FLOOD-RESILIENT MASS TRANSIT PLANNING IN OUAGADOUGOU 1 CONTENTS CONTENTS ............................................................................................................................................................................................................. 2 ACKNOWLEDGMENTS ........................................................................................................................................................................................ 3 ABBREVIATIONS AND ACRONYMS ................................................................................................................................................................. 4 GLOSSARY OF KEY TECHNICAL TERMS ........................................................................................................................................................ 5 EXECUTIVE SUMMARY........................................................................................................................................................................................ 7 1. Context ......................................................................................................................................................................................................12 1.1. Ouagadougou’s growth patterns ............................................................................................................................................12 1.2. Major flooding events of this century ....................................................................................................................................13 1.3. Impacts on the transport system .............................................................................................................................................14 2. Addressing the flood risk to improve the resilience of Ouagadougou’s transport system ................................................16 2.1. The planned mass transit system ............................................................................................................................................16 2.2. Flood modelling analysis ...........................................................................................................................................................17 3. Identifying and prioritizing interventions to mitigate flood risk ................................................................................................22 3.1. Flood risk vis-à-vis the future mass transit system ............................................................................................................22 3.1. Developing a “long list” of potential solutions ....................................................................................................................24 3.2. Learning from global best practices .......................................................................................................................................24 3.3. Prioritizing measures for Ouagadougou through multi-criteria analysis ......................................................................36 3.4. Results of the multicriteria analysis ........................................................................................................................................38 3.5. Sensitivity analysis .......................................................................................................................................................................42 REFERENCES.......................................................................................................................................................................................................45 ANNEX 1: DRONE-BASED IMAGERY COLLECTION TO CONSTRUCT A DIGITAL ELEVATION MODEL .........................................47 ANNEX 2: FLOOD RISK MODELING ..............................................................................................................................................................48 ANNEX 3: COMPARISON OF FLOOD RISK ..................................................................................................................................................53 ANNEX 4: IMPACT OF FLOOD HAZARD ON TRAFFIC FLOWS ..............................................................................................................54 ANNEX 5: IDENTIFYING SOLUTIONS TO ADDRESS FLOOD RISK .........................................................................................................67 2 ACKNOWLEDGMENTS The study was led by Aiga Stokenberga (Transport Economist, IAWT4) with overall guidance from Soukeyna Kane (Country Director, AFCW3), Aurelio Menendez (Practice Manager, IAWT4), Maimouna Mbow Fam (Country Manager, AWMBF), and Kofi Nouve (Manager, Operations, AWCW3). The following World Bank staff and consultants provided valuable technical input: Komlan Kounetsron, Vivien Deparday, Natalia Romero Lane, Nathalie Andrea Wandel, Nicolas Desramaut, Van Anh Vu Hong, and Lukas Loeschner. We also acknowledge the extensive administrative assistance provided by Lisa Warouw. Valuable feedback was provided by peer reviewers Vincent Vesin (Sr. Transport Specialist, IAWT4) and Cecile Lorillou (Disaster Risk Management Specialist, SAWU1). The team would like to thank several external partners. Flood risk modelling for Ouagadougou was conducted by Yves Kovacs, Quentin Strappazzon, and Camille Rogeaux from SEPIA Conseils based on aerial drone imagery collected by Espace Geomatique. Julien Prachay, Karim Selouane, Sandy Kumar, Camille Vignote, and Philippe Sohouenou from Louis Berger International / AGEIM / Resallience carried out the analysis aimed at identifying priority interventions to mitigate the flood risk affecting Ouagadougou’s future mass transit system . Dr. Yoshinori Fukubayashi at the Department of Civil and Environmental Engineering, University of Miyazaki, provided valuable input to prepare the case study on Japanese city experience with addressing flood risk. Jennifer Mannix, Ran Goldblatt, Daynan Crull, Ghermay Araya, and Larry Curran at New Light Technologies designed the ESRI StoryMap communication tool for this activity. Finally, the team would like to acknowledge the generous funding from the Global Facility for Disaster Reduction and Recovery (GFDRR) and the Government of Japan, and to thank the colleagues at GFDRR – Niels B. Holm-Nielsen, Jared Phillip Mercadante, Akiko Urakami, and Akiko Toya – for guidance. 3 ABBREVIATIONS AND ACRONYMS ANAC National Civil Aviation Agency AOI Area of Interest cm centimeter GFDRR Global Facility for Disaster Reduction and Recovery DGTTM General Directorate of Land and Maritime Transport DTM Digital Terrain Model DSM Digital Surface Model IDF intensity-duration-frequency IRD Institut de Recherche pour le Developpement mm millimeter OPTIS Ouagadougou Public Transport Implementation Study POI Point of Interest RP Return Period UAV Unmanned aerial vehicle US$ United States Dollar VTOL Vertical take-off and landing 4 GLOSSARY OF KEY TECHNICAL TERMS bathymetry The measurement of depth of water in oceans, seas, or lakes Vegetated, shallow, landscaped depressions designed to capture, treat, and infiltrate stormwater runoff and convey stormwater at a slow, controlled rate; the flood-tolerant bioswale vegetation and soil act as a filter medium, cleaning runoff and allowing infiltration. Bioswales are generally installed within or near paved areas (e.g. parking lots, roads and sidewalks). Large open conduits conveying water on the surface; they form the primary drainage network, canals collecting water from secondary structures and channeling it to the natural outlet Structures that form the secondary and tertiary drainage networks; typically rectangular section concrete structures, with dimensions ranging from 50cm to a few meters wide; channels are generally used to collect rainwater along the road surface. When playing this function of longitudinal drainage, they are generally called “caniveaux” in French (open channels rectangular street gutters). Larger channels collecting water from several smaller ones and leading to a canal may be called “collecteurs”. In this report, both types will be referred to as channels, since they are represented by similar rectangular concrete structures. In Ouagadougou, longitudinal channels are often partially covered with concrete slabs. Buried structures, either circular (pipe culverts – “buses” in French) or rectangular (box culverts culverts – “dalots” in French), whose function is to ensure hydraulic transparency of a road by channeling water under the road surface. They are also referred to as transverse/cross-section structures. A rain event, either observed or synthetic, which is chosen as the basis for the design of a design storm hydraulic structure A representation of the bare ground (bare earth) topographic surface of the Earth  excluding Digital Elevation trees, buildings, and any other surface objects; it is as a subset of the Digital Terrain Model, Model which also represents other morphological elements. Both models are three-dimensional models representing, in digital form, the relief of a portion of land. But, while the DSM represents the altitude of the first surface observed from the sky Digital Terrain Model (tree top, roofs, ground, etc.), the DTM aims at representing the altitude of the ground. To / Digital Surface elaborate a dynamic hydraulic model, one needs a DTM (to model water circulation on the Model ground). The result obtained from an aerial topographic survey, whether LIDAR of photogrammetric, is a DSM, which requires data treatment to access a digital model offering information as close as possible to the ground elevation. A dynamic hydraulic model simulates the evolution of flows/heights/speeds over time, during the whole duration of the design storm. Such a model requires more input data and more calculation capacities than a stationary model, but in return offers more information on the dynamic hydraulic hydraulic system functioning. Dynamic models are necessary when the simple information on model peak flows is not sufficient, and in complex situations which exceed the capabilities of stationary models: when water storage/retention occurs, or when a downstream condition exists for example (when the outlet of the hydraulic system is a lake or ocean, for example). Gray solutions Solutions based on hard, human-engineered infrastructure that uses concrete and steel Solutions that use soils and vegetation to utilize, enhance and/or mimic the natural Green solutions hydrological cycle processes of infiltration, evapotranspiration and reuse A policy and land-use zone designation used in land-use planning to retain areas of largely greenbelt undeveloped, wild, or agricultural land surrounding or neighboring urban areas 5 A graph showing the rate of water flow in relation to time, given a specific point or cross hydrograph section; often used to evaluate stormwater runoff on a particular site hyetograph A graphical representation of the distribution of rainfall intensity over time intensity-duration- A mathematical function that relates the rainfall intensity with its duration and frequency of frequency curve occurrence Hydraulic model aiming only at calculating the peak situation caused by the design storm non-dynamic / (peak flow / water height / velocity). Such models use simplification hypotheses, by stationary hydraulic considering the rainfall as stationary during a given duration corresponding to the highest model sensibility of the watershed. Stationary models generally offer enough information for simple scenarios. A garden of native shrubs, perennials, and flowers planted in a small depression, which is generally formed on a natural slope; designed to temporarily hold and soak in rain water rain garden runoff that flows from roofs, driveways, patios or lawns; dry most of the time and typically holds water only during and following a rainfall event; unlike bioretention swales, they do not convey stormwater runoff. Smaller public parks (generally occupying less than one acre of land) that represent ideal locations for green infrastructure (vegetated bioretention cells) that treats and captures pocket parks stormwater through bio-filtration and infiltration. Pocket parks are opportunistic, often sited on whatever land is available, and might be constructed to revitalize unused or underused land (e.g. decommissioned railroad tracks). An average time or an estimated average time between events such as earthquakes, floods, landslides, or a river discharge flows to occur; also known as recurrence interval or repeat interval. A storm intensity with a return period of 10 years has a 1 in 10 chance of being observed each year. Return periods are used to define the level of risk that a project owner return period can accept, depending on the criticality of the infrastructure to be protected. For example, a road is typically designed to be protected against flooding of its surface by storms with a 10 year return period, whereas a large dam or a nuclear plant would have to be designed to