0 × ℎ × 2+ — Density of water ρ = 1,000 kg/m³ — Acceleration of gravity g = 9.81 m/s² — Depth of water h — Energy gradient Ie The critical bed shear stress τcr is based on empirical values (Schneider Bautabellen, chapter 13.36). The soil material within the different gullies varies with some being pure sand, some indicating gravel and some showing a vegetation cover. The critical shear stress differs dependent on the conditions: • sand/ fine gravel: τcr = 10 N/m² • grass, long time overflow: τcr = 18 N/m² For the situation within the gully in catchment ID09 the following can be stated. In the middle of the gully is sand which implies that shear stress values above 10 N/m² leads to erosion, values below indicate that there is no or only minor erosion. As there is a certain (minor) vegetation cover along the edges of the gully shear stress values higher than 18 N/m² indicate erosion within all parts of the cross-section. For the three models – existing situation, construction of detention ponds and check dams – the above-mentioned formula was applied for all cross sections computed in the model using the water depth and energy gradient in the sections. The results are summarized and presented in the summa- rized diagram below. Due to uncertainties regarding inflow and outflow boundary conditions within the models the results for the first and last 50m of the gully have been excluded from the analysis. In case of construction of detention ponds (which serve as check dams after silting up) the following assumptions have been made as complete avoidance of erosion is unrealistic without additional measures: — Discharge from small side gullies into the modelled main gully cannot be improved by detention ponds as there is little to no space for the ponds and resettlement to construct these ponds on this smaller scale was not considered. Hence, a certain amount of sediments entering the gully cannot be avoided. — Reduction of flow velocities in the main gully to values with no erosion in all sections and for all Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 95 return periods is unrealistic as the average distance of two adjacent ponds might be too close — The station downstream of a detention pond usually shows supercritical flow (will be protected with scour protection) — Minor sediment transport in the middle of a gully can be accepted as natural process. If the side slopes will be protected sufficiently (= vegetation, retaining walls etc.) the gully will be stable. Additional Remark: Compared to the results submitted within the draft version of this report the num- ber of retention ponds was increased from 15 to 18 based on discussions with technicians from the municipality of Nacala and impressions at the most recent site visit. Due to the above-mentioned assumptions the exceedance for the modelled bed shear stresses in the model with detention ponds and check dams was chosen to 10% (those are mainly the stations with scouring protection downstream of a pond)). Within the following table the average bed shear stresses for all three models – including the assump- tions for the detention pond model and check dam model – is indicated: Table 2-27 Bed Shear Stress Analysis – 5yr Return Period Average Bed Shear Model Maximum Discharge Remarks Stress m³/s N/m² Existing Situation 15.1 71.3 - Detention Pond Model 15.1 2.9 scour protection downstream of pond Check Dams Model 15.1 9.1 required Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 96 130 120 110 100 90 Shear Chan (N/m²) 80 70 Existing Situation 60 Detention Ponds 50 Check Dams 40 30 20 10 0 HQ2 HQ5 HQ10 HQ20 HQ50 Q total (m³/s) Figure 2-50 Bed Shear Stress vs. Q Discussion of results: — The results indicate bed shear stresses for the existing situation of more than 40 N/m² for the shortest return period of 2 years. Major erosion and sediment transport take place as to be ob- served during heavy rainfall events. — The results for the model with detention ponds indicate the lowest bed shear stresses for all re- turn periods. Only minor sediment transport can be expected. The ponds will silt up most likely due to additional sediment input from the sides. — Check dam model: After silting up of the detention ponds the longitudinal slope is significantly reduced compared to the existing situation. The bed shear stresses rise slightly compared to the detention pond model. Return intervals of more than 5 years will lead to erosion within the gully. Due to the increased embankment stabilisation this erosion will take place in the middle of the gully without endangering further growth of the gully. Construction of detention ponds / check dams lead to heavily reduced bed shear stresses and further erosion can be avoided with this proposed measure for rainfall events with a 5-year return period. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 97 Additional Remarks for Technical Analysis All results for the hydrological, hydraulic and sedimentological analysis of the gully presented in this chapter must be discussed in the perspective of the data available. As the results heavily depend on the base data which must be collected, compiled and there are certain uncertainties regarding the quality and the reliability of the model results. In addition, the overall impact of climate change was discussed and adds additional uncertainties. In principal, the data required for a detailed hydrological, hydraulic and sedimentological is compre- hensive. Considering the scope of this project several assumptions had to be made in order to tech- nically assess the situation. Although these assumptions have been made with greatest caution, the results will be different if more data (and time) would have been available. In principal, the consultant selected assumptions on the safe side while still trying to stick as close as possible to the situation observed in Nacala. The general goal of the assessment is to identify if nature-based solutions – as discussed in this report – can help to solve some of the drainage problems in Nacala. The results of this assessment show that there are lots of potential benefits and that the situation can be improved applying these methods. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 98 AQUATIC ECOSYSTEM MAPPING AND ASSESSMENT OF PROPOSED SOLUTIONS - NACALA The advantage of nature-based solutions is that they can effectively combine the positive aspects of natural ecosystems with built infrastructure, in order to maximise the benefits for people. The following sections provide details of the extent and condition of the natural, aquatic ecosystems of the City of Nacala, the ecosystem services they provide, and how these could be improved, supported or im- pacted by the proposed flood and erosion protection measures identified by this project. Ecosystem mapping The natural capital of the City of Nacala is a combination of aquatic (rivers and wetlands, marine environment) and terrestrial ecosystems (soils, geology, dryland vegetation). For this project, the riv- ers, wetlands and vegetation were mapped, and are shown in Figure 2-51. Figure 2-51 Map of the main watercourses, wetlands and sub basins in Nacala. Natural ecosystems are subject to many pressures and continue to be degraded world-wide, resulting in a loss of the ecosystem services which they could provide (Turpie et al., 2010; Snaddon et al., 2018). Climate change will exacerbate ecosystem degradation, and this will have an impact on eco- system services, and the people that rely on them. In order to provide ecosystem services, natural ecosystems need to be secured and appropriately managed or rehabilitated. Healthy ecosystems deliver a wealth of ecosystem services, including both climate change mitigation (e.g. disaster risk reduction though through flood attenuation and sea surge protection, carbon sequestration, etc) and adaptation (e.g. through localised cooling effects, or buffers surrounding urban areas that protect land from storm impacts, and that provide water infiltration areas). The incorporation of wetlands, rivers and landscapes into nature-based or hybrid solutions for flooding Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 99 and erosion provides cost-effective, long-term options for service delivery to people that can supple- ment or even substitute built infrastructure, especially in areas where the latter is limited or non-exist- ent, such as in informal parts of the City. For this study, the aim was to understand how the City’s natural ecosystems and built infrastructure interact and support each other, and to improve this rela- tionship through nature-based and hybrid solutions. Description of the aquatic ecosystems of Nacala Unlike the City of Quelimane, Nacala is located in an area where the rivers are short and the catch- ments relatively small (see Figure 2-52). As described in earlier sections of this report, Nacala lies in a semi-arid part of the country. This has an influence on the quantities and frequencies (i.e. the hy- drological regimes) of base and high flows that reach the City, and the risk of and exposure to flooding and erosion. In Nacala, there are few rivers carrying flow from large, upstream catchments and also few large wetlands and, consequently, few ecosystems with the capacity to hold flow from rainfall, especially during high discharge events. There are a number of short, steep rivers flowing westwards through the City to- wards Baia de Bengo, and fewer, longer rivers flowing along more gentle gradients eastwards of the City to the open ocean, and a few others that lie to the south, also flowing into Baia de Bengo. There is a wa- tershed between the westward and east- ward flowing rivers, which runs from south to north through the City, more or less fol- lowing the route of the EN8. Historically (before the establishment of the City of Nacala), the short, westward-flowing rivers probably would have flowed as fairly small grassland systems (a few metres across) with flashy discharge regimes (short peri- ods of high flow, then longer periods with little or no flow). During high rainfall events, much of the water falling in the area would have run off as sheet flow (diffuse flow over large areas), in addition to the water con- centrated in the river channels themselves. Some of these river channels would have formed short waterfalls over the sandstone cliffs along the coast of Baia de Bengo. Figure 2-52 The main wetland systems of Mozambique (source: Chabwela, 1991 and Saket, 1994). The rivers to the east of the City are meandering systems, flowing off the high ground of the City, and towards a wide coastal wetland and an inlet off the Indian Ocean. Wide swathes of mangrove trees grow along this section of the coast. Within the City, the rivers have been channelized in some areas, and stabilised with gabions in others. Gabion walls have been constructed in several locations within Nacala already. Still, in some locations the stones have been stolen shortly after completion of construction. In other locations gabion walls have been in place for almost 20 years. If gabion walls and stone packings are used community shall be informed that those constructions are installed to protect their properties and stealing of stones is Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 100 endangering people living next to the gabion walls. A few wetlands are located along the coastline to the west of the City, and also to the east of the City. These wetlands occur where the water table is close to or at the ground surface. Some of these wetlands are cultivated as highly productive vegetable gardens (Figure 2-53). Here, the soils are fer- tile, and the water supply plentiful. There are no wetlands located within the elevated part of the City, due to the steep topography and permeable soils, which together reduce the capacity to hold water and sustain wetlands. The vegetation occurring in the wetlands around the City of Nacala includes the bul- rush (Typha capensis), various sedges (Cyperaceae), grasses (Poaceae, such as Pennisetum purpureum), reeds (Phrag- mites australis). To the east of the City, the wetlands and coastline support mangrove trees, which are most likely Avicennia ma- rina, but possibly also other species. This part of the City was not visited during the field trip. Figure 2-53 Vegetable gardens in a wetland to the east of the City Ecosystem services provided by aquatic ecosystems Wetlands and rivers provide a range of ecosystem services. The following sections list these services, as derived from Kotze et al. (2009) and McInnes and Everard (2017). Supporting services underpin all of the other services. They include: — Maintenance of biodiversity — Soil formation — Primary production — Nutrient cycling — Water cycling The provisioning services of aquatic ecosystems cover the products that are derived directly from wetlands and rivers. In Nacala, these include: — Water provision; — Cultivated and wild animal and plant foods; — Building materials such as sand, clay, wood, reeds and sedges; — Wood for fuel; — Grazing for livestock, and — Medicinal plants. The provision of grazing for livestock and the cultivation of wetlands are particularly important services in a semi-arid environment, especially during dry periods or seasons (e.g. Fynn et al., 2015). Regulating services are the process benefits that people receive from wetlands. These include: — Flow regulation; infiltration and recharge — Flood attenuation; Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 101 — Erosion control; — Carbon storage — Sediment retention; and — Water quality improvement. Cultural services relate to the relationship between aquatic ecosystems and people. They include: — Cultural and religious experiences; — Tourism and recreation; and — Education and research. Ecological Evaluation of Proposed Measures The ecological evaluation of the proposed measures has not been done with a complete solution in mind, but rather by assessing the individual measures that make up the „toolkit“ proposed in this report. The individual measures assessed include: — Revegetation of terrestrial areas and riparian zones of watercourses (i.e. along the banks of rivers and man-made channels); — Detention ponds; — Bank protection measures — Creation of additional recreational areas (parks) for retention purposes. The criteria used for the ecological assessment are provided in Table 2-28, with a more detailed assessment presented under Annex III - Ecological Evaluation of Proposed Measures. The aim of nature-based or hybrid solutions is to minimize the negative impacts of such interventions, and to maximize their positive impacts. Table 2-28 Evaluation Criteria for Environmental Consequences of Measures Criterion Description Impact on biodiversity Positive or negative impact on biodiversity, which includes hab- itat, species, communities, ecological processes Impact on ecosystem ser- Positive or negative impact on ecosystem services, as de- vices scribed in Section 2.5.3 Spatial extent of the influ- Limited to the site itself, local area (i.e. limited to within 2 km of the ence of the measure measure), regional (within 30 km of the measure), national (e.g. af- fecting a national priority area or key biodiversity area) or global. Duration of the measure Predict whether the lifespan of the measure will be: Temporary (less than 2 years, ), Short term (2 to 5 years); Medium term (5 to 15 years); Long term (longer than 15 years), or Permanent. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 102 Criterion Description Possible negative ecologi- Summary statements regarding the possible negative ecological im- cal impacts pacts associated with each measure. Table 2-29 Ecological Evaluation of Proposed Measures Measure Impact on biodiversity Revegetation Improves biodiversity by introducing plant species and the fauna associated with them Increases the diversity of habitats available for fauna Provides refuge, feeding and breeding areas for fauna Supports ecological processes such as pollination, fertilisation, genetic diversity. Detention ponds May reduce biodiversity due to replacement of natural areas with hardened sur- faces (weirs and lined ponds) Provides pond habitat during wet months Will support revegetation measures downstream by reducing flooding and erosion in downstream reaches and vegetated areas Bank protection measures Stabilises bank habitat for flora and fauna Supports revegetation measures by stabilising banks, and ensuring an appropri- ate gradient for revegetation Creation of additional rec- Depending on plant species planted, these areas may introduce plant and faunal reational areas (parks) for diversity retention purposes Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 103 3 QUELIMANE REPORT Chapter 3 summarises the results gained from the work performed under Task 3 of the ToR for the city of Quelimane. FIELD VISITS, DATA COLLECTION AND ASSESSMENT OF CURRENT SITUATION Chapter 3.1 provides an overview on the available data base, the observations made during the field visit and a comprehensive assessment of the current situation prevailing at the city of Quelimane. Desktop Review and Preparatory Works After the consulting team has familiarized itself with the data, the field visit took place in August 2018. Based on the compiled data, a desktop mapping within the project area was elaborated, showing the location and rough extent of wetlands and rivers. The mapping included the analysis of the satellite imagery and remote-sensed data grids. Furthermore, urban expansion was reviewed so that future de- velopments in the area are also included in the measures (see Annex 4). The community mapping campaign was planned before the IC’s first field visit to Quelimane and conducted during this visit. Field Visit to Quelimane The visit of the Consultant’s team to Quelimane took place from 13.08. to 17.08.2018. Various locations found to be characteristic of the prevailing conditions in and around the city were inspected and docu- mented by photographs on August 14th, 15th and 16th 2018. Moreover, a workshop was held on August 16th together with local stakeholders and representatives of various authorities aimed at the collection of more specific data on representative sites as well as more detailed information from the local stake- holders on the existing problems. The results of the field visit and the workshop were used as a basis to categorize the investigated sites and to develop appropriate concepts for the implementation of nature based countermeasures. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 104 Figure 3-1 Overview of the site locations Observed Problems in Quelimane: • Sand and wave blocking drainage channels. • Removal of vegetation, leading to instability of soils and erosion of river banks. • Roads without any stormwater drainage systems. • Houses very close to wetland or within wetland. • Silting up of stormwater channels and pipes. • Missing drainage channels in large areas of Quelimane. • Outlets without flap gates. For a complete documentation of the field visit including indication of locations and photos please refer to the following cross–sections. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 105 Site 1: Airport / Aeroporto Site 1, the area around the Airport of Quelimane (and Sraguar B / Pequeno Brasil) is characterized by a small but growing population, small settlements including an institute of higher education and access roads. The population growth can clearly be seen when comparing aerial photographs taken between 2002 and 2018 (see Annex VI, Drawing S1-2). The population actually affected in this area by natural disasters was estimated to approx. 6,000. Especially the area east of the airstrip is prone to additional housing, starting with scattered dwellings to the northwest of the original city limits (e.g. in 2012) and continuously densifying to become a compound urban district (e.g. in 2018). The major problem of this site is the unobstructed drainage of surface waters after heavy rainfalls. The absence of appropriate drainage channels or their pollution and blockage regularly leads to flooding of the surrounding neigh- bourhoods. The impact of the uncontrolled discharge of floodwaters into the undercut bank area of Rio Dos Bons Sinais south of the airport which is subject to informal settlements (Site 6, Figure 3-28) has also to be taken into account. The overall risk of flooding occurs since +/- 15 years and was classified as being very high. However, as it does effect a large and raising number of inhabitants, the need to improve the current situation is considered to be high, see Table 3-1. Figure 3-2 City district east of airport Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 106 Site 2: Port and Bank Wall / Porto e Murro de Protecçáo Site 2, Port and Bank Wall, is located directly at the shore of the Rio Dos Bons Sinais, approx. 0.5 km from the city centre. Site 2: Port and Bank Wall Mangrove belt Rio Dos Bons Sinais Southern edge of bank wall Figure 3-3 Location of Site 2 (Google Earth) In general, the bank wall with the mangrove belt forms a good protection against flooding and erosion, see Figure 3-3 and Figure 3-4. While mangroves east of the container terminal remained nearly un- touched over more than a decade (see Annex VI, Drawing S2-2), westwards of the container terminal, mangroves have been cut which may lead to erosion problems at that site in the future, see Figure 3-5. Also, the comparison of aerial photographs taken from 2002 to 2018 (Drawing S2-2) reveal that the wetlands east of the bank wall attracted several settlers although it is known that the area is prone to regular tidal flooding. In the course of the project it was assumed that ca. 1,500 people are affected by flooding and problems occurring from progressive erosion at Site 2. Figure 3-4 Shore line with mangroves in front of bank wall at Site 2 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 107 Figure 3-5 Cut mangroves west of the container terminal Another point of concern observed in the port area is the dewatering pipes from the city that run through the concrete wall. Most of them are equipped with flap gates which prevent the sea water and storm surges from entering the city. However, in some dewatering pipes flap gates are missing and sea water may enter, causing flooding during high spring tides or storm surges. Thus, there is a moderate risk for flooding due to tides, storm surges and sea level rise. Pipes without flap gates Figure 3-6 Water outlets without flap gates at Site 2 Furthermore, current-induced erosion takes place at the southern edge of the concrete wall as no man- groves are present in front of the wall. Settlements of the pavement behind the quay indicate erosion at the waterside toe. At this location, the risk of coastal erosion is high. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 108 Figure 3-7 Erosion-induced settlements behind bank wall at the south-eastern end of Site 2 Erosion may occur, if an embankment is unprotected against high flow velocities and erosion protection measures have already been installed in the area, see Figure 3-8. Figure 3-8 Erosion protection measures at the south-eastern end of the bank wall Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 109 Site 3: Primeiro de Maio Site 3, Primeiro de Maio, is located in the north of Quelimane, approx. 2.3 km from the city centre. The population affected by flooding is currently estimated to about 25,100 (including the evaluated popula- tion of Manhaua, Santagua and Mapiazua, see Table 3-1) and further population growth is anticipated for the entire area. Existing and added infrastructure mainly comprises housing and access roads. The main problem of the area is the insufficient capacity of sewers which has not been enlarged in accord- ance with the development as well as the discontinuous maintenance - leading to blockage or deterio- ration. The severity of impact of the area is considered to be high. However, as Site 3 is only one of several inner-city districts that consolidate and enhance flooding throughout the entire urban area, the need to find appropriate mitigation matters is classified as being high, see Table 3-1. In the course of the workshop, flooding events were estimated to occur since +/- 10 years. Site 3: Primeiro de Maio Rio Dos Bons Sinais Figure 3-9 Location of Site 3 (Google Earth) Figure 3-10 Disconnected or blocked pipe in urban area Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 110 Site 4: Acordo de Lusaka and existing drainage network Site 4, Acordo de Lusaka, is located northeast of Site 3 in approx. 3 km distance from the city centre. Similar to Site 3, the area of Acordo de Lusaka mainly suffers from flooding caused by blocked or dete- riorated drainage channels. Approx. 6,000 inhabitants are presumed to be affected by these events in this area. Typical infrastructure buildings include dwellings, access roads and standposts. The exposi- tion to recurring flooding is high and flooding of the area is observed since >5 years. The importance to find appropriate mitigation matters is considered to be high. Site 4: Acordo de Lu- saka Rios Dos Bons Sinais Figure 3-11 Location of Site 4 (Google Earth) Figure 3-12 High ground water level in urban areas Sites 3 and 4 are representative for all other inner-city districts discussed during both workshops. Added up with the populations estimated for Barrio Mapiazua, Santagua and Manhaua, the total number of people effected by flooding in the northern part of Quelimane sums up to approx. 31,500. Aerial snap- shots from 2002, 2009, 20012, 2015 and 2018 (see Annex VI, Drawing S3/4-2) show that the amount of undeveloped and thus unsealed land within the city of Quelimane has constantly diminished over Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 111 time. Besides housing and access roads, the area also encompasses school buildings and small infra- structures as shops and workshops. The problems to be encountered in this part of the city are mainly resulting from an insufficient development and maintenance of the drainage and sewage system (Figure 3-12 High ground water level in urban areas (Figure 3-12). Attention should also be paid on the potential risk of drinking water contamination or disruption of its supply during flooding, as the construction of subsurface pipeline networks is uncommon in the area and potable water is mainly available from standposts. Operation and maintenance (O&M) of existing channels and inlets/outlets can be improved. Also a proper waste management and waste collection would improve the drainage capacity (Figure 3-15, Fi- gure 3-16). Figure 3-13 Map of drainage channels and outlets in the city centre Figure 3-14 Drainage inlets and manholes in the city centre Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 112 Figure 3-15 Uncleaned channels Figure 3-16 Uncleaned and unprotected inlet Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 113 Figure 3-17 Lack of maintenance of drainage channels Figure 3-18 Drainage channel inlet Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 114 Site 5: Inhangome Site 5, Inhangome, is located opposite of Site 6, southwest of Quelimane’s airport with a distance of approx. 3 km from the city centre. The area is devided by river distributaries and backwaters and existing infrastructure comprises housing, access roads, agricultural land and a hospital. Due to the missing shoreline protection (partly missing mangrove belt) this site has a high risk of flooding due to tides, storm surges and sea level rise as well as coastal erosion. Since about 10 years, repeatedly occur- ring damage caused by flooding and associated coastal erosion can be observed and it was assumed that about 1,500 people are regularly hit by these incidents. According to local stakeholders, mitigation matters to attenuate the key problems of the area should be discussed with priority. Airport Site 5: Inhangome Wetlands Rio Dos Bons Sinais Figure 3-19 Location of Site 5 – Inhangome (Google Earth) Figure 3-20 Wetlands and shore line without mangroves at Site 5 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 115 Some wetlands are located southwest of the airport on the landside of the access road to Inhangome. These wetlands already serve as water retention areas. Figure 3-21 Wetlands southwest of the airport at Site 5 The wetlands west of Inhangome could not be visited because existing bridges have either collapsed or are in a very bad condition. Erosion was observed along the tributary of the Rio Dos Bons Sinais at Inhangome. Figure 3-22 Wetlands at Inhangome Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 116 Figure 3-23 Condition of the temporary bridge crossing the tributary at Inhangome Figure 3-24 Wooden bridge crossing the tributary at Inhangome / Insufficient erosion pro- tection with thin concrete layer Figure 3-25 Cut mangroves at Inhangome Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 117 Figure 3-26 Wetlands at Inhangome Generally, the main causes of loss of mangroves are • Harvesting for building materials • Harvesting for charcoal • Clearing for agricultural land The natural hydrology and condition of the sediment (in which the mangroves grow and would be replanted) has not been altered significantly. Areas where mangroves have been replanted, around Incidua for instance, seem to be restored relatively successfully. Figure 3-27 Re-growing trees at Incidua. Trees are establishing well Prerequisite for successful restoration lies in having due regard of the species / plant community foreseen to re-grow. This includes: • Substrate height (where to plant the seedlings – best to be planted in ditches) • Water flow (plantation shall be provided with natural hydrology) • Selection of appropriate species It is presumed from the field observations that these requirements can be met in and around Quelimane. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 118 Site 6: Chuabo Dembe Site 6, Chuabo Dembe, is located south of Quelimane’s airport, approx. 2.2 km from the city centre. It is subject to informal settlements and encompasses dwellings, dirt roads and small infrastructures. The dewatering system of the airport runs through this site and visible water outlets seem to be too small to accommodate heavy rainfall. Thus, there is a moderate risk of flooding due to insufficient drainage. Furthermore, the embankments of the outlets are prone to watercourse erosion (moderate to high risk). The population affected by flooding and erosion in this area is estimated to approx. 2,000. Aerial photographs taken between 2002 and 2018 clearly depict how informal dwellings have pro- gressed into the wetlands at Chuabo Dembe (see Annex VI, Drawing S5/6-2). Airport Site 6: Chuabo Dembe Informal settlement in wetlands Existing Mangrove Belt Rio Dos Bons Sinais Figure 3-28 Location of Site 6 (Google Earth) Figure 3-29 Drainage channel downstream of water outlet at Site 6 (SE of the airport) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 119 Figure 3-30 Erosion-prone embankment downstream of water outlet at Site 6 (southwest of the airport) The settlement which develops into the wetland and former mangrove area is flooded twice a day during high tide, in particular during spring tides. There is a high risk of flooding due to tides, storm surges and sea level rise as well as coastal erosion. To counter against the periodical flooding, some land plots and most pathways have been elevated. Figure 3-31 Flooded settlement during high tide and elevated pathways to avoid flooding at Site 6 Figure 3-32 Flooded settlement during high tide at Site 6 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 120 Site 7: Torone Velho Site 7, Torone Velho, is located at the eastern periphery of Quelimane, approx.1.1 km from the city centre, bordering the wetlands a major tidal creek. The area is prone to tidal flooding (especially during spring tides) storm surges and sea level rise. It is assumed that approx. 6,000 inhabitants are impacted by regular flooding. From the succession of aerial photographs taken in between 2002 and 2018 (see Annex VI, Drawing S7-2), a continuous agglomeration of housing - and thus population - can clearly be seen; accompanied by a constant reduction of undeveloped or greened space. Typical for the eastern border of the city of Quelimane the photographs also show that natural barriers, especially natural drain- age channels, seem to bring uncontrolled growth of residential areas to a stop. Rio Site 7: Torone Velho Informal settlement in wetlands Rios Dos Bons Sinais Figure 3-33 Location of Site 7 (Google Earth) Figure 3-34 Commercial area on elevated ground and flooded wetlands / settlements dur- ing spring tide at Site 7 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 121 Figure 3-35 Drainage channel / tidal stream along the settlement boundary at Site 7 Figure 3-36 Flooded plots and settlements during spring tide at Site 7 Figure 3-37 (Informal) harbor at tidal stream at Site 7 and Figure 3-38 Playground upstream of drainage channel at Site 7 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 122 Site 8: Incidua Site 8, Incidua, lies on a peninsular between two tributaries of the Rio Dos Bons Sinais south-east of the city, approx. 2.2 to 3.8 km from the city centre. The ground elevation is the same as of the surround- ing wetlands. Wide areas of the wetlands are prone to erosion as mangroves providing natural protection have been removed. Consequently, a moderate to high risk of flooding due to tides, storm surges and sea level rise as well as coastal erosion can be attributed to this site. Wetlands Rio Rio Emergency bridge Site 8: Incidua Wetlands Rio Dos Bons Sinais Broken bridge Figure 3-39 Location of Site 8 (Google Earth) Figure 3-40 Wetlands without mangroves at Site 8 The peninsula of Incidua is bordered by two tributaries of the Rio Dos Bons Sinais. Bridges spanning over these watercourses are an important infrastructural element for the local community. The flow ve- locities in both riverbeds are high during the tides. In combination with the narrowing of the river’s cross section by bridge abutments and the erosion-prone soils, a very high risk of watercourse erosion can Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 123 be stated. At both rivers, watercourse erosion already led to the collapse of the original bridges. To prevent ongoing erosion at the bridge site, planning for better protection of the bridge abutments should be taken into account. The succession of aerial photographs taken in between 2002 and 2018 (see Annex VI, Drawing S8-2) clearly reveal how the wetlands and river bank south-east of Incidua forms a natural boundary that is not crossed by informal settlements and how population density grew over time within the peninsula. Emergency bridge Insufficiently protected bridge abutment Ongoing erosion at unpro- tected embankment Figure 3-41 Emergency bridge with ongoing erosion in the west of Site 8 Figure 3-42 Existing bearings of emergency bridge (with risk of scouring) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 124 Figure 3-43 Insufficient erosion protection measures with geotextiles at Site 8 Figure 3-44 Collapsed bridge in the east of at Site 8 Figure 3-45 Aerial views of collapsed bridge at Site 8 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 125 Site 9: Ivagalane Site 9, Ivagalane, is located approx. 5 to 7.5 km east of the city centre. To the west, the area is devided from the urban centre by Rio Murropue while to the east it is flanked by a tributary river of Rio Dos Bon Sinais. Only a small patch of land close the bridge spanning over Rio Murropue and bordered by wet- lands to the north and south seems to attract settlers as only a small growth in population can be derived from the aerial/satellite photographs taken in between 2002 and 2018 (see Annex VI, Drawing S9-2). Typical infrastructure comprises access roads, housing, power supply and standposts. The ground ele- vation at Ivagalane is nearly the same as of the surrounding wetlands implicating a high risk of flooding due to tides, storm surges and sea level rise. It was estimated during the workshop that approxi- mately 2,000 people would be affected by floodwaters. Rio Murropoe wetlands wetlands Site 9 Ivagalane Emergency bridge Figure 3-46 Location of Site 9 (Google Earth) Figure 3-47 Settlement at the edge of the wetlands at Site 9 The area of Site 9 is connected to the city of Quelimane by a bridge spanning over Rio Murropue. Flow velocities in the river bed are high, especially during the tides. In combination with the narrowing of the river’s cross section caused by the bridge abutments and the erosion-prone soils, there is a very high risk of watercourse erosion. The original bridge spanning over Rio Murropue was damaged by erosion of the bridge abutments and replaced by an emergency bridge (see Figure 3-48). The span of the emergency bridge is much wider than of the original construction. It is obvious that the riverbed has widened due to erosion of unprotected Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 126 embankments which are still not well protected against erosion, although again forming the abutments of the emergency bridge. Moreover, original bridge foundations are used as intermediate supports for the emergency bridge. An inclination of these supports can already be observed, presumably caused by local scouring, Figure 3-48. If no measures are going to be undertaken to stabilize both, abutments and interme- diate supports, a repeated collapse of the bridge may occur. Emergency bridge Inclined pillar of origi- nal bridge Ongoing erosion at unprotected embankment Figure 3-48 Emergency bridge with ongoing erosion at Site 9 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 127 Site 10: Murropue Site 10, Murropue, is located approx. 6 km northeast of the city centre of Quelimane directly at the western bank of Rio Murropue. Terrestrial and watercourse erosion are the key problems of this site. Erosion effects have been observed since about 5 years and the severity of its impact is considered by locals to be high. To extenuate the erosion effects, groins were constructed by USAID in a prominent river loop (see Figure 3-49 and Figure 3-50). The aerial picture of the existing groins (see Figure 3-51) reveals that the stone filling of some of the groins is missing. Presumably it has been removed by local population for house building or similar construction works. Also, aerial pictures taken from 2002 to 2018 show that the area constantly de- velops since 2012 (see Annex VI, Drawing S10-2). The local population effected by problems resulting from progressive erosion was estimated to 1,275. Infrastructure prevailing in the area comprises streets, housing, electrical supply network, standposts and agricultural land. Site 10 Murropue Wetlands Rio Murropue Figure 3-49 Location of Site 10 (Google Earth) Figure 3-50 Existing groins (constructed by USAID) at Site 10 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 128 Figure 3-51 Aerial picture of existing groins (constructed by USAID) at Site 10 Figure 3-52 Erosion upstream and downstream of existing groins at Site 10 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 129 Site 11: Tivire / Lago Segunda Site 11 was discussed during the workshop in August. However, this site is not part of the project scope. This was also agreed during the workshop in May 2019. Figure 3-53 Location of Site 11 (Google Earth) Figure 3-54 Wetlands at Site 11 northeast of Quelimane Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 130 Site 12: Micajune / Floresta B & A Site 12, Micajune / Floresta B & A, is located at the northern rim of the city, approx. 4.4 km from the city centre. In this region, the groundwater is very high, even during dry season. As such, insufficient drainage of rain waters and flooding of the coastal areas also affects approximately 5,000 inhabitants of this area and corresponding observations have been made since about 5 years. However, during the workshop the impact of any incident was considered to be moderately severe. Drawing S12-2 (see Annex 6) shows that settlement progresses towards the wetlands which will result in a rising impact of flooding on existing and newly established households. Affected infrastructure comprises school build- ings, access roads, standposts and residential houses. Site 12: Micajune Tidal river / Wetlands Figure 3-55 Location of Site 12 (Google Earth) Figure 3-56 Informal settlements in wetlands at Site 12 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 131 Figure 3-57 Wetlands / riverbed at Site 12 Figure 3-58 Wetlands and drainage channel at Site 12 Table 3-1 and Table 3-2 summarize the findings and observations gathered during the field visits and workshops in August 2018 and May 2019. Table 3-1 gives an overview of the representative sites and their categorisation. Table 3-2 summarizes the biophysical characteristics recorded at the field sites. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 132 Table 3-1 Overview of representative sites and their categorization Site No. Field Visit Key Category Characterisation/Key information (Workshop) Problem Affected Population: 6,000 Infrastructure: Airport, houses, access roads, schools, graveyard, market places, drainage pipes and outlets, small Site 1: Airport / Aero- FD bridges, churches porto Severity of Impact: very high to severe Importance of Mitigation: high Airport Issue since when: 15 years Affected Population: ca. 2,500, population influx Infrastructure: housing, access roads, Institute of higher education, churches, market places, no ditches/trenches, Sraguar B FD blockage of water channels (Pequeno Brasil)* Severity of Impact: high Importance of Mitigation: high Issue since when: +/- 10 years Affected Population: 1,500 Infrastructure: Fishing port and port, cool houses, drainage Port and Bank system, residence of the Governor, access roads and roads, Site 2: flood protection wall, slaughterhouse, houses, restaurants, FD, EC, FS Port and Bank Wall churches Severity of Impact: medium (flooding) to high (erosion) Wall Importance of Mitigation: high Issue since when: > 30 years Affected Population: ca. 2,000, population influx Infrastructure: housing, access roads, ditches/trenches Site 3: missing or blocked FD Primeiro de Maio Severity of Impact: medium Importance of Mitigation: high Issue since when: +/- 10 years Affected Population: ca. 6,000 Infrastructure: settlements, access roads and standposts Site 4: FD Severity of Impact: high Acordos de Lusaka Importance of Mitigation: high Issue since when: >5 years Affected Population: ca. 3,000 Drainage Channels Infrastructure: settlements/dwellings, access roads, in the City school, church, standposts (minor tradition of using sub- Bairro Mapiazua* FD merged water lines) Severity of Impact: high Importance of Mitigation: high Issue since when: >5 years Affected Population: ca. 6.500 Infrastructure: access roads, settlements, standposts, market place, church Santagua* FD Severity of Impact: high Importance of Mitigation: high Issue since when: 5 years Affected Population: ca. 13.600 Infrastructure: access roads, standposts, school building and settlements, church Manhaua* FD, FS Severity of Impact: high Importance of Mitigation: high to very high Issue since when: >5 years Affected Population: 1,500 Infrastructure: housing, access roads and roads, agricul- tural land, hospital, market place, borehole for water supply Wetlands / Bridges Site 5: Inhangome FS, EC Severity of Impact: high Importance of Mitigation: high Issue since when: +/- 10 years Divided by river and sea water distributaries/backwaters Affected Population: ca. 2,000 Infrastructure: streets, dwellings and small infrastructures, Site 6: FS , FD , small mooring places for fishing boats Chuabo Dembe EC , ET Severity of Impact: medium to high Importance of Mitigation: medium Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 133 Site No. Field Visit Key Category Characterisation/Key information (Workshop) Problem Issue since when: +/- 5 years Devided by Rio Kwakwa (=Rio Dos Bons Sinais) Affected Population: 6,000 Infrastructure: Churches, houses, schools, access roads and roads, market places, Site 7: Torone Velho FS, FD, Ec Severity of Impact: medium Importance of Mitigation: high Issue since when: > 35 years Affected Population: 7,000 Infrastructure: Hospital, school, access roads and roads, houses, bridges, boreholes for water supply and water Site 8: Incidua FS, EC, ET pipes, market places Severity of Impact: medium to high Importance of Mitigation: high Issue since when: +/- 10 years Affected Population: ca. 2,000 Infrastructure: bridge, school, access roads, power supply, standposts/boreholes for water supply Site 9: Ivagalane F S , ET Severity of Impact: medium to high Importance of Mitigation: high Issue since when: +/- 10 years Devided by Rio Murropue, Affected Population: > 1,275 Infrastructure: streets, housing, electrical supply network, standposts/boreholes for water supply, agricultural land Site 10: Murropue ET Severity of Impact: high Importance of Mitigation: high to very high Issue since when: 5 years Devided by Rio Murropue Site 11: Tivire/ ET No information given; site is not part of the project scope Lago Segunda** Affected Population: ca. 5,000 Infrastructure: school buildings, access roads, stand- post/borehole for water supply, health care centre, ceme- Site 12: Micajune/ F D , ET tery, houses, market place, soccer field Floresta B & A Severity of Impact: high Importance of Mitigation: high Issue since when: + 5 years (site outside study area) Not considered any fur- Affected Population: ca. 10,000 Infrastructure: bridges, access roads, housing Severity of Impact: medium ther Namuinho** Erosion Importance of Mitigation: medium Issue since when: 5 years Devided by Rio Namuinho (site outside study area) Not considered any fur- Affected Population: ca. 160,000 Infrastructure: boreholes for freshwater supply Severity of Impact: medium ther Nicoadala** Erosion Importance of Mitigation: high Issue since when: >5 years Separated by outskirts/undeveloped outskirt areas *Sites that were discussed during the workshop but which are not documented by photographs are presented in italics. **Sites that were not considered any further as they lie outside the study area are marked grey Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 134 Table 3-2 Summary table of biophysical characteristics recorded at the field sites in Quelimane Fauna and flora Site number Bairro Field notes observed at the Problems observed Photos site Thin strip of Mangroves: mangroves along • Avicennia ma- the edge of the rina river, providing Litter and blocked storm- Site 2 Port (Liberdade) • Lumnitzera bank stabilisa- water channels. tion. racemosa Large banks of • Rhizophera Mix of mangrove species at the Port. Litter is dumped between the litter. mucronata mangroves and the wall. Mangrove strip along the bank of the Rio Dos Bons Sinais. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 135 Fauna and flora Site number Bairro Field notes observed at the Problems observed Photos site • Toads Densely popu- (couldn’t iden- Stormwater channels are lated bairro in the tify species) full of litter, and blocked centre of town. • Pistia strati- with rubble. Site 3 Primeiro de Maio Some formalised otes (exotic In some cases, houses stormwater chan- invasive spe- have been built in the nels. cies of water stormwater drains. lettuce) Blocked pipe in 1 de Maio. Flooding occurs around the Acordo de Lu- • Cyperaceae Site 4 stormwater channels. saka • Kikuyu grass Litter dumped. Pond, part of drainage system in Acordo de Lusaka. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 136 Fauna and flora Site number Bairro Field notes observed at the Problems observed Photos site Large areas de- Inundation of areas at high Site 5 Inhangome nuded of man- - tide. groves. Area cleared of mangroves at Inhangome. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 137 Fauna and flora Site number Bairro Field notes observed at the Problems observed Photos site • Phragmites mauritianus Houses have (growing in been built on in- canal from the undated area, on road) fill that is placed • Little egret within a raised (Egretta gar- earth boundary. zetta) Small ponds are • Crabs – only sometimes cre- saw mud Flooding; lack of or poor in- Low tide ated in order to crabs here frastructure; no hard edge Site 6 Chuabo Dembe trap fish on the (Scylla ser- to housing encroachment outgoing tide. rata) into wetlands/mangroves Most “formal” toi- • Mudskippers lets have a con- (Periophthal- servancy tank mus kalolo) that is emptied • Mangrove by the municipal- ity, and air pipe; whelk (Tere- less formal are bralia pallus- tris) pit latrines. • Avicennia ma- rina High tide Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 138 Fauna and flora Site number Bairro Field notes observed at the Problems observed Photos site Informal settle- ment is en- croaching below the high tide • Black kite mark. (Milvus mi- grans) Flooding; lack of or poor in- Mounds of litter High tide and litter at Torone Velho. frastructure; no hard edge Site 7 Torone Velho along the edge of • Grey heron to housing encroachment the water, likely • Striated heron into wetlands/mangroves to float off at high (Butorides tide. striata), Some formal stormwater chan- nels. Formalised stormwater channel at Torone Velho. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 139 Fauna and flora Site number Bairro Field notes observed at the Problems observed Photos site Road and bridge are vulnerable to flooding and ero- sion (bridge foundations are Flooding of houses, and vulnerable). roads, leading to isolation Mangroves have of parts of the City. Houses next to wetlands and channels in Incidua. been removed • Cyperus tex- Mangrove replanting pro- from several ar- tilis (sedge) ject seems successful but Site 8 Incidua eas. • Avicennia ma- there is concern about fol- Mangrove reha- rina. low-up, and policing the re- bilitation project moval of the replanted and resilient trees. housing (using Litter in drainage channels. local materials, such as coconut trunks for build- ing) located here Mangrove rehabilitation project, Incidua. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 140 Fauna and flora Site number Bairro Field notes observed at the Problems observed Photos site Bridge washed away over a year • Mudskippers Bridge failed, has not been ago, not re- (Periophthal- replaced. Site 8 Incidua placed. Bridge mus kalolo) collapsed first, Flow of water in the river is fast when the level is high. and was then washed away. Bridge washed away past Incidua. Very flat, so floodwaters Flooding of houses along come very close the edge of the settlement. to houses. • Mangroves Bridge has been replaced, Bridge was Site 9 Ivagalane along the river but is just resting on the washed away, banks. old piles/abutments. replaced with emergency struc- Bank erosion which makes ture, but struc- the abutments vulnerable. ture is not stable. Wetlands at Ivagalane. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 141 Fauna and flora Site number Bairro Field notes observed at the Problems observed Photos site Slumping of bank at bridge in Ivagalane. Emegency replacement bridge resting on old piles. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 142 Fauna and flora Site number Bairro Field notes observed at the Problems observed Photos site Groynes are be- ing built here (USAID?) to stablise the bank erosion and al- Site 10 Morropue - Bank erosion. low build-up of sediments. Groynes are be- ing built using lo- cal materials. Groynes being built at Morropue. Settlement is en- croaching into • Painted reed frog (Hypero- the wetland. lius mar- Wetland is well- moratus) – Flooding of houses and Site 12 Micajune vegetated but no this is the roads. mangroves, so eastern-most likely to be a observation of freshwater sys- this species. tem. Wetland at Micajune / Inhangome. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 143 Fauna and flora Site number Bairro Field notes observed at the Problems observed Photos site Painted reed frog on a reed. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 144 Data Collection Received Data Based on the meetings and contacts held with the Client, the Storm Water Drainage team, the Munici- pality and other stakeholders, the consultant collected the data as listed in Table 3-3 and subsequent paragraphs. Table 3-3 Received data and information Information Source Comments Basemap / Vector data OpenStreetMap (OSM) Vector data DIVA-GIS Vector data WFP Mapping Elevation Grids SRTM, ASTER Orthophotos ESRI Imagery, Google Imagery Hydrological Data INAM Precipitation, Temperature R5 Hazard Data World Bank Bathymetry INAHINA, Port of Quelimane Landcover CCI Climate Data WorldClim Water Levels INAHINA Geological Map Grantham et al., 2008 In addition to the data listed in Table 3-3, the following sources were used to extend the knowledge on key figures of the area under investigation. Additional Data Collection USAGE OF UNMANNED AERIAL VEHICLE (UAV) UAV were used to collect high-resolution aerial imagery of Sites 5, 7, 8, 10, 11 and 12. LOCAL INFORMATION Through interviews and workshops in Quelimane the IC gathered information from the communities and local stakeholders on existing erosion, drainage and flooding problems. Based on the outcome of the mapping activities, the IC's team identified flooding/erosion zones in and around the city of Quelimane and collected additional data. SOIL CONDITIONS To assess the soil conditions in the project area, the IC acquired a geological map of the area [Grantham et al., 2008]. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 145 ACQUIRED HYDROGRAPHICAL INFORMATION The topography in Quelimane ranges from 3.66 to 8.14 above the mean-sea level [CCAP (2015): “Vul- nerability Mapping in Quelimane and Pemba]. Nautical charts for the project area and drawings of existing flood protection structures in Quelimane were reviewed. Bathymetric data of the tidal bights / navigational channels to the ports of Quelimane were provided by Instituto Nacional de Hidrografia E Navegação, Mozambique (INAHINA), (Figure 3-61). The hydrographical data is available as analogous maps and thus only allows for limited interpre- tive approach. Bathymetric measurement data date from 2011 and were gathered around the commer- cial quay and the bank wall south of Quelimane. Figure 3-59 Extract of nautical charts for Quelimane Information about chart datum is given in the nautical charts as follows: Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 146 The following pictures show the depths in the tidal river Rio dos Bons Sinais along the bank wall and port area. Figure 3-60 Bathymetric Measurements 2011 [INAHINA, 2011] Figure 3-61 Bathymetric Measurements 2011 [INAHINA, 2011] HYDROLOGICAL AND TIDAL DATA Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 147 Hydrological data were collected from gauging stations in Quelimane and tidal calenders (Figure 3-61, Figure 3-62 and Figure 3-63). The Chart Datum in Quelimane is 2.60 m below medium water level. Figure 3-62 Predicted tide curves for Quelimane for August 2018 [Wxtide, 2018] Figure 3-63 Tide curves for Quelimane for the year 2018 [Wxtide, 2018] Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 148 Figure 3-64 Tide tables for Quelimane [INAHINA, 2018] Figure 3-65 Tide values for Quelimane [INAHINA, 2018] Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 149 Gauging stations and benchmarks in and around Quelimane do exist but are in parts inadequately main- tained and data does not seem to be regularly recorded and collected by a responsible authority. Thus, target-specific data analyses are only limited. Figure 3-66 Radar gauge (defective) and water level gauge at pier of fishing port Figure 3-67 Tidal levels for Quelimane according to sea chart 16402 Figure 3-68 Existing benchmark at collapsed bridge Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 150 Figure 3-69 Normal spring tide situation in Quelimane (13th of August 2018) PREVIOUS FLOOD EVENTS Tropical storm 'Desmond' brought heavy rain and strong winds of up to 70km/h to Mozambique in Jan- uary 2019. Parts of Quelimane were flooded after the storm had made landfall. Figure 3-70 shows those areas around Quelimane that were effected the most by flooding during Desmond. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 151 Figure 3-70 Flooded areas / Delineation map (Storm Desmond, 01/2019) [https://emer- gency.copernicus.eu] Figure 3-71 Flooded areas / Delineation map (Storm IDAI, 03/2019) [https://emergency.co- pernicus.eu] Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 152 VULNERABILITY MAPPING BY USAID D) Exposure Map for Natural Disasters based B) Sensitivity Map for Natural Disasters based on popu- on topography, vegetation and mangrove lation density, schools, housing type and sanitation C) Adaptation Capacity Map for Natural Disastersbased on A) Vulnerability Map for Natural Disasters proximity to roads, schools, churches and markets as well aggregation of Maps B, C and D as vegetation index Figure 3-72 Vulnerability (A), Exposure (B), Sensitivity (C) and Adaption Capacity (D) for Natural Disaster at Quelimane Municipality with indexes ranging from very low (green) to very high (red) [USAID, 2015] Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 153 LITERATURE REVIEW A literature research was conducted to identify and evaluate the recent internationally published litera- ture related to “Building or Engineering with Nature” and similar projects currently conducted in Quelimane and Mozambique. References used are listed in Chapter 6. However, internationally recog- nized standards and guidelines for the design, evaluation, and construction of nature-based structures are not yet available. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 154 Community Based Mapping Campaign Methodology Please refer to section 2.1.4.1 Questionnaire Please refer to section 2.1.4.1 Results Summary The main goal of the community mapping campaign was to identify sites and gather first information – especially photos and locations. Furthermore, the free-text comments were important for a first assessment. The gathered information was used to identify sites to visit during the consultant’s team field visit. Questions 4 and 5 were collecting mostly subjective information. Most issues recorded were considered to have a high to very high severity of impact and also a high importance of mitigation. The mapping also shows that the answers regarding the affected population and timing of the issue are rather inconsistent. Therefore, the information gathered in these steps is considered to have a low significance and has been verified on site during the field visit and during both workshops in August 2018 and May 2019. Figure 3-73 Quelimane, Community Mapping Results for Questions 4, 5 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 155 In conclusion, the Community Based Mapping Campaign provided the following inputs: — Community Mapping Campaign helped to identify sites and gather lots of information — The results of the campaign gave a positive input for the further advancement of the project — Preparation of the questionnaire and selection of applicable answers have a high influence on the significance of the results and must be prepared carefully A complete documentation of the results of the Community Mapping Campaign is given in Annex II - Community Based Mapping Campaign – Collected Data. Summary and Assessment of Current Situation - Quelimane Table 3-1 and Table 3-2 provide an overview of all sites visited and / or discussed during the workshop. The key problems observed in these areas are classified as following: • FD: Flooding due to insufficient drainage / waste management • FS: Flooding due to tides, storm surges and / or sea level rise • EC: Coastal erosion • ET: Terrestrial and watercourse erosion The main challenges in the City of Quelimane relate to flooding of infrastructure (houses, roads, airport, agricultural areas, etc), deforestation (which exacerbates the flooding), and lack of maintenance of the stormwater infrastructure. 1. Flooding: Houses, industries and road infrastructure are seen to be encroaching into areas of in- creasing flood risk. There appears to have been repeated encroachment of legal and illegal housing into high flood risk areas within the City, for instance into parts of the City that are inundated daily at high tides. Industrial complexes have also been constructed within wetlands and river floodplains, leading to an increased risk of flooding of these areas (see Figure 3-74). In addition, the proliferation of buildings, industries and roads in the City has led to hardening of the catchments, through soil sealing, and this has the direct result of changing the hydrology (i.e. the volume, velocity and fre- quency) of surface flow throughout the City. This may contribute to flooding, and may exacerbate the lack of capacity of the stormwater management system. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 156 Figure 3-74 Industrial development (within the red outline) in a wetland. 2. Deforestation: The removal of mangroves and other indigenous and even exotic trees for use as building materials and for charcoal production, causes instability of the soils by removing stabilising root matter from the soil, and changing the manner in which water falls on the soil – rain falling on bare soil has a higher erosive capacity than the same rain falling on leaves before it hits the soil. The rain falls with greater intensity on bare soil, washing the finer soil particles away and leading to greater soil instability. Trees, shrubs and grasses also get rid of water through transpiration, thereby reducing the total volume of water exiting the catchment. 3. Stormwater management: The management and maintenance of the stormwater and flood pro- tection drainage system within Quelimane pose some real challenges. The formalised stormwater infrastructure in the City appears to be blocked or damaged in several areas. Sand, rubble and litter blocks drainage channels and pipelines, and accumulates in detention and retention basins, and in the streets. These blockages and damaged infrastructure lead to the switching of flow pathways, from the desired route to alternative routes through residential or industrial areas, causing flooding in these areas and isolation of parts of the City during these floods. The stormwater infrastructure is not keeping pace with the urbanisation and densification of houses, roads and infrastructure in Quelimane, and does not cater for the predicted increases in the fre- quency and intensity of storms due to climate change across the African continent. This does not allow for the systematic and effective design of stormwater management systems within the City, which would be able to cope with the volume and velocity of runoff experienced during the high rainfall months, especially during storms and cyclones. Table 3-1 provides information on the estimated number of people affected by any natural disasters as discussed during the workshop and site visit. For comparison and as data base of subsequent risk estimation, the population of the various sites was again estimated based on the surface area and the number of houses of each site (area and houses were measured/counted on the base of Google Earth satellite images). Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 157 Table 3-4 Estimated population per site (based on surface area and counted no. of houses) Surface area No. of inhabit- No. of inhabit- Surface area No. of Site with houses ants ants [m²] houses [m²] (5/house) (10/house) Quelimane 30,086,737 30.086.737 34,152 170,760 341,520 Site 1 2,141,932 255,418 660 3,300 6,600 Site 2 337,505 127,423 72 360 720 Site 3 & 4 5,094,018 5,094,018 9,6360 48,180 96,360 Site 5 1,577,203 479,436 511 2,555 5,110 Site 6 635,988 319,622 516 2,580 5,160 Site 7 291,464 167,316 421 2,105 4,210 Site 8 2,882,806 1,653,645 1,681 8,405 16,810 Site 9 2,933,343 678,775 93 465 930 Site 10 497,066 330,640 84 420 840 Site 12 473,118 149,227 145 725 1,450 Total 16,864,443 9,255,519 13,819 69,095 138,190 For the risk assessment the data given by local stakeholders at both workshops were considered. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 158 IDENTIFICATION OF POSSIBLE MEASURES - QUELIMANE Overview of Possible Measures to be Applied in Quelimane According to the Preliminary City Risk Profiles from the CityCORE Africa Project provided with the ToR and after evaluation of the visited sites, following strategic objectives were identified for Quelimane: • Expand and improve the city's drainage system • Improve the city's water capture and supply system • Introduce Coastal Protection Measures taking into account sea level rise, marine intrusion and storms • Introduce erosion protection measures Allocation of Possible Measures to Sites In the following, suitable flood and erosion measures are described for each site. The categorisation of each site is according to Table 3-1. Moreover, Table 3-5 gives an overview of the concepts. The con- cepts are also visualized in the drawings in Annex V for each site. General Remarks In this stage of planning, only a very rough topography is available. Thus, the layout of the retention ponds, dikes and drainage channels are preliminary concepts, only. They will have to be adapted in next planning phase. In order to minimize the risk and the extent of the required measures, further development of settlements in the wetlands and in the areas where measures are proposed should be prevented. Site 1: Airport / Aeroporto In order to reduce the risk of flooding due to heavy rainfalls, several retention basins and new drainage channels shall be provided at site 1. The drainage channels shall be equipped with flap gates to prevent backwater during high tides and storm surges. Figure 3-75 Proposed flood and erosion measures at Site 1 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 159 Site 2: Port and Bank Wall / Porto e Murro de Protecçáo At the south edge of the Bank Wall, a revetment will be required to prevent further erosion due to high current velocities. This solution cannot be a sole green revetment, as the flow velocities are too high at this location as site visits have shown. Figure 3-76 Proposed flood and erosion measures at Site 2 Sites 3 and 4: Primeiro de Maio / Acordo de Lusaka At sites 3 and 4, new drainage channels and retention basins shall be constructed in order to reduce the flooding during heavy rainfalls. Also, the existing drainage channels shall be rehabilitated and main- tained. With regard to the large numbers of houses and dense development, the new drainage channels will have to be constructed mainly as underground pipes. Figure 3-77 Proposed flood and erosion measures at Site 3 and 4 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 160 Site 5 and 6: Inhangome and Chuabo Dembe At site 6, flooding due to tides and storm water surges will be prevented by a green dike which acts as enclosure for the present (informal) settlement. However, in order to ensure the drainage of rain water, numerous outlets equipped with flap gates will be required. At site 5, the shore line will be protected by a green revetment. Furthermore, the remaining part of the wetland between sites 5 and 6 (brown coloured area) shall be used to re-grow mangroves, which act as a natural protection against waves during storm surges. The success rate for the mangrove seedlings of Avicennia marina species is far higher than for other species, so it is recommended to be used as pioneer species. Other species can be planted during a later phase to gain higher diversification. Figure 3-78 Proposed flood and erosion measures at Sites 5 and 6 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 161 Site 7: Torone Velho At site 7, the shore line needs to be protected by either a green slope or dike, depending on the topog- raphy. As the dike or slope will need to be located between the existing drainage channel and the set- tlement, it is most likely that some houses will have to be relocated. Furthermore, the existing drainage channels will have to be rehabilitated and maintained and retention basins shall be provided in suitable locations. The Consultant has identified areas in the vicinity where retention basins could be constructed. However, the property owners would have to be consulted in next planning phases. Figure 3-79 Proposed flood and erosion measures at Site 7 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 162 Site 8: Incidua Here, the existing settlements will be protected by re-growing of the mangroves in the perimeter. The shore line directly adjacent to the tidal river in the south will require a revetment. Presumably, this revetment cannot be constructed as a green revetment due to the high flow velocities. The abutments at the two bridges shall also be protected by stone revetments to avoid further erosion. Figure 3-80 Proposed flood and erosion measures at Site 8 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 163 Site 9: Ivagalane At site 9, the abutments at the bridge shall be protected by stone revetments. Figure 3-81 Proposed flood and erosion measures at Site 9 Site 10: Murropue Here, further erosion along the banks will be prevented by a revetment. This revetment cannot be con- structed as a green revetment due to the high flow velocities. As described before, the existing measures financed by USHD are not as effective as they should be. Figure 3-82 Proposed flood and erosion measures at Site 10 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 164 Site 12: Micajune At this Site, the flooding shall be prevented by maintenance and rehabilitation of existing drainage sys- tems in combination with a retention basin. Figure 3-83 Proposed flood and erosion measures at Site 12 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 165 Table 3-5 Description of measures possibly applied in Quelimane Main Prob- Category Method Characterization / Key Information Pros (+) and Cons (-) Examples lem • One of the most effective bridge protections + location of potential where a road embankment crosses floodplain damage is moved away from the bridge Guide Banks • Consisting of sand fill with stone riprap • Reduces rapidly diverging flow turbulences and + natural resources scour -> smooth flow, less erosion + easy to repair Bridges and Erosion Bank Wall • Stone rip rap and gabions at high current veloc- Reinforcement ities + mostly natural re- of the Bridge sources • Possible that loose stones will be taken (for Abutments building material) + easy to repair • Possible solution: superficial grouting of the stones Flooding + high durability, even due to at high flow velocities • Dissipation vs. Reflection storm “Wave Steps” surges and • Research project of Leibniz University, Hano- - no nature-based solu- erosion ver, Germany tion „Wave steps“, Hamburg Bank Wall + rainwater can drain Flooding and tidal water is kept due to in- outside Flap gates sufficient • Fit drainage pipes with flap gates drainage - rainfall and tidal high water: no discharge of rainwater possible Flap gates Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane Main Prob- Categories Method Characterization / Key Information Pros (+) and Cons (-) Examples lem + local resources + easy to repair Flooding due to + logistic, easy to Wetlands and Geotextile storm transport (sand filling Bank Wall bags • Filled with sand surges and on site) erosion - a damaged, bag influ- ences position of other bags + mostly natural re- sources • Materials: sand, silt / clay Dikes + easy to repair • Covered with reinforced vegetation + experience - large footprint Flooding due to + local resources storm Wetlands surges + easy to repair - large amount of back- fill material (sand re- Landfill / Back- quired) fill • Backfill the regularly flooded zones - only suitable for (up to now) uninhabited ar- eas - erosion protection re- quired Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane Main Prob- Characterization / Key Information Pros (+) and Cons (-) Categories Method Examples lem • Vertical elements (of timber or steel) bring shear Vegetation re- forces into the ground. inforced + natural resources slopes and o Length 1 to 3 m embankments + easy to repair o Diameter 20 to 200 mm o In between: vegetation Erosion Wetlands (cont.) + nature-based solution Geochutes • Cells are filled with earth and planted or filled with cement. + can be planted B Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane Main Prob- Pros (+) and Cons (-) Categories Method Characterization / Key Information Examples lem • Geotextile and establish vegetation + natural resources • Robust and adaptable plants necessary (dry and wet + easy to repair season) + natural resources + local products • Coconut fibre geotextiles will be covered by soil and grass and thus stabilized + inexpensive • natural revetment of coconut mats, enhancing + job creation Vegetation re- growths of vegetation + Involvement of local Wetlands inforced population Erosion • Research project of the Leibniz University Han- (cont.) slopes and nover, Germany + easy to repair embankments - still under research • Covering Vegetation (sea weed and / or man- groves) • Reduction of wave height: (dissipation of en- ergy in percentage according to Leibniz Univer- + plants which can sur- sity) vive in salt water Seaweed: 25-45% + natural resources o strong rooting - time to grow required o sensitive against water pollution - prevention of erosion of seeds during Salt marsh (grass): 62-79% growth process re- Mangroves: 25-37% (width :800 m–1500 m of mangrove quired (geotextiles or belt) coconut fibre mats) Seaweed below water level o 3 to 5 times cheaper than conventional wave breakers o adaptable: grow with rising sea level Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane Main Categories Method Characterization / Key Information Pros (+) and Cons (-) Examples Problem + combination: public places (football, parking places) during dry season and flood protection • Targeted drainage into basins during wet season Retention Basins • Enhance infiltration by use of rub- - risk of malaria if water is stag- ble drains etc. nant too long - large areas required + larger discharge volumes pos- Flooding • Deepening of the existing chan- sible Airport and due to in- nels City sufficient drainage • Diverting the drainage system + decentralisation of discharge into the surrounding (unoccupied) wetlands Rehabilitation of exist- + natural resources ing drainage channels and / or new drainage + easy to repair channels • Using vegetation as covering - vegetation cover: difficult to re- (embankment, trapezoidal shape move waste with reinforced vegetation) - larger area required than with concrete solution - maintenance of vegetation re- Embankment with vegetation quired Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane RISK ASSESSMENT - CITY RISK PROFILE QUELIMANE Introduction Based on the available information on existing site and environmental conditions at the sites investigated within the present study, the potential risks due to flooding have been assessed. Flooding risks comprise flooding due to heavy rainfall events, either due to insufficient capacity of formal (canalised) or informal (streams and tributaries), or indirectly due to erosion related to enhanced flows resulting in bank erosion. Furthermore, flooding risks may be caused by high sea levels, as a combina- tion of high tidal levels, storm surges and future sea level rise, and associated indirect risks due to bank erosion resulting from enhanced flows during such high sea level events. These risks, as a combination of their estimated probabilities of occurrence as well as their impacts on the local socio-economics of the local and wider population, have been evaluated, in order to identify sites with higher risks. This may inform decision-making on prioritisation of measures to reduce flood and erosion risks, as proposed within this report. Approach Risk is defined as the product of the probability of risk events with their impacts on the local community. Within the context of the present study, such risk events comprise flood events due to the failure, insuf- ficient capacity or absence of the local flood protection system. These events can be driven by high sea levels (as combination of tides and storm surges) or by strong and/or persistent rainfall events. The flood protection system consists of the various urban drainage canals and gullies, shoreline protec- tion (e.g. mangroves or other protective vegetation, groynes, revetments and other engineered water frontages), as well as an protective array of (informal) bunds, dikes and other elevated areas. The probabilities can be estimated from the available information on site conditions and historic datasets and statistics on sea levels and strong precipitation events. Estimation of the impacts of flood events is of high complexity, as it may comprise a variety of indirect impacts, such as spread of diseases, long-term negative socio-economic impacts and direct or indirect casualties. Within the framework of this study, such impacts have been simulated by assessing potential impacts on per capita income within defined affected areas, whilst taking into account other assets of wider socio-economic value. Therefore, the following approach has been adopted to conduct the risk assessment for Quelimane: 1. Assessment of Probabilities: a. Identification of key risk factors for flooding (high sea level or strong rainfall events). b. Estimation of critical values leading to occurrence of flooding (e.g. maximum allowable sea level or maximum precipitation event as combination of rate and duration). c. Estimation of probabilities of conditions exceeding critical values. 2. Assessment of Impacts: a. Derive annual income of affected area by multiplying: i. Affected area ii. Population density iii. GDP b. Possible additional value due to socio-economically valuable assets. 3. Estimation of risks: Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 171 a. Multiplication of estimated probabilities and impacts These steps and their outcomes are described in detail in the following sections. Assessment of Probabilities of Risk Events Introduction To assess the probability of flooding events at each of the investigated sites, as a first step the key risk factors are identified. These may consist of high sea levels, strong rainfall events, or a combination of the two. Where erosion is experienced, this is either the result of currents induced by the tides, storm surges and sea level rise (i.e. sea level-related) or by currents resulting from heavy precipitation and runoff (i.e. rainfall-related). Erosion risks are therefore attributed to these underlying hydrological/ hy- drodynamic source risk factors in this assessment. As a second step, the available site information has been reviewed to estimate critical levels, i.e. levels that, when exceeded, would likely induce risk events (flooding and/or erosion). Due to the limited infor- mation available (e.g. absence of detailed topography and information on capacities of existing drainage system and flow capacity), this could only be carried out at a qualitative level. With aid of the established critical levels, the available information on the key risk factors, in terms of rainfall rates and sea levels, was made. In this manner indicative probabilities of recurrence of critical risk events were derived for each of the investigated sites. Identification of Key Risk Factors As a basis for the identification of project measures to mitigate the occurring problems and risks, the sites were categorised by the following site characteristics (see Table 3-1 and Table 3-6): • Airport • Port and Bank Wall • Drainage Channels in the City • Wetlands / Bridges Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 172 Table 3-6 Overview of problems / risks observed in Quelimane Flooding Flooding due to Terrestrial due to in- tides, storm Coastal ero- and water- Category Site sufficient surges, sea sion course ero- drainage level rise sion Site 1: Airport / Ae- Airport + -- -- -- roporto Site 2: Port and Port and Bank ++ (locally at Bank Wall / Porto e -- + southern edge -- Wall Murro de Pro- of bank wall) tecçáo Site 3: + -- -- -- in the City Primeiro de Maio Channels Drainage Site 4: Acordos de Lu- ++ -- -- -- saka Site 5: Inhan- -- ++ ++ -- gome Site 6: Chuabo + ++ ++ + Dembe Site 7: Torone + + -- -- Wetlands / Bridges Velho Site 8: Incidua -- + + ++ (bridge) Site 9: Ivagalane -- + -- ++ (bridge) Site 10: Murro- -- -- -- ++ pue Site 11: Tivire / Lago Se- -- -- -- + gunda Site 12: Micajune + -- -- + Key: (--) No risk, (-) Low risk, (+) Moderate risk, (++) High risk Based on the available information obtained from the site visits, the workshops and the community- based mapping campaign the following key risk factors are derived: • The main problem of Site 1 is the obstructed drainage of surface water after heavy rainfalls. The absence of appropriate drainage channels and lack of proper maintenance leads to flooding of surrounding neighbourhoods. The overall risk of flooding is classified as moderate. The key risk factor is hence rainfall. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 173 • Site 2 is prone to regular tidal flooding at wetlands east of the bank wall. Some flap gates on dewatering pipes are missing, causing flooding during high spring tides or storm surges. Shore- line erosion is noted towards the southerly end of the bank wall. The key risk factor is hence sea level. • The main problem of Site 3 is insufficient capacity of sewers which has not been enlarged in accordance with the development as well as the discontinuous maintenance – leading to block- age or deterioration. Severity of impact is considered to be intermediate. The key risk factor is hence rainfall. • Site 4 suffers from flooding caused by blocked or deteriorated drainage channels. Exposition to recurring flooding is high. The key risk factor is hence rainfall. • Due to the missing shoreline protection, Site 5 has a high risk of flooding due to tides, storm surges and sea level rise, as well as shoreline erosion along the river and its tributary. Problems were suggested to have priority. The key risk factor is hence sea level. • Site 6 is affected by the dewatering system of the airport, which is insufficient to accommodate heavy rainfall. Local settlements are flooded twice daily at high tide. Key risk factors therefore comprise both sea level as well as rainfall. • The area of Site 7 is prone to flooding due to high sea level, as combination of tides, storm surges and sea level rise. The key risk factor is hence sea level. • Site 8 comprises a peninsula, enclosed by two tributaries of the main river, surrounding wet- lands and includes two bridge sites across the tributaries. The site is subject to flooding during high sea levels due to the low elevation, as well as erosion induced by tidal currents. Erosion has already caused collapse of the original bridges and threatens the abutments of an emer- gency bridge. The key risk factor is therefore sea level. • A tributary to the main river flanks Site 9. Due to the low elevation of the area, this area is at risk of flooding due to high sea levels. The site comprises a bridge connecting the city to the sur- rounding area. The original bridge has been damaged due to erosion and has been replaced by an emergency bridge. The stability of the latter is threatened by erosion, resulting mainly from tidal flows. Key risk factor is therefore sea level. • Whereas ground levels at Site 10 are sufficient to avoid serious flooding problems, the banks of the tributary along the site are threatened by erosion due to runoff currents in combination with the curved alignment of the stream. The key risk factor at this site is hence rainfall. • Site 11 is not considered in this assessment. • Site 12 includes wetlands with informal settlements as well as other dwellings at low elevation. Groundwater levels remain high throughout the year in this area. Rainfall and erosion due to runoff are the main problems experienced. Rainfall therefore constitutes the key risk factor at this site. Estimation of Critical Values for Risk Events The map with the city’s drainage channels shows that the storm water drainage system is largely limited to the city centre, between the airport to the west and the wetlands to the east. Capacities and further details, apart from the observations made during the site visits, are unknown. During the site visits it was observed that various channels lack maintenance and clearance of debris and sediments, which impede on the drainage capacity and efficiency of the system. For the present assessment it is therefore crudely assumed that regular maintenance and clearance will be carried out in the future, maximising the capacity of the presently existing system. Away from the drainage system’s area, it is assumed that insufficient drainage and temporal storage (retention) areas are available. In addition, low-lying areas will receive surface runoff from adjacent higher areas, exacerbating local flooding problems. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 174 It is not possible to derive critical rainfall intensities for the sites investigated in the present study, as the capacity of the various parts of the drainage system is not known. To obtain some indication, it is crudely assumed that the system, when properly maintained, is able to cope with the average annual and monthly rainfall rates over the past years (2010-2017, refer to Table 3.10). The average annual rainfall rate is equal to 1,104.4 mm/year and the average monthly rate of the wettest month (January) is equal to 292.2 mm/month. Clearly, shorter duration events will have a much higher rainfall rate than the monthly averages, as the monthly averages include variation between days (including days without significant rainfall), and varia- tion during a day (including hours without significant rainfall). Since the average annual rainfall rate is similar to that derived for Lumbo (950 mm/year), and lies within the same rainfall region in Mozambique, the intensity-distribution-frequency (IDF) curve values for this station have been used as first order indi- cation for Quelimane, as listed in Table 3.7. Table 3.7 IDF-Curve Values for Quelimane Precipitation Intensity (mm/hr) Rainfall Duration (min) 20 60 120 160 200 260 T = 2yr 129.8 66.6 43.7 36.7 32.0 27.3 T = 5yr 175.9 91.6 60.7 51.2 44.8 38.3 T = 10yr 206.2 108.2 72.0 60.8 53.4 45.8 T = 20yr 235.2 124.1 82.9 70.1 61.6 52.9 T = 25yr 244.8 129.3 86.4 73.1 64.2 55.1 T = 50yr 273.0 144.8 97.0 82.2 72.2 62.1 Since critical rainfall intensities for the investigated sites cannot be derived based on the available data and information, in this assessment the return period has been derived based on the anecdotal indica- tion on risk levels obtained during the site visits and workshops. Where high risks, impacts or recurrence were indicated, a medium return period (T=10yr) is adopted, whereas for moderate or intermediate in- dications, the lowest return period in Table 3.7 (T=2yr) is assumed. More extreme precipitation, associ- ated with larger return periods, is deemed beyond the scope of this assessment. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 175 Table 3.8 Critical Precipitation Intensity No. Site Recurrence based on site visit reports Critical Return Period “moderate risk” 1 Airport / Aeroporto T=5yr “importance of mitigation is high” Port & Bank Wall / Porto 2 Rainfall not considered a key risk factor e Murro de Protecçáo “intermediate risk” 3 Primeiro de Maio T=5yr “need for mitigation is high” “high recurrence of flooding” 4 Acordos de Lusaka T=5yr “importance for mitigation is high” 5 Inhangome Rainfall not considered a key risk factor “moderate risk due to insufficient drainage” 6 Chuabo Dembe T=2yr “importance of migigation is medium” 7 Torone Velho Rainfall not considered a key risk factor 8 Incidua Rainfall not considered a key risk factor 9 Ivagalane Rainfall not considered a key risk factor “impact considered high” 10 Murrope T=5yr “importance of mitigation is high to very high” 11 Not considered in this assessment 12 Micajune/Floresta B/A “groundwater is very high, even during dry season” T=5yr The results of graphical modelling of potential flooding is shown in Figure 3-84, based on satellite im- agery available from the project’s database. This data is available as fairly coarse 30x30 m gridded data. Small scale features are therefore not captured. It is furthermore noted that the satellite data appears not to have been filtered for structures or vegetation. Therefore, actual levels may be lower than included in the model as the levels represent house and tree tops rather than surface levels. Relative levels derived from the satellite imagery were referenced to the known elevation of the port area (6 mCD). Three indicative high sea level states, namely 5 mCD, 6 mCD and 7 mCD, were subse- quently imposed on this terrain model to identify areas with potential flood risks due to high sea levels. For example, where yellow is shown, land levels are below 6 mCD and such areas could be flooded in events with higher sea levels, subject to flood duration, flooding pathways and any existing flood pro- tection (e.g. embankments, surrounding elevated areas, dykes, etc.). The critical level for such areas would be 5 mCD, on the condition the area does not show substantial green areas. The results on the one hand confirm the status quo, i.e. that high tidal states (5 mCD) cause regular flooding of the low-lying (wetland) areas surrounding the Quelimane city centre and on the other hand reveal that more elevated city districts are also at risk in case of more extreme tidal levels (6 mCD and 7 mCD). Furthermore, in combination with the available information on the city’s storm water drainage system (Figure 3-85), an indication can be obtained of the potential for certain areas to collect rather than dis- perse surface runoff from surrounding areas due to strong rainfall events. This is however subject to uncertainty as the functionality and carrying capacity of the various drainage structures (canals, reten- tion areas, flap gates) is unknown, and this capacity strongly depends on the level of maintenance (e.g. clearance of debris and sediment from the existing canals) at the time of the event. These results, based on moderately detailed topographic data, provide a first order indication of local ground levels and hence to their susceptibility to risk events related to high sea levels. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 176 Figure 3-84 Potential Inundation Areas for Selected Sea States (mCD) Figure 3-85 Drainage Channels in the City Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 177 Table 3.9 Critical Sea Level per Site (indicative only) No. Site Critical Sea Level (mCD) 1 Airport / Aeroporto <5 Port & Bank Wall / Porto e Murro de Pro- 2 >7 tecçáo 3 Primeiro de Maio 6 4 Acordos de Lusaka 5 to 6 5 Inhangome 5 6 Chuabo Dembe 5 7 Torone Velho 6 to 7 5 in wetlands 8 Incidua > 7 in centre 5 in wetlands 9 Ivagalane > 7 in centre 10 Murrope >7 11 Not considered in this assessment 12 Micajune/Floresta B/A <5 Estimation of Probabilities of Risks Events Rainfall Events The Statistical Yearbooks by the National Institute of Statistics report average monthly rainfall rates per year (Table 3.10). These records illustrate monthly rainfall rates during the rainy season (November to April) to reach up to roughly 620 mm (January 2012). In the dry season (May to October) monthly rainfall rates are generally below 100 mm, with a maximum of approximately 120 mm in October 2013. Figure 3-86 illustrates these records. The records illustrate that the maximum recorded monthly rainfall in Quelimane between 2010 and 2017 was 618.7 mm, as recorded in January 2012. The average annual rainfall equals 1,104.0 mm. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 178 Table 3.10 Monthly Rainfall in Quelimane (mm) (Source: Instituto Nacional de Estatísta) 2017 2016 2015 2014 2013 2012 2011 2010 Average Jan 181.4 264.7 458.5 29.9 524.1 618.7 174.3 85.9 292.2 Feb 238.6 98.3 306.1 45.3 351.9 131.1 296.3 174.2 205.2 Mar 383.5 176.3 91.6 4.5 122.2 272.1 247.8 105.4 175.4 Apr 219.7 73.2 120.1 6.5 91.3 121.5 237.9 88.6 119.9 May 38.2 38.2 29.5 5.5 64.2 12.6 48.3 44.5 35.1 Jun 29.5 34.0 22.3 5.2 36.0 60.4 12.8 118.5 39.8 Jul 24.4 51.3 38.4 5.4 51.7 5.7 107.9 70.8 44.5 Aug 26.3 10.4 5.1 1.1 8.6 0.5 48.6 17.5 14.8 Sep 8.5 0.0 4.8 2.6 1.0 1.2 9.9 0.8 3.6 Oct 9.2 6.4 16.1 0.1 120.5 12.0 26.5 1.5 24.0 Nov 139.7 85.2 30.4 3.2 0.0 95.1 32.3 32.6 52.3 Dec 352.3 134.9 38.8 7.5 - 146.7 97.3 - 97.2 Total 1,651.3 972.9 1,161.7 116.8 1,371.5 1,477.6 1,339.9 740.3 1,104.0 Note: maximum values per month in bold; absolute maximum in bold and underlined 700 2017 2016 2015 2014 2013 2012 2011 2010 600 500 Monthly Rainfall (mm) 400 300 200 100 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 3-86 Monthly Rainfall in Quelimane (Source: Instituto Nacional de Estatísta) These monthly rates are well exceeded by, shorter duration, rainfall rates during regular cyclones and tropical storms reaching Mozambique. In the past months, Mozambique has been hit by several such Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 179 events, including the cyclones Desmond (January 2019), Idai (March 2019) and Kenneth (April 2019). Particularly cyclone Idai caused a large number of casualties, and mass havoc and destruction1. Although data is sparse and only available from unconfirmed public sources, available reports indicate rates as high as 257 mm/day during cyclone Kenneth (e.g. World Meteorological Organization (WMO) website2 on recent cyclone Kenneth). Subject to the speed and development of such storms, these rates may persist over one or more days. WMO also reports rainfall of 106 mm and 150 mm per day on two consecutive days during cyclone Dineo in February 2017. Cyclone Desmond is reported to have led to some 277 mm of rainfall in 24 hours in Beira3. A publication by the United States National Oceanic and Atmospheric Administration (NOAA) on heavy rainfall events in Southern Africa reports up to 339 mm/24 hrs rainfall at Quelimane during cyclone Favio in January 20074. The frequency of occurrence, development and strength of cyclones is subject to many factors, including the interaction between the local and wider oceanographic and meteorological systems. Data on prob- abilities of occurrence is not available. Furthermore, associated rainfall rates are highly dependent on the storm’s path and landfall location. Whereas it may be assumed that several severe storms occur each year, no return periods can be associated with specific storm conditions. High Sea Level Events High sea levels are the combined result of astronomic tidal levels, estimated sea level rise due to climate change and storm surges. Astronomical Tides Average tidal levels vary between 0.7 mCD and 4.5 mCD (spring tides) and between 2.0 mCD and 3.1 mCD (neap tides), around a mean sea level of 2.6 mCD. Equinoctial spring tidal levels are between 0.3 mCD and 4.8 mCD. These tidal curves are visualised schematically in Figure 3-87. 1 https://public.wmo.int/en/media/news/tropical-cyclone-idai-hits-mozambique, accessed 02/05/2019 2 https://public.wmo.int/en/media/news/another-unprecedented-tropical-cyclone-and-flooding-hits-mozambique, accessed 02/05/2019 3 https://www.aljazeera.com/news/2019/01/flooding-mozambique-tropical-cyclone-desmond-landfall-190122093541944.html, accessed on 02/05/2019 4 A Study Of Heavy Rainfall Events During The 2006/2007 Southern Africa Summer, NCEP-NOAA, 2007 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 180 6.00 Equinoctial Spring Tide 5.00 Average Spring Tide Average Neap Tide 4.00 Sea Level (mCD) 3.00 2.00 1.00 0.00 0.00 3.00 6.00 9.00 12.00 15.00 18.00 21.00 24.00 Time (hrs) Figure 3-87 Neap and Spring Tidal Curves Sea Level Rise Estimates of global sea level rise due to climate change effects vary subject to the selected scenario. Furthermore, local sea level rise is expected to vary against the global average. Regarding the predic- tions for sea level rise in Mozambique, three scenarios were considered by the World Bank (2010), termed low, medium, and high. The associated estimates for sea level rise range between 0.40 m and 1.26 m by 2100 (Figure 3-88). These scenarios project a global mean sea level rise of 0.16 to 0.38 m by 2050. However, all projections of climate change are subject to uncertainties arising from limitations in knowledge. In consideration of a study horizon up to 2050 for this assessment, a sea level rise of 0.3 m is assumed. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 181 Figure 3-88 Global Mean Sea Level Rise Scenarios relative to 1990 Levels (Source: World Bank, 2010) Storm Surges With available long-term water level records, it would be possible to compare actual sea levels to astro- nomic tidal levels, and derive surge levels with associated probabilities on this basis. Such long-term records are however not available at Quelimane. Hence, only a broad estimate of storm surges and their probabilities can be made. The basis for such estimate is limited to information on surge levels during extreme cyclone events, which is relatively sparse. For the cyclone events described above, surge levels were reported within the range of up to 3- 5 m (Cyclone Kenneth) and around 2.5 m (Cyclone Idai). Surge levels due to storm events are, however, strongly dependent on the storm’s path, location of landfall, travelling speed, wind speed and direction, and persistence of the severe wind conditions. As the most frequently occurring storms would be of lesser strength than the few cyclones for which surge estimates are available, so would be the associated surge levels. Publicly available wind climate information was assessed to gain an impression of the general wind conditions at Quelimane. Although these data are based on a modelled 30-year, hourly history of global wind climates, with unconfirmed accuracy, this is deemed acceptable for the purpose of this study, in the absence of long-term records of local wind measurements. These data are listed in Table 3.11 and Table 3.12, and visualised in Figure 3-89 and Figure 3-90. It is clearly seen from the wind rose that the strongest winds blow from the sector south to west. This is also the sector faced by the Quelimane coastline and hence these directions would give rise to the largest possible surge for a given wind speed. Wind speeds in this dataset are, however, fairly mild, and gen- erally below 38 km/h. As the global character of the data source, with a 30 km resolution, does not allow for a detailed repre- sentation of local storm events, it is likely that moderate to strong storm events would have higher wind speeds, although well below those occurring during the cyclone events (with speeds up to and above 200 km/h). Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 182 To simulate a sort of intermediate, moderate to strong, storm climate, it is here assumed that wind speeds during such storms would be between twice and triple to those reported in the wind climate, and storm surges for winds from the sector south to west would lead to storm surges up to 1 to 1.5 m, linearly scaled to wind speeds up to 150 km/h. The probability of occurrence of such storms are assumed to be at roughly one-tenth of the conditions in the full wind climate. Table 3.11 Monthly Wind Data for Quelimane (Source: www.meteoblue.com) Month 0 >1 >5 > 12 > 19 > 28 > 38 > 50 > 61 Jan 0.0 0.1 5.8 14.2 10.0 0.8 0.1 0.0 0.0 Feb 0.0 0.1 6.4 13.8 7.3 0.6 0.1 0.0 0.0 Mar 0.0 0.1 5.6 19.5 5.6 0.2 0.0 0.0 0.0 Apr 0.0 0.1 4.8 18.3 6.5 0.4 0.0 0.0 0.0 May 0.0 0.0 7.3 19.3 4.4 0.0 0.0 0.0 0.0 Jun 0.0 0.0 7.5 17.1 4.9 0.5 0.0 0.0 0.0 Jul 0.0 0.0 4.9 18.1 7.5 0.5 0.0 0.0 0.0 Aug 0.0 0.0 1.0 16.8 12.7 0.5 0.0 0.0 0.0 Sep 0.0 0.0 0.0 5.8 22.9 1.3 0.0 0.0 0.0 Oct 0.0 0.0 0.2 4.0 25.0 1.9 0.0 0.0 0.0 Nov 0.0 0.0 0.4 6.3 22.0 1.3 0.0 0.0 0.0 Dec 0.0 0.0 2.9 14.3 13.5 0.3 0.0 0.0 0.0 Data in days per month Table 3.12 Distribution of Wind Speed and Direction for Quelimane (Source: www.mete- oblue.com) Month 0 >1 >5 > 12 > 19 > 28 > 38 > 50 > 61 N 5 67 73 20 6 90 0 0 0 NNE 3 64 159 104 20 0 0 0 0 NE 1 64 239 226 24 0 0 0 0 ENE 4 107 344 265 52 0 0 0 0 E 0 72 301 297 132 1 0 0 0 ESE 4 96 322 308 160 5 0 0 0 SE 1 88 333 298 104 3 0 0 0 SSE 0 78 387 402 165 6 0 0 0 S 10 133 504 503 149 7 0 0 0 SSW 1 83 322 307 75 4 0 0 0 SW 4 89 233 170 55 3 0 0 0 WSW 0 59 124 72 26 4 0 0 0 W 5 79 68 25 11 3 1 0 0 WNW 1 43 25 6 2 0 0 0 0 NW 3 50 22 2 0 0 0 0 0 NNW 0 38 27 4 0 0 0 0 0 Data in hours per year Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 183 Figure 3-89 Monthly Wind Data for Quelimane (Source: www.meteoblue.com) Figure 3-90 Wind Rose for Quelimane (Source: www.meteoblue.com) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 184 The above derivation leads to first order estimates, based on a 24-hr storm duration to allow surges to build up. Relating this estimate to the outcomes of the coastal flooding hazard assessment by Deltares (2017), the resulting number of occurrences is however deemed to be overly conservative. For Quelimane, Deltares reports a 99 percentile (i.e. 0.01 occurrences per year) exceedance peak surge level of 1.12 m, a 95 percentile value (0.05 occurrences per year) of just below 0.5 m and a mean value (1 occurrence per year) of approximately 0.25 m. Reciprocally, this can be roughly translated into 0.05, 0.02 and 0.005 occurrences per year for respectively a surge height of 0.5 m, 1.0 m and 1.5 m. These estimates, listed in Table 3.13, can only be confirmed and/or improved with long-term local rec- ords, possibly in combination with detailed meteorological and hydrodynamic model studies. Table 3.13 Estimated Yearly Occurrences for Storm Surge levels Wind Speed (km/h) Surge Height (m) Occurrences per Year (-) 30 - 50 0.5 0.05 50 - 100 1.0 0.02 100 - 150 1.5 0.005 Based on the above, in the further assessment a typical storm surge of 1 m with a return period of fifty years would be considered as representative. This excludes more extreme events, including cyclones, for which estimates of return periods involve a good deal of uncertainty (Deltares, 2017). Combined Probabilities for High Sea Level Risk Events Storm surges would come on top of the instantaneous tidal level at the time of the storm event. In an adverse case, the surge would occur simultaneously with a high spring tide high water after a sea level rise of 0.3 m has materialised. In such a case, a storm surge of 1.5 m would lead to a combined high water level of up to 4.8 mCD + 0.3 m sea level rise + 1.5 m surge = 6.6 mCD. The same surge at a neap high water would only lead to a combined level of 3.1 mCD + 0.3 m sea level rise + 1.5 m surge = 4.9 mCD, which is similar to present day spring high water level (4.8 mCD). In Table 3.14, various combinations of tide, sea level rise and storm surge are given, as well as their potential to flood sites with certain land levels. Clearly, higher land levels could be subject to flooding in case of higher storm surges, as reported for past cyclone events, subject to the tidal phase at the time of occurrence. Table 3.14 Overview of Occurrence of Flooding Events due to High Sea Level Flooding due to Flooding due to Flooding due to Land Level Tide Tide + SLR Tide + SLR + Surge (mCD) N S E.S. N S E.S. N S E.S. 4.0 - X X - X X X X X 4.5 - - X - X X - X X 5.0 - - - - - X - X X 5.5 - - - - - - - X X 6.0 - - - - - - - - X 6.5 - - - - - - - - - 7.0 - - - - - - - - - N = Neap Tide; S = Spring Tide; E.S. = Equinoctial Spring Tide; SLR = Sea Level Rise of 0.3m; 24hr Storm surge of 1.0m Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 185 From the above it can be derived that sites with land or flood protection measures below 5.0 mCD are likely to be subject to regular or occasional (but at least annual) flooding due to combined tidal levels and sea level rise. In case of a storm surge up to 1.0 m, not unlikely to occur at least once every 50 years, the required land or flood protection level would be at least 6.0 mCD. It is noted that flooding could still occur in case of an adverse combination of high spring tide with the occurrence of the storm surge, although the probability of such a combination would be less of the order of 1% per year. Overview of Estimated Probabilities of Risk Events The results of the assessment of the probability per year of risk events per site is given in Table 3.15, excluding extreme (cyclone) events with conditions beyond those considered in the assessment. The gradual development of shoreline erosion cannot be statistically derived, but is occurring with cer- tainty, as observed during the site visits. Such a risk is hence included at 100%, although no time scale can be associated. Table 3.15 Summary of Probabilities per Year of Risk (Rainfall and High Sea Level) Events Critical Sea Critical Precipitation Probability of Occurrence No. Site Level (mCD) Return Period per Year (%) 1 Airport / Aeroporto N/A T=5yr 20 Port & Bank Wall / 2 Porto e Murro de Pro- >7 N/A 2 tecçáo 3 Primeiro de Maio N/A T=5yr 20 4 Acordos de Lusaka N/A T=5yr 20 5 Inhangome 5 N/A 2 5 2 6 Chuabo Dembe T=2yr 50 7 Torone Velho 6 to 7 N/A <1 5 in wetlands 2 8 Incidua N/A > 7 in centre <1 5 in wetlands 2 9 Ivagalane N/A > 7 in centre <1 10 Murrope N/A T=5yr 20% 11 Not considered in this assessment 12 Micajune/Floresta B/A N/A T=5yr 20% Assessment of Socio-Economic Impacts of Risk Events Introduction To assess the potential socio-economic impacts of the identified risk events, information from the site visits and workshops, in terms of size of the affected population and presence of any socio-economic valuable infrastructure and buildings at the sites, has been considered. In combination with other publicly available statistical socio-economic information on the city of Quelimane and of Mozambique, a first order attempt at valuating the possible impacts has been made. The purpose of this evaluation is to arrive at indicative costs associated with the occurrence of a risk event. Gross Domestic Product value has been combined with the approximated number of affected people, and added to any additional socio-economic values at each of the sites. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 186 Population Affected by Risk Event Based on the site visits and workshops with local representatives, the following information is available on the local population affected by previous flooding events at the sites: • At Site 1 the number of affected inhabitants is fairly large and raising, estimated at approxi- mately 6,000. • The affected population at Site 2 was reported to be approximately 1,500. • The affected population of Site 3 is 25,100 and growing. This includes the affected population of Bairro Mapiazua, Santagua and Manhaua. • The affected population of Site 4 is approximately 6,000. • The affected population at Site 5 was reported to be approximately 1,500. • The affected population at Site 6 was estimated to be approximately 2,000. • The affected population at Site 7 was reported to be approximately 6,000. • The affected population at Site 8 was reported to be approximately 7,000. • At Site 9 the affected population is approximately 2,000. • The affected population at Site 10 is above 1,275. • Site 11 is not considered in this assessment. • At Site 12 the affected population is approximately 5,000. The total affected population at the identified sites would hence be about 63,500. The Statistical Yearbook for 2017 (National Institute of Statistics) reports a population of 28,862,000 for Mozambique. The population of Quelimane in 2017 was approximately 350,000. Unconfirmed publicly available information indicates the population of Quelimane to have grown from roughly 150,000 in 1997 to 195,000 in 2007 and up to 350,000 in 2017. The growth rate in the latter decade (80%) is hence over 2.5 times as high as in the former (30%). The estimated affected population at the sites subject of this study is, based on information obtained during the site visits and workshops, therefore approximately 18% of the overall Quelimane population. Gross Domestic Product Local GDP per capita data for Quelimane could not be sourced. As best estimate, the latest national values for Mozambique are applied. The Statistical Yearbook for 2017 by the National Institute of Statistics reports a Gross Domestic Product (GDP) of 687.116 billion MT, which equates to 12.649 billion US Dollar. With a population of 28,862,000, this results in a GDP per capita of USD 428. The World Bank reports a GDP of 12.646 billion USD for Mozambique and a population of 29,668,834 in 2017 (https://data.worldbank.org/country/mozambique). This equates to a GDP per capita of approx- imately USD 426. The Trading Economics website indicates a steady growth of the per capita GDP over the past decade, from about 400 USD in 2009 to 519 USD in 2017, with a declining growth rate in later years (https://tradingeconomics.com/mozambique/gdp-per-capita). Whereas these figures are not entirely consistent, a value of USD 428 GDP per capita is applied in this assessment, based on the data by the National Institute of Statistics. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 187 Combined with the 2017 population of 350,000, the GDP of Quelimane is estimated at roughly 150 mil- lion USD5. Other Socio-Economically Valuable Assets Assessment of the available information on the identified sites reveals the following with regard to any additional valuable assets, beyond typically average value assets including dwellings, roads and small infrastructures6: • Site 1 comprises the airport, which serves the entire city and its surroundings and hence con- stitutes a socio-economic importance beyond its local perimeter. It is however reported not to be directly affected by flooding. In addition, an institute of higher education is located at this site, as well as further schools, a graveyard, market places and churches. The impact value for the graveyard is nominally included at USD 50,000, and all others at USD 200,000 each. • The bank wall at Site 2 serves as river frontage to the city centre and city in its entirety, and hence constitutes a socio-economic importance beyond its local perimeter. The site also in- cludes a fishing port with cool houses, the governor’s residence and city centre with restaurants and slaughterhouse. The impact value, due to loss of socio-economic activity and for rehabili- tation of affected buildings and materials, is nominally included at USD 200,000 for the fishing port, USD 100,000 for the governor’s residence and USD 200,000 for the city centre. • Site 3, including Bairro Mapiazua, Santagua and Manhaua, comprises regular infrastructure, such as housing, roads and stand posts. In addition, a church is present in each of three areas, a school is present at Bairro Mapiazua and Manhaua, and a market place at Santagua, of which the impact value is nominally included at USD 100,000 per area. • Site 4 contains dwellings, access roads and stand posts. • At Site 5, infrastructure comprises housing, roads, agricultural land and a hospital, as well as a market place. Of these, the hospital and market place would constitute additional socio-eco- nomic value. The impact value, due to loss of socio-economic activity and for rehabilitation of affected buildings and materials, is nominally included at USD 500,000 for the hospital and at USD 50,000 for the market place. • Site 6 comprises informal settlements with dwellings, dirt roads and small infrastructures, as well as small mooring places for fishing boats. These are not considered as significant additional socio-economic value. • Churches, houses, schools, roads and market places exist at Site 7. The impact value is nom- inally included at USD 300,000. • Sites 8 comprises a bridge site, with a destructed and/or emergency bridge, a hospital, a school and market places. The bridge site forms part of the regional road network and carries a wider socio-economic value, connecting Quelimane with the surrounding rural areas. Its impact value is nominally included at USD 200,000. The impact value of the hospital is nominally included at USD 200,000, the school at USD 50,000 and the market places at USD 100,000. 5 It is noted that this national average GDP is likely an overestimation, as per the 2015 poverty assessment Zambezia ranks the third poorest province (56.5) out of 11 provinces in Mozambique behind Niassa (60.6%) and Nampula (57.1%), and far from the national poverty mean of 46.1%. In the absence of specific data for Quelimane, the national average is however adopted in this study as indicative value. 6 The various other socio-economic assets have been assessed based on the collated information provided by the local workshop attendees, combined with impressions obtained during the site visits. All objects identified in this manner have been included, without any filtering, selection or prioritisation procedures applied. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 188 • At Site 9, the bridge site, with a destructed and/or emergency bridge, as well as a school exist. These are nominally included at USD 200,000 for the bridge and USD 50,000 for the school. • Site 10 contains dwellings, access roads stand posts and power supply. • Site 11 is not considered in this assessment. • At Site 12, school buildings, a health care centre and a cemetery are present in addition to dwellings, access roads and stand posts. No information on the size and scope of the school(s) is available. Its impact value, due to loss of provision of educational services and for rehabilita- tion of affected buildings and materials, is estimated at USD 100,000. The impact value for the health care centre and cemetery are nominally included at USD 200,000 and USD 50,000, re- spectively. Summarising, it is seen that, except for Sites 4, 6 and 10, each site comprises various sorts of added socio-economically valuable assets that have been represented in this risk assessment. Overview of Potential Socio Economic Impacts The results of the assessment of potential socio-economic impacts, on an annual basis, of risk events is given in Table 3.16. Table 3.16 Summary of Estimated Potential Socio-Economic Impacts Additionally af- Affected Popu- Affected GDP fected Socio- Total Impact No. Site lation (based on (USD) Economic Value (USD) Workshops) (USD) 1 Airport / Aeroporto 6,000 2,568,000 750,000 3,318,000 Port & Bank Wall / 2 Porto e Murro de Pro- 1,500 642,000 500,000 1,142,000 tecçáo 3 Primeiro de Maio 25,100 10,742,800 300,000 11,042,800 4 Acordos de Lusaka 6,000 2,568,000 N/A 2,568,000 5 Inhangome 1,500 642,000 550,000 1,192,000 6 Chuabo Dembe 2,000 856,000 N/A 856,000 7 Torone Velho 6,000 2,568,000 300,000 2,868,000 8 Incidua 7,000 2,996,000 850,000 3,846,000 9 Ivagalane 2,000 856,000 250,000 1,106,000 10 Murrope >1,275 545,700 N/A 545,700 11 Not considered in this assessment 12 Micajune/Floresta B/A 5,000 2,140,000 400,000 2,540,000 Note 1: Loss of life is not valuated Note 2: Site 3 includes the affected population of Bairro Mapiazua, Santagua and Manhaua Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 189 Estimation of Risks The performance and reduction in estimated exceedance probabilities depend to a significant extent on e.g. the present design and performance as well as topographic settings, for which no reliable data is available, as well as on the detailed design of the measures to be implemented. The outcomes of this risk assessment should hence be interpreted with due care. The risks, defined as the product of the risk event probability and their socio-economic impact, have been calculated as show in Table 3.17. Risk values have been colour-coded to indicate their ranking. Table 3.17 Risk Estimates for Existing Conditions Key Risk Socio-economic Risk Value (‘000 No. Site Probability (%) Event Impact (USD) USD) 1 Airport / Aeroporto Rainfall 20 3,318,000 664 Port & Bank Wall / 2 Porto e Murro de Pro- Sea level 2 1,142,000 23 tecçáo 3 Primeiro de Maio Rainfall 20 11,042,800 2,209 4 Acordos de Lusaka Rainfall 20 2,568,000 514 5 Inhangome Sea level 2 1,192,000 24 6 Chuabo Dembe Sea level 2 856,000 17 7 Torone Velho Sea level <1 2,868,000 29 8 Incidua Sea level 2 3,846,000 77 9 Ivagalane Sea level 2 1,106,00 22 10 Murrope Rainfall 20 545,700 109 11 Not considered in this assessment 12 Micajune/Floresta B/A Rainfall 20 2,540,000 508 Evaluation of Risk Reduction Due to Implementation of Proposed Measures The performance and reduction in estimated exceedance probabilities depend to a significant extent on e.g. the present design and performance as well as topographic settings, for which no reliable data is available, as well as on the detailed design of the measures to be implemented. The outcomes of this risk assessment should hence be interpreted with due care. Design criteria obviously have a bearing on the implementation costs. In case the proposed measures to mitigate the various flooding problems at Quelimane are imple- mented, it can be assumed that a significant reduction of the probability of occurrence of the risk events can be achieved. The level of reduction is subject to more detailed planning, for which additional data and information is required. The required information comprises in particular details on the local topography and the pre- sent state and capacities of the drainage network. Detailed planning will provide details on retention volumes and capacities of the various drainage canals. In combination with the existing system, the design capacities of the integral drainage system can sub- sequently be determined. On this basis, the reduction in exceedance probabilities of severe rainfall events can be assessed with a higher level of confidence. Similarly, the detailed planning of flood protection against higher sea levels can be used to derive the reduction in exceedance probabilities for high sea level events. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 190 For this assessment it is assumed that exceedance probabilities of rainfall and high sea level events would reduce to approximately 1% for properly designed drainage and sea level protection systems, which can cope with all but severe tropical storms and cyclones. A reduction in the probability of occurrence of the defined events to, say, 1% is considered to be tech- nically feasible. Whether such measures are financially feasible, or whether it is realistic to assume these may be implemented, depends on the assessment, criteria and drivers of the financing institutions, which are not studied within the scope of the present assessment. In essence, this section therefore merely provides an example of how a reduction in occurrence proba- bilities would lead to a reduction in risk levels. Subject to the above design factors and the willingness for funding of mitigation measures, the design probabilities may need to be adapted in this estimation. This would be a straightforward exercise that can be carried out once more detailed data and designs are available. On this basis these remaining probabilities, the estimated risk values reduce to those listed in Table 3.18. Table 3.18 Risk Estimates after Implementation of Proposed Measures Key Risk Socio-economic Risk Value(‘000 No. Site Probability (%) Event Impact (USD) USD) 1 Airport / Aeroporto Rainfall 1 3,318,000 33 Port & Bank Wall / 2 Porto e Murro de Pro- Sea level 1 1,142,000 11 tecçáo 3 Primeiro de Maio Rainfall 1 11,042,800 110 4 Acordos de Lusaka Rainfall 1 2,568,000 26 5 Inhangome Sea level 1 1,192,000 12 6 Chuabo Dembe Sea level 1 856,000 9 7 Torone Velho Sea level 1 2,868,000 29 8 Incidua Sea level 1 3,846,000 38 9 Ivagalane Sea level 1 1,106,00 11 10 Murrope Rainfall 1 545,700 5 11 Not considered in this assessment 12 Micajune/Floresta B/A Rainfall 1 2,540,000 25 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 191 ECOSYSTEM MAPPING AND ASSESSMENT OF PROPOSED SOLUTIONS - QUELIMANE Mapping of Ecological Infrastructure The natural capital of the City of Quelimane is a combination of aquatic (rivers and wetlands, mangrove swamps) and terrestrial ecosystems (soils, geology, dryland vegetation). For this project, the rivers, wetlands and mangroves were mapped, and are shown in Figure 3-91. Figure 3-91: Map of the main tidal rivers, wetlands and mangroves in and around Quelimane Natural ecosystems are subject to many pressures, and continue to be degraded world-wide, resulting in a loss of the ecosystem services which they could provide (Turpie et al., 2010; Snaddon et al., 2018). Climate change will exacerbate ecosystem degradation, and this will have an impact on ecosystem services, and the people that rely on them. In order to provide ecosystem services, natural ecosystems need to be secured and appropriately managed or rehabilitated. Healthy ecosystems deliver a wealth of ecosystem services, including both climate change mitigation (e.g. disaster risk reduction though through flood attenuation and sea surge protection, carbon sequestration, etc) and adaptation (e.g. through localised cooling effects, or buffers surrounding urban areas that protect land from storm im- pacts, and that provide water infiltration areas). The incorporation of wetlands, rivers and landscapes into nature-based or hybrid solutions for flooding provides cost-effective, long-term options for service delivery to people that can supplement or even substitute built infrastructure, especially in areas where the latter is limited or non-existent, such as in informal parts of the City. For this study, the aim was to understand how the City’s natural ecosystems and built infrastructure interact and support each other, and to improve this relationship through nature- based and hybrid solutions. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 192 Description of the aquatic ecosystems of Quelimane The City of Quelimane is located to the north of the extensive Zambezi delta, in the Province of Zambé- zia. The City itself is located on the banks of the Rio Dos Bons Sinais, also known as Rio Quelimane or Rio Quá-qua. The area is rich in floodplain wetlands, wetland depressions and mangroves. The vege- tation type is that characteristic of formations on alluvium. Soils are sandy in the drier parts of the City, tending towards organic silt in the wetter areas and wetlands. A number of mangrove species were observed to occur in and around Quelimane, especially along the banks of the various rivers flowing around the City. These species include the white mangrove, Avicen- nia marina, the stilt mangrove, Rhizophora mucronata, and the black mangrove, Lumnitzera racemosa. The mangroves are very effective at erosion control, as a result o their stabiliseing root structure, and they also play an important role in regulating flood waters, as they slow down flow considerably where the plant is allowed to grow in dense stands. Extensive freshwater wetlands surround the City of Quelimane. The freshwater systems are well veg- etated, with a number of grass species, sedge species (such as Cyperus textilis), and some floating macrohytes. The wetlands support a variety of fauna – during the field visit the following were observed: Grey and Striated Herons (Ardea cinerea and Butorides striata), Black Kite (Milvus migrans), and the Painted Reed Frog (Hyperolius marmoratus). Table 3-19 provides an overview of species for planting in Quelimane. In Table 3-20 and Table 3-21 some notes about the features on Vetiver Grass and Ele- phant grass are attached. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 193 Figure 3-92 The main wetland systems of Mozambique (source: Chabwela, 1991 and Saket, 1994) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 194 Table 3-19 Species list for planting in Quelimane Common Scientific Height Habitat Photo name name Any, good for sta- bilising soils and Vetiver Chrysopogon 0.6 m green slopes (see grass zizanioides Appendix) Vetiver grass in rows on a steep gully bank Any, good for sta- Elephant Pennisetum Up to bilising soils (see grass purpureum 3m Appendix) Buffalo Stenotaphrum Up to Damp areas grass secondatum 0.4m Dactyloctenium Up to LM grass Drier areas austral 0.5m Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 195 Common Dactyloctenium Up to Grows along the crowfoot aegyptium 0.5m coast Basket Up to Wet areas, edges Cyperus textilis grass 3m of ponds Miniature Up to Wet area – mar- Cyperus prolifer papyrus 1.2m gins of ponds Cyperus papy- Up to Wet areas, edges Papyrus rus 5m of ponds Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 196 Themeda trian- Up to Red grass Drier areas dra 1.5m Bolboschoenus Alkali bul- maritimus or Up to Wet areas, edges rush Bolboschoenus 1.2m of ponds glaucus Table 3-20 Vetiver grass Feature Advantage • Slows down runoff Dense tufted clump grass • Increased density of leaves means higher transpiration • Intercepts rainfall and reduces its erosive capacity • Stabilise soils • The roots improve the shear strength of soil by between 30 Relatively deep roots and 40% • Allows infiltration of water to depth, and increased soil mois- ture locally encourages the growth of other plants • The roots break up compacted soils, allowing water to infil- Strong roots trate along the roots • Slows down runoff by increasing roughness (i.e. increasing friction) • Encourages deposition of sediment • Can withstand high flow velocities and water depths Stiff overlapping leaf bases, • Can retain water behind a line of vetiver (as long as height strong erect stems of water does not exceed ~30 cm behind the vetiver) • Blocks the passage of soils and debris, ensuring that this is not washed away and gradually builds up a soil terrace • If planted in a dense hedge, this functions as an effective sediment fence • Easy to cultivate, grows easily in most soils (as long as Propagates vegetatively there is little frost in winter) • Grows easily and establishes well from runners Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 197 • Invasive potential is low • New shoots grow up from the base, even if this is buried in sediment, so gradually builds up over time (in Fiji – built up 2m vertically, over 30 years on a 20% slope) Does not invade • Will not outcompete local plants Ecological climax species, • Unlikely to invade and outcompete local plants not a pioneer Tolerates high levels of pes- ticides, herbicides, toxins • Will grow well even in stormwater drains and heavy metals (i.e. urban stormwater runoff) Drought and flood resistant • Withstands fluctuations in conditions Withstands high flows • Will grow in erosion gullies, stormwater drains etc Medicinal properties • Reduces stomach parasites in livestock Table 3-21 Elephant grass Feature Advantage Tall clumped perennial with • Stems can be used for fencing, building materials and fod- tough stems der (and for silage) • Stabilise soils, but may invade into adjacent areas Shallow spreading runners, • Easily cultivated from cuttings or runners but also deep roots • Grows quickly Stems not very dense, so cannot be planted in a sin- gle line Fire tolerant Inhabits a wide ecological • Easy to establish in different soil conditions throughout the range, in different soils, alti- catchment tudes etc Well adapted to drought • Will survive dry periods conditions Medicinal qualities • Plant extracts used as a diuretic in Africa (www.cabi.org) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 198 Ecosystem services provided by aquatic ecosystems Wetlands and rivers provide a range of ecosystem services. The following sections list these services, as derived from Kotze et al. (2009) and McInnes and Everard (2017). Supporting services Supporting services underpin all of the other services. They include: • Maintenance of biodiversity • Soil formation • Primary production • Nutrient cycling • Water cycling Provisioning services The provisioning services of aquatic ecosystems cover the products that are derived directly from wet- lands and rivers. In Quelimane, these include: • Water provision; • Cultivated and wild animal and plant foods; • Building materials such as sand, clay, wood, reeds and sedges; • Wood for fuel; • Grazing for livestock, and • Medicinal plants. Regulating services Regulating services are the process benefits that people receive from wetlands. These include: • Flow regulation, infiltration and recharge; • Flood attenuation; • Erosion control; • Carbon storage; • Sediment retention; and • Water quality improvement. Cultural services Cultural services relate to the relationship between aquatic ecosystems and people. They include: • Cultural and religious experiences; • Tourism and recreation; and • Education and research. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 199 Ecological Evaluation of Proposed Measures The ecological evaluation of the proposed measures has not been done with a complete solution in mind, but rather by assessing the individual measures that make up the toolkit proposed in this report. The individual measures assessed include: • Wetlands and bank wall protection: geotextile bags, geochutes, dikes, backfill, revegetation • City and airport area: Retention ponds; • Bridges and Bank Wall: Guide banks and reinforcement of bridge abutments, wave steps The criteria used for the ecological assessment are provided in Table 2-28. The aim of nature-based or hybrid solutions is to minimise the negative impacts of such interventions, and to maximise their positive impacts. Table 3-22 Evaluation Criteria for Environmental Consequences of Measures Criterion Description Positive or negative impact on biodiversity, which includes habitat, Impact on biodiversity species, communities, ecological processes Impact on ecosystem ser- Positive or negative impact on ecosystem services, as described in vices Section 2.5.3 Limited to the site itself, local area (i.e. limited to within 2 km of the Spatial extent of the influ- measure), regional (within 30 km of the measure), national (e.g. af- ence of the measure fecting a national priority area or key biodiversity area) or global. Predict whether the lifespan of the measure will be: • Temporary (less than 2 years, ), • Short term (2 to 5 years); Duration of the measure • Medium term (5 to 15 years); • Long term (longer than 15 years), or • Permanent. Possible negative ecologi- Summary statements regarding the possible negative ecological im- cal impacts pacts associated with each measure. The following table (Table 3-23) summarises the ecological evaluation of the proposed measures. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 200 Table 3-23 Ecological evaluation of the proposed measures for Quelimane. Categories Main Problem Method Guide Banks Bridges and Erosion Bank Wall Reinforcement of the Bridge Abutments Flooding due to storm “Wave Steps” surges and erosion Bank Wall Flooding due to insufficient Flap gates drainage Flooding due to storm Wetlands and surges and Geotextile bags Bank Wall Erosion Flooding due to storm Dikes surges Wetlands Backfill Erosion Vegetation reinforced slopes and embankments Retention Basins Flooding due to insufficient Airport and City drainage Rehabilitation of existing drainage channels and / or new drainage channels Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 201 4 COMMUNITY-BASED MEASURES NBS for flood and erosion protection can provide sustainable and low-cost alternatives, which can partly be implemented by local communities. Looking at the solutions proposed under section 2.3, the involve- ment of community members and groups is recommended particularly for (re)vegetation measures, as well as for small works within the affected neighbourhoods (e.g. bank stabilisation). Involving communities is an essential aspect of creating ownership over (green) infrastructure, thereby preventing ill-usage such as littering, theft and vandalism, increasing the benefits for the community and as a result the sustainability of the infrastructure. Specific community-based measures to be applied in combination with the NBS presented above are described in this section. AWARENESS RAISING CAMPAIGNS The general term ‘awareness raising’ (AR) stands for a number of activities that are directed to inform a group of people about a specific issue, in order to change negative and/or establish new attitudes and practices. Awareness raising is a main instrument to improve situations, where people’s lack (or igno- rance) of information causes social and/ or environmental problems. In the following, three main types of AR activities are shortly presented. Organized with individual volun- teers, community groups (women, children, DRR Committees, AR Cam- etc.) and/ or with paigns schools. Visibility through (social) media is im- portant to create broader aware- Figure 4-1 Beach cleaning by ‘Cooperativa Repensar’ ness. Source: Repensar, facebook Involving com- munity groups and schools in workshops that AR Train- reduce problems ings and and provide a Workshops benefit at the same time, such as upcycling of waste. Figure 4-2 Community Recycling workshop, Beira Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 202 Informative ses- sions, in the for- mat of presenta- tions, theatre, community walks AR Ses- and others. En- sions tire communities to be involved, organized by NGOs, local gov- ernment entities or community groups. Figure 4-3 Community awareness session, Beira Distribution of in- formation, educa- tional contents and key mes- Publications sages through and Media brochures/ leaf- lets, social media postings, TV/ra- dio spots, as well as information points. Figure 4-4 Chiveve colouring book It is highly recommended to set up an awareness raising programme as part of any investment into flood and erosion protection. In Nacala, the use of the land by residents is a crucial factor for erosion risks so that community members need to be informed about hazardous practices and preventive measures. In Quelimane, the urbanisation in low-lying areas along tidal rivers or into wetlands is a factor for flood risk. Furthermore, an engagement by individuals or groups should be fostered in order to im- prove problematic conditions in place, e.g. when it comes to littering. This should focus on the protection of the land and environment, particularly the vegetation of erosion-prone areas. The planting of trees in and by local communities should be a key measure, illustrated below. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 203 TREE PLANTING One of the main goals of nature-based erosion protection in Quelimane and Nacala is to reduce the runoff in case of heavy rainfall events and to reduce ongoing erosion along tidal rivers. An easy and highly sustainable measure is to permanently increase vegetation in affected areas. As a possibility to increase vegetation a tree planting programme to sensitize, encourage and capacitate the local popu- lation could be elaborated. Especially in the rather densely populated areas where large-area measures are not possible (e.g. catchment ID 09 in Nacala) small scale tree planting campaigns can help to reduce the runoff. Several wetland areas or banks along tidal streams in Quelimane could be used for re-planting of mangroves. Tree planting campaigns already have some history in Africa and in Mozambique. Kenya’s ‘Green Belt Movement’ was one of the first environmental movements in Africa, founded in 1977 by Nobel Prize winner Wangari Maathai. Until today over 51 million trees were planted by GBM communities, particu- larly women. Similar projects started around the globe in the past decades, many resulting from the United Nations Environment Programme’s (UNEP) “Billion Tree Campaign”, where more than 14 billion trees have been planted from 2006 to 2016. Other exemplary projects in neighbouring countries in Africa are as follows: — “The Green Earth Appeal”: Tree Planting Campaigns in Kenya, Tanzania, Uganda and other Afri- can nations (https://greenearthappeal.org/one-child-one-tree) — “Ripple Africa”: Tree Planting in Malawi (https://www.rippleafrica.org) — “One Tree Planted”: Tree Planting Campaign in Ethiopia, Kenya, Rwanda and Tanzania (https://onetreeplanted.org) Also in Mozambique, tree-planting campaigns have been conducted by governmental and non-gov- ernmental actors at different levels until today. A recent national tree-planting competition at schools, organized with private partners, shows the existing public awareness and environmental education. Based on the various experiences, community tree planting can be organized in different ways: — Grassroots engagement: community groups (e.g. women) organize themselves in establishing tree nurseries, planting and protecting trees. They may receive assistance at the beginning, through equipment, seedlings and training. — External engagement: government institutions or NGOs conduct campaigns and lead the plant- ing of trees, ideally with community members. — Individual engagement: Based on awareness campaigns, competitions and/ or rewards, indi- vidual households, schools, institutions, companies, etc. realise tree planting. A local nursery needs to be available. Figure 4-5 Mangrove reforestation through the NGO ADEL in Beira Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 204 In Quelimane and Nacala, a combination of tree planting campaigns and re-vegetation measures is highly recommended. On the one side, planting of trees, bushes and grasses at erosion ‘hot-spots’ shall be implemented with and monitored by local NGOs or community organisations under technical guidance (see section 2.3.2). This shall follow a revegetation programme for the affected neighbourhoods, to be set up ‘externally’ within a green/ hybrid infrastructure investment or through the Municipality, ideally with support by MITADER/ DPTADER. On the other side, individual tree planting will be key for a community-wide engagement. This may follow the “One Child – One Tree” approach, involving schools in the AR campaign to distribute plants, teach children basic gardening skills and monitor the growth of trees. With a local public-private- partnership (PPP), a small competition could be started to increase the motivation of children to take care of their trees. The extension of the existing municipal tree nursery in Nacala and an establishment of tree nurseries in Quelimane will be a pre-condition for both measures. Besides improvements of equipment and plant species, capacity building measures should also be provided. COMMUNITY WORKS The above examples show that implementation of nature-based solutions is possible with or through the local community with the aim of reducing costs of implementation while creating ownership, aware- ness and thereby more sustainable results. This involvement is also viable for technically simple hybrid solutions for flood and erosion protection. Community works can be carried out in different formats, such as “cash for work”, “food for work” or voluntary works, depending on the location, type and extent of works. An important component of com- munity engagement in infrastructure works is the involvement of existing community groups from the planning stage until the operation and maintenance of the built infrastructure. During the planning stage, the local knowledge can be of importance for the choice and design of project solutions. The implemen- tation of works through community members then provides a number of benefits, incl.: — Low construction costs — Better coordination and acceptance of works activities within community — Ownership of community in regard to maintenance and protection against vandalism/ ill-uses — Awareness raising of flood/ erosion problems, causes and solutions within community — Income-generation for local people Nevertheless, all works must be planned and carried out under technical guidance in order to guarantee good quality and prevent or reduce adverse effects. This also includes health and safety aspects during the implementation of works, as well as environmental and social aspects. For Nacala, the implementation of community works should be further assessed during the planning stage of erosion protection measures such as embankment protection of erosion gullies and construc- tion of detention ponds as described under section 3.3.3. Figure 4-6 Community works for Goto drainage ditches in Beira Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 205 Figure 4-7 Technical staff and farmers coordinating works in Somaliland Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 206 5 COST BENEFIT ASSESSMENT A cost benefit assessment will be elaborated for Quelimane and Nacala. COST ESTIMATION FOR PROJECT MEASURES IN QUELIMANE Table 5-1 gives an overview about unit prices and total costs for project measures in Quelimane in- cluding investment costs and maintenance costs. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 207 Table 5-1 Cost estimation for project measures in Quelimane Total Price per Fact Unit Price Total Price Site No. Description Position Site No. Qty. Unit Site or US $ US $ US $ Construction of drainage channel/ pipes (airport) D1 1 2012 m $90 $181.080 D2 1 619 m $60 $37.140 Construction of drainage channels/ pipes D3 1 802 m $60 $48.120 R1 1 1 1,2 - $295.515 1 Construction of retention basins/ ponds R2 1 1 1,2 - $186.609 $980.073 R3 1 1 1,2 - $186.609 O/F6 1 1 ls - $15.000 Construction of outlets/ flap gates O/F7 1 1 ls - $15.000 O/F8 1 1 ls - $15.000 2 Construction of shore protection, revetment SP1 2 300 m $2.500 $750.000 $750.000 D4 3/4 1382 m $60 $82.920 D5 3/4 633 m $60 $37.980 D6 3/4 633 m $60 $37.980 D7 3/4 1400 m $60 $84.000 D8 3/4 861 m $60 $51.660 Construction of drainage channels/ pipes D9 3/4 422 m $60 $25.320 D10 3/4 58 m $60 $3.480 D11 3/4 50 m $60 $3.000 D12 3/4 103 m $60 $6.180 3/4 D13 3/4 144 m $60 $8.640 $724.449 D14 3/4 17 m $60 $1.020 R4 3/4 ls 1,2 - $21.166 R5 3/4 ls 1,2 - $24.499 R6 3/4 ls 1,2 - $54.173 R7 3/4 ls 1,2 - $58.368 Construction of retention basin/ ponds R8 3/4 ls 1,2 - $69.717 R9 3/4 ls 1,2 - $99.617 R10 3/4 ls 1,2 - $24.484 R11 3/4 ls 1,2 - $30.245 5 Construction of green revetment GRS1 5 3149 m $15 $47.235 $47.235 Construction of drainage channel/ pipes (airport) D15 6 1055 m $90 $94.950 D16 6 168 m $60 $10.080 D17 6 478 m $60 $28.680 Construction of drainage channels/ pipes D18 6 490 m $60 $29.400 D19 6 292 m $60 $17.520 O/F1 6 1 ls - $15.000 O/F2 6 1 ls - $15.000 6 $3.397.991 Construction of outlets/ flap gates O/F3 6 1 ls - $15.000 O/F4 6 1 ls - $15.000 O/F5 6 1 ls - $15.000 Construction of dike with green revetment DGR1 6 1980 m $639 $1.265.715 Protection and re-growing of mangroves PM1 6 938323 m2 $2 $1.876.646 Construction of outlets/ flap gates O/F9 7 1 ls - $15.000 Construction of retention basin/ ponds R12 7 1 ls 1,2 - $29.969 7 $1.156.829 Construction of dike with green revetment DGR2 7 1740 m $639 $1.111.860 Protection and re-growing of mangroves PM2 8 114904 m2 $2 $284.962 Construction of revetment RV1 8 836 m $425 $355.300 PB1 8 1 ls - $25.500 8 $742.262 PB2 8 1 ls - $25.500 Construction of protection of bridge abutment (revetment) PB3 8 1 ls - $25.500 PB4 8 1 ls - $25.500 PB5 9 1 ls - $25.500 9 Construction of protection of bridge abutment (revetment) $51.000 PB6 9 1 ls - $25.500 10 Construction of revetment RV2 10 1403 m $425 $596.275 $596.275 12 Construction of retention basin/ ponds R13 12 1 ls 1,2 - $230.652 $230.652 EC1 817 m $5 $4.085 EC2 1130 m $5 $5.650 EC3 1709 m $5 $8.545 EC4 2619 m $5 $13.095 EC5 2664 m $5 $13.320 EC6 1378 m $5 $6.890 various Rehabilitation of existing drainage channels $71.300 EC7 1362 m $5 $6.810 EC8 1517 m $5 $7.585 EC9 268 m $5 $1.340 EC10 380 m $5 $1.900 EC11 238 m $5 $1.190 EC12 178 m $5 $890 in general maintenance cost green revetment (1.5 years) 1 ls $11.000 $11.000 $11.000 $8.748.066 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 208 COST - BENEFIT ASSESSMENT NACALA COST - BENEFIT ASSESSMENT QUELIMANE Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 209 6 LIST OF REFERENCES HITs and Misses in Mozambique’s climate change Action Plans, Africa Blythe, J. 