cope with extreme events with return periods of several hundreds or even thousands of years. Also referred to as drainage ratio, it is the ratio between the height of water runoff at the exit of a specific surface (called "net rain") and the height of water precipitated (called "gross runoff coefficient rain"). This ratio is affected by factors such as evapotranspiration, impermeability of surfaces, soils infiltration capacities. It has a lower value for permeable, well vegetated areas (forest, flat land). Soft solutions Solutions that use institutions and technology services The watershed / catchment area of a watercourse is defined by the area receiving the waters feeding this watercourse. In a broader use, a watershed can be defined for any point in space: watershed it is formed by all the areas receiving water that runs off to that point (all the areas located upstream of that point). Areas where water covers the soil, or is present either at or near the surface of the soil all wetlands year or for varying periods of time during the year, including during the growing season 6 EXECUTIVE SUMMARY Ouagadougou, the largest city in Burkina Faso, is in the AOI representing different flood types, for which growing rapidly, with the annual rate reaching 9 percent hydrographs were generated showing the evolution of by some estimates, and with commensurate challenges water height over time during the 2-year return period for ensuring efficient mobility for its residents. Like (RP) flood and the 2009 flood. The analysis of the many urban areas in Sahelian West Africa, ramping up and down of water heights associated with Ouagadougou is also highly vulnerable to extreme rainfall intensities allowed to characterize the type of hydro-meteorological events. In Sahel countries, the flooding associated with various situations and causes. frequency of extreme storms tripled in the last 35 years; This analysis helped characterize the severity of floods between 1991-2009 alone Burkina Faso experienced in the set of points, from which the insights were 11 major floods. The flooding events of the last few extrapolated to other areas presenting the same decades directly affected the functionality of characteristics (areas next to a canal flooded by Ouagadougou’s transport system, especially overflow, street parallel to the slope, street considering the sparsity of the climate-resilient (paved) perpendicular to the slope and flooded by transverse road network and the dominance of poorly maintained streets, low points / basins). dirt roads. Moreover, urban growth, extreme weather events, and climate change are expected to continue to To further classify the road and future mass transit drive an upward trend in flooding risk in the future, sections in order to prioritize interventions, the analysis highlighting the urgent need for flood-resilient applied the criteria of an “area priority score” and a infrastructure development in Ouagadougou. “flood criticality score”, which together combine into an overall “impact score”: In the context of the plans to develop an efficient, bus- based mass transit system in Ouagadougou in the ▪ The area priority score was assigned based on the medium term, the study aimed to characterize the projected mass transit traffic and urban issues. For spatial distribution and severity of flood risk affecting example, the section of 28.257 St. on Line 4 was the planned system; and to identify, evaluate and assigned a low priority score because this section prioritize interventions that would increase its is at the end of a future planned bus line, where resilience. The study focuses on a pilot sector of 67 the travel demand and therefore the impacts of km2, covering a large part of central Ouagadougou and service disruptions would be limited. On the its strategic infrastructures, at the intersection of the contrary, the sections of Avenue du Capitaine future planned mass transit system and the areas of the Thomas Sankara and Avenue Nelson Mandela city a priori considered more flood prone (e.g., near the shared by most future bus lines were given a high major dams). score because they serve the City Centre and attract the highest travel demand of the planned By working with a local drone operator and an mass transit system. international flood modelling firm, the study constructed high spatial resolution Digital Elevation ▪ The flood criticality score was assigned based on and Digital Terrain Models for the area of interest (AOI), the flood mechanism and consequences (flood which served as inputs for developing a hydrological depth and duration); sections flooded for an model. The main output of the flood modeling are maps extended period with a high depth of water were showing maximum water heights and speeds in the AOI, given the maximum criticality score. under four return periods, summarized as follows: The analysis allowed concluding that the traffic on ▪ Frequent stormwater event (2-year return period) Avenue Nelson Mandela and the Nations Unies ▪ Rare stormwater event (10-year return period) roundabout, used by most future bus lines, could be ▪ Very rare stormwater event (50-year return period) interrupted by the 2-year RP rainfall. This area is the ▪ Exceptional stormwater event (historical flood of most critical in terms of not only flood criticality (flood September 1st 2009, with a return period higher depth and duration) but also urban area priority than 100 years) (presents the highest projected mass transit traffic and serves the city center). Outside of this area, the traffic In general, axes 2, 3, 6 and 8 of the future planned on the roads used by several lines could be interrupted mass transit network are not found to be subject to high by the 2-year RP rainfall at specific locations of their flood hazards, as compared to axes 1, 4, 5, and 7. To respective itinerary. further understand the flood risk affecting the planned layout of the bus system, a set of points was selected 7 Area priority score Flood criticality score Impact score ▪ Future mass transit ▪ Flood mechanism line affected ▪ Road/infrastructure ▪ Projected future assets affected mass transit traffic ▪ Consequences of 2- ▪ Urban issues, land and 10-year RP use floods on traffic Next, a methodology was developed to identify the 2- and 10-year-return-period models to find solutions for improving the flood resilience of the structural solutions that would protect the planned transport system of Ouagadougou. First, a infrastructures and bus operation against such events. “long list” of solutions was identified based on The use of structural solutions to protect the preliminary criteria of relevance for Ouagadougou and infrastructures and bus operation against the 50-year considered vis-à-vis the specific sections of the planned RP floods and floods such as the one that occurred in transport system exposed to flood risk. The analysis 2009 (whose RP is superior to 100 years) would be explicitly considers not only structural infrastructure costly and ineffective as these events are rare. Instead, (“gray” measures) but also ecosystem-based soft adaptation solutions (e.g. development of pre- approaches (“green” measures), hybrid measures, and disaster and business continuity action plans) could be non-structural, or “soft”, measures (e.g. risk monitoring, used to reduce the impacts on the transport operation. territorial planning, etc.). To compare and rank the measures and arrive at a The analysis focused on the main streets, intersections “short list”, the team developed a multicriteria-analysis and obstacles (e.g. canals) used by the planned transit methodology. To this end the following criteria and system. The analysis considered the consequences of weights were selected in the base scenario: Maintenance Environmental and socio- Investment cost Flood reduction benefits economic benefits cost (weight: 30) (weight: 30) (co-benefits) (weight: 10) (weight: 30) COSTS BENEFITS The multicriteria analysis was used to compare the apply to limited sections. Finally, as expected, green performance of the different types of measures (green, and mixed solutions are associated with the highest gray, soft). To this end, the 26 measures were classified environmental and socio-economic benefits (co- into groups of measures applying similar solutions. benefits). Soft measures are associated with the lowest The multicriteria analysis allowed the identification of investments and maintenance costs. Gray measures are the top measures that should be prioritized based on associated with medium upfront costs and low the total score. However, to evaluate how the choice of maintenance costs. In contrast, the initial costs of green weights impacts the rank of the measures, we measures greatly vary. Finally, the highest upfront and performed a sensitivity analysis by considering three maintenance costs are associated with hybrid measures scenarios, each representing a different set of priorities that mix green and gray elements. for the decision makers. These different sets of priorities are represented by different sets of weights attached to In terms of the flood-impact-reduction benefits, soft the criteria used in the analysis. solutions tend to be associated with medium to very high benefits as they generally apply to the whole AOI. Scenario 1 is the base scenario, with a balanced The flood-impact-reduction-benefit score of the gray distribution of priorities between costs, flood impact solutions is generally medium to high, as these reduction benefits and environmental and socio- measures are effective in reducing flood impacts but economic co-benefits. 8 Scenario 2 represents a situation where the decision- points at strategic locations collecting trucks, landfill / makers consider the reduction of flood impacts as the incineration sites absolute priority and the other criteria as secondary. 17: Build a rain garden on the Nations Unies Scenario 3 puts higher priority on the maintenance cost, roundabout and a relatively high priority on co-benefits. This could represent the preoccupations of the Ouagadougou 24: Develop a flood monitoring and disaster prevention system for the city: Pre-disaster action plan (timeline) Municipality, who could more easily obtain external that aims to prevent damage and allow public transport funding to pay for the investment but will be in charge of the maintenance of the infrastructure and have to to resume operation at an early stage. The actions should be triggered depending on the information consider the acceptability of the measures. provided by a flood monitoring tool (considering current and predicted rainfall and water level in the The top-10 rankings resulting from the three scenarios are close, as they tend to mostly include the same dams) — the system should be managed by the city authorities and the relevant information passed on to measures although in different order. For example, the the bus operating company first seven measures of the base scenario also appear among the ten highest ranked measures of scenarios 2 22: Enforce regulations aiming at limiting water run-off and 3. All in all, seven measures rank among the ten generated by new constructions. A simple way is to highest ranked measured in all three scenarios and impose a limit on the flow a newly developed area can should therefore be prioritized (see Figure 1): discharge to the drainage network. This engages developers to integrate stormwater management in 21: Set up a dedicated maintenance plan and team in their design and implement solutions to reduce flows charge of the periodic and systematic cleaning and sent to the network (e.g. green roofs, permeable maintenance of flood related structures (canals, culverts, pavement over parking lots). rain gardens, pockets parks, etc.) dedicated to the mass transit system, in particular before the rainy season 11: Build a drainage system to protect Ave Oumarou Kanazoe and Place du Rail roundabout 25: Reinforce the solid waste collection system: organize awareness-raising activities, and most 2: Install a bioswale in the median of the road on Ave importantly, provide an efficient collection system, Kwame Nkrumah and Ave de l'UMOA concrete baskets at the bus stops, garbage collection Figure 1: Location of the proposed flood-resilience and adaptation measures 9 Figure 2 summarizes the investment and maintenance collection, beautifying the surrounding landscape, and costs associated with each priority measure, as well as improving stormwater retention efficiency of existing the flood reduction benefits and co-benefits. The infrastructure and natural ecosystems. In contrast, the graphs illustrate much greater relative range in terms of enforcement of regulations aiming at limiting water run- costs than benefits, albeit the benefits have not yet off generated by new constructions is deemed to have been monetized at this stage and are assessed on a relatively low co-benefits (albeit also very low costs), qualitative scale. The variation in co-benefits is given that this measure would increase the cost of somewhat greater, with measures such as the construction projects in the city. To ensure the reinforcement of the solid waste collection system acceptability of this measure, therefore, a first step having the highest score, due to the benefits it would might be to apply it only to development projects by generate also in terms of job creation in waste professionals, excluding individual owners. Figure 2: Comparison of the costs and benefits of the prioritized measures Enforce regulations aiming at 25,000 Enforce regulations aiming at limiting 3 limiting water run-off generated by water run-off generated by new new constructions 0 constructions 8 Dedicated maintenance plan and Dedicated maintenance plan and team in charge of the periodic and 150,000 team in charge of the periodic and 7 systematic cleaning and systematic cleaning and maintenance maintenance of flood related 80,000 of flood related structures dedicated 9 structures dedicated to the mass… to the mass transit system 8,400 6 Build a rain garden on the Build a rain garden on the roundabout Nations Unies roundabout Nations Unies 7 84,000 Develop a flood monitoring and Develop a flood monitoring and disaster prevention system for the 25,000 disaster prevention system for the 6 city: Pre-disaster action plan that city: Pre-disaster action plan that aims to prevent damage and allow 150,000 aims to prevent damage and allow 6 public transport to resume… public transport to resume… 75,000 8 Reinforce the solid waste collection Reinforce the solid waste collection system system 8 450,000 Drainage system to protect Ave 12,250 Drainage system to protect Ave 5 Oumarou Kanazoe and Place du Rail Oumarou Kanazoe and Place du Rail roundabout 490,000 roundabout 7 Bioswale in the median of the road 195,000 Bioswale in the median of the road on 7 on Ave Kwame Nkrumah and Ave de Ave Kwame Nkrumah and Ave de l'UMOA 1,185,000 l'UMOA 8 Maintenance cost for 5 years (US$) Investment costs (US$) Co-benefits Flood reduction benefits Moreover, several caveats apply to the prioritized in the Sahelian climate might require irrigation for part measures and should be considered in the actual design of the year, and a follow-up study might be necessary and implementation process. For example, with respect to determine what plant species would be best adapted to measure 17, it should be noted that creating green to the climate in Ouagadougou. spaces is not in itself sufficient to improve infiltration but that efforts should also be made to increase the Which of the measures prioritized above are strictly presence of absorbing soil (e.g., using underground technical and which ones are more a matter of good structures such as rainwater collection systems 1 ). urban planning and political will? As noted in the study, Hence, rain gardens and other green spaces should be flooding in Ouagadougou is more a result of unplanned specifically designed with technical specifications to urban growth rather than the frequency of extreme maximize absorption and soil water retention. Similarly, rainfall. Hence, good urban planning and integrated regarding measure 2, it should be noted that a bioswale policies across sectors are crucial. Among others, the 1 The underground rainwater collection systems, in turn, could Ouagadougou by allowing the collected water to be reused for also provide the benefit of mitigating drought impacts in agricultural purposes or be purified and used as potable water. 10 city’s urban mobility plan should not be segregated in the top: the development of a flood monitoring and from the flood risk management strategy and should disaster prevention system for the city, the enforcement include measures to counteract the impacts of flooding. of regulations aiming at limiting water runoff generated An example that emphasizes the links between the by new constructions, and the development of a risk mobility plan and stormwater management is the awareness culture in the bus management company to potential development of much needed bicycle and manage the residual flood risk. motorcycle lanes that could be implemented along with the recommended permeable pavement. Their actual ranking notwithstanding, some of the soft measures – such as developing a flood monitoring and The importance of good planning and policy and disaster prevention system for the city – should likely regulatory actions vis-à-vis more structural engineering be implemented in parallel to infrastructure investments solutions is underlined by the fact that the top two under any scenario, given their role in reinforcing the measures singled out by the multicriteria analysis are sustainability and commitment of the local authorities so-called “soft” solutions – related to the maintenance to a long-term vision. and cleaning of the flood-related structures and the reinforcement of the waste collection system. These solutions are ranked highly because they would Note: An interactive online tool was prepared along effectively contribute to the flood resilience of the entire with this report to allow stakeholders and decision- future mass transit system, are associated with large makers to more closely interact with the data and the long-term environmental and socio-economic benefits, findings. and require limited initial and maintenance investments. In addition to these, several other “soft” measures rank 11 1. Context 1.1. Ouagadougou’s growth patterns This territory is planned by means of a master plan for the development of Greater Ouaga (SDAGO) of 2010. Ouagadougou currently has 3 million inhabitants with a population growth rate of 7-9 percent per year due to The urban area of Ouagadougou has different types of the natural growth rate and migration, and the land use but is dominated by built-up areas of varying population is expected to reach nearly 4 million by density, constrained by roads and waterways. Some 2025. The city is composed of 12 districts and 55 rare areas are still little built-up or covered with sectors. The average density of the conurbation is low, vegetation. The built-up areas are dominated by the around 50 inhabitants per hectare, but can exceed 100 traditional housing estates, forming rectangular blocks inhabitants per hectare in the non lotis (non-parceled, in which the building stock is very dense. The houses precarious) districts (AFD, 2019). In terms of are located in enclosed concessions, with openings at governance, Ouagadougou is an "urban commune with the base of the walls to ensure rainwater drainage. The special status" led by an elected central mayor and 12 concession yards are either concreted or natural. The district mayors. roads that delimit the blocks are generally unpaved, except for the main roads. Undeveloped housing is also Greater Ouaga covers an area of 3,300 km² within a 30 very present in the peripheral areas, with a lower km radius of Ouagadougou. It includes the urban density of buildings and an irregular structure. The commune of Ouagadougou and seven rural communes network of structuring urban roads takes a radial form around Ouagadougou: Pabré, Tanghin-Dassouri, from the city center, and only the main road network is Komki-Ipala, Komsilga, Koubri, Saaba and Loumbila. paved. Figure 3: View of an outlying neighbourhood, traditional housing The morphological elements that characterize the urban territory of Ouagadougou are the three dams located to the north of the city center as well as the greenbelt that is delimited across a horizontal plane and which includes the Bangr Weogo urban park to the northeast, commonly known as “The Forest”. However, this greenbelt is regularly threatened by various developments (housing, quarries, landfills, household waste). Its role in terms of urban agriculture must also be stressed: approximately 5,000 people work in over 100 agricultural sites within an area of 750 ha. This ecosystem is fragile and vulnerable because it is threatened by various types of pollution: industrial waste, household waste dumps, massive digs, etc. Source: Google Earth Figure 4 Ouagadougou structure Figure 5 Ouagadougou water system Source: AFD (2019) 12 1.2. Major flooding events of this century of one part (the most intense) of the rainfall, the return period of this rainfall is more likely to be of the order of Like many urban areas in Sahelian West Africa, 100 years. The flood most severely affected Ouagadougou is highly vulnerable to extreme hydro- neighborhoods near the dams and marigots crossing meteorological events. In the region the frequency of the city, notably Kouritenga, Pissy, Dapoya, Paspanga, extreme storms tripled in the last 35 years; between Ouidi, Larlé, Tanghin, and Bissiguin. 1991-2009 alone, Burkina Faso experienced 11 major floods. During the September 2009 West Africa floods, Heavy flooding in Ouagadougou in August 2015 Burkina Faso was one of the most affected countries: affected 20,000 people, destroyed several thousand 150,000 people fled their homes, mostly in homes, and left parts of the city under water for days. Ouagadougou, where a record-breaking 263 mm In July of 2018, heavy rains resulted in significant rainfall was recorded in less than 12 hours, affecting flooding and caused road traffic disruption throughout around half of the city’s territory. According to the capital. evaluation by Traoré (2012), who analyzed the record Figure 6: Floods extent map in September 2009 Source: Cited in Bazoun et al. (2010) Hangnon et al. (2015) list the major floods that occurred between 1983 and 2012, between June and 24 June 2015: Heavy rain fell on Ouagadougou September. The results show that the rainfall that and some localities in Burkina Faso. With a volume causes flooding is often normal, with a return period of of between 67 millimetres and 79.8 millimetres, less than 6 years. Flooding in Ouagadougou is this rainfall caused four deaths, including three therefore a result of unplanned urban growth and children, as well as extensive material damage. uncoordinated planning rather than just the frequency 10 July 2016: Heavy rain fell on Ouagadougou or intensity of extreme rainfall. A review of press articles and several regions of Burkina Faso. At the and other documents shows that after 2012, floods occur almost every year: Yalgado Ouédraogo University Hospital and the 13 General Directorate of Land and Maritime maintenance of rain collectors, an accumulation of solid Transport (DGTTM), this rain caused enormous deposits (waste, load products) and an inherently low material damage. In several other areas of the water carrying capacity which increases flood risk. capital, 1,488 people were forced to abandon their homes invaded by rainwater. The city’s rainwater drainage network is essentially limited to the main collectors, which drain water to the 19-20 July 2016: Heavy rain fell on Ouagadougou dams' reservoirs, and to a secondary network bordering and some localities in Burkina Faso, causing the main secondary roads. Almost all of the network is extensive damage. 51.4 mm was recorded in open on the surface, sometimes covered with slabs. The Ouagadougou Airport, 55.3 mm in Somgandé and collectors are often very clogged or deteriorated. 97.6 mm in Pô (~150 km from Ouaga). 9 August 2016: 88 mm in less than 12 hours; The evolution of rainfall under the effect of climate most affected districts were 1, 2, 3, 4, 6, 9, 12, change has been addressed at the Sahel scale, including and Rimkieta neighborhood in Ouagadougou (Panthou et al., 2014). The main results show that the annual number of rain storms 18 May 2017: 96.7 mm in less than 12 hours; tends to decrease over the whole Sahel, and that the affected districts 4 and 12 and the Kouritenga, intensities of the heaviest showers tend to increase. Bissighin, and Rimkieta neighborhoods Climate change scenarios based on the CP4 regional 25 July 2018: Around 100 mm of water fell on model have also been provided for Ouagadougou, as Ouagadougou in the space of 24 hours; eight part of the AMMA 2050 project, led by CEH (UK) with secteurs (1, 2, 3, 6, 7, 10, 11 and 12) were the collaboration of Institut de Recherche pour le affected by flooding Developpement (IRD) based in Burkina Faso. In the same project, Taylor et al. (2017) show that the July 4, 2019: 89 mm in Ouaga Aero, 75 mm in frequency of heavy rainfall has tripled since 1982 in the Somgandé, 2-hour episode Sahel, based on cloud surface temperature measured by Meteosat. Flooding poses a recurrent threat to Ouagadougou for a number of reasons: (i) the city is naturally prone to Analysis rainfall for the prediction period 2021-2050 seasonal flooding, given its flat topography, its network in comparison with the 1971-2000 baseline period of riverside channels, and the soils’ poor water shows various trends for the evolution of annual rainfall: retention and infiltration capacity; (ii) the three dams a significant downward trend for two models and a that contribute so much to the identity of the city are significant upward trend for two other models. Across silted up and can no longer fulfil their role as an outflow, five climatic models comparing the reference period and buffer as well as potable water supply for the the prediction period, also the anticipated increase in