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Eastern Africa Coastal Forest programme, 2007 Swiss Agency for Devel- Climate Change, Swiss Fast Start Financing for Developing Countries, opment and Cooperation 2012 (SDC) Economics of Adaptation to Climate Change, Mozambique, EACC The World Bank Publications and Reports, World Bank, Washington, 2010 Implementing nature-based flood protection, Principles and implemen- The World Bank tation guidance, 2017 Theron, A. & Analysis of potential coastal zone climate change impacts and possi- Rossouw, M. ble response option in the southern African region, 2008 Coastal Planning and Adaptation to Mitigate Climate Change Impacts, Theron, A. et al. INGC Phase 2 report, 2012 United Kingdom Hydro- Africa Pilot Volume III, South and east coasts of Africa, Thirteenth Edi- graphic Office tion, Taunton, 1980 United Kingdom Hydro- Africa Pilot Volume III, South and east coasts of Africa, Fourteenth graphic Office Edition, Taunton, 2006 An Assessment of Hydrological and Land use Characteristics affecting United Nations Environ- River-Coast Interactions in the West Indian Ocean Region, United Na- ment Programme (UNEP) tions Environment Programme (UNEP)/Nairobi Convention Secretar- iat, ISBN 9987-8977-6-2, Nairobi, 2009 United Nations Human Climate Change Assessment for Maputo, Mozambique: A Summary, Settlements Programme ISBN Number:978-92-1-132247-7, Nairobi, 2012 (UN-HABITAT) United States Agency for International Develop- Mozambique: The impact of climate change on water and the coast- ment (USAID), Council line, Mozambique training course on the use of weather and climate for Scientific and Indus- information in decision-making, Presentation, Maputo, 2012 trial Research (CSIR), KULIMA United States Agency for Explanatory Note of Vulnerabilty Mapping for Natural Disasters in the International Develop- Municipalities of Quelimane and Pemba – Coastal City Adaption Pro- ment (USAID) ject (CCAP), 2015 Pre-identification Mission to Mozambique, 30 May – 4 June 2010, Mis- Water Mondiaal sion Report, 2010 Mozambique: largest cities and towns and statistics of their population. World Gazetteer Available at: http://world-gazetteer.com (accessed: 13/06/2013) Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 212 Wright,T. ; Tomlinson, J.; Schueler, Direct and Indirect Impacts of Urbanization on Wetland Quality”, in T.; Cappiella, K.; Wetlands & Watersheds Article N° 1, Ellicott City, December 2006 Kitchell, A.; Hirschman, D. https://www.aljazeera.com/news/2019/01/flooding-mozambique-tropi- cal-cyclone-desmond-landfall-190122093541944.html, accessed on 02/05/2019 https://public.wmo.int/en/media/news/another-unprecedented-tropical- cyclone-and-flooding-hits-mozambique, accessed 02/05/2019 https://public.wmo.int/en/media/news/tropical-cyclone-idai-hits- mozambique, accessed 02/05/2019 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 213 7 ANNEX I – FIELD VISIT REPORT NACALA Table 7-1 Summary table of characteristics and challenges recorded at the field sites in Nacala. Site Bairro Field notes Problems observed Photos num- ber Site 1 Just north of Watercourse #1: Insufficient capacity in the re- Ribaue – area tention ponds that were built west of the air- “Airport” watercourse (#1) runs west- for the airport. port. wards from the road. Sand blocking drainage chan- Four water- • Main erosion occurred here in Oc- nels. courses (1 – 4) tober 2017. Removal of vegetation, lead- • A retention pond was built to the ing to instability of the soils, west (downstream) of the road, to and erosion of gullies across hold runoff from the airport. Runoff is conveyed into the pond via a 1m- the landscape. diameter pipe under the road. Riprap below retention pond. • Gabions and riprap were installed in and downstream of the pond, but some of these were washed away during high rainfall. • Erosion gully deepens (max. 8m deep) and widens with distance downstream. • A double-storey house was washed away downstream, close to the Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 214 Site Bairro Field notes Problems observed Photos num- ber coast – here the erosion has re- 8m-deep gully. moved all fines/sand, down to sandstone. • Groundwater now surfaces within the eroded gully. Watercourse #3: Finnish project installed gabions in the channel. Water flowed along this chan- nel until October 2017, when sand blocking channel forced water along an alternative route. Site where house was washed away. Erosion has re- Mesquite has been planted to stabilise moved all fine sand, down to sandstone. soils – fairly effective (but very invasive, and uses groundwater) Formalised drainage channel blocked with sand, forc- ing water to flow along alternative routes, and further Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 215 Site Bairro Field notes Problems observed Photos num- ber erode the landscape. Site 2 Ribaue Watercourse / gully has several Trees are being removed in branches. Deepest erosion gully is up to the area, making soils unsta- 1 watercourse 15 m deep. All were formed recently ble. (early 2017; Oct 2017; January 2018). Roads without any stormwater Gully is filled with tyres, sand and burnt drainage systems turn into wa- litter, in an attempt to halt erosion. tercourses during heavy rains, as water just flows along them. Some of the gullies were originally roads, providing a favourable pathway for water. Litter that is dumped into the gullies Watercourse goes over a sheer sand- stone cliff at the beach / coastline. A seep wetland is located just to the south of the sediment deposit at the mouth of the river. Deeply incised erosion gullies going down towards the coast. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 216 Site Bairro Field notes Problems observed Photos num- ber Site 3 Mocone Watercourse #1: Sand-mining. 2 water- Several gullies join to flow into the next Houses very close to steep courses barrio (Cidade Baixa). banks of gullies; some have collapsed into the gully (see Gully is wide and deep, with very steep photo to right). banks. Children play on the steep banks of the gully, which is dangerous. Confluence of a few gullies. Brickmaking activities in the watercourse. Material is taken from the bed and banks of the gullies. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 217 Site Bairro Field notes Problems observed Photos num- ber Site 3 Mocone Watercourse #2: Poor stormwater management in the upper parts of the catch- 2 water- Main watercourse through centre of ment leads to water flowing courses town. Flows under a new hotel develop- along pathways and tracks, ment site into stormwater channels and causing erosion. pipes, and out to sea just south of the Port. Construction of a hotel across the watercourse – dangerous The erosion gullies form due to small location for the hotel, and also pathways and roads within the residen- necessitates taking water un- tial areas becoming preferential path- derground in pipes to the sea. ways for water during high rainfall. Wide erosion gully in Mocone. This gully ends up at the These join to become wider, deeper gul- hotel development site. lies. (At pt 1410) Water is collected from res- idential areas and roads, forming a big gully on a steep gradient. Gabions have been constructed, but the stones have been taken. Municipality has planted vetiver and ele- phant grass along the edges of the ero- sion gullies. This seems to be partially successful, but insufficient in the face of heavy rainfall. Erosion gully widens into what was a retention basin, Erosion worsened dramatically in Janu- but is now completely sedimented up. This is just ary 2018. upstream of the hotel development site. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 218 Site Bairro Field notes Problems observed Photos num- ber House partially collapsed into gully Vetiver grass planted along the edges of the erosion gully. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 219 Site Bairro Field notes Problems observed Photos num- ber Site 4 Cidade Baixa Watercourse #1: Road crossing over Water- course#1 becomes very dan- 5 water- Lower section of watercourse that be- gerous during high discharge. courses gins in Mocone. Slightly older erosion Four people drowned at this gully, which has been stabilised to some crossing in 2017/2018. extent with gabions (stones missing) and Prosopis. Illegal housing, and houses collapsing into the gully. The channel was draining well, with no erosion (gabions), until recently (2017). Mesquite planted in the watercourse, and eroded The gully is much wider further upstream banks of the gully. (see photo to right). Road crossing and pipe in Watercourse #1. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 220 Site Bairro Field notes Problems observed Photos num- ber Watercourse#1 looking downstream from road cross- ing. Watercourse#1 further upstream of road. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 221 Site Bairro Field notes Problems observed Photos num- ber Gabion in a now abandoned channel, Watercourse #1. Site 4 Cidade Baixa Watercourse #2 and #3: n/a 5 water- Short, well-vegetated channels with little courses erosion Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 222 Site Bairro Field notes Problems observed Photos num- ber Site 4 Cidade Baixa Watercourse#4: Outlet pipe is damaged, par- tially blocked with sand. 5 water- This is the downstream section of the courses watercourse in Mocone that ends at the Sand deposited in the port hotel site. area. The port is due to expand into this area. Outlet of Watercourse #4 onto the beach. Site 4 Cidade Baixa Watercourse #5: Sand deposited in the port area. 5 water- This watercourse is fairly stable, and courses well vegetated as it flows through the Litter from upstream is depos- residential area. ited on the beach. The sediment deposit in the port area is Polluted water discharged substantial. The port is due to expand from industries along the into this area, to a point just south of coast. Some treat the water. here. Silting up of stormwater chan- Outlet of Watercourse#5 onto the sediment deposit. There are several industrial sites along nels and pipes. the coastline, where buildings have been located on top of drainage lines. Water is now diverted into channels or pipes. Some of these pipes and chan- nels are now silted up. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 223 Site Bairro Field notes Problems observed Photos num- ber Site 5 South of Watercourse #1: Polluted water discharged Cidade from industries along the There are several industrial sites along coast. Some treat the water. 5 water- the coastline, where buildings have courses (more been located on top of drainage lines. Silting up of stormwater chan- or less) and 4 Water is now diverted into channels or nels and pipes. coastal wet- pipes. Some of these pipes and chan- lands nels are now silted up. The water table is high along the coast, so water seeps out at various points Lower section of Watercourse#1 as it flows out to sea. (and must also be diverted away from buildings and infrastructure), and also leads to the formation of the seepage wetlands. Watercourse#1 receives a substantial volume of stormwater. A new storm- water diversion and channel have been constructed on this system. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 224 Site Bairro Field notes Problems observed Photos num- ber Culvert under railway, Watercourse#1. New stormwater drainage channel and culvert under the railway. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 225 Site Bairro Field notes Problems observed Photos num- ber Site 5 South of Watercourses #2 and #3: Silting up of drainage chan- Cidade nels, retention ponds and Short drainage systems, receiving water pipes. 5 water- from various industrial sites and the courses (more roads. #3 receives some water from the Truck parking area next to fuel or less) and 4 fuel depot retention pond and the truck depot was built on top of the coastal wet- parking area. drainage line, diverting water lands towards the fuel tanks. Watercourse#2. Watercourse #3 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 226 Site Bairro Field notes Problems observed Photos num- ber Site 5 South of Watercourse #4 (fuel depot): Location of fuel tanks and truck Cidade parking area in the water- Watercourse should flow through the course means that drainage 5 water- fuel depot, but was diverted to flow will always be a problem. courses (more northwards. However, the diversion was or less) and 4 blocked with sand when the truck depot Diversion channels and drop coastal wet- was built, and so water flowed under the inlet structure (upstream of the lands tanks. road) have silted up or been blocked with sand, and so wa- ter now flows through the tank depot. Sand blocking the diversion channel in the left of the photo. Water has lifted one of the tanks. See water level on the side of the tank, and sandbags in the foreground. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 227 Site Bairro Field notes Problems observed Photos num- ber Watercourse #5: Stormwater channels make several sharp turns which may Watercourse starts as various channels not handle the volume of water on the slopes. in rainy months. Gradient is very steep. The road crossing the top section of the watercourse was washed away some time ago (at pt. 1419) and was replaced (tarred) recently, also with a 1m-diame- ter pipe under the road and a drop inlet structure above the road. Top section of Watercourse #5, showing channel diverting around electricity infrastructure. Below the road, the new stormwater channel makes a few 90° turns, and may overflow into residential areas again (see photo to right). Road also washed away further down the river (mid-section), not replaced as yet. At lower end, the watercourse drains through the BP industrial site. Outlet below the road. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 228 Site Bairro Field notes Problems observed Photos num- ber Road washed away in middle section of Watercourse #5. Site 6 Micajone Watercourses in this Bairro are longer The main risk in this part of the than those flowing through the City. Ero- City is that the roads connect- 2 water- sion is less, and the systems are fairly ing with Micajone are flooded courses well vegetated. Also fewer houses and and washed away during high settlements. rainfall months, isolating the villages to the south. View of upper catchment of Watercourse #1. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 229 Site Bairro Field notes Problems observed Photos num- ber Small channel within the residential area of Micajone flowing into Watercourse #2, showing vegetation along the margins of the channel – good for stabilisation. View of Watercourse #2, flowing away from houses towards the ocean. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 230 Site Bairro Field notes Problems observed Photos num- ber Site 7 Ontupaia Watercourses flow in an easterly direc- Erosion gullies start between tion in this bairro. Rivers are long, flow- the houses. Water flows along 2 water- ing along a gradient that is far gentler the streets, and gathers mo- courses than those in the City. However, erosion mentum and sediment, caus- gullies are still deep and dangerous. ing massive erosion gullies. Some gullies have been partly filled with People place obstructions in litter and rubble. This has been success- the older gullies, which causes ful in some cases, but it likely to lead to the water to merely find an- litter being washed downstream, and other route. possibly into the mangroves along the coastline. Walls, houses, parts of school property, and infrastructure (FIPAG pipeline) have washed away in places. Wall washed away by an erosion gully, Watercourse #1. During the high rainfall event that caused the more serious gullies, the water was flowing 10m deep apparently. This whole property was washed away, except for the water Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 231 Site Bairro Field notes Problems observed Photos num- ber tank (Watercourse #1). Top of Watercourse #2, flowing past school, showing destruction of formal stormwater channel. This gully collects water from about 5 drainage lines. These stormwater channels were part of a pilot project some time back. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 232 Site Bairro Field notes Problems observed Photos num- ber FIPAG pipeline affected by erosion, Watercourse #2. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 233 Site Bairro Field notes Problems observed Photos num- ber Two channels meet to form massive erosion gully, Watercourse #2. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 234 Site Bairro Field notes Problems observed Photos num- ber Site 8 Matapue (this Similar to Site 7 – longer rivers flowing Substantial erosion gullies is a resettle- eastwards towards the coastline. have formed along roads – the ment area for bigger the road, the bigger the Triangulo, late gully. 1980s) Roads are eroding, which 1 main water- threatens infrastructure such course as pipeline carrying water from FIPAG boreholes. View eastwards to coastline, catchment of Watercourse #1. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 235 Site Bairro Field notes Problems observed Photos num- ber Watercourse #1 – litter, soil and rubble placed in erosion gullies to fix them. Watercourse #1 (north of photo above) – vegetable gardens in the watercourse. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 236 Site Bairro Field notes Problems observed Photos num- ber Watercourse #1 where it flows along the road, causing erosion around FIPAG pipeline. Site 9 Short water- Short steep watercourses flowing west- Short, steep erosion gullies courses (at wards towards the coastline. causing damage to houses least 4 differ- and infrastructure. ent water- courses) Erosion gully below the road along the coast, Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 237 Site Bairro Field notes Problems observed Photos num- ber Watercourse #2. Stabilised (older) erosoin gully, Watercourse #3. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 238 8 ANNEX II - COMMUNITY BASED MAPPING CAMPAIGN – COLLECTED DATA In the following table, the answers are represented using abbreviations as shown in Figure 2-5: — F – Flooding — E – Erosion — D – Drainage — W – Waste Management — O – Other Table 8-1 Collected Data – Community Mapping Campaign Quelimane and Nacala Since when is the issue present? Number of Affected People Importance of Mitigation Additional Comments Severity of Impact Type of Issue # O comitê tem conhecimento do problema e toma medidas de miti- 1 I; E; D - o o >5 gação,clgrc nhaherengue,nacala Comitê capacitado e trabalha para comunidade,tem mapa de ame- aça tem kit de emergência,fica 16km de nacala, maior ciclone foi 2 I; E - o o 1-2 94 e ciclone joker,foi feita simulação neste comitê dia 2.8 com mu- nicípio e ingc Comitê de teteriane nacala.precisa capacitação melhorar kit de 3 I; E - o o 2-5 emergencia Cidade de nacala.mapas e cenários climáticos.ainda envio mais 4 E; W o + + 2-5 documentos.ceers Mapa indica cenários de Nacala.fiz encontros com agricultura e pesca director,município ur- 5 I; E; D; W o + + 2-5 banização e resíduos solidos,ambiente e E, .presidente municipal e vereadores e vereadores.visitei 3 comités com ingc.charifo 6 E; D; W + + >5 E, de mais de 3m, estrada completamente destruída e intranaitavel 7 E; D; W + + + 1-2 Destruição do sistema de barreiras naturais agravou a situacao 8 E; D; W o + + >5 Destruição dos gaviões de proteção contra a erosao Cursos diferentes de água cruzam-se neste ponto e criam este tipo 9 E; D; W - + + 1-2 de destruicao Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 239 Since when is the issue present? Number of Affected People Importance of Mitigation Additional Comments Severity of Impact Type of Issue # 10 E; D o + + >5 Falta de drenagens exacerbaram a situacao 11 E; D; W o + + 1-2 Problemas de falta de Drainagem, Reassentamentos informais obstruem ruas e bloqueiam o curso 12 E; D o + + 1-2 normal das águas. Falta de sistemas de D, Ponto intervencionado pelo município, onde a cratera era de mais 13 E; D o + + 1-2 de 10m. Grave problema de falta de D, Invasão populacional em zonas proibidas/de risco... Surgimento de 14 E; D o + + 2-5 um assentamento urbano e prática de uso de terra para fins co- merciais (produção de tijolos) 15 E; D; O o + + <1 16 E; D o + + <1 17 E; D o + + <1 18 E; D + + + >5 19 E; D o + + 2-5 No bairro Outupaia, a E, de grande Magnitude aconteceu na última 20 E; D; W o + + <1 época chuvosa 2017/2018. Os moradores afirmam que a falta de valas de Drainagem, condiciona este tipo de situacoes 21 E; D; W + + + 1-2 22 E; D o + + >5 Maior cratera da cidade, todos gaviões removidos (6). Existe a 23 E; D + + + >5 mais de 20 anos Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 240 Since when is the issue present? Number of Affected People Importance of Mitigation Additional Comments Severity of Impact Type of Issue # 24 E; D + + + >5 Mais de 10anos, mas a última época chuvosa foi a pior de todas 25 E; D + + + >5 Substacao Teemo_electrica da EDM inundada de área em mais de 26 E; D o + + >5 3m Quelimane valas obstruídas,resíduos sólidos,corte de mangal,ca- 27 I; E; D; W o o + >5 sas em perigo.vereador urbano e técnicos . 28 I; E + + + 2-5 Comitê de morocue,quelimane.dique para reabilitar 29 E + + + 2-5 E, severa casa destruída do Sr Pinto castiano.mororue Vulnerável área,called tecane zone,quelimane.have comitê oficial 30 I; E; D + + + 2-5 desaster 31 I; E; D 1-2 Tudo 32 D; W + + + 2-5 House on the Bridgestone.micajune.quelimane Projectos de compostagem e biogás para famílias não cortarem 33 I; E; W o o o 1-2 mangal em curso no município de Quelimane. Comitê de macajuni,Quelimane com problemas de inundações e 34 I; D + + + 2-5 construção sobre Drainagem, .estamos fazer levantamento do pro- blema com líder local. 35 E; D; W + + + 2-5 Colecta de dados Quelimane técnicos emusa. 36 I; E o + + 2-5 E, e reposição deangal.morropue,quelimane Bairro. Incidia,Quelimane,Erosao, da ponte,inundações e infraes- 37 I; E; D + + + 2-5 truturas afectadas Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 241 Since when is the issue present? Number of Affected People Importance of Mitigation Additional Comments Severity of Impact Type of Issue # 38 I; E; W + + + >5 Não tem água potavel I; E; D; W; 39 + + + >5 População cortou mangal para habitar O 40 I; E; D; W + + + >5 O bairro não tem canalização de água... 41 I; D; W + + + >5 I; E; D; W; As coisas acontecem mais porquê as pessoas fazem corte do 42 + + + >5 O mangal 43 I; E; W + + + >5 Não há acesso para entrada do camião para reconher o lixo 44 I; E + + + >5 45 I; E; D; W + + + >5 Falta água potável e saneamento do meio 46 I; D; O o + + >5 47 I; D; O + + + >5 48 I; D; W + + + >5 Zona de inundação.grave 49 I; O + + + >5 50 I; D + + + >5 Zona sem água... 51 I; O + + + >5 Mares altas Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 242 Since when is the issue present? Number of Affected People Importance of Mitigation Additional Comments Severity of Impact Type of Issue # As pessoas continuam construindo suas casas mesmo sabendo 52 I; + + + >5 dos riscos que as mesmas corem Plantadas barreiras de proteção e retenção de sementes de man- 53 E o + + 2-5 gal para sua reposição natural 54 E; D; W + + + >5 O povo não tem como obter água potável a curta distância 55 E; D; W + + + >5 As casas tem sofrido por causa de vendavais. Projecto do CCAP, que termina dia 22 de Agosto, reposicao do 56 I; E o + + 2-5 mangal 57 I; E; W + + + >5 Zona propensa a vendavais e inundacao. 58 I; W o + + na Ocorrência de inundacao assim como riscos de corrente elétrica 59 I; - o o 2-5 A ponte foi aumentada dedido ao aumento da largura do rio Mua- rua. Os sacos que tinham sido colocados estao a desaparecer, 60 I; E - + + 2-5 muitos ja nao existem, foram aos poucos levados pela agua. Este fenomeno comecou a verificar-se depois de 2013 61 I; D; W + + + >5 . I; E; D; W; 62 + + + >5 Houve corte das árvores para fabrico de carvão O Um bairro que esta a ser criado numa zona altamente vulneravel 63 I; D + + + >5 de mangal I; E; D; W; 64 + + + >5 O reasentamento e informal O Pessoas que vivem na area em alusao, tem falta de agua para o 65 I; E; D; W + + + >5 consumo. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 243 Since when is the issue present? Number of Affected People Importance of Mitigation Additional Comments Severity of Impact Type of Issue # 66 E - + + <1 Por causa da construção de um novo condomínio e falta de Drai- 67 E; D - + + <1 nagem,, as águas invadiram e destruíram a estrada Erosao na estrada, de cerca de 6m, nas laterais não afectou por- 68 E; D - + + <1 que tinha uma vala de Drainagem,, e a plantação de prosopos aju- dou a conter a erosao no lado direito da estrada Casa de 2 andares arrastada pela fúria das aguas, destruiu es- trada, sistemas de abastecimento de água e agora é um local onde 69 E; D o + + <1 apareceu um lençol freático e há aguas a correrem e a população usa para pequenas machambas, lavar a roupa e tomar banho 70 E; D + + <1 Sistema de abastecimento de água da FIPAG danificado O Município enche as crateras do lixo que recolhe na cidade e de- 71 E; D - + + <1 pois compacta com cerca de 1m de areia Iniciou no princípio de 2017 e a medida que vamos caminhando a 72 E; D + + 2-5 sua profundidade aumenta, até a mais ou menos 8-10m Cratera formada em 2018 (Janeiro) e chega a juntar_de com a for- 73 E; D + + <1 mada em 2017 na zona em que esta última é muito profunda, cri- ando problemas de intransitabilidade nas ruas a juzante O município tentou aplicar algumas acções para mitigar a E, neste local: colocação de pneus, gaviões e plantio de capim vetivel. Mas 74 E; D o + + 1-2 o problema persiste tende a crescer, devido a expansão urbanas em zonas já identificadas como sendo de risco. Os pequenos caminhos criados pelas pessoas, tem se tornado li- nhas de água que ao longo do seu percurso, por causa da veloci- 75 E; D o + + na dade das águas ela começa a criar erosao e no final todos estes caminhos se encontram e criam graves problemas Isto já acontece há muitos anos. As águas que vem da zona alta e zona baixa, encontram-se neste ponto e descem para um mesmo ponto. Por falta de valas de D, estas águas correm a grandes velo- 76 E; D o + + na cidades causando erosao ao longo do seu percurso. Isto é exacer- bado pela vandalização dos gaviões colocados nestes percursos (a população rouba para construir suas casas). O município tam- bém tentou plantar prosopos para conter a fúria das aguas 77 E; D + + + na Linhas de erosao a encontrar-se... Linhas de junção das linhas de água a jusante, onde pela fúria das 78 E; D + + + na águas, muitas casas são destruídas e a areia levada para a baixa da cidade e inundado toda baixa de areia e edifício. Bairro Mocone Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 244 Since when is the issue present? Number of Affected People Importance of Mitigation Additional Comments Severity of Impact Type of Issue # Algumas medidas que o município tem implementado para mitigar 79 E; D + + + na a erosoa. Bacia de retenção próximo aos tanques da BP e Petromoc, rompe- ram com as últimas cheias e criou problemas nos tanques 41 e 60 80 E; D - + + <1 da Petromoc. A EDM, BP e Petromoc sugeriram que o município apresentasse um plano de reestruturação desta bacia de retenção para eles financiarem Alguns exemplos de uma vala de Drainagem, construída depois 81 E; D o + + 1-2 das cheias. E um mau exemplo de construção em lugares proibi- dos A população vive nesta zona ilegalmente, tendo sido interditada 82 E; D + + + >5 mas vivem a revelia e ainda cavam para tirar areia para a constru- cao Foi construída uma bacia para retenção de água para reduzir a força de águas e passar directo para a estrada mas com as cheias 83 I; E; D + + <1 deste ano trouxeram os solos e assoriou. Mas isso deu-se por te- rem roubado os gaviões (existiam 8: zona do triângulo) Em muitos destes locais onde temos maior nível de erosao, pode- mos verificar fontes de águas subterrâneas muito próximas a su- perfície e as populações usam estas águas. Acredita-se que a 84 E; D o + + 1-2 combinação da força das águas e a existência destas fontes sub- terrâneas possam contribuir para a retirada destas grandes cama- das de areia. Esta estrada foi destruída na época chuvosa 2016/17 Nesta vala de D,, existe uma caixa de 2x3m, que faz com que a água caia quando ela entra da vala acima e depois caia e vai para a vala de saída. Esta estrada esteve cortada na época 2017/18 por causa da fúria das águas. E quando estás águas saem, encontram 85 I; E; D + + + <1 outra linha de água e se juntam, passando pelo local onde a es- trada cortou-se e depois vão por uma vala que passa de baixo da linha féria e vão para a bacia de retenção que fica perto da BP e Petromoc, que tambem ficou destruída na última época chuvosa pelos mesmos motivos A reposição dos solos nos locais de erosao é feita aterrando lixo colhido na cidade e por fim terra. Neste local há 2 ravinas, mas a mais recente, que criou muitos problemas, diz-se que surgiu este 86 I; E; D + + + 1-2 ano, depois que um morador criou uma barreira de proteção de água com sacos de areia que desviou o fluxo normal das águas mas criando uma outra cratera no sítio por onde passou. Na outra, a água não passou 87 E + + + 2-5 E, matou 4 pessoas 2017 pelas correntes de chuva. Bairro micajune propensa a inundações.casas e pessoas afecta- 88 I; + + + >5 das. 89 E; D + + + >5 Rio mulwahipa E, alta e 4 mortos 2017 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 245 Since when is the issue present? Number of Affected People Importance of Mitigation Additional Comments Severity of Impact Type of Issue # 90 E + + + >5 E, humana.nacala 91 E + + + >5 E, humana.nacala rio mulwahipa. 