conurbation; in addition, the drainage system is poorly daily maximum rainfall in Burkina Faso varies widely maintained, while the uncovered gutters tend to be depending on the model, from 0.7 to 17.4%. Thus, used as waste disposal, and clogging by litter reduces although the literature indicates that climate change is the network drainage capacity; (iii) rapid settlement very likely to cause an increase in rainfall intensities in growth and soil sealing are additionally straining the the next decades, the results of climate models show drainage capacity, while uncontrolled urban significant differences from one model to another, and development results in an increase in direct flood these predictions apply to maximum daily rainfall, hazard exposure, especially in non-lotis neighborhoods, whereas the concentration time of the watersheds whose population accounted for two-thirds of victims composing the Area of Interest (AOI) spans from 30 during the most recent floods. In sum, the frequency of minutes to 3 hours. There is thus not enough evidence flooding in Ouagadougou has increased since 2000, to generate an estimation of the increase in rainfall due to the effect of exceptional rainfall but also because intensities for short, intense events. of rapid and poorly controlled urbanization. Floods occur every year, after rains of only a few dozen 1.3. Impacts on the transport system milimeters. The flooding events of the last few decades directly Stormwater is another threat. The city was built on a affected the functionality of the city’s transport system, site that could be described as marshy (the presence of especially considering the sparsity of the climate- marigots, or river side channels, was a natural resilient (paved) road network and the dominance of protection against invaders), with a series of flat areas poorly maintained dirt roads. Physical impacts of the that slope gently (between 0.5% and 1%) from south floods, with the associated operational and to north, without any elevated points. The soil has a maintenance impacts, are summarized in Table 1. An limited capacity for water infiltration and conservation. illustration of the impacts of floods on the traffic in In addition, rainfall episodes, while generally decreasing Ouagadougou is illustrated in Figure 7, showing the over the last 30 years (due to the phenomenon of dry Nations Unies roundabout in Ouagadougou city center spells), can also be particularly intense (up to 180 under normal versus flooded conditions. mm/h), especially with the lack of upkeep and 14 Urban growth, extreme weather events, and climate urgent need for flood-resilient infrastructure change are expected to continue to drive an upward development in Ouagadougou. trend in flooding risk in the future, highlighting the Figure 7: The Nations Unies roundabout in Ouagadougou: under normal versus flooded conditions Sources: © Tiphaine Brunet, 2013; ©RFI, 2009 Table 1: Impact of past floods on the transport system Past flood event Rainfall Impacted areas3 Physical impacts Operation impacts Maintenance impacts classification2 1st September Very exceptional Mostly impacted Flooded roads, Public and private Serious maintenance 2009 i.e. return districts: 1 bridges and transport delays operations required period > 100 (Kouritenga); 2 scuppers and disruptions due for inspecting, years (Dapoya, Ouidi, Larlé, to speed reductions cleaning and Paspanga); 4 Major damage and impassable planning repair and (Tanghin), 6 (Pissy); 8 observed on over roads (the delays reconstruction works (Bissighin) 15 structures and disruptions (mainly lasted for several Repair and embankments and days - and even reconstruction work covering of weeks in some required for over 15 bridges and areas) structures scuppers)4 24 June 2015 Normal to Certain districts are Flooded roads, Public and private Maintenance 10 July 2016 severely always impacted (e.g. bridges and transport delays operations required 19-20 July 2016 abnormal i.e. 12), while others are scuppers and disruptions due for inspecting, 9 August 2016 return periods more or less to speed reductions cleaning and 18 May 2017 between 1 and impacted depending Minor damage of and impassable planning repair works 25 July 2018 50 years on the flood events structures roads (e.g. 7 and 11) 2 Tazen et al. (2018) 3 SEPIA Conseils. (2021) 4 World Bank. (2010) 15 2. Addressing the flood risk to improve the resilience of Ouagadougou’s transport system 2.1. The planned mass transit system transit system as well as the location of key urban zones in Ouagadougou. The proposed system is expected to Over the medium- to long run, a bus-based mass transit carry about 150,000 passengers per day, with the system (possibly a Bus Rapid Transit, or BRT, network busiest segments of the system – located in the city with feeder services) is planned to be developed in the center – carrying over 14,000 passengers per day per city to improve urban mobility along high priority direction (Transitec, 2021). corridors. Figure 8 shows the planned layout of the Figure 8: Situation map of the planned mass transit network Source: Elaborated by authors, based on Transitec (2021) In light of the past extensive flood damage to the city’s design specifications that enhance robustness and transport connectivity and the potentially growing flexibility of infrastructure severity of flood risks in the future, there is a need for applying a “climate lens” to prioritizing the future Operations & Maintenance: Inventory and mapping of planning of the public transport system in transport infrastructure, development and imple- Ouagadougou, through a comprehensive, life-cycle mentation of sound asset management and mainte- approach to risk that addresses the following aspects, nance systems, improving institutional and financial as detailed in the guidance developed by the Global arrangements for infrastructure maintenance; inte- Facility for Disaster Reduction and Recovery (GFDRR) gration of climate and disaster risk considerations in the (Figure 9): prioritization of investments in new infrastructure, rehabilitation, and restoration Systems Planning: Shifting deployment of long-lived infrastructure away from disaster-prone areas to avoid Contingency Planning: Developing policy and development lock-in; consideration of integration and institutional frameworks, communication protocols, and redundancy on critical infrastructure to offer investments in emergency preparedness and response; alternatives alignment of transport systems and flows with local and regional evacuation, response, and recovery needs Engineering & Design: Improving design standards of transport infrastructure to maintain connectivity and Institutional Capacity & Coordination: Centralizing of reduce disaster risk; use of innovative materials and disaster risk information and data comprehensively by enhancing strong coordination among central 16 governments, line ministries and agencies, and local The current study focuses primarily on the first two municipalities; upstream planning of transport systems aspects of the life-cycle approach with the objective to to reduce the hazard exposure of the infrastructure that improve the resilience of the future urban transport results in greater disaster risk; mitigation of institutional system in Ouagadougou. and regulatory challenges, which are crosscutting in nature, to utilize the life cycle approach effectively Figure 9: The Life-cycle approach to addressing risk Source: GFDRR 2.2. Flood modelling analysis this dam, the water level varies every year with an amplitude of up to 4 meters between the minimum and Building on the available rainfall data and past flood the maximum level. modeling studies available for Ouagadougou, the current study focuses on a pilot sector of 67 km 2, The average annual rainfall in Ouagadougou over the covering a large part of Ouagadougou city center and 1983-2012 period was 733 mm. RainCell data its strategic infrastructures. The flood risk modeling acquired in 2016 and 2017 on a set of fourteen rain conducted for the purposes of the current study was gauges give an overview of the rainfall characteristics based on aerial imagery collected by an unmanned leading to floods on 9 August, 2016, and 18 May, aerial vehicle (UAV, or “drone”) with vertical take-off 2017. Rainfall totals varied from 40 to 140 mm, with a and landing (VTOL) capability over the period of several duration of about 2h30 for the intense part of the rain. weeks in July-August 2020. The imagery was then used The episode was centred on the south-eastern part of to construct a Digital Elevation Model (DEM) at a Ouagadougou, where the rainfall exceeded 100 mm. resolution of 10 cm covering the study area, a key input According to the local IDF curves, the rainfall into the flood risk modeling. The detailed description of corresponds to a return period of more than 50 years the process and the challenges encountered in the if we refer to an accumulation of 120 mm in 3 hours, process, including in developing the DEM, is provided and to a return period of much less than 1 year if we in Annex 1. refer to an accumulation of 20 mm in 15 minutes. These examples can be used as project rainfall associated with In addition to the drone imagery collection, a field return periods of between 2 and 10 years. Indeed, campaign was undertaken to survey the dimensions of although daily rainfall in excess of 100 mm was the main canals in the city: Gounghin (Moro Naba) canal, observed in both 2016 and 2017 at least at one of the University canal, United Nations canal, Wemtemga measuring stations, it should be considered that the canal, and Somgandé canal. Four of the five canals probability of observing such rainfall at a given point is (Gounghin, University, United Nations, Wemtemga) are more rare. within the AOI (part of the city crossed by the planned future mass transit system). A geomorphological rule The runoff coefficients for Ouagadougou tend towards was then adopted to estimate channel dimensions values around 0.70 for the two basins monitored in (Bouvier et al., 2017), relating the width of flow sections 1979, around 0.40 for the two basins monitored in to the slope and area of the upstream basin. Depths 1992-1993, and around 0.10-0.20 for the three basins were estimated at 1 meter. monitored in 2016-2017. For the study area, considering that the urbanization is relatively dense, it The flows are regulated by three dams as they pass was considered best to adopt runoff coefficients of the through the city. Bathymetry and water levels are order of 0.7-0.8 for rainfall reaching 50 mm. These available for dam 3. The water level in this dam is coefficients are reduced to 0.40-0.50 for any areas still particularly important for estimating the possible undergoing urbanization. downstream influence of the flows in the study area. In 17 Figure 10: Location of the two study catchments Source: SEPIA Conseils The study area is divided into two sub-catchments by the network of secondary canals, coming from the (Figure 10). The river system in the West is mainly districts or bordering the main roads. Along the tarmac composed of the Gounghin Canal, which is the longest roads, the canals are built, while on the dirt roads, canal in the study area. Its upstream part has long gullies are dug naturally. These storm drains are also remained undeveloped, and was a natural gully. often deteriorated and clogged with rubbish, which However, since 2016, strong urbanization has taken prevents them from functioning properly. place, and this canal is now fully developed. The hydrographic network in the East is denser, composed Numerous structures ensure the hydraulic continuity of of three main canals: Nations-Unies, University, and flows within the canals under roads, railways and paths. Wemtenga. The Nations-Unies canal is partly buried. They vary in size from small passage structures on Together with the University canal, they meet at the secondary canals to large structures on the main canals, entrance to Bagr Weaogo Park. In this area the canals particularly downstream. The latter are often made up are no longer developed. The University, Wemtenga, of several pillars, which favors the retention of waste Gounghin and United Nations canals are partially built and the creation of logjams. Thus, they are often in poor of concrete, and their depth and width increase as one condition and obstructed, reducing the flow and thus moves downstream of the canals. They are often very increasing the frequency and risk of channel overflows. congested or deteriorated. These main canals are fed Figure 11: Examples of rainwater collectors in Ouagadougou Source: Louis Berger 18 Figure 12: Structure located on the Gounghin canal (Rue 17.250) on the left obstructed by a lot of rubbish and structure on the university canal on the right with congestion at the bridge piers. Source: Louis Berger The study area is almost completely urbanized. In order supplemented by systematic work based on aerial to determine the current land use, the work carried out photographs in order to take into account the changes in this study is based on data from Bonnet and Nikiema that have taken place since 2013. Overall, most land (2013) which provide contours of the built-up areas, use corresponds to dense grouped housing (Figure 13). Figure 13: Land use Source: SEPIA Conseils 19 Figure 14: Relief of the studied territory made from the DTM Source: Espace Geomatique The elevation of the study area varies between 285 and 340 m in altitude. The low points of concentration of the flows are found at the level of the canals and dams downstream. The water of the agglomeration is drained towards the latter, then towards their outlets to the north. In the south-west of the study area there are areas that reach altitudes of over 340 m. The study area is characterized by low slopes, generally between 1 and 5%. Only a few embankment roads and the banks of the canals have slopes of over 5%. As a result, rainwater runoff can take a long time to drain away to the canals and dams. The main output of the flood modeling are maps ▪ Very rare stormwater event (50-year return period) showing maximum water heights and speeds in the AOI, ▪ Exceptional stormwater event (historical flood of under four return periods, summarized as follows:5 September 1st 2009, with a return period higher than 100 years) ▪ Frequent stormwater event (2-year return period) ▪ Rare stormwater event (10-year return period) Table 2: Characteristics of the reference rainfall events 2-year return 10-year return 50-year return Historical rain storm period period period of 2009 Occurrence Frequent Rare Very rare Exceptional Cumulative intensity in 1hour (mm) 54 70 97 112 Cumulative total in 3 hours (mm) 70 106 142 173 Source: SEPIA Conseils The two-dimensional (2D) modelling of flows carried - Understand the flooding dynamics at the out in this study makes it possible to: block/neighborhood scale: taking into account the general urban topography, the obstacle - Evaluate the impact of the occurrence of effect of buildings, and the preferential different rainfall ranges, from a frequent drainage axes on roads; rainfall with a return period of 2 years to an exceptional rainfall of the September 2009 - Provide useful information for crisis type with a return period of more than 100 management: identification of cut-off traffic years; routes in particular; - Delimit the main risk areas for people and - Evaluate the stakes at risk exposed to runoff property: flow axes and main accumulation phenomena. areas with an estimate of the order of magnitude of the potential submersion heights On the other hand, this model does not allow to and flow speeds; simulate the functioning of underground drainage axes within built-up areas; accurately represent flooding at 5All the stormwater events where simulated over a few hours period, with a 5-second time step. 20 the scale of a building; or simulate the dynamic roads and railways, i.e., the presence of bulky items interactions between the three dams downstream of the and rubbish trapped at the level of the bridge piers, study area. creating logjams. The purpose of this scenario is to assess the risk of overflow in the event of The study modeled three scenarios for the four obstruction of these structures. reference rainfall events (2-year, 10-year, 50-year) and the historical rainfall of 2009: ▪ Scenario 3 is based on the reference scenario but with a high downstream condition. It corresponds ▪ Scenario 1 is the reference scenario. The to the hypothesis of dams being overflooded, with downstream condition of the model corresponds to a water level of 289.5 m. In this scenario, hydraulic the average level of the dams, i.e., a height of continuity is ensured at the level of the structures. 286.5 m. Hydraulic continuity is ensured at the various hydraulic structures. For the canals that are The comparison of the floodable areas for the 2-year in water all year round, only the effective hydraulic return period for Scenario 1 vs. Scenario 3 is shown in capacity is taken into account. Annex 3. A comparison of the size of the area within the study area of interest that is flooded with a height ▪ Scenario 2 is scenario 1, to which is added the of more than 3 cm under scenarios 1 and 3 is hypothesis of a total obstruction of the flows at the summarized in Figure 15. level of the canal passage structures under the 2,113 Figure 15: Flooded area for scenarios 2,031 1 and 3 and for different return 1,812 periods (hectares) 1,714 1,149 1,157 2-year return period 10-year return period 50-year return period Scenario 1 Scenario 3 Source: SEPIA Conseils The comparison of the modelling results with existing Flood risk was conceptualized along two main axes, historical data, such as on the spatial distribution of the namely flood height (in cm) and flow velocity (in meters 2009 flood events and the less dramatic 2016 and per second). Thus, the flood risk analysis distinguishes 2018 flood events, allows checking the capacity of the between : model to reproduce past floods, as well as to highlight its strengths and weaknesses. Overall, the modelling ▪ Low risk areas with water heights < 15 cm results reproduce the main flood and disaster areas observed in 2009 and even allow refining them and ▪ Zones at risk with water height between 15 and understanding the flood origin mechanisms 50 cm and velocity below 0.5 m/s. (overflowing of the canals, formation of a basin or runoff ▪ Strong hazard zones due to the presence of on the road). significant water heights (> 50 cm) which induce a risk of drowning Looking at the surface and point disorders observed during the 2016 and 2018 rainfall events, secteur 12, ▪ Strong hazard zones due to the presence of for example, appears to have been affected during each significant velocities (>0.5 m/s) which induce risk intense rainfall episode, while others are affected more of being washed away occasionally, such as secteurs 7 and 11. In turn, for each ▪ Very strong hazard zones, with both high affected secteur, more detailed analysis was carried out, submersion heights and velocities (H > 50 cm and focusing in on the overlaps between flood risk and the V > 0.5 m/s), associated with very high risk of future mass transit network. drowning and being swept away. 21 Figure 16: Areas flooded by the 10-year return period storm in the downstream Mogho Naaba watershed Source: SEPIA Conseils 3. Identifying and prioritizing interventions to mitigate flood risk 3.1. Flood risk vis-à-vis the future mass The flood modeling results, further, allow to identify the transit system areas along the projected bus lines where floods would slow down traffic and where traffic would become Prior to proposing a specific set of solutions and impossible. Based on the existing literature, the weighing their relative costs and benefits, the potential potential water heights were related to vehicle speed vulnerability of the planned future mass transit network (see Pregnolato et al., 2017), allowing to define 15 cm was analyzed based on the results of the modelling of as the threshold water height leading to an interruption scenario 1 (the reference scenario) in order to of traffic when it is reached or exceeded. It is considered determine in detail the degree of exposure of each that below this threshold, for areas where heights are branch of the future network. The analysis focuses on between 3 cm and 15 cm, traffic would be slowed down the main streets, intersections and obstacles (e.g. the and traffic speed reduced, resulting in a reduced canals) used by the planned transit system. The analysis operation of the transport network. considers the consequences of the 2- and 10-year-RP models to find structural (or hard) solutions that would To better understand the flood events affecting the protect the infrastructures and bus operation against planned bus system, a set of points was selected in the such events. AOI, representing different flood types, for which hydrographs were generated showing the evolution of The use of structural solutions to protect the water height over time during the 2-year RP flood and infrastructures and bus operation against the 50-year the 2009 flood. The analysis of the ramping up and RP rain and 2009 flood (whose RP is superior to 100 down of water heights associated with rainfall years) models would be costly and ineffective as these intensities allowed to characterize the type of flooding events are rare. Instead, soft adaptation solutions (e.g. associated with various situations and causes. The development of pre-disaster and business continuity detailed findings from the individual points-of-interest action plans) could be used to reduce the impacts of are presented in Annex 4. rare events on the transport operation. This analysis helped characterize the severity of floods Axes may be subject to high water levels, high flow in the set of points, from which the insights were velocities, or a combination of both. In general, future extrapolated to other areas presenting the same axes 2, 3, 6 and 8 are not subject to high flood hazards, characteristics (areas next to a canal flooded by as compared to axes 1, 4, 5, and 7. overflow, street parallel to the slopes, street perpendicular to the slope and flooded by transverse 22 streets, low points / basins). The results of the analysis maintain transport services), assigned based on the are summarized in Table 4.1. in Annex 4. projected mass transit traffic and urban issues. For example, the section of 28.257 St. on Line 4 was The analysis led to the following conclusions concerning assigned a priority score of 1 because this section the flood impacts on the future mass transit system: is at the end of the bus line, where the travel demand (5,506,893 passengers per year) and ▪ The traffic on Avenue Nelson Mandela and the therefore the impacts of service disruptions would Nations Unies roundabout (in the city center), be limited. On the contrary, the sections of Avenue used by most future bus lines could be du Capitaine Thomas Sankara and Avenue Nelson interrupted by the 2-year RP rainfall. Mandela shared by most bus lines were given a score of 3 because they serve the City Centre and Outside of this area, the traffic on the roads used by attract the highest travel demand (16,400,643 several lines (except lines 2, 3, 6 and 9) could be passengers per year) of the planned mass transit interrupted by the 2-year RP rainfall at specific locations system. of their respective itinerary. More specifically: ▪ Road/intersection and infrastructure assets affected by the 2- and 10- year RP rain events ▪ The operation of line 1 could be interrupted in Nongremasson St. due to an overflow of the ▪ Consequences of the 2-year RP rainfall model and canal. consequences of the 10-year RP rainfall model - whether the traffic would be unimpacted, slowed ▪ The operation of line 4 could be interrupted (water height below 15 cm) or blocked (water on 28.257 St., Ave de l'Indépendance and height above 15cm) under these conditions. Blvd Charles De Gaulle. ▪ Description of the flood - the probable cause and ▪ The operation of line 5 could be interrupted in characteristics of the flood in the sections. For multiple locations, i.e. 14.09 St., Ave des Arts, example, Nongremasson street on Line 1 is flooded Ave Houari Boumedienne and Ave de la because of an overflow of the Canal central (or Grande Mosquée. Nations Unies) while 25.257 St. on Line 4 is ▪ The operation of line 7 could be interrupted flooded because of the run off from the on Ave Oumarou Kanazoe, Ave Ouezzin perpendicular streets in the south-east, which act Coulibaly and Simon Compaoré St. as canals and convey water to the north-west. ▪ The operation of lines 7 and 8 could be ▪ Flood criticality score, assigned based on the flood interrupted on the roundabout de la Bataille mechanism and consequences (from 1 to 4, du Rail. depending on the flood depth and duration); the section flooded for an extended period with a high depth of water were given the maximum criticality To further classify the road/future mass transit sections score (4). in order to prioritize interventions, the following criteria ▪ Impact of flood issues on the planned mass transit were applied: system: this score combines the area priority score and the flood criticality score. To this end, the area ▪ Future mass transit network line affected priority score and the flood criticality score were ▪ Projected future mass transit traffic – extracted multiplied, and the resulting number scaled in [0,4] from the OPTIS Study (2021) and rounded to the nearest 0.5 number. Hence the lowest non-zero score is 0.5, identifying sections ▪ Urban issues for transport system (transit area vs. that are neither critical in terms of urban transit catchment area, land use, etc.) – assessed using issues nor in terms of flood criticality (flood depth land-use data (including from Google Maps) and and duration). The maximum score of 4 indicates the knowledge of the local consultants. areas that are critical both in terms of the flood ▪ Area priority score (from 1 to 3, with 3 criticality (flood depth and duration) and urban representing a section on which it is crucial to issues priority (high projected mass transit traffic and service to critical urban areas). 