92 E + + + >5 Grande E, Nacala Este é o local onde a D, desagua no mar. Depois da estrada des- truída, em caminho ao mar, nota-se uma espécie de bacia natural de retenção, mas que depois a água e guiada em canais naturis 93 E; D o + + 2-5 pequenos para o mar, onde caem em forma de cascata, isto de- vido a altura deste ponto ao mar. A população vive ao lado deste canal, o que a torna vulnerável. Invasão da população em zonas identificadas como sendo de risco. Anualmente regista-se perdas de vidas humanas pela força 94 E; D + + + >5 das águas. Antes tinha uma proteção de gaviões e a montante, ti- nham sido plantadas mezquite há 10 anos Erosao exacerbada pela remoção dos gaviões ao longo do curso do caminho das águas, isto por parte da população, para a cons- 95 E; D o + + 2-5 trução de casas. Reportado que anualmente morem pessoas neste curso Encontro dos caminhos de água, que causam a erosao e destruí- ram a estrada antiga. Nota-se claramente a população a extrair ar- 96 E; D + + + 2-5 reia para a construção das suas casas, em zonas identificadas como proibidas 97 E; D o + + 1-2 98 E; D o + + 1-2 A população recorre ao plantio de estacas de Munhengueira para 99 E; D; W o + + 1-2 evitar a estrada expansão da erosao para os seus quintais. E esta medida tem funcionado. Para combater esta situação, a escola desenhou um projecto de 100 E; D + + + 2-5 combate a erosao que consistia em plantar acácias ao longo desta D, para combater esta situação. 101 E; D; W + + 2-5 102 E; D; W + + + 2-5 Ravina que afecta o murro da escola secundária Ravina que cerca a escola secundaria e que já assoriou 2 furos de 103 E; D; W + + + 2-5 água... Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 246 Since when is the issue present? Number of Affected People Importance of Mitigation Additional Comments Severity of Impact Type of Issue # D, sobre o rio maiaia que nasce na baixa da cidade na zona do 104 E; D - + + na porto e que não tem causado inundações e nem transportado sedi- mentos D, subterania (colector de descarga) que leva toda todas águas da 105 E; D + + na cidade alta e baixa para o mar... Este é o colector principal da ci- dade e mostra-se meio acoriado até a metade. Por causa do acoriamento das drenagens as fábricas na zona do o 106 D + + na Porto tiverem de abrir novas valas para auxiliar nas que já existiam 107 D + na Local onde desagua o rio maiaia 108 D + + na Canais de D, construídos por empresas ao longo da costa 109 D + + na 110 D + + na 111 D + + na 112 D + + na 113 D + + na 114 D + + 1-2 As marcas nos tanques mostram o nível em que a areia e as águas alcancaram na última época chuvosa. Mas isto deveu-se a 115 D + + 1-2 barramento de um canal de água para construir um parque de es- tacionamento de camiões Neste ponto a estrada ficou cortada, tendo sido tb destruídas ca- sas, e algumas instâncias turísticas. Houve um trabalho de reposi- 116 E; D o + + 1-2 ção da estrada é montada uma conduta para drenar as águas para o mar, mas a mesma não foi montada até ao mar e esta a causar mais erosao a sua jusante. Estrada erodida afectando o principal sistema de abastecimento de 117 E; D + + 1-2 água da Cidade de Nacala e algumas pontecas ao logo da mesma estrada Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 247 Since when is the issue present? Number of Affected People Importance of Mitigation Additional Comments Severity of Impact Type of Issue # Estrada erudida com tubagem da FIPAG em alto risco. Esta tuba- 118 E; D + + 1-2 gem sai do furo principal de água que abastece toda cidade de Na- cala Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 248 9 ANNEX III - ECOLOGICAL EVALUATION OF PROPOSED MEASURES Measure Impact on biodiver- Impact on ecosystem services Spatial extent of Duration of Possible sity measure measure negative Type of ecosystem Type of influence ecological service impacts Revegetation Improves biodiversity by Maintenance of biodiver- Increases diversity of In the case of urban Medium- to long- May cause introducing plant species sity plants and fauna asso- gardening: limited to term short-term and the fauna associated ciated with them the site mobility of with them soils during In the case of large- establish- Increases the diversity of Soil formation Planting an area in- scale revegetation of ment phase. habitats available for creases the organic terrestrial areas and ri- fauna content of the soil, parian areas: local im- May fail, thereby improving the pact, due to influence causing fur- Provides refuge, feeding condition of the soil on downstream hydrol- ther erosion and breeding areas for ogy fauna Primary production Increases productivity Supports ecological pro- through planting cesses such as pollina- tion, fertilisation, genetic Cultivated and wild foods Revegetation is pro- diversity. and Medicinal plants posed to include edible and useful species Grazing Plants can be edible for livestock Building materials and Plants should be use- Wood for fuel ful for building, for wood, and for charcoal Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 249 Measure Impact on biodiver- Impact on ecosystem services Spatial extent of Duration of Possible sity measure measure negative Type of ecosystem Type of influence ecological service impacts Flow regulation, water in- Improves soil condition filtration and recharge and so improves infil- tration and recharge Vegetation intercepts rainfall and decreases the intensity of rainfall and runoff Erosion control Vegetation intercepts rainfall and decreases the intensity of rainfall and runoff Carbon storage Increased organic con- tent of the soils means that the planted areas are carbon sinks, es- pecially in the wetter areas Sediment retention Vegetation traps sedi- ment, causing accu- mulation over time, fix- ing steep gradients and banks Water quality improve- Plants take up nutri- ment ents, thereby reducing nutrient loading in the Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 250 Measure Impact on biodiver- Impact on ecosystem services Spatial extent of Duration of Possible sity measure measure negative Type of ecosystem Type of influence ecological service impacts water, and improving water quality. Tourism and Recreation A well-vegetated land- scape has greater aes- thetic value for tour- ism, and can be designed to accommo- date sporting and rec- reational activities Detention ponds May reduce biodiversity Water provision Ponds may store suffi- Local Long-term May accumu- due to replacement of cient water for commu- late sediment natural areas with hard- nity use at such a ened surfaces (weirs and rapid rate, lined ponds) Flow regulation, water in- Detention ponds will that the ca- filtration and recharge change the manner in pacity is re- Provides pond habitat which water flows duced, during wet months through the catchment, thereby re- Will support revegetation slowing it down, and ducing the measures downstream reducing the erosive ecosystem by reducing flooding and capacity of the runoff. services of- erosion in downstream fered by the Detention ponds will reaches and vegetated measure. slow water down and areas There is evi- store water, allowing dence of this for local infiltration of all over water. Nacala. Flood attenuation Ponds have the capac- ity to hold floodwaters Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 251 Measure Impact on biodiver- Impact on ecosystem services Spatial extent of Duration of Possible sity measure measure negative Type of ecosystem Type of influence ecological service impacts Erosion control Ponds will reduce the erosive capacity of the runoff Sediment retention Ponds will effectively trap sediment Water quality improve- If detention ponds are ment planted, then it is likely that water exiting the ponds will be of better quality than the water entering it. Cultural and religious ex- The ponds may be- periences come the focal point of cultural or religious ceremonies Bank protection measures Stabilises bank habitat Maintenance of biodiver- Stabilises the banks Local Long-term May cause for flora and fauna sity for revegetation short-term mobility of Supports revegetation Soil formation Stabilised banks that soils during measures by stabilising have a gentle gradient contruction banks, and ensuring an and which can be phase. appropriate gradient for planted will improve revegetation the state of the soil Erosion control Protected banks are less likely to erode, and soles likely to Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 252 Measure Impact on biodiver- Impact on ecosystem services Spatial extent of Duration of Possible sity measure measure negative Type of ecosystem Type of influence ecological service impacts cause sedimentation downstream Sediment retention Depending on the de- sign of the bank pro- tection measures, these can lead to build up of sediment within the watercourses, and not just export down- stream. Water quality improvement Banks that are planted up with vegetation will contribute to the im- provement of water quality in the river. Creation of additional rec- Depending on plant spe- Flow regulation, water infil- Retention areas will Local Permanent May cause reational areas (parks) for cies planted, these areas tration and recharge change the manner in short-term retention purposes may introduce plant and which water flows mobility of faunal diversity through the catchment, soils during slowing it down, and contruction reducing the erosive phase. capacity of the runoff. Retention areas will slow water down, al- lowing for local infiltra- tion of water. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 253 Measure Impact on biodiver- Impact on ecosystem services Spatial extent of Duration of Possible sity measure measure negative Type of ecosystem Type of influence ecological service impacts Flood attenuation Retention areas have the capacity to hold floodwaters, to some extent Erosion control Retention areas will slow water down and reduce the erosive ca- pacity of the runoff Sediment retention All retention areas will effectively trap sedi- ment Water quality improve- If recreational areas to ment be used for retention are planted, such as with grass, then it is likely that water exiting these areas will be of better quality than the water entering them. Cultural and religious ex- These recreational ar- periences eas could be used for cultural or religious events Tourism and recreation Recreational areas can be used for sport and Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 254 Measure Impact on biodiver- Impact on ecosystem services Spatial extent of Duration of Possible sity measure measure negative Type of ecosystem Type of influence ecological service impacts other recreational ac- tivities, or just provide green spaces in the ur- ban environment. Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 255 10 ANNEX IV – RESULTS OF HYDRAULIC MODEL NACALA RESULTS OF HYDRAULIC MODEL - EXISTING SITUATION River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 1191.28 8.20 87.77 88.83 88.83 89.36 0.014118 3.24 2.53 2.40 1.01 1186.28 8.20 87.51 88.27 88.54 89.22 0.031856 4.33 1.90 2.51 1.59 1181.28 8.20 87.25 87.92 88.24 89.03 0.039969 4.66 1.76 2.62 1.81 1176.28 8.20 86.99 87.60 87.95 88.80 0.045871 4.85 1.69 2.73 1.97 1171.28 8.20 86.72 87.30 87.67 88.56 0.050049 4.96 1.65 2.84 2.08 1166.28 8.20 86.46 87.01 87.38 88.30 0.053083 5.02 1.63 2.95 2.16 1161.28 8.20 86.20 86.73 87.09 88.03 0.055184 5.05 1.62 3.06 2.21 1156.28 8.20 85.93 86.44 86.81 87.75 0.056728 5.06 1.62 3.17 2.26 1151.28 8.20 85.67 86.17 86.53 87.46 0.057362 5.04 1.63 3.28 2.28 1146.28 8.20 85.41 85.89 86.25 87.16 0.057493 5.00 1.64 3.39 2.30 1141.28 8.20 85.14 85.62 85.96 86.87 0.057462 4.96 1.65 3.50 2.30 1136.28 8.20 84.93 85.44 85.77 86.57 0.047933 4.71 1.74 3.40 2.10 1131.28 8.20 84.72 85.26 85.57 86.33 0.043250 4.58 1.79 3.30 1.98 1126.28 8.20 84.51 85.07 85.38 86.11 0.040444 4.51 1.82 3.20 1.91 1121.28 8.20 84.29 84.88 85.18 85.91 0.039199 4.49 1.83 3.10 1.87 1116.28 8.20 84.08 84.69 84.99 85.72 0.038483 4.49 1.83 3.00 1.84 1111.28 8.20 83.87 84.50 84.80 85.53 0.037803 4.49 1.83 2.90 1.81 1106.28 8.20 83.66 84.31 84.61 85.34 0.037310 4.50 1.82 2.80 1.78 1101.28 8.20 83.45 84.12 84.42 85.15 0.036690 4.50 1.82 2.70 1.75 1096.28 8.20 83.23 83.93 84.23 84.97 0.036273 4.51 1.82 2.60 1.72 1091.28 8.20 83.02 83.75 84.05 84.78 0.035875 4.51 1.82 2.50 1.69 1086.28 8.20 82.67 83.14 83.51 84.51 0.064405 5.20 1.58 3.37 2.43 1081.28 8.20 82.32 82.68 83.04 84.14 0.085474 5.35 1.53 4.24 2.84 1076.28 8.20 81.97 82.27 82.61 83.66 0.095639 5.22 1.57 5.11 3.01 1071.28 8.20 81.62 81.89 82.19 83.14 0.095796 4.96 1.65 5.98 3.01 1066.28 8.20 81.27 81.52 81.79 82.63 0.091827 4.66 1.76 6.85 2.94 1061.28 8.20 80.91 81.16 81.40 82.15 0.087892 4.41 1.86 7.72 2.87 1056.28 8.20 80.56 80.79 81.02 81.69 0.084500 4.19 1.96 8.59 2.80 1051.28 8.20 80.21 80.43 80.64 81.25 0.081990 4.01 2.05 9.46 2.75 1046.28 8.20 79.86 80.07 80.26 80.82 0.080083 3.85 2.13 10.33 2.71 1041.28 8.20 79.51 79.71 79.89 80.41 0.078588 3.71 2.21 11.20 2.67 1036.28 8.20 79.48 79.77 79.86 80.11 0.023278 2.58 3.17 10.93 1.53 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 256 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 1031.28 8.20 79.44 79.93 79.83 80.06 0.004396 1.56 5.26 10.66 0.71 1026.28 8.20 79.41 79.91 80.04 0.004395 1.57 5.21 10.39 0.71 1021.28 8.20 79.37 79.88 80.01 0.004357 1.58 5.18 10.12 0.71 1016.29 8.20 79.34 79.86 79.99 0.004407 1.60 5.11 9.85 0.71 1011.29 8.20 79.30 79.83 79.97 0.004438 1.62 5.05 9.58 0.71 1006.29 8.20 79.27 79.80 79.94 0.004591 1.66 4.95 9.31 0.72 1001.29 8.20 79.23 79.77 79.92 0.004817 1.70 4.83 9.04 0.74 996.29 8.20 79.20 79.73 79.65 79.89 0.005183 1.75 4.68 8.77 0.77 991.29 8.20 79.17 79.62 79.62 79.85 0.009167 2.12 3.87 8.50 1.00 986.74 8.20 79.03 79.39 79.49 79.77 0.021148 2.75 2.98 8.45 1.48 982.20 8.20 78.90 79.24 79.36 79.67 0.024503 2.89 2.84 8.41 1.58 977.65 8.20 78.77 79.10 79.23 79.55 0.026187 2.95 2.78 8.36 1.63 973.10 8.20 78.64 78.97 79.10 79.43 0.027376 3.00 2.74 8.32 1.67 968.56 8.20 78.51 78.84 78.97 79.30 0.027699 3.01 2.72 8.27 1.68 964.01 8.20 78.38 78.71 78.84 79.17 0.027994 3.03 2.71 8.23 1.69 959.47 8.20 78.25 78.58 78.71 79.05 0.028217 3.04 2.69 8.18 1.69 954.92 8.20 78.11 78.44 78.58 78.92 0.028493 3.06 2.68 8.14 1.70 950.37 8.20 77.98 78.31 78.45 78.79 0.028232 3.06 2.68 8.09 1.69 945.83 8.20 77.85 78.18 78.32 78.66 0.028443 3.07 2.67 8.05 1.70 941.28 8.20 77.72 78.05 78.19 78.53 0.028168 3.07 2.67 8.00 1.69 936.28 8.20 77.59 77.87 78.02 78.37 0.035558 3.13 2.62 9.20 1.88 931.28 8.20 77.45 77.71 77.85 78.18 0.036898 3.03 2.71 10.40 1.90 926.28 8.20 77.32 77.56 77.69 77.99 0.035493 2.88 2.85 11.60 1.85 921.28 8.20 77.18 77.42 77.53 77.80 0.033653 2.73 3.01 12.80 1.80 916.28 8.20 77.05 77.27 77.38 77.62 0.032451 2.61 3.15 14.00 1.76 911.28 8.20 76.92 77.13 77.23 77.45 0.031532 2.50 3.27 15.20 1.72 906.28 8.20 76.78 76.99 77.08 77.29 0.031007 2.42 3.39 16.40 1.70 901.28 8.20 76.65 76.85 76.93 77.13 0.030581 2.35 3.50 17.60 1.68 896.28 8.20 76.51 76.71 76.78 76.97 0.030179 2.28 3.60 18.80 1.66 891.28 8.20 76.38 76.56 76.64 76.82 0.030487 2.23 3.68 20.00 1.66 886.28 8.20 76.32 76.62 76.59 76.73 0.007014 1.46 5.60 18.80 0.86 881.28 8.20 76.27 76.59 76.70 0.005882 1.42 5.76 17.60 0.79 876.28 8.20 76.21 76.57 76.67 0.004764 1.37 5.99 16.40 0.72 871.28 8.20 76.15 76.55 76.65 0.004016 1.34 6.13 15.20 0.67 866.28 8.20 76.09 76.54 76.63 0.003473 1.32 6.22 14.00 0.63 861.28 8.20 76.04 76.52 76.61 0.003151 1.32 6.21 12.80 0.61 856.28 8.20 75.98 76.50 76.59 0.003082 1.36 6.04 11.60 0.60 851.28 8.20 75.92 76.47 76.58 0.003221 1.43 5.74 10.40 0.61 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 257 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 846.28 8.20 75.86 76.43 76.56 0.003830 1.57 5.22 9.20 0.67 841.28 8.20 75.80 76.28 76.28 76.52 0.009222 2.17 3.79 8.00 1.01 836.28 8.20 75.32 75.56 75.78 76.36 0.069578 3.96 2.07 8.50 2.57 831.28 8.20 74.84 75.05 75.27 75.96 0.091232 4.22 1.94 9.00 2.90 826.28 8.20 74.35 74.55 74.77 75.48 0.100582 4.26 1.93 9.50 3.02 821.28 8.20 73.87 74.06 74.28 74.96 0.102251 4.20 1.95 10.00 3.03 816.28 8.20 73.38 73.57 73.78 74.44 0.102671 4.13 1.99 10.50 3.03 811.28 8.20 72.90 73.08 73.28 73.92 0.102385 4.05 2.02 11.00 3.02 806.28 8.20 72.41 72.59 72.78 73.40 0.101768 3.98 2.06 11.50 3.00 801.28 8.20 71.93 72.10 72.29 72.88 0.101561 3.91 2.10 12.00 2.99 796.28 8.20 71.44 71.61 71.80 72.37 0.101076 3.84 2.13 12.50 2.97 791.28 8.20 70.96 71.13 71.30 71.86 0.100957 3.78 2.17 13.00 2.96 786.72 8.20 70.81 71.03 71.16 71.49 0.043826 2.99 2.75 12.45 2.03 782.17 8.20 70.66 70.91 71.03 71.30 0.031714 2.75 2.98 11.91 1.76 777.61 8.20 70.51 71.03 70.89 71.13 0.003284 1.39 5.88 11.36 0.62 773.05 8.20 70.36 71.05 71.11 0.001519 1.11 7.36 10.82 0.43 768.49 8.20 70.22 71.05 71.10 0.000885 0.96 8.59 10.27 0.33 763.94 8.20 70.07 71.05 71.09 0.000594 0.85 9.60 9.73 0.27 759.38 8.20 69.92 71.06 71.09 0.000436 0.78 10.45 9.18 0.24 754.82 8.20 69.77 71.06 71.09 0.000344 0.74 11.11 8.64 0.21 750.26 8.20 69.62 71.06 71.08 0.000288 0.71 11.61 8.09 0.19 745.71 8.20 69.47 71.06 71.08 0.000254 0.69 11.95 7.55 0.17 741.15 8.20 69.32 71.06 71.08 0.000235 0.68 12.12 7.00 0.16 736.16 8.20 69.45 71.06 71.08 0.000228 0.66 12.47 7.75 0.17 731.18 8.20 69.57 71.06 71.08 0.000232 0.65 12.63 8.50 0.17 726.19 8.20 69.69 71.05 71.08 0.000248 0.65 12.59 9.25 0.18 721.20 8.20 69.82 71.05 71.07 0.000278 0.66 12.37 10.00 0.19 716.22 8.20 69.94 71.05 71.07 0.000329 0.69 11.94 10.75 0.21 711.23 8.20 70.06 71.04 71.07 0.000415 0.73 11.31 11.50 0.23 706.24 8.20 70.18 71.04 71.07 0.000564 0.78 10.46 12.25 0.27 701.25 8.20 70.31 71.02 71.06 0.000860 0.88 9.34 13.00 0.33 696.27 8.20 70.43 71.00 71.06 0.001573 1.04 7.89 13.75 0.44 691.28 8.20 70.55 70.87 70.87 71.03 0.009604 1.78 4.62 14.50 1.01 686.73 8.20 70.45 70.71 70.77 70.96 0.019918 2.23 3.67 14.27 1.40 682.19 8.20 70.35 70.60 70.67 70.87 0.020917 2.28 3.60 14.04 1.44 677.64 8.20 70.24 70.50 70.57 70.77 0.021017 2.30 3.57 13.82 1.44 673.09 8.20 70.14 70.40 70.47 70.67 0.020809 2.30 3.56 13.59 1.44 668.55 8.20 70.04 70.30 70.37 70.58 0.021053 2.33 3.53 13.36 1.45 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 258 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 664.00 8.20 69.93 70.20 70.27 70.48 0.020804 2.33 3.52 13.14 1.44 659.46 8.20 69.83 70.10 70.18 70.38 0.020630 2.34 3.50 12.91 1.44 654.91 8.20 69.73 70.00 70.08 70.29 0.020896 2.37 3.46 12.68 1.45 650.36 8.20 69.63 69.90 69.98 70.19 0.020612 2.37 3.46 12.46 1.44 645.82 8.20 69.52 69.80 69.88 70.10 0.020823 2.40 3.42 12.23 1.45 641.27 8.20 69.42 69.70 69.78 70.00 0.020540 2.40 3.41 12.00 1.44 636.27 8.20 69.36 69.71 69.73 69.92 0.011226 2.02 4.06 11.65 1.09 631.27 8.20 69.31 69.67 69.69 69.88 0.011025 2.03 4.04 11.30 1.08 626.27 8.20 69.26 69.66 69.64 69.83 0.008137 1.87 4.38 10.95 0.94 621.27 8.20 69.20 69.62 69.59 69.79 0.007809 1.87 4.39 10.60 0.93 616.28 8.20 69.15 69.57 69.55 69.75 0.007648 1.88 4.37 10.25 0.92 611.28 8.20 69.09 69.53 69.50 69.71 0.007564 1.89 4.33 9.90 0.91 606.28 8.20 69.04 69.49 69.46 69.67 0.007092 1.88 4.36 9.55 0.89 601.28 8.20 68.98 69.45 69.41 69.64 0.007069 1.90 4.31 9.20 0.89 596.28 8.20 68.93 69.41 69.37 69.60 0.006841 1.91 4.30 8.85 0.87 591.28 8.20 68.87 69.33 69.33 69.56 0.009167 2.12 3.87 8.50 1.00 586.28 8.20 68.44 68.69 68.89 69.41 0.060664 3.76 2.18 8.75 2.41 581.28 8.20 68.02 68.24 68.45 69.06 0.077365 4.01 2.05 9.00 2.69 576.28 8.20 67.59 67.80 68.02 68.66 0.084891 4.08 2.01 9.25 2.80 571.28 8.20 67.16 67.37 67.58 68.22 0.087816 4.08 2.01 9.50 2.84 566.28 8.20 66.73 66.94 67.15 67.78 0.088587 4.06 2.02 9.75 2.84 561.28 8.20 66.30 66.51 66.71 67.33 0.088317 4.01 2.04 10.00 2.84 556.28 8.20 65.88 66.08 66.28 66.88 0.088335 3.98 2.06 10.25 2.83 551.28 8.20 65.45 65.65 65.84 66.44 0.088250 3.94 2.08 10.50 2.83 546.28 8.20 65.02 65.22 65.41 65.99 0.088182 3.90 2.10 10.75 2.82 541.28 8.20 64.59 64.79 64.98 65.55 0.088152 3.87 2.12 11.00 2.82 536.72 8.20 64.47 64.72 64.86 65.20 0.040850 3.08 2.66 10.82 1.98 532.16 8.20 64.35 64.63 64.75 65.02 0.028581 2.78 2.95 10.64 1.69 527.60 8.20 64.23 64.52 64.63 64.89 0.025582 2.70 3.03 10.45 1.60 523.04 8.20 64.11 64.41 64.51 64.78 0.025135 2.71 3.03 10.27 1.59 518.48 8.20 63.99 64.29 64.40 64.67 0.024916 2.72 3.02 10.09 1.59 513.92 8.20 63.87 64.18 64.28 64.55 0.024261 2.71 3.02 9.91 1.57 509.36 8.20 63.75 64.06 64.17 64.44 0.024035 2.72 3.01 9.73 1.56 504.80 8.20 63.63 64.23 64.05 64.33 0.003023 1.44 5.69 9.55 0.60 500.24 8.20 63.51 64.24 64.31 0.001664 1.20 6.83 9.36 0.45 495.68 8.20 63.39 64.25 64.30 0.001052 1.04 7.86 9.18 0.36 491.12 15.10 63.27 63.93 63.93 64.26 0.008549 2.55 5.92 9.00 1.01 486.14 15.10 63.18 63.63 63.78 64.17 0.021659 3.28 4.61 10.20 1.56 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 259 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 481.15 15.10 63.08 63.47 63.64 64.05 0.026910 3.37 4.48 11.40 1.72 476.17 15.10 62.99 63.35 63.51 63.91 0.028932 3.32 4.55 12.60 1.77 471.18 15.10 62.89 63.23 63.38 63.75 0.028043 3.18 4.74 13.80 1.73 466.20 15.10 62.79 63.12 63.26 63.60 0.027120 3.05 4.94 15.00 1.70 461.22 15.10 62.70 63.02 63.14 63.45 0.026023 2.93 5.15 16.20 1.66 456.23 15.10 62.60 62.91 63.03 63.32 0.025656 2.84 5.32 17.40 1.64 451.25 15.10 62.51 62.81 62.91 63.19 0.024848 2.74 5.51 18.60 1.61 446.26 15.10 62.41 62.70 62.80 63.06 0.024405 2.66 5.67 19.80 1.59 441.28 15.10 62.32 62.60 62.69 62.94 0.023590 2.58 5.86 21.00 1.56 436.28 15.10 62.24 62.60 62.63 62.82 0.011132 2.10 7.20 19.80 1.11 431.28 15.10 62.15 62.53 62.56 62.77 0.011270 2.16 7.00 18.60 1.12 426.28 15.10 62.07 62.47 62.50 62.71 0.010489 2.16 6.98 17.40 1.09 421.28 15.10 61.99 62.46 62.44 62.66 0.007084 1.97 7.67 16.20 0.91 416.28 15.10 61.91 62.45 62.63 0.005422 1.86 8.10 15.00 0.81 411.28 15.10 61.83 62.43 62.60 0.004466 1.81 8.35 13.80 0.74 406.28 15.10 61.74 62.41 62.58 0.003961 1.80 8.40 12.60 0.70 401.28 15.10 61.66 62.38 62.56 0.003890 1.85 8.18 11.40 0.70 396.28 15.10 61.58 62.34 62.53 0.004236 1.97 7.68 10.20 0.72 391.28 15.10 61.50 62.16 62.16 62.49 0.008547 2.55 5.92 9.00 1.01 386.28 15.10 61.32 61.73 61.92 62.39 0.029390 3.59 4.21 10.30 1.79 381.28 15.10 61.14 61.49 61.70 62.21 0.039643 3.77 4.00 11.60 2.05 376.28 15.10 60.96 61.27 61.48 62.00 0.045279 3.78 4.00 12.90 2.17 371.28 15.10 60.78 61.07 61.27 61.76 0.046106 3.67 4.12 14.20 2.17 366.28 15.10 60.60 60.88 61.06 61.51 0.045285 3.53 4.28 15.50 2.14 361.28 15.10 60.42 60.69 60.86 61.27 0.043764 3.39 4.46 16.80 2.10 356.28 15.10 60.24 60.50 60.65 61.04 0.042708 3.27 4.62 18.10 2.06 351.28 15.10 60.06 60.31 60.46 60.82 0.041852 3.16 4.78 19.40 2.03 346.28 15.10 59.88 60.12 60.26 60.60 0.041051 3.07 4.93 20.70 2.01 341.28 15.10 59.70 59.93 60.07 60.39 0.040651 2.99 5.06 22.00 1.99 336.28 15.10 59.56 59.82 59.93 60.20 0.027439 2.71 5.58 20.80 1.67 331.28 15.10 59.41 59.69 59.80 60.07 0.025428 2.71 5.58 19.60 1.62 326.28 15.10 59.26 59.85 59.67 59.95 0.002612 1.38 10.91 18.40 0.57 321.28 15.10 59.11 59.86 59.93 0.001398 1.17 12.92 17.20 0.43 316.28 15.10 58.97 59.87 59.92 0.000904 1.05 14.44 16.00 0.35 311.28 15.10 58.82 59.87 59.92 0.000658 0.97 15.55 14.80 0.30 306.28 15.10 58.67 59.87 59.91 0.000522 0.93 16.31 13.60 0.27 301.28 15.10 58.52 59.87 59.91 0.000448 0.91 16.68 12.40 0.25 296.28 15.10 58.38 59.87 59.91 0.000411 0.90 16.70 11.20 0.24 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 260 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 291.28 15.10 58.23 59.86 59.91 0.000404 0.92 16.34 10.00 0.23 286.28 15.10 58.33 59.86 59.90 0.000410 0.91 16.56 10.80 0.24 281.28 15.10 58.43 59.86 59.90 0.000428 0.91 16.63 11.60 0.24 276.28 15.10 58.52 59.86 59.90 0.000462 0.91 16.52 12.40 0.25 271.28 15.10 58.62 59.85 59.90 0.000516 0.93 16.23 13.20 0.27 266.28 15.10 58.72 59.85 59.89 0.000598 0.96 15.76 14.00 0.29 261.28 15.10 58.82 59.84 59.89 0.000727 1.00 15.07 14.80 0.32 256.28 15.10 58.92 59.83 59.89 0.000932 1.06 14.18 15.60 0.36 251.28 15.10 59.02 59.81 59.88 0.001302 1.16 13.00 16.40 0.42 246.28 15.10 59.12 59.78 59.87 0.002081 1.32 11.42 17.20 0.52 241.28 15.10 59.22 59.63 59.63 59.84 0.008828 2.03 7.45 18.00 1.01 236.28 15.10 59.12 59.47 59.54 59.77 0.015777 2.43 6.20 17.70 1.31 231.28 15.10 59.03 59.38 59.45 59.69 0.016515 2.48 6.08 17.40 1.34 226.28 15.10 58.93 59.28 59.36 59.60 0.016855 2.52 6.00 17.10 1.36 221.28 15.10 58.84 59.19 59.27 59.52 0.017071 2.54 5.94 16.80 1.37 216.28 15.10 58.74 59.10 59.18 59.43 0.016876 2.55 5.92 16.50 1.36 211.28 15.10 58.65 59.01 59.09 59.35 0.016941 2.57 5.87 16.20 1.36 206.28 15.10 58.55 58.92 59.00 59.26 0.016938 2.59 5.83 15.90 1.36 201.28 15.10 58.46 58.83 58.91 59.17 0.016938 2.61 5.79 15.60 1.37 196.28 15.10 58.36 58.74 58.83 59.09 0.016558 2.61 5.79 15.30 1.35 191.28 15.10 58.27 58.65 58.74 59.00 0.016627 2.63 5.74 15.00 1.36 186.28 15.10 58.27 58.80 58.71 58.95 0.004914 1.76 8.60 16.30 0.77 181.28 15.10 58.27 58.78 58.92 0.004487 1.66 9.09 17.60 0.74 176.28 15.10 58.27 58.77 58.90 0.004251 1.59 9.49 18.90 0.72 171.28 15.10 58.27 58.75 58.87 0.004045 1.53 9.87 20.20 0.70 166.28 15.10 58.27 58.74 58.85 0.003956 1.48 10.17 21.50 0.69 161.28 15.10 58.26 58.72 58.83 0.003900 1.45 10.44 22.80 0.68 156.28 15.10 58.26 58.71 58.81 0.003928 1.42 10.64 24.10 0.68 151.28 15.10 58.26 58.69 58.79 0.004094 1.41 10.72 25.40 0.69 146.28 15.10 58.26 58.66 58.76 0.004546 1.43 10.58 26.70 0.72 141.28 15.10 58.26 58.57 58.57 58.73 0.009442 1.75 8.64 28.00 1.01 136.28 15.10 58.03 58.23 58.34 58.62 0.040522 2.75 5.50 27.20 1.95 131.28 15.10 57.79 58.00 58.11 58.40 0.042808 2.82 5.35 26.40 2.00 126.28 15.10 57.56 57.77 57.89 58.19 0.043502 2.87 5.26 25.60 2.02 121.28 15.10 57.33 57.53 57.66 57.97 0.043820 2.92 5.18 24.80 2.04 116.28 15.10 57.09 57.30 57.43 57.75 0.043845 2.95 5.11 24.00 2.04 111.28 15.10 56.86 57.08 57.21 57.53 0.043323 2.98 5.06 23.20 2.04 106.28 15.10 56.62 56.85 56.98 57.31 0.042915 3.01 5.01 22.40 2.03 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 261 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 101.28 15.10 56.39 56.62 56.76 57.09 0.043020 3.06 4.94 21.60 2.04 96.28 15.10 56.15 56.39 56.53 56.88 0.043068 3.10 4.86 20.80 2.05 91.28 15.10 55.92 56.16 56.31 56.66 0.042319 3.14 4.82 20.00 2.04 86.28 15.10 55.52 55.72 55.90 56.38 0.069979 3.58 4.22 21.00 2.55 81.28 15.10 55.13 55.32 55.49 56.00 0.080100 3.66 4.12 22.00 2.70 76.28 15.10 54.73 54.91 55.08 55.59 0.083161 3.64 4.15 23.00 2.74 71.28 15.10 54.34 54.51 54.68 55.17 0.083054 3.58 4.22 24.00 2.73 66.28 15.10 53.94 54.11 54.27 54.75 0.083043 3.52 4.28 25.00 2.72 61.28 15.10 53.55 53.71 53.87 54.33 0.082730 3.47 4.36 26.00 2.70 56.28 15.10 53.15 53.32 53.47 53.91 0.082279 3.41 4.43 27.00 2.69 51.28 15.10 52.76 52.92 53.06 53.49 0.082282 3.36 4.49 28.00 2.68 46.28 15.10 52.36 52.52 52.66 53.07 0.081287 3.30 4.57 29.00 2.66 41.28 15.10 51.96 52.12 52.26 52.66 0.081984 3.27 4.62 30.00 2.66 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 262 RESULTS OF HYDRAULIC MODEL - INCLUDING DETENTION PONDS River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 1191.28 8.2 87.77 90.48 88.83 90.57 0.00127 1.26 6.5 2.4 0.24 1186.28* 8.2 87.51 90.49 90.55 0.000885 1.1 7.48 2.51 0.2 1181.28* 8.2 87.25 90.5 90.55 0.00063 0.96 8.55 3.87 0.17 1176.28* 8.2 86.99 90.52 87.95 90.53 0.000361 0.57 14.27 15.35 0.19 1176.2 Inl Struct 1171.28* 8.2 86.72 88.61 88.73 0.001935 1.53 5.36 2.84 0.36 1166.28* 8.2 86.46 88.63 88.71 0.001429 1.26 6.53 4.45 0.33 1161.28* 8.2 86.2 88.66 88.69 0.000603 0.84 9.81 8.36 0.25 1156.28* 8.2 85.93 88.67 88.68 0.000233 0.61 13.49 9.47 0.16 1151.28* 8.2 85.67 88.67 88.68 0.000129 0.52 15.8 8.7 0.12 1146.28* 8.2 85.41 88.67 86.25 88.68 0.000112 0.52 15.64 6.6 0.11 1146.2 Inl Struct 1141.28 8.2 85.14 87.15 87.22 0.000932 1.17 7.02 3.5 0.26 1136.28* 8.2 84.93 87.17 87.21 0.000423 0.87 9.47 5.43 0.21 1131.28* 8.2 84.72 87.17 87.2 0.000304 0.72 11.37 7.24 0.18 1126.28* 8.2 84.51 87.18 87.2 0.000265 0.64 12.75 8.91 0.17 1121.28* 8.2 84.29 87.18 87.2 0.000258 0.6 13.61 10.37 0.17 1116.28* 8.2 84.08 87.18 87.19 0.000273 0.59 13.98 11.57 0.17 1111.28* 8.2 83.87 87.18 87.19 0.000311 0.59 13.89 12.39 0.18 1106.28* 8.2 83.66 87.17 87.19 0.000376 0.61 13.38 12.69 0.19 1101.28* 8.2 83.45 87.17 87.19 0.000471 0.65 12.53 12.21 0.21 1096.28* 8.2 83.23 87.15 87.19 0.000397 0.8 11.44 10.35 0.13 1091.28 8.2 83.02 87.15 84.05 87.18 0.000402 0.79 10.91 11.31 0.12 1091.2 Inl Struct 1086.28* 8.2 82.67 84.65 84.73 0.001066 1.23 6.68 3.37 0.28 1081.28* 8.2 82.32 84.68 84.71 0.000362 0.82 10.01 4.24 0.17 1076.28* 8.2 81.97 84.69 84.71 0.00015 0.59 13.91 5.11 0.11 1071.28* 8.2 81.62 84.7 82.19 84.7 0.000083 0.39 20.84 13.1 0.1 1071.2 Inl Struct 1066.28* 8.2 81.27 82.23 82.31 0.001404 1.24 6.61 6.85 0.4 1061.28* 8.2 80.91 82.26 82.29 0.000391 0.79 10.39 7.72 0.22 1056.28* 8.2 80.56 82.27 82.29 0.000157 0.54 15.16 10.56 0.14 1051.28* 8.2 80.21 82.28 82.28 0.00006 0.38 21.41 12.47 0.09 1046.28* 8.2 79.86 82.28 82.28 0.000028 0.3 27.2 12.55 0.07 1041.28 8.2 79.51 82.28 82.28 0.000019 0.26 30.99 11.2 0.05 1036.28* 8.2 79.48 82.28 82.28 0.00002 0.27 30.62 10.93 0.05 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 263 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 1031.28* 8.2 79.44 82.28 79.83 82.28 0.00002 0.27 30.23 10.66 0.05 1031.2 Inl Struct 1026.28* 8.2 79.41 80.73 80.75 0.000206 0.6 13.78 10.39 0.16 1021.28* 8.2 79.37 80.73 80.75 0.000202 0.6 13.77 10.12 0.16 1016.29* 8.2 79.34 80.73 80.75 0.0002 0.6 13.72 9.85 0.16 1011.29* 8.2 79.3 80.73 80.75 0.000198 0.6 13.67 9.58 0.16 1006.29* 8.2 79.27 80.73 80.75 0.000198 0.6 13.59 9.31 0.16 1001.29* 8.2 79.23 80.73 80.75 0.000198 0.61 13.5 9.04 0.16 996.29* 8.2 79.2 80.73 80.75 0.000199 0.61 13.38 8.77 0.16 991.29 8.2 79.17 80.73 80.75 0.000201 0.62 13.26 8.5 0.16 986.74* 8.2 79.03 80.73 80.74 0.00016 0.57 14.3 8.45 0.14 982.20* 8.2 78.9 80.73 80.74 0.000129 0.53 15.35 8.41 0.13 977.65* 8.2 78.77 80.73 80.74 0.000107 0.5 16.36 8.36 0.11 973.10* 8.2 78.64 80.73 80.74 0.00009 0.47 17.38 8.32 0.1 968.56* 8.2 78.51 80.73 80.74 0.000076 0.45 18.37 8.27 0.1 964.01* 8.2 78.38 80.73 80.74 0.000065 0.42 19.36 8.23 0.09 959.47* 8.2 78.25 80.73 80.74 0.000057 0.4 20.32 8.18 0.08 954.92* 8.2 78.11 80.73 80.74 0.00005 0.39 21.29 8.14 0.08 950.37* 8.2 77.98 80.73 80.74 0.000044 0.37 22.23 8.09 0.07 945.83* 8.2 77.85 80.73 80.74 0.000039 0.35 23.17 8.05 0.07 941.28 8.2 77.72 80.73 78.19 80.74 0.000035 0.34 24.09 8 0.06 941.2 Inl Struct 936.28* 8.2 77.59 78.92 78.94 0.000268 0.67 12.26 9.2 0.19 931.28* 8.2 77.45 78.92 78.94 0.00015 0.54 15.3 10.4 0.14 926.28* 8.2 77.32 78.93 78.94 0.000089 0.44 18.65 11.6 0.11 921.28* 8.2 77.18 78.93 78.93 0.000055 0.37 22.32 12.8 0.09 916.28* 8.2 77.05 78.93 78.93 0.000036 0.31 26.3 14 0.07 911.28* 8.2 76.92 78.93 78.93 0.000024 0.27 30.59 15.2 0.06 906.28* 8.2 76.78 78.93 78.93 0.000017 0.23 35.2 16.4 0.05 901.28* 8.2 76.65 78.93 78.93 0.000012 0.2 40.15 17.6 0.04 896.28* 8.2 76.51 78.93 78.93 0.000009 0.18 45.41 18.8 0.04 891.28 8.2 76.38 78.93 76.64 78.93 0.000006 0.16 50.99 20 0.03 891.2 Inl Struct 886.28* 8.2 76.32 77.78 77.78 0.000041 0.3 27.36 18.8 0.08 881.28* 8.2 76.27 77.78 77.78 0.000042 0.31 26.61 17.6 0.08 876.28* 8.2 76.21 77.78 77.78 0.000044 0.32 25.74 16.4 0.08 871.28* 8.2 76.15 77.78 77.78 0.000047 0.33 24.71 15.2 0.08 866.28* 8.2 76.09 77.78 77.78 0.00005 0.35 23.56 14 0.09 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 264 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 861.28* 8.2 76.04 77.77 77.78 0.000056 0.37 22.27 12.8 0.09 856.28* 8.2 75.98 77.77 77.78 0.000064 0.39 20.83 11.6 0.09 851.28* 8.2 75.92 77.77 77.78 0.000075 0.43 19.26 10.4 0.1 846.28* 8.2 75.86 77.77 77.78 0.000092 0.47 17.54 9.2 0.11 841.28 8.2 75.8 77.77 77.78 0.000118 0.52 15.68 8 0.12 836.28* 8.2 75.32 77.77 77.78 0.000054 0.39 20.82 8.5 0.08 831.28* 8.2 74.84 77.77 75.27 77.78 0.000028 0.31 26.42 9 0.06 831.2 Inl Struct 826.28* 8.2 74.35 75.17 75.23 0.001104 1.05 7.82 9.5 0.37 821.28* 8.2 73.87 75.2 75.22 0.000222 0.62 13.3 10 0.17 816.28* 8.2 73.38 75.2 75.21 0.000077 0.43 19.13 10.5 0.1 811.28* 8.2 72.9 75.21 75.21 0.000034 0.32 25.4 11 0.07 806.28* 8.2 72.41 75.21 72.78 75.21 0.000018 0.26 32.14 11.5 0.05 806.2 Inl Struct 801.28* 8.2 71.93 72.29 72.29 72.47 0.009401 1.89 4.33 12 1.01 796.28* 8.2 71.44 72.18 71.8 72.22 0.000882 0.9 9.15 12.5 0.33 791.28 8.2 70.96 72.19 72.21 0.000155 0.51 16.05 13 0.15 786.72* 8.2 70.81 72.19 72.21 0.000113 0.46 17.75 13.43 0.13 782.17* 8.2 70.66 72.2 72.2 0.000094 0.43 18.94 13.59 0.12 777.61* 8.2 70.51 72.2 72.2 0.000084 0.42 19.72 13.39 0.11 773.05* 8.2 70.36 72.2 72.2 0.000076 0.41 20.16 12.73 0.1 768.49* 8.2 70.22 72.2 72.2 0.000068 0.4 20.42 11.42 0.1 763.94* 8.2 70.07 72.2 72.2 0.000058 0.4 20.69 9.73 0.09 759.38* 8.2 69.92 72.19 72.2 0.000055 0.39 20.9 9.18 0.08 754.82* 8.2 69.77 72.19 72.2 0.000053 0.39 20.93 8.64 0.08 750.26* 8.2 69.62 72.19 72.2 0.000053 0.39 20.81 8.09 0.08 745.71* 8.2 69.47 72.19 72.2 0.000054 0.4 20.52 7.55 0.08 741.15 8.2 69.32 72.19 72.2 0.000057 0.41 20.08 7 0.08 736.16* 8.2 69.45 72.19 72.2 0.000049 0.39 21.28 7.75 0.07 731.18* 8.2 69.57 72.19 72.2 0.000044 0.37 22.3 8.5 0.07 726.19* 8.2 69.69 72.19 72.2 0.000041 0.35 23.13 9.25 0.07 721.20* 8.2 69.82 72.19 72.2 0.000039 0.34 23.79 10 0.07 716.22* 8.2 69.94 72.19 72.2 0.000043 0.33 24.53 12.89 0.08 711.23* 8.2 70.06 72.19 72.2 0.000043 0.32 25.36 14.61 0.08 706.24* 8.2 70.18 72.19 72.2 0.000041 0.32 25.91 15.4 0.08 701.25* 8.2 70.31 72.19 72.2 0.00004 0.32 25.92 15.54 0.08 696.27* 8.2 70.43 72.19 72.2 0.000042 0.32 25.26 15.2 0.08 691.28 8.2 70.55 72.19 70.87 72.2 0.00005 0.34 23.8 14.5 0.09 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 265 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 686.73* 8.2 70.45 72.19 72.2 0.000043 0.33 24.9 14.27 0.08 682.19* 8.2 70.35 72.19 72.2 0.000038 0.32 25.95 14.04 0.07 677.64* 8.2 70.24 72.19 72.2 0.000033 0.3 26.94 13.82 0.07 673.09* 8.2 70.14 72.19 72.2 0.000029 0.29 27.9 13.59 0.07 668.55* 8.2 70.04 72.19 72.2 0.000026 0.28 28.81 13.36 0.06 664.00* 8.2 69.93 72.19 72.2 0.000024 0.28 29.67 13.14 0.06 659.46* 8.2 69.83 72.19 72.2 0.000022 0.27 30.49 12.91 0.06 654.91* 8.2 69.73 72.19 72.2 0.00002 0.26 31.26 12.68 0.05 650.36* 8.2 69.63 72.19 72.2 0.000019 0.26 31.97 12.46 0.05 645.82* 8.2 69.52 72.19 72.2 0.000017 0.25 32.65 12.23 0.05 641.27 8.2 69.42 72.19 69.78 72.2 0.000016 0.25 33.28 12 0.05 641.2 Inl Struct 636.27* 8.2 69.36 69.75 69.73 69.92 0.00827 1.84 4.46 11.65 0.95 631.27* 8.2 69.31 69.7 69.69 69.88 0.008005 1.84 4.46 11.3 0.94 626.27* 8.2 69.26 69.67 69.64 69.84 0.007529 1.83 4.49 10.95 0.91 621.27* 8.2 69.2 69.64 69.8 0.006527 1.77 4.64 10.6 0.85 616.28* 8.2 69.15 69.62 69.76 0.00552 1.7 4.83 10.25 0.79 611.28* 8.2 69.09 69.6 69.74 0.004625 1.63 5.04 9.9 0.73 606.28* 8.2 69.04 69.59 69.71 0.003879 1.56 5.27 9.55 0.67 601.28* 8.2 68.98 69.58 69.69 0.003304 1.5 5.47 9.2 0.62 596.28* 8.2 68.93 69.57 69.67 0.002836 1.45 5.67 8.85 0.58 591.28 8.2 68.87 69.56 69.66 0.002485 1.4 5.84 8.5 0.54 586.28* 8.2 68.44 69.6 69.63 0.000462 0.81 10.12 8.75 0.24 581.28* 8.2 68.02 69.61 69.63 0.000164 0.57 14.36 9 0.14 576.28* 8.2 67.59 69.62 69.63 0.000076 0.44 18.76 9.25 0.1 571.28* 8.2 67.16 69.62 69.62 0.000041 0.35 23.35 9.5 0.07 566.28* 8.2 66.73 69.62 67.15 69.62 0.000024 0.29 28.15 9.75 0.05 566.2 Inl Struct 561.28* 8.2 66.3 66.75 66.71 66.92 0.00699 1.84 4.45 10 0.88 556.28* 8.2 65.88 66.84 66.87 0.000573 0.83 9.86 10.25 0.27 551.28* 8.2 65.45 66.85 66.87 0.000169 0.56 14.72 10.5 0.15 546.28* 8.2 65.02 66.86 66.86 0.000071 0.42 19.72 10.75 0.1 541.28 8.2 64.59 66.86 66.86 0.000036 0.33 24.91 11 0.07 536.72* 8.2 64.47 66.86 66.86 0.000032 0.32 25.8 10.82 0.07 532.16* 8.2 64.35 66.86 66.86 0.000029 0.31 26.64 10.64 0.06 527.60* 8.2 64.23 66.86 66.86 0.000026 0.3 27.45 10.45 0.06 523.04* 8.2 64.11 66.86 66.86 0.000024 0.29 28.21 10.27 0.06 518.48* 8.2 63.99 66.86 64.4 66.86 0.000022 0.28 28.92 10.09 0.05 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 266 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 516.2 Inl Struct 513.92* 8.2 63.87 65.12 65.15 0.000273 0.66 12.41 9.91 0.19 509.36* 8.2 63.75 65.13 65.15 0.000215 0.61 13.37 9.73 0.17 504.80* 8.2 63.63 65.13 65.14 0.000174 0.57 14.27 9.55 0.15 500.24* 8.2 63.51 65.13 65.14 0.000144 0.54 15.13 9.36 0.14 495.68* 8.2 63.39 65.13 65.14 0.000121 0.51 15.95 9.18 0.12 491.12 15.1 63.27 65.09 65.14 0.000374 0.92 16.41 9 0.22 486.14* 15.1 63.18 65.1 65.13 0.000236 0.77 19.64 10.2 0.18 481.15* 15.1 63.08 65.11 65.13 0.000156 0.65 23.1 11.4 0.15 476.17* 15.1 62.99 65.11 65.13 0.000107 0.56 26.77 12.6 0.12 471.18* 15.1 62.89 65.11 65.12 0.000076 0.49 30.66 13.8 0.11 466.20* 15.1 62.79 65.11 65.12 0.000055 0.43 34.8 15 0.09 461.22* 15.1 62.7 65.11 65.12 0.000041 0.39 39.14 16.2 0.08 456.23* 15.1 62.6 65.12 65.12 0.000031 0.35 43.7 17.4 0.07 451.25* 15.1 62.51 65.12 65.12 0.000023 0.31 48.5 18.6 0.06 446.26* 15.1 62.41 65.12 65.12 0.000018 0.28 53.54 19.8 0.05 441.28 15.1 62.32 65.12 62.69 65.12 0.000014 0.26 58.79 21 0.05 441.2 Inl Struct 436.28* 15.1 62.24 63.59 63.6 0.000158 0.56 26.77 19.8 0.15 431.28* 15.1 62.15 63.59 63.6 0.00015 0.57 26.65 18.6 0.15 426.28* 15.1 62.07 63.59 63.6 0.000147 0.57 26.32 17.4 0.15 421.28* 15.1 61.99 63.58 63.6 0.000146 0.59 25.81 16.2 0.15 416.28* 15.1 61.91 63.58 63.6 0.000149 0.6 25.1 15 0.15 411.28* 15.1 61.83 63.58 63.6 0.000156 0.62 24.19 13.8 0.15 406.28* 15.1 61.74 63.58 63.6 0.000168 0.65 23.09 12.6 0.15 401.28* 15.1 61.66 63.57 63.6 0.000187 0.69 21.76 11.4 0.16 396.28* 15.1 61.58 63.57 63.6 0.000216 0.75 20.26 10.2 0.17 391.28 15.1 61.5 63.56 63.59 0.000261 0.81 18.55 9 0.18 386.28* 15.1 61.32 63.57 63.59 0.000146 0.65 23.16 10.3 0.14 381.28* 15.1 61.14 63.57 63.59 0.000087 0.54 28.21 11.6 0.11 376.28* 15.1 60.96 63.58 63.59 0.000055 0.45 33.73 12.9 0.09 371.28* 15.1 60.78 63.58 63.58 0.000036 0.38 39.71 14.2 0.07 366.28* 15.1 60.6 63.58 61.06 63.58 0.000024 0.33 46.14 15.5 0.06 366.2 Inl Struct 361.28* 15.1 60.42 60.86 60.86 61.07 0.008745 2.07 7.29 16.8 1 356.28* 15.1 60.24 60.89 60.65 60.97 0.002029 1.29 11.72 18.1 0.51 351.28* 15.1 60.06 60.91 60.96 0.000725 0.91 16.51 19.4 0.32 346.28* 15.1 59.88 60.92 60.95 0.000332 0.7 21.53 20.7 0.22 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 267 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 341.28 15.1 59.7 60.93 60.94 0.000172 0.56 26.96 22 0.16 336.28* 15.1 59.56 60.93 60.94 0.000135 0.53 28.55 20.8 0.14 331.28* 15.1 59.41 60.93 60.94 0.000111 0.51 29.81 19.6 0.13 326.28* 15.1 59.26 60.93 60.94 0.000096 0.49 30.69 18.4 0.12 321.28* 15.1 59.11 60.93 60.94 0.000085 0.48 31.23 17.2 0.11 316.28* 15.1 58.97 60.93 60.94 0.000079 0.48 31.4 16 0.11 311.28* 15.1 58.82 60.93 60.94 0.000076 0.48 31.21 14.8 0.11 306.28* 15.1 58.67 60.93 60.94 0.000075 0.49 30.68 13.6 0.1 301.28* 15.1 58.52 60.93 60.94 0.000077 0.51 29.78 12.4 0.1 296.28* 15.1 58.38 60.92 60.94 0.000083 0.53 28.54 11.2 0.11 291.28 15.1 58.23 60.92 60.94 0.000093 0.56 26.93 10 0.11 286.28* 15.1 58.33 60.92 60.94 0.000086 0.54 28.02 10.8 0.11 281.28* 15.1 58.43 60.92 60.94 0.000081 0.52 28.96 11.6 0.11 276.28* 15.1 58.52 60.92 60.94 0.000078 0.51 29.73 12.4 0.1 271.28* 15.1 58.62 60.92 60.94 0.000076 0.5 30.35 13.2 0.1 266.28* 15.1 58.72 60.92 60.93 0.000076 0.49 30.81 14 0.11 261.28* 15.1 58.82 60.92 60.93 0.000076 0.49 31.1 14.8 0.11 256.28* 15.1 58.92 60.92 60.93 0.000078 0.48 31.24 15.6 0.11 251.28* 15.1 59.02 60.92 60.93 0.000082 0.48 31.22 16.4 0.11 246.28* 15.1 59.12 60.92 60.93 0.000087 0.49 31.05 17.2 0.12 241.28 15.1 59.22 60.92 60.93 0.000094 0.49 30.69 18 0.12 236.28* 15.1 59.12 60.92 60.93 0.000082 0.47 31.87 17.7 0.11 231.28* 15.1 59.03 60.92 60.93 0.000073 0.46 32.96 17.4 0.11 226.28* 15.1 58.93 60.92 60.93 0.000065 0.44 34.02 17.1 0.1 221.28* 15.1 58.84 60.92 60.93 0.000059 0.43 35.03 16.8 0.1 216.28* 15.1 58.74 60.92 60.93 0.000053 0.42 35.95 16.5 0.09 211.28* 15.1 58.65 60.92 60.93 0.000049 0.41 36.84 16.2 0.09 206.28* 15.1 58.55 60.92 60.93 0.000045 0.4 37.66 15.9 0.08 201.28* 15.1 58.46 60.92 60.93 0.000042 0.39 38.44 15.6 0.08 196.28* 15.1 58.36 60.92 60.93 0.000039 0.39 39.13 15.3 0.08 191.28 15.1 58.27 60.92 60.93 0.000037 0.38 39.79 15 0.07 186.28* 15.1 58.27 60.92 60.93 0.00003 0.35 43.25 16.3 0.07 181.28* 15.1 58.27 60.92 60.93 0.000025 0.32 46.73 17.6 0.06 176.28* 15.1 58.27 60.92 60.93 0.000021 0.3 50.18 18.9 0.06 171.28* 15.1 58.27 60.92 60.93 0.000018 0.28 53.67 20.2 0.06 166.28* 15.1 58.27 60.92 60.93 0.000016 0.26 57.13 21.5 0.05 161.28* 15.1 58.26 60.92 60.93 0.000014 0.25 60.6 22.8 0.05 156.28* 15.1 58.26 60.92 60.93 0.000012 0.24 64.09 24.1 0.05 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 268 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 151.28* 15.1 58.26 60.92 60.93 0.000011 0.22 67.55 25.4 0.04 146.28* 15.1 58.26 60.92 60.93 0.00001 0.21 71.01 26.7 0.04 141.28 15.1 58.26 60.92 58.57 60.93 0.000009 0.2 74.49 28 0.04 141.2 Inl Struct 136.28* 15.1 58.03 58.73 58.76 0.000672 0.79 19.08 27.2 0.3 131.28* 15.1 57.79 58.74 58.76 0.000272 0.61 24.91 26.4 0.2 126.28* 15.1 57.56 58.74 58.75 0.000141 0.5 30.23 25.6 0.15 121.28* 15.1 57.33 58.74 58.75 0.000084 0.43 35.14 24.8 0.12 116.28* 15.1 57.09 58.74 58.75 0.000055 0.38 39.67 24 0.09 111.28* 15.1 56.86 58.74 58.75 0.000039 0.34 43.79 23.2 0.08 106.28* 15.1 56.62 58.74 58.75 0.000029 0.32 47.53 22.4 0.07 101.28* 15.1 56.39 58.75 58.75 0.000023 0.3 50.9 21.6 0.06 96.28* 15.1 56.15 58.75 58.75 0.000019 0.28 53.9 20.8 0.06 91.28 15.1 55.92 58.75 56.31 58.75 0.000016 0.27 56.51 20 0.05 91.2 Inl Struct 86.280* 15.1 55.52 56.64 56.66 0.000256 0.64 23.46 21 0.19 81.280* 15.1 55.13 56.65 56.66 0.000087 0.45 33.42 22 0.12 76.280* 15.1 54.73 56.65 56.66 0.000038 0.34 44.11 23 0.08 71.280* 15.1 54.34 56.65 56.66 0.000019 0.27 55.54 24 0.06 66.280* 15.1 53.94 56.65 54.27 56.66 0.000011 0.22 67.77 25 0.04 66.2 Inl Struct 61.280* 15.1 53.55 53.87 53.87 54.03 0.009328 1.79 8.42 26 1.01 56.280* 15.1 53.15 53.32 53.47 53.89 0.077779 3.35 4.5 27 2.62 51.280* 15.1 52.76 52.92 53.06 53.49 0.08188 3.36 4.5 28 2.67 46.280* 15.1 52.36 52.52 52.66 53.08 0.081895 3.31 4.56 29 2.67 41.28 15.1 51.96 52.12 52.26 52.66 0.081902 3.27 4.62 30 2.66 1191.28 8.2 87.77 90.48 88.83 90.57 0.00127 1.26 6.5 2.4 0.24 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 269 RESULTS OF HYDRAULIC MODEL - FINAL SITUATION / CHECK DAMS River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 1191.28 8.2 88.99 90.38 90.04 90.69 0.006735 2.45 3.35 2.4 0.66 1186.28* 8.2 88.99 90.36 90.65 0.00619 2.38 3.45 2.51 0.65 1181.28* 8.2 88.99 90.34 90.61 0.005709 2.31 3.56 2.62 0.63 1176.28* 8.2 88.99 90.48 90.08 90.53 0.001607 0.99 8.28 14.75 0.42 1176.2 Inl Struct 1171.28* 8.2 87.41 88.79 89.01 0.004371 2.09 3.93 2.84 0.57 1166.28* 8.2 87.41 88.82 88.97 0.004176 1.74 4.72 6.1 0.63 1161.28* 8.2 87.41 88.88 88.93 0.001037 1.01 8.12 9.73 0.35 1156.28* 8.2 87.41 88.88 88.92 0.000574 0.85 9.63 9.85 0.28 1151.28* 8.2 87.41 88.87 88.92 0.000728 0.97 8.47 8.3 0.31 1146.28* 8.2 87.41 88.84 88.18 88.91 0.001079 1.19 6.89 6.24 0.36 1146.2 Inl Struct 1141.28 8.2 85.14 87.18 87.25 0.000895 1.15 7.13 3.5 0.26 1136.28* 8.2 85.02 87.2 87.24 0.000438 0.88 9.33 5.46 0.21 1131.28* 8.2 85.02 87.2 87.23 0.000364 0.78 10.57 7.31 0.21 1126.28* 8.2 85.02 87.2 87.23 0.000356 0.72 11.33 9.01 0.21 1121.28* 8.2 85.02 87.2 87.23 0.000388 0.71 11.61 10.51 0.21 1116.28* 8.2 85.02 87.2 87.22 0.000466 0.72 11.42 11.73 0.23 1111.28* 8.2 85.02 87.19 87.22 0.000617 0.76 10.77 12.57 0.26 1106.28* 8.2 85.02 87.18 87.22 0.00091 0.85 9.68 12.82 0.31 1101.28* 8.2 85.02 87.16 87.21 0.001513 1 8.19 12.08 0.39 1096.28* 8.2 85.02 87.08 87.2 0.001875 1.53 6.07 8.52 0.34 1091.28 8.2 85.02 87.05 86.05 87.18 0.002268 1.62 5.11 4.52 0.36 1091.2 Inl Struct 1086.28* 8.2 83.62 84.46 84.46 84.88 0.011219 2.89 2.84 3.37 1 1081.28* 8.2 83.62 84.57 84.34 84.78 0.004559 2.04 4.02 4.24 0.67 1076.28* 8.2 83.62 84.6 84.74 0.00261 1.63 5.04 5.11 0.52 1071.28* 8.2 83.62 84.65 84.22 84.7 0.001227 0.99 8.32 12.65 0.39 1071.2 Inl Struct 1066.28* 8.2 81.44 82.23 82.35 0.002616 1.52 5.39 6.85 0.55 1061.28* 8.2 81.44 82.24 82.33 0.001943 1.34 6.14 7.72 0.48 1056.28* 8.2 81.44 82.25 82.31 0.001417 1.11 7.38 10.47 0.42 1051.28* 8.2 81.44 82.25 82.3 0.000955 0.96 8.58 11.67 0.36 1046.28* 8.2 81.44 82.25 82.3 0.000852 0.92 8.9 11.64 0.34 1041.28 8.2 81.44 82.25 82.29 0.000821 0.91 9.03 11.2 0.32 1036.28* 8.2 81.44 82.24 82.29 0.00089 0.94 8.74 10.93 0.34 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 270 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 1031.28* 8.2 81.44 82.23 81.83 82.28 0.00097 0.97 8.44 10.66 0.35 1031.2 Inl Struct 1026.28* 8.2 79.72 80.77 80.8 0.00042 0.75 10.93 10.39 0.23 1021.28* 8.2 79.72 80.77 80.8 0.000451 0.77 10.61 10.12 0.24 1016.29* 8.2 79.72 80.76 80.8 0.000485 0.8 10.28 9.85 0.25 1011.29* 8.2 79.72 80.76 80.79 0.000524 0.82 9.95 9.58 0.26 1006.29* 8.2 79.72 80.75 80.79 0.000567 0.85 9.62 9.31 0.27 1001.29* 8.2 79.72 80.75 80.79 0.000616 0.88 9.29 9.04 0.28 996.29* 8.2 79.72 80.74 80.78 0.000673 0.92 8.96 8.77 0.29 991.29 8.2 79.72 80.73 80.78 0.000737 0.95 8.62 8.5 0.3 986.74* 8.2 79.72 80.73 80.78 0.000756 0.96 8.54 8.45 0.31 982.20* 8.2 79.72 80.73 80.77 0.000776 0.97 8.46 8.41 0.31 977.65* 8.2 79.72 80.72 80.77 0.000797 0.98 8.37 8.36 0.31 973.10* 8.2 79.72 80.72 80.77 0.000818 0.99 8.29 8.32 0.32 968.56* 8.2 79.72 80.71 80.76 0.000841 1 8.2 8.27 0.32 964.01* 8.2 79.72 80.71 80.76 0.000864 1.01 8.12 8.23 0.32 959.47* 8.2 79.72 80.7 80.76 0.000889 1.02 8.03 8.18 0.33 954.92* 8.2 79.72 80.7 80.75 0.000915 1.03 7.95 8.14 0.33 950.37* 8.2 79.72 80.69 80.75 0.000943 1.04 7.86 8.09 0.34 945.83* 8.2 79.72 80.69 80.74 0.000973 1.06 7.77 8.05 0.34 941.28 8.2 79.72 80.68 80.19 80.74 0.001004 1.07 7.67 8 0.35 941.2 Inl Struct 936.28* 8.2 78.38 78.92 79.06 0.004526 1.65 4.96 9.2 0.72 931.28* 8.2 78.38 78.92 79.03 0.00344 1.46 5.62 10.4 0.63 926.28* 8.2 78.38 78.92 79.01 0.002739 1.31 6.27 11.6 0.57 921.28* 8.2 78.38 78.92 78.99 0.002259 1.19 6.88 12.8 0.52 916.28* 8.2 78.38 78.92 78.98 0.001904 1.09 7.49 14 0.48 911.28* 8.2 78.38 78.91 78.97 0.001632 1.01 8.09 15.2 0.44 906.28* 8.2 78.38 78.91 78.96 0.001418 0.95 8.68 16.4 0.41 901.28* 8.2 78.38 78.91 78.95 0.001248 0.89 9.26 17.6 0.39 896.28* 8.2 78.38 78.9 78.94 0.001108 0.83 9.84 18.8 0.37 891.28 8.2 78.38 78.9 78.64 78.93 0.00099 0.79 10.41 20 0.35 891.2 Inl Struct 886.28* 8.2 76.84 77.81 77.82 0.000149 0.45 18.26 18.8 0.15 881.28* 8.2 76.84 77.8 77.82 0.000173 0.48 17.05 17.6 0.16 876.28* 8.2 76.84 77.8 77.82 0.000204 0.52 15.84 16.4 0.17 871.28* 8.2 76.84 77.8 77.81 0.000242 0.56 14.63 15.2 0.18 866.28* 8.2 76.84 77.79 77.81 0.000294 0.61 13.4 14 0.2 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 271 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 861.28* 8.2 76.84 77.79 77.81 0.000364 0.67 12.18 12.8 0.22 856.28* 8.2 76.84 77.78 77.81 0.000464 0.75 10.94 11.6 0.25 851.28* 8.2 76.84 77.77 77.8 0.000611 0.85 9.7 10.4 0.28 846.28* 8.2 76.84 77.75 77.8 0.000848 0.97 8.43 9.2 0.32 841.28 8.2 76.84 77.73 77.79 0.001269 1.15 7.11 8 0.39 836.28* 8.2 76.84 77.72 77.78 0.001111 1.09 7.55 8.5 0.37 831.28* 8.2 76.84 77.72 77.27 77.78 0.000981 1.03 7.99 9 0.35 831.2 Inl Struct 826.28* 8.2 74.41 75.18 75.24 0.001402 1.13 7.24 9.5 0.41 821.28* 8.2 74.41 75.17 75.23 0.001267 1.08 7.6 10 0.4 816.28* 8.2 74.41 75.17 75.22 0.001152 1.03 7.95 10.5 0.38 811.28* 8.2 74.41 75.17 75.22 0.001053 0.99 8.3 11 0.36 806.28* 8.2 74.41 75.17 74.78 75.21 0.000967 0.95 8.65 11.5 0.35 806.2 Inl Struct 801.28* 8.2 71.93 72.29 72.29 72.47 0.009401 1.89 4.33 12 1.01 796.28* 8.2 71.44 72.34 71.8 72.37 0.000462 0.73 11.21 12.5 0.25 791.28 8.2 71.42 72.34 72.37 0.00039 0.68 11.97 13 0.23 786.72* 8.2 71.42 72.34 72.36 0.000396 0.69 11.87 13.39 0.23 782.17* 8.2 71.39 72.34 72.36 0.000411 0.69 11.86 13.88 0.24 777.61* 8.2 71.42 72.33 72.36 0.000486 0.73 11.27 13.86 0.26 773.05* 8.2 71.42 72.32 72.36 0.000611 0.79 10.44 13.38 0.28 768.49* 8.2 71.42 72.31 72.35 0.000775 0.87 9.47 12.27 0.31 763.94* 8.2 71.42 72.3 72.35 0.000883 0.95 8.59 10.23 0.33 759.38* 8.2 71.42 72.29 72.34 0.001 1.03 7.99 9.18 0.35 754.82* 8.2 71.42 72.27 72.34 0.001209 1.11 7.38 8.64 0.38 750.26* 8.2 71.42 72.26 72.33 0.001503 1.21 6.76 8.09 0.42 745.71* 8.2 71.42 72.23 72.32 0.001939 1.34 6.1 7.55 0.48 741.15 8.2 71.42 72.19 72.31 0.002672 1.52 5.39 7 0.55 736.16* 8.2 71.42 72.19 72.29 0.002094 1.37 6 7.75 0.5 731.18* 8.2 71.42 72.2 72.27 0.001695 1.24 6.59 8.5 0.45 726.19* 8.2 71.42 72.22 72.25 0.000749 0.77 10.66 17.26 0.31 721.20* 8.2 71.42 72.22 72.25 0.000712 0.77 10.69 16.69 0.31 716.22* 8.2 71.42 72.22 72.25 0.000673 0.76 10.75 16.19 0.3 711.23* 8.2 71.42 72.21 72.24 0.000634 0.76 10.83 15.75 0.29 706.24* 8.2 71.42 72.21 72.24 0.000596 0.75 10.94 15.37 0.28 701.25* 8.2 71.42 72.21 72.24 0.000559 0.74 11.07 15.03 0.28 696.27* 8.2 71.42 72.21 72.23 0.000527 0.73 11.22 14.74 0.27 691.28 8.2 71.42 72.21 71.74 72.23 0.000513 0.72 11.39 14.5 0.26 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 272 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 686.73* 8.2 71.42 72.2 72.23 0.000538 0.73 11.16 14.27 0.27 682.19* 8.2 71.42 72.2 72.23 0.000565 0.75 10.93 14.04 0.27 677.64* 8.2 71.42 72.19 72.22 0.000595 0.77 10.7 13.82 0.28 673.09* 8.2 71.42 72.19 72.22 0.000627 0.78 10.47 13.59 0.29 668.55* 8.2 71.42 72.19 72.22 0.000662 0.8 10.23 13.36 0.29 664.00* 8.2 71.42 72.18 72.22 0.0007 0.82 10 13.14 0.3 659.46* 8.2 71.42 72.18 72.21 0.000742 0.84 9.76 12.91 0.31 654.91* 8.2 71.42 72.17 72.21 0.000789 0.86 9.52 12.68 0.32 650.36* 8.2 71.42 72.17 72.2 0.00084 0.88 9.28 12.46 0.33 645.82* 8.2 71.42 72.16 72.2 0.000898 0.91 9.03 12.23 0.34 641.27 8.2 71.42 72.15 71.78 72.2 0.000963 0.93 8.78 12 0.35 641.2 Inl Struct 636.27* 8.2 69.36 69.75 69.73 69.92 0.008377 1.85 4.44 11.65 0.95 631.27* 8.2 69.31 69.71 69.69 69.88 0.007894 1.83 4.48 11.3 0.93 626.27* 8.2 69.26 69.67 69.64 69.84 0.007298 1.81 4.53 10.95 0.9 621.27* 8.2 69.2 69.64 69.8 0.006241 1.74 4.71 10.6 0.84 616.28* 8.2 69.15 69.63 69.77 0.005232 1.67 4.91 10.25 0.77 611.28* 8.2 69.09 69.61 69.74 0.004366 1.6 5.14 9.9 0.71 606.28* 8.2 69.04 69.6 69.72 0.003658 1.53 5.36 9.55 0.65 601.28* 8.2 68.98 69.59 69.7 0.003118 1.47 5.57 9.2 0.6 596.28* 8.2 68.93 69.58 69.68 0.002682 1.42 5.77 8.85 0.56 591.28 8.2 68.87 69.57 69.67 0.002355 1.38 5.94 8.5 0.53 586.28* 8.2 68.73 69.59 69.65 0.001173 1.1 7.48 8.75 0.38 581.28* 8.2 68.73 69.58 69.64 0.001118 1.07 7.66 9 0.37 576.28* 8.2 68.73 69.58 69.64 0.001067 1.05 7.84 9.25 0.36 571.28* 8.2 68.73 69.58 69.63 0.001019 1.02 8.02 9.5 0.36 566.28* 8.2 68.73 69.57 69.15 69.62 0.000975 1 8.19 9.75 0.35 566.2 Inl Struct 561.28* 8.2 66.3 66.79 66.93 0.005374 1.7 4.83 10 0.78 556.28* 8.2 65.99 66.85 66.9 0.000815 0.93 8.81 10.25 0.32 551.28* 8.2 65.99 66.85 66.89 0.000781 0.91 8.99 10.5 0.31 546.28* 8.2 65.99 66.85 66.89 0.000749 0.89 9.18 10.75 0.31 541.28 8.2 65.99 66.84 66.88 0.00072 0.88 9.36 11 0.3 536.72* 8.2 65.99 66.84 66.88 0.000761 0.9 9.15 10.82 0.31 532.16* 8.2 65.99 66.83 66.88 0.000806 0.92 8.94 10.64 0.32 527.60* 8.2 65.99 66.83 66.87 0.000856 0.94 8.72 10.45 0.33 523.04* 8.2 65.99 66.82 66.87 0.000911 0.96 8.51 10.27 0.34 518.48* 8.2 65.99 66.81 66.4 66.86 0.000973 0.99 8.29 10.09 0.35 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 273 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 516.2 Inl Struct 513.92* 8.2 64.32 65.32 65.36 0.000538 0.82 9.96 9.91 0.26 509.36* 8.2 64.32 65.32 65.36 0.000568 0.84 9.74 9.73 0.27 504.80* 8.2 64.32 65.31 65.35 0.0006 0.86 9.51 9.55 0.28 500.24* 8.2 64.32 65.31 65.35 0.000636 0.88 9.29 9.36 0.28 495.68* 8.2 64.32 65.3 65.35 0.000676 0.91 9.06 9.18 0.29 491.12 15.1 64.32 65.02 64.98 65.31 0.00687 2.38 6.34 9 0.91 486.14* 15.1 64.32 65.05 65.26 0.004621 2.02 7.47 10.2 0.75 481.15* 15.1 64.32 65.06 65.22 0.003431 1.78 8.5 11.4 0.66 476.17* 15.1 64.32 65.07 65.2 0.002686 1.59 9.49 12.6 0.59 471.18* 15.1 64.32 65.07 65.18 0.002177 1.45 10.44 13.8 0.53 466.20* 15.1 64.32 65.08 65.17 0.001808 1.33 11.38 15 0.49 461.22* 15.1 64.32 65.08 65.15 0.001534 1.23 12.3 16.2 0.45 456.23* 15.1 64.32 65.08 65.14 0.001321 1.14 13.2 17.4 0.42 451.25* 15.1 64.32 65.08 65.13 0.00115 1.07 14.1 18.6 0.39 446.26* 15.1 64.32 65.08 65.13 0.001012 1.01 15 19.8 0.37 441.28 15.1 64.32 65.07 64.69 65.12 0.000898 0.95 15.89 21 0.35 441.2 Inl Struct 436.28* 15.1 62.6 63.7 63.73 0.000305 0.69 21.76 19.8 0.21 431.28* 15.1 62.6 63.7 63.72 0.000355 0.74 20.34 18.6 0.23 426.28* 15.1 62.6 63.69 63.72 0.000417 0.8 18.91 17.4 0.24 421.28* 15.1 62.6 63.68 63.72 0.000499 0.86 17.47 16.2 0.27 416.28* 15.1 62.6 63.67 63.72 0.000607 0.94 16.02 15 0.29 411.28* 15.1 62.6 63.66 63.71 0.000759 1.04 14.54 13.8 0.32 406.28* 15.1 62.6 63.64 63.71 0.00098 1.16 13.04 12.6 0.36 401.28* 15.1 62.6 63.61 63.7 0.001329 1.31 11.49 11.4 0.42 396.28* 15.1 62.6 63.57 63.69 0.001948 1.54 9.84 10.2 0.5 391.28 15.1 62.6 63.48 63.67 0.003427 1.91 7.91 9 0.65 386.28* 15.1 62.6 63.5 63.64 0.002362 1.63 9.28 10.3 0.55 381.28* 15.1 62.6 63.51 63.62 0.001747 1.43 10.58 11.6 0.48 376.28* 15.1 62.6 63.52 63.6 0.001353 1.27 11.86 12.9 0.42 371.28* 15.1 62.6 63.53 63.59 0.001083 1.15 13.12 14.2 0.38 366.28* 15.1 62.6 63.53 63.06 63.59 0.000889 1.05 14.36 15.5 0.35 366.2 Inl Struct 361.28* 15.1 60.42 61.28 61.33 0.000965 1.05 14.38 16.8 0.36 356.28* 15.1 60.24 61.29 61.32 0.000428 0.79 19 18.1 0.25 351.28* 15.1 60.26 61.29 61.32 0.000396 0.76 19.93 19.4 0.24 346.28* 15.1 60.26 61.29 61.32 0.000344 0.71 21.28 20.7 0.22 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 274 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 341.28 15.1 60.26 61.29 61.32 0.000302 0.67 22.62 22 0.21 336.28* 15.1 60.26 61.29 61.31 0.000346 0.71 21.29 20.8 0.22 331.28* 15.1 60.26 61.28 61.31 0.000399 0.76 19.95 19.6 0.24 326.28* 15.1 60.26 61.28 61.31 0.000467 0.81 18.6 18.4 0.26 321.28* 15.1 60.26 61.27 61.31 0.000554 0.88 17.24 17.2 0.28 316.28* 15.1 60.26 61.26 61.3 0.000669 0.95 15.87 16 0.31 311.28* 15.1 60.26 61.24 61.3 0.000827 1.04 14.47 14.8 0.34 306.28* 15.1 60.26 61.22 61.29 0.001056 1.16 13.04 13.6 0.38 301.28* 15.1 60.26 61.2 61.28 0.00141 1.31 11.56 12.4 0.43 296.28* 15.1 60.26 61.16 61.27 0.002029 1.51 9.98 11.2 0.51 291.28 15.1 60.26 61.08 61.25 0.003475 1.86 8.13 10 0.66 286.28* 15.1 60.26 61.08 61.23 0.002913 1.72 8.8 10.8 0.61 281.28* 15.1 60.26 61.08 61.21 0.002488 1.6 9.46 11.6 0.56 276.28* 15.1 60.26 61.08 61.19 0.002157 1.49 10.11 12.4 0.53 271.28* 15.1 60.26 61.08 61.18 0.001896 1.41 10.75 13.2 0.5 266.28* 15.1 60.26 61.08 61.17 0.001683 1.33 11.37 14 0.47 261.28* 15.1 60.26 61.07 61.16 0.001506 1.26 11.99 14.8 0.45 256.28* 15.1 60.26 61.07 61.15 0.001357 1.2 12.61 15.6 0.43 251.28* 15.1 60.26 61.07 61.14 0.001229 1.14 13.23 16.4 0.41 246.28* 15.1 60.26 61.07 61.13 0.00112 1.09 13.84 17.2 0.39 241.28 15.1 60.26 61.07 61.12 0.001026 1.05 14.45 18 0.37 236.28* 15.1 60.26 61.06 61.12 0.001099 1.07 14.06 17.7 0.38 231.28* 15.1 60.26 61.05 61.11 0.001183 1.11 13.66 17.4 0.4 226.28* 15.1 60.26 61.04 61.11 0.00128 1.14 13.25 17.1 0.41 221.28* 15.1 60.26 61.03 61.1 0.001393 1.18 12.83 16.8 0.43 216.28* 15.1 60.26 61.02 61.09 0.001526 1.22 12.39 16.5 0.45 211.28* 15.1 60.26 61 61.08 0.001686 1.26 11.94 16.2 0.47 206.28* 15.1 60.26 60.99 61.07 0.001882 1.32 11.47 15.9 0.5 201.28* 15.1 60.26 60.97 61.06 0.00213 1.38 10.96 15.6 0.52 196.28* 15.1 60.26 60.94 61.05 0.002475 1.45 10.39 15.3 0.56 191.28 15.1 60.26 60.91 61.04 0.002987 1.55 9.74 15 0.61 186.28* 15.1 60.26 60.91 61.02 0.002517 1.43 10.57 16.3 0.57 181.28* 15.1 60.26 60.91 61 0.002168 1.33 11.37 17.6 0.53 176.28* 15.1 60.26 60.91 60.99 0.001893 1.24 12.16 18.9 0.49 171.28* 15.1 60.26 60.9 60.97 0.001671 1.17 12.94 20.2 0.47 166.28* 15.1 60.26 60.9 60.96 0.001489 1.1 13.72 21.5 0.44 161.28* 15.1 60.26 60.9 60.95 0.001337 1.04 14.49 22.8 0.42 156.28* 15.1 60.26 60.9 60.95 0.00121 0.99 15.24 24.1 0.4 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 275 River Sta- Froude # Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width tion Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) 151.28* 15.1 60.26 60.89 60.94 0.001101 0.94 15.99 25.4 0.38 146.28* 15.1 60.26 60.89 60.93 0.001008 0.9 16.74 26.7 0.36 141.28 15.1 60.26 60.89 60.57 60.93 0.000925 0.86 17.49 28 0.35 141.2 Inl Struct 136.28* 15.1 58.03 58.75 58.78 0.000619 0.77 19.56 27.2 0.29 131.28* 15.1 57.92 58.75 58.77 0.000412 0.69 21.92 26.4 0.24 126.28* 15.1 57.92 58.75 58.77 0.000447 0.71 21.15 25.6 0.25 121.28* 15.1 57.92 58.74 58.77 0.000486 0.74 20.38 24.8 0.26 116.28* 15.1 57.92 58.74 58.77 0.00053 0.77 19.6 24 0.27 111.28* 15.1 57.92 58.73 58.76 0.000581 0.8 18.83 23.2 0.28 106.28* 15.1 57.92 58.73 58.76 0.000641 0.84 18.04 22.4 0.3 101.28* 15.1 57.92 58.72 58.76 0.000711 0.88 17.25 21.6 0.31 96.28* 15.1 57.92 58.71 58.75 0.000795 0.92 16.44 20.8 0.33 91.28 15.1 57.92 58.7 58.31 58.75 0.000896 0.97 15.63 20 0.35 91.2 Inl Struct 86.280* 15.1 55.94 56.63 56.68 0.001242 1.05 14.38 21 0.41 81.280* 15.1 55.94 56.62 56.68 0.001144 1.01 15 22 0.39 76.280* 15.1 55.94 56.62 56.67 0.001058 0.97 15.61 23 0.37 71.280* 15.1 55.94 56.62 56.66 0.000982 0.93 16.22 24 0.36 66.280* 15.1 55.94 56.62 56.27 56.66 0.000913 0.9 16.84 25 0.35 66.2 Inl Struct 61.280* 15.1 53.55 53.87 53.87 54.03 0.009328 1.79 8.42 26 1.01 56.280* 15.1 53.15 53.32 53.47 53.89 0.077779 3.35 4.5 27 2.62 51.280* 15.1 52.76 52.92 53.06 53.49 0.08188 3.36 4.5 28 2.67 46.280* 15.1 52.36 52.52 52.66 53.08 0.081895 3.31 4.56 29 2.67 41.28 15.1 51.96 52.12 52.26 52.66 0.081902 3.27 4.62 30 2.66 1191.28 8.2 88.99 90.38 90.04 90.69 0.006735 2.45 3.35 2.4 0.66 Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 276 11 ANNEX V – PROPOSED PROJECT MEASURES FOR QUELIMANE Table of Contents General Layout – Sites and Categories General Layout – Possible Measures Site 1: Airport – Possible Measures Site 2: Port and Bank Wall – Possible Measures Site 3: Primeiro de Maio & Site 4: Acordo de Lusaka – Possible Measures Site 5: Inhangome & Site 6: Chuabo Dembe – Possible Measures Site 7: Torone Velho – Possible Measures Site 8: Incidua – Possible Measures Site 9: Ivagalane – Possible Measures Site 10: Murropue – Possible Measures Site 12: Micajune / Floresta B & A – Possible Measures Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 277 12 ANNEX VI – SITE DEVELOPMENT OF PROJECT SITES IN QUELIMANE Table of Contents Site 1: Airport – Site Development Site 2: Port and Bank Wall – Site Development Site 3: Primeiro de Maio & Site 4: Acordo de Lusaka – Site Development Site 5: Inhangome & Site 6: Chuabo Dembe – Site Development Site 7: Torone Velho – Site Development Site 8: Incidua – Site Development Site 9: Ivagalane – Site Development Site 10: Murropue – Site Development Site 12: Micajune / Floresta B & A – Site Development Urban Flood and Erosion Risk Assessment and Potential Nature-Based Solutions for Nacala and Quelimane 278