23 Figure 17: Approach to prioritizing transport infrastructure segments Area priority score Flood criticality score Impact score ▪ Future mass transit ▪ Flood mechanism line affected ▪ Road/infrastructure ▪ Projected future assets affected mass transit traffic ▪ Consequences of 2- ▪ Urban issues, land and 10-year RP use floods on traffic 3.1. Developing a “long list” of potential The adaptability to Ouagadougou was assessed using solutions three levels: high (for solutions already implemented in Ouagadougou), medium (solutions already In order to identify priority solutions for improving the implemented in a similar context (African or other flood resilience of the planned transport system, first, a developing countries) and low (solutions only long list of solutions was identified based on implemented in developed countries, with a low preliminary criteria of relevance for the case of readiness level and adaptability to the local context). Ouagadougou. Next, the analysis focused on the specific sections of the planned transport system The long list of solutions is presented in a Table 5.2. in exposed to floods. Finally, a multicriteria analysis was Annex 5, where they are ordered depending on the developed to prioritize among the flood resilience and implementation scale. There are 23 solutions in the list, adaptation measures. The analysis explicitly considers including preserving flood expansion areas, using not only structural infrastructure (“gray” measures) but permeable pavements and building rain gardens, also ecosystem-based approaches (“green” measures), amongst others. The list includes six soft, four gray and hybrid measures, and non-structural, or “soft”, eight green solutions but also solutions that mix green, measures (e.g. risk monitoring, territorial planning, etc.). gray and soft elements. To gather a list of possible solutions, the different Table 5.1. in Annex 5 shows a breakdown of the experts of the project team (i.e. hydraulic engineer, sources for the tentative costs. The costs included in the urban planner, green Infrastructure and nature-based present report are indicative and a more detailed solutions expert, etc.) relied on their experience from assessment would be required to reduce the past projects and on the literature. To analyze and uncertainty around those costs. classify the possible solutions, the project team selected the following criteria: 3.2. Learning from global best practices ▪ Solution type ▪ Advantages Combining green and gray infrastructure can provide ▪ Disadvantages lower-cost, more resilient, and more sustainable ▪ Implementation scale (street, neighborhood, infrastructure solutions (Browder et al., 2019). A or watershed) noteworthy example of such integration of the water ▪ Adaptability to Ouagadougou cycle with city infrastructure is China’s sponge cities ▪ Situations for which the solution is adapted (State Council of China, 2015). Under this ambitious program, the country seeks to reduce the effects of The solution type refers to whether the solution is green flooding through a mix of low-impact development (uses soils and vegetation to utilize, enhance and/or measures and urban greenery and drainage mimic the natural hydrological cycle processes of infrastructure, and to have 80 percent of urban areas infiltration, evapotranspiration and reuse), gray (hard, reuse 70 percent of rainwater by 2020. This approach human-engineered infrastructure that uses concrete is similar to what Australia’s Cooperative Research and steel) or soft (use institutions and technology Centre for Water Sensitive Cities calls its vision of the services). The project team ensured that the long list of “city as a water catchment” (Hallegatte et al., 2019). solutions included different types and implementation scales, as different solutions should be combined to Based on the flood risk assessment, the study analyzed effectively improve the flood-resilience of the future technical solutions that could be applied to reduce the transport system and city. vulnerability and exposure of the planned future mass transport routes. This analysis was guided by the global 24 experience on the topic. The team reviewed the to the point of reducing vegetated areas to literature to identify case studies of cities (outside only a few parks, located mainly in the Burkina Faso) that successfully implemented design peripheral areas. It is precisely through the improvements of urban transport systems to strengthen development of "linear parks" that allow their climate resilience. The chosen criteria for selecting vegetation to be reinstated in the city and the case studies resulted from the team’s experience water infiltration to be increased that São and knowledge of Ouagadougou, other cities in Africa, Paulo can be interesting as an example for and other countries, where climate risks and mobility Ouagadougou. At the same time, São Paulo's are challenging. These included: public transport network is complex and dense. The demand for public service has been ▪ Mobility (transport system, its operation and growing for several years. The main modes of intramodality) transportation are buses, which are also ▪ Physical geography (geographic location, impacted by the flooding phenomena. hydrology and typology of floods) ▪ Urban environment (demographic and Shanghai (China). The rapid urbanization of socioeconomic, urban morphology, infrastructures) Shanghai has created complex impacts on the ▪ Mobility governance and disaster risk water cycle and hydrology. For this reason, the management city is strongly exposed to urban flooding characterized by runoff, just as in Five cities, covering both high- and low-income Ouagadougou. The limitation of permeable contexts, were selected based on these criteria and due spaces and the problems of pollution of urban to their best practices in flood risk management: water bodies are also issues addressed by the authorities and can be inspiring for Singapore, which highlights nature-based Ouagadougou. This city is one of the Chinese solutions on the scale of a city-state subject to "sponge cities" whose integrated approach to flooding issues with impacts on its public flood management can serve as a good transport systems. Like Ouagadougou, practice. Thus, all sectors, including transport Singapore has also experienced rapid (buses, cabs, metro, etc.) benefit from this urbanization in recent years and has a very strategy. high population density. Moreover, the city's preferred means of public transport is a Nairobi (Kenya): The city has developed surface rail system combined with a dense bus different types of transports (bus, train, local network. Concerned about the flooding issues, bus) to respond to the rapid population this city has been working for several years on growth. The city faced recurrent flash floods innovative technologies for flood risk during the last years that are more and more management: new technologies, use of nature- intensive because of climate change. During based solutions, etc. It is a particularly flood- the flooding periods, all the transports stop in resilient city. some neighborhoods. Nairobi shares the same challenges as Ouagadougou. The lack of Toyooka (Japan), exemplifying the Japanese sanitation infrastructures and the insufficient disaster preparedness and business continuity maintenance of the existing canal system due approach. This approach is specific because it to solid waste, the urban morphology of the relies on the principle that despite all city led to devastations in the city during a protection measures aiming at avoiding flash flood. natural hazards, uncontrollable natural disasters will happen, and the society should Moreover, urban sprawl (whose main be prepared to adapt to these events in order component is informal settlements like in to minimize disruptions and destructions. Ouagadougou, and increase of residential neighborhoods in the suburbs), has reinforced São Paulo (Brazil). This city also faces frequent the need to manage and mitigate floods. flooding during the wet season and shares Therefore, concerned about the issue, public with Ouagadougou the problem of maintaining and private stakeholders have developed rivers and canals that receive diffuse pollution resilient initiatives such as: governance tools, from storm water run-off and solid litter. São green infrastructures, urban policies, etc. Paulo has also experienced rapid urbanization 25 Case Study 1: Bio-swales and rain gardens in Singapore Similarities with Ouagadougou • Public transport affected by flooding events • Projected • Rapid urbanization despite its land constraints • increase in the intensity of weather variability • Densely- Water-related vulnerabilities ranging from flooding populated city to supply scarcity • Population growth and biophysical limitations. Urban transport system The railway network can be regarded as the backbone of The Singaporean government has prioritized railway the public transport network, and is supported by the bus system over bus services as bus services could not services. be the solution for a compact city like Singapore. Climate hazards affecting the city • Singapore receives about 2,400 mm of rainfall annually • Many impervious surfaces (e.g. roofs, parking lots, • Increase in occurrence of flash flooding coinciding with and streets) prevent stormwater from infiltrating localized storm events • Floods caused by a combination into the ground and generate increased runoff that of heavy rainfall, high tides and drainage problems, enters the stormwater drainage system • Singapore especially in low-lying area has experienced several major floods that have resulted in widespread devastation, as well as destruction to life and property Orchard Road (2010) Upper Changi Road (2018) Solutions implemented to enhance the system’s resilience Context. In the 1960s and 1970s, Singapore witnessed This programme proposes to manage storm water frequent flooding (especially in the low-lying city center), runoff in a more sustainable manner via the which caused widespread disruption and damage. To implementation of ABC Waters design features 6 , reduce the risk of flooding, Singapore traditionally relied natural systems (i.e. plants and soil) able to detain on a network of canals and rivers to channel water into and treat rainwater runoff before discharging the reservoirs and the sea. The city launched major projects cleansed runoff into the downstream drainage to enlarge natural waterways (e.g. the Kallang River) and system. There are various types of ABC Waters line riverbanks with concrete to improve conveyance of design features, this case study focuses on water and reduce bank erosion. Singapore’s national bioretention systems including bioswales (vegetated water agency (PUB), launched in 2006 the Active, and bioretention swales) and rain gardens. Beautiful, Clean Waters (ABC Waters) Programme. 6 ABC Design Guidelines 4th Edition 26 Bioswales convey stormwater at a slow, controlled rate, Bioretention basins or rain gardens are vegetated and the flood-tolerant vegetation and soil act as a filter land depressions designed to detain and treat medium, cleaning runoff and allowing infiltration. stormwater runoff. Their treatment process is the Bioswales generally are installed within or near paved same as bioretention swales; the runoff is filtered areas (e.g. parking lots, roads and sidewalks). In locations through densely planted surface vegetation and with low infiltration rates, underdrains can be used to then percolated through a prescribed filter media collect excess water and discharge the treated runoff to (soil layers). Unlike bioretention swales, they do not another green infrastructure practice or storm sewer convey stormwater runoff. system. The difference between vegetated and bioretention swales is that the latter have a bioretention systems located within the base. Bioretention swale along a road with Rain garden captures stormwater Vegetated swale in carpark at the standard kerbs (with slots) in Faber Hills runoff from adjacent Holland Plain Singapore Botanic Gardens Estate Road. Financial & organizational arrangement. Stormwater Economic cost (design, construction & maintenance). management often entails a municipal-level program of Design features are green infrastructures that mimic infrastructure development and coordination. Revising natural systems. They are cost effective, sustainable, building codes to encourage green infrastructure and environment friendly. Costs vary greatly depending approaches can facilitate their adoption. Their on size, plant material, and site considerations. implementation can benefit from collaboration between Bioswales are generally less expensive when used in communities, designers, climate scientists and place of underground piping. Maintenance costs and governments to ensure sustainability in design, time are higher initially and then taper off once management and maintenance. Government incentives established. The estimated cost of a bioretention area can also encourage landowners to install bioswales on is between $5 and $30 per square foot7. their land. Maintenance: • Maintain good vegetation growth Design & construction. Bioswales should be carefully (remove weeds, prune vegetation, etc.) • Routine designed to integrate with the characteristics of the inspection of vegetation • Inspect inlet and outlet surrounding landscape. Vegetation plays a key role in points for scour and blockage, etc. • Remove litter and maintaining the porosity of the soil media of the debris • Maintenance should be conducted after major bioretention system and also in the taking up of storm event nutrients from the percolating surface runoff. The plants selected must be able to withstand wet and dry Benefits & co-benefits (economic, environmental, conditions. Infiltration-based design features for social): • Reduce stormwater volume and flow velocity bioretention swales should be sited at least 1 m above and increase groundwater recharge • Settles coarse the seasonal high groundwater table. A study showed sediments
• Encourages habitat creation and that underground gravel layers for storage and orifice promotes biodiversity
• Filters and cleanses water outlets significantly improve the runoff control naturally without the use of any chemicals • Ease of effectiveness of rain gardens and bioswales. design • Provide aesthetic appeal 7 Massachusetts Clean Water Toolkit 27 Case Study 2: Renaturing the city with Linear Parks and Pocket Parks in São Paulo Similarities with Ouagadougou • Climate projections indicate a likely increase in the • Nature is restricted to some parks, mainly in the number of days with heavy storms • Public transport peripheral areas • Occupation of risky areas such as affected by flooding events • Rivers and canals receive slopes and banks of watercourses • Lack of urban diffuse pollution from stormwater run-off and solid planning • Water shortages litter • Intense and rapid urbanization over the landscape with the eradication of the original ecosystems Urban transport system São Paulo’s surface transport suffers from slow speeds SPTrans, is arguably the world’s most complex, with and long commute times, due to heavy congestion. The over 14,500 buses, 1300 lines and 500km worth of modal share for public transport has remained constant exclusive bus lanes. Public transport options: over the years, despite growth in passenger numbers. Commuter rail, light rail including metro and monorail, São Paulo’s immense public-transport system, run by bus, bike. Climate hazards affecting the city • São Paulo is vulnerable to urban heat-island effects towards periods of drought • Reservoirs and and recurrent severe floods • Strong storms caused by watercourses encounter severe damage, as they are land-cover change • Climate change could lead to not designed to exclude garbage and other forms of increases in the intensity of rainfall events, potential contamination floods and landslides, as well as an increased tendency A metropolitan train is stuck at a flooded rail track running Heavy rains flooded the city, causing its main river to overflow along the Pinheiros river. Feb. 10, 2020 its banks. Feb. 10, 2020 Solutions implemented to enhance the system’s resilience Context. In Brazil, nature-based solutions (the re- Linear parks offer a wealth of pervious surface that can naturalization of rivers with greenways, rain gardens be used to absorb rainwater and runoff from adjacent and bio-swales, green roofs and walls, urban forests, developed landscapes that currently drain directly to detention and retention naturalized basins, pervious piped collection systems. The city of Sao Paulo pavements, as well as linear parks) are used to municipal green plan proposes 49 linear parks. The concurrently solve multiple challenges. This case study Tiquatira Linear Park (320,000 m²) was the first linear focuses on linear parks and pocket parks. park in the city of São Paulo, built along the river Tiquatira to assist in the preservation and conservation of stream bed, and to provide a safe range of landscaping and greening between the stream and urban roads. The Tietê River Valley Park (currently under construction) will be the largest linear park in the world stretching for roughly 75 km to the source of the Tietê River. 28 Pocket parks are smaller public parks (generally Pocket parks are opportunistic, often sited on whatever occupying less than one acre of land) that represent land is available, and might be constructed to revitalize ideal locations for green infrastructure (vegetated unused or underused land (e.g. decommissioned bioretention cells) that treats and captures stormwater railroad tracks). In Sao Paulo, Araucárias Square is a through bio-filtration and infiltration. pioneer public pocket garden (which includes a rain garden) that collects, filters and infiltrates the run-off of impervious land cover. Linear Park Tiquatira between the stream and the road Araucárias Square: pocket garden and rain garden and Financial & organizational arrangement. The Taubaté Maintenance: • Include all the work necessary to keep and Santa Lúcia linear parks, are being implemented the park’s natural ecosystems functioning and ensure by the municipality with funds from a federal that the area is safe, clean, and operating efficiently to government programme. Forty-three parks are in the serve the needs of its visitors • For ewample, final phase of the preliminary study, financed by the maintaining good vegetation growth (remove weeds, municipal fund for the environment. The Araucárias prune vegetation, etc.), routine inspection of Square was a community planting effort which included vegetation, removal of litter and debris, maintaining several green grassroots movements with basic signage and fencing, etc. support of local city administration. The Tiete Park is being built with the support of the state government, Benefits & co-benefits (economic, environmental, as an environmental offset to the reconstruction of social): • Green space in urban areas has a role in Marginal Tiete, an important highway along the Tiete balancing the water cycle system, reducing heat, River that crosses the urban area. providing habitats for wildlife, and controlling the local climate • Increase biodiversity of flora and fauna • Design & construction. The city developed an index of Contribute to urban climate by reducting air social green areas to identify the regions that had more temperature and urban heat • Improve air quality • or less accessibility to green areas, mapping the Decrease air pollution and carbon sequestration • priority neighborhoods. The mapping indicated where Contribute to social and human wellbeing • Mitigate new linear parks should be planned, designed and the production of GHG emissions implemented. Economic cost (design, construction & maintenance). The costs vary greatly depending on the characteristics of the green space and planned intervention. The spatial system of the urban green space may require conservation, restoration, maintenance, improvements and protection of existing and planned spatial forms. Creating partnerships for urban green spaces offers opportunities for coordination of environmental regeneration programs at potentially low financial costs. Among the different categories of expenditure (maintenance, operating and capital costs), maintenance costs constitute the major part of the total cost, ranging from 75% to 95%8. 8 Tempesta (2015) 29 Case Study 3: China’s Sponge Cities an Integrated Approach: The case of Lingang in Shanghai Similarities with Ouagadougou • Rapid urbanization has had complex impacts on • Increased risk of urban flood disasters • Limited the regional hydrological cycle • Changes in natural green spaces • Water stress • Serious pollution of hydrological processes on natural river basins urban water bodies. Urban transport system Extensive public transport system, largely based on It is the longest metro system in the world with more buses, trolley buses, taxis, and a rapidly expanding than 673 km of lines. metro system. Climate hazards affecting the city • Shanghai is one of the top 20 cities exposed to flood • Threat is posed by tidal waves, storms caused by disasters • The geological and climatic conditions of monsoon winds, as well as fluctuations in the level of Shanghai make it sensitive to flooding risks during the rivers flowing through the city (the Yangtze and heavy rainfall events • Exposed to typhoon induced Huangpu) storm surges People get on the bus at a flooded section of the Xietu Road Typhoon Fitow brought heavy rain to the city, flooding more in Shanghai, July 30, 2009. than 50 roads in October 2013. Solutions implemented to enhance the system’s resilience Context. China is investing nearly US$300 billion Lingang, Shanghai’s “sponge city”, is the largest city through 2020 to create 30 “sponge city” projects in among the 30 pilot cities. Its wide streets are built Beijing, Shanghai, Shenzhen, Wuhan, and other areas. with permeable pavements, allowing water to drain to A sponge city is an integrated approach that involves the soil. Central reservations are used as rain gardens, a broad range of concepts such as multi-scale filled with soil and plants. The manmade Dishui Lake conservation and water system management, multi- helps control the flow of water, and buildings feature function of ecological systems, urban hydrology and green rooftops and water tanks. A total of 36 km of runoff control frameworks, and impacts of roads have been renovated and concrete sidewalks urbanization and human activities on the natural replaced with water-absorbent bricks to reduce water environment. By 2030, China aims to install sponge pooling during heavy rainfall. Instead of going directly city projects in 80 % of urban areas across the to drainage, the bulk of the rainwater is absorbed by country and reuse at least 70 % of rainwater. the soil in grass ditches alongside the roads. Retrofits have been completed in Lingang at 26 residential neighborhoods covering 200 hectares. The city has also implemented pocket parks. 30 Permeable pavements in Lingang help stop buildup of surface Wetland areas help to absorb rainwater in Lingang water during heavy rain. Financial & organizational arrangement. The central Economic cost (design, construction & maintenance). government is providing US$59 to US$88 million per China's central government is providing a significant year to each of its 30 pilot cities for 3 consecutive amount of funding for the pilot cities, but the years as start-up capital to help them devise and subsidies are far from enough to fully fund sponge construct nature-based solutions. This investment is city construction. Estimates vary but suggest that intended to inspire the creation of public-private sponge city construction could require investments of partnerships (PPP). China’s Ministry of Finance 100 to 150 million yuan (US$15 to US$23 million per created a strategy to support the PPP model by km2). The total area under construction in the first 16 soliciting private investment in construction projects pilot cities is more than 450km2.11 and formalizing its procurement process for PPPs. 9 For those cities by which PPP are introduced that Maintenance. There must be planning and legal reach a certain scale, additional subsidies of up to frameworks, and tools in place to implement, 10% of the initial funding are added as a bonus. maintain, and adapt the infrastructure systems to However, this subsidy is far from enough to collect, store, and purify excess rainwater. accomplish the whole project and most funds are Benefits & co-benefits (economic, environmental, expected to be raised by local municipalities. The social). • Absorb and reuse flood water • Improve commitment of funding from local municipalities is flood and sediment control as well as water one of the preconditions to apply for a sponge city purification • Minimize the burden on the city drainage project.10 and water networks reducing water treatment and Design & construction. Unlike traditional cities, where equipment maintenance costs • Create better quality impermeable roadways, buildings and sidewalks of life in the areas • Mitigate the production interfere with the natural water cycle, sponge cities of GHG emissions mimic and support the natural water cycle. A Sponge City is more than just its infrastructure. It is a city that makes urban flood risk management central to its urban planning policies and designs. Constructions and renovations should be carried out in an integrated way. Since 2015, all newly planned urban districts, all types of industrial parks, development zones and living community should be designed and built according to the new standard. 9 Ozment et al (2019) 10 Embassy of the Kingdom of the Netherlands, I&M department (2016) 11 World Resources Institute (2018) 31 Case Study 4: Nairobi’s green and hybrid infrastructures and participative risk management approach Similarities with Ouagadougou • Urban sprawl and rapid urbanization have affected • Poor solid waste and wastewater management • the hydrological cycle and reduced the nature in the Informal settlements city • Lack of drainage infrastructures and blockage of existing drains due to solid waste Urban transport system Large choices of urban transport system (private) as well as taxis, taxi moto, cycle taxi and tuktuk based on buses, Matatus (minibus), Commuters train Climate hazards affecting the city • Intensive rainfalls thereby to flood • “flash” floods • Nairobi has experienced several major floods and and extensive flooding for days. • Exposed to the drought pockets during the last years, which led to phenomenon of El Nino, which affected the intensity of widespread devastation, as well as destruction to life rainfalls. Climate change could reinforce the intensity and property of rainfall periods and landslides in informal settlements Nairobi CBD after the severe rainfalls, October, 2018. Flooding along the railway in the slum of Kibera, Nairobi, 2015. Solutions implemented to enhance the system’s resilience Context. Nairobi County Council in conjunction with Green Infrastructures are hybrid public spaces along the national government has decided to invest in the Nairobi River such as the restored Michuki structural and non-structural measures to tackle Memorial Park. The park now provides locals with flooding risks. The government has decided to expand pavement and plants for water retention and its strategy of green infrastructures to the entire city, infiltration, recreational space, while also improving to use technology to alert the inhabitants, and to the microclimate and habitat quality. Other projects design and maintain resilient solutions using such as Green corridors along Ngong Road or urban participative methods. forests are projected. Green infrastructures are implemented differently in slums: in Kibera, one of the world’s largest informal settlements, they implement sustainable drainage techniques (planted revetments, bamboo plantation for erosion control, structured detention and infiltration, using soft drink crates instead of expensive “stormblocs”, and rainwater harvesting). 32 The use of the technology: The government has - Promotion of and Participation in regular enhanced the use of telecommunication and social clean-ups, thereby freeing the drains of network to alert and share the information of rainfalls. garbage The Trilogy Emergency Relief Application SMS platform - The Participatory flood modelling, e.g. in works across the country and helps communities to be Kibera slum in Nairobi: in 2015 a program better prepare for potential floods (advice, safety named ‘Building Integrating Community rules). Perspectives’ was undertaken, to involve residents in flood risk management, get Pedagogy and design of green solutions through a information during workshops and support bottom up approach: Various participative actions the design of the flood model, but also have been held by the City Council, the Kenya Red involve and train institutional stakeholders. Cross Society, and international stakeholders: This collaboration between the inhabitants, experts and institutional actors allowed the - Pedagogy by raising awareness among the prioritization of needs in the slum. inhabitants Michuki Memorial Park, 2020 At Andolo in Kibera, green public space combined hard soft and social solutions, in 2017 Financial & organizational arrangement. The national Maintenance: Different actors are mobilized, government and the Nairobi city council, experts of depending on the situation. Social initiatives are Kenya Red Cross Society, Kenya’s Meteorological suggested: inhabitants and unemployed inhabitants department, National Disaster Operational Centre can contribute to the cleaning and maintenance at the work together. There is not a city organization. Other request of the Nairobi City Council. In Kibera, the stakeholders get involved for local study, NGOs, community actively contributes to the maintenance. International institutions and academics. Nairobi takes The government and the Nairobi City Council allocated part into international programs such as Resilient a budget to clean drains and canals. Cities Network. Benefits & co-benefits (economic, environmental, Design & construction. The green infrastructures are social). • Improve the water cycle system and the air built in critical areas of the city that could be affected quality • Support the city drainage system • Participate by hazards (settled along the rivers). They are in solid waste management • Green spaces in dense composed of endogenous plants and infiltrating urban areas • Inclusive projects that raise awareness pavements. In the slum, the green infrastructures are among the population • Mitigate the material hybrid. The solutions combine hard (e.g. drains), and destruction and the production of GHG emissions soft and social aspects. They use local materials and distinct techniques. The flooding problems were Economic cost (design, construction & maintenance). resolved thanks to wire mesh gabions filled with rocks In 2018/2019, the government allocated 60.4 billion collected by residents, porous drainage channels Kenyan shillings (around US$607 million as of March made from recycled perforated plastic pipes, 2019), 2.4% of the national budget, to environmental bioswales that allow water to drain naturally, and by protection, water harvesting and flood control. For the reclaiming and refilling land by the river bank. They Michuki Memorial Park, the rehabilitation cost 30 built a pavilion, used as offices for a local finance million Kenyan shillings (US$278,000). program, craft co-operative and compost businesses. Its roof captures rainwater used on the garden dedicated to the production of vegetables. 33 Case Study 5: Japan’s disaster preparedness and business continuity approach Similarities with Ouagadougou • Public transport affected by flooding events • variability • Densely-populated cities Projected increase in the intensity of weather Urban transport system • Japan has a modern and efficient public • Due to land constraints and congestion, subways transportation network, especially within are usually the fastest and most convenient transport metropolitan areas and between the large cities • The system in major cities urban transport systems include buses, trains, subways, trams, taxis, ride-sharing apps, etc. Climate hazards affecting the cities • Heavy rains, high wind and typhoons leading to widespread devastation, service disruptions, as well floods, overflowing rivers and landslides • Large - as damage to property and loss of lives • Flood scale water-related disasters caused by typhoons are vulnerability is increasing due the increased becoming more frequent and severe • Japanese cities concentration of population and socio-economic (e.g. Toyooka in October 2004 and Hiroshima in July functions, as well as the advanced use of 2018) experienced major floods that resulted in underground space. A parking flooded in Toyooka (2004) Flooded road in Hiroshima (2018) Solutions implemented to enhance the system’s resilience Context. Japanese cities (such as Toyooka) that were Timelines: The timeline of Toyooka city was severely damaged by typhoons decided not only to developed to strengthen crisis management implement structural resilience measures (e.g. capabilities in the event of a large-scale flood disaster excavating river channels and strengthening levees), to help prevent and mitigate disasters. A total of 17 but also to implement soft measures. The soft organizations, including Toyooka City, Hyogo measures implemented include the preparation of Prefecture, Kobe District Meteorological Observatory, pre-disaster action plans (timelines), and more Hyogo Prefectural Police, local railroad and bus generally measures towards the creation of “flood operators, telephone companies, and electric power prevention and disaster awareness society". companies, participated in the meeting to discuss "what will be done, when, and by whom" in case of a large-scale disaster, and to formulate a timeline to clarify each entity and person's actions12. 12 http://www.bousai.go.jp/kohou/kouhoubousai/h29/87/special_01.html 34 This timeline was developed with the assumption that When the cumulative rainfall plus the projected typhoons and rainfall can be predicted using cumulative rainfall for the next 6 hours exceeds a monitoring tools. The latter combine information planned threshold, the disaster prevention and about the accumulated rainfall, the forecasted mitigation actions corresponding to this threshold is accumulated rainfall for the next six hours and river triggered. levels (the main river in Toyooka is the Maruyama River). The timeline includes three scenarios In the transportation sector, a system was put in place composed of different stages defined according to to ensure that station staff and crew are informed and the amount of rainfall (current and projected rainfall), that standards are set for suspension and post- taking into consideration the characteristics of the hazard recovery of service. Bus companies also Maruyama River, and the probable location, level and prepared relocation plans and parking spaces to evolution of the water. evacuate their vehicles to areas with higher elevations, which allowed the vehicles to escape the damage and resume operation at an early stage. Buses evacuating from a flooded area Emergency Sandbag Storage Area on Itabashi Ward Station (Tokyo, 2017)14 (Nagano, 2019)13 Flood prevention and disaster awareness society: The Design & construction: There must be planning and Japanese authorities seek to rebuild a "flood legal frameworks, and tools in place to implement, prevention and disaster awareness society" in which maintain, and adapt soft measures. In general, the the entire society is always prepared for floods by developments of local emergency plans and timelines sharing goals for disaster mitigation and promoting involves a wide range of stakeholders from the public structural and soft measures in an integrated and sector (the national government, local governments, systematic system that involves neighboring meteorological office, police and fire department) but municipalities, prefectures, the national government, also the private sector (bus, telecommunication and etc., in preparation for large-scale flood damage. electricity companies) as well as the inhabitants of the Toyooka City not only distributed flood hazard maps areas prone to flooding. to all households, but also conducted proactive educational programs such as visiting lectures to Maintenance: The plan should also be regularly directly reach out to residents. To complement the reviewed and tested for continuous improvements. conventional disaster prevention radio system, the Benefits & co-benefits (economic, environmental, city has established a system to send faxes social): Raise awareness among inhabitants • Soft simultaneously to community leaders and people measures are cheaper and faster to implement than with hearing disabilities. structural measures • Complement structural measures. 13 https://www.mlit.go.jp/unyuanzen/content/torikumi_jirei177.pdf 14 https://www.mlit.go.jp/river/bousai/shinsuihigai/pdf/171225_zentai_lo.pdf 35 3.3. Prioritizing measures for All these solutions appear in the list of proposed Ouagadougou through multi-criteria measures except the dyke that would not be effective analysis in protecting Nongremasson street that is perpendicular to the canal. The results of the analysis of the flood-exposed sections were used to propose appropriate measures (i.e. Another example is 28.257 St. on Line 4, which is application of one or several solutions to a flood-prone flooded due to runoff from the perpendicular streets in area). To this end, the project team associated each the south-east that acts as canals and convey water to section with an appropriate solution based on the type the north-west. This area is located at the top end of of flood in the section and availability of land in the the watershed, with relatively high slopes, and the area. For example, the flood in Nongremasson Street on perpendicular streets lack proper drainage. Hence, the Line 1 is due to an overflow of the canal Central, solutions that can be considered are: causing floods in the adjacent areas and perpendicular streets, such as Nongremasson St. In this case, several ▪ Solutions aiming at improving the drainage solutions can be considered: capacity of the perpendicular streets: build a drainage system; ▪ Solutions aiming at increasing the canal's capacity ▪ Solutions aiming at improving the water retention (next to the bridge and downstream): clearing capacity of the area: permeable pavements, porous obstructions, refurbishment, removing silt, asphalt, and rain gardens. improving the outlet; ▪ Solutions aiming at reducing the peak flows in the Both types of solutions were included in the list of canal (targeting the upstream watershed): retention proposed measures. In particular, it is proposed to basin, local storage, parks, swales. implement a rain garden in the open space close to ▪ Solutions aiming at preventing overflows from the Saint Paul chapel. A set of soft and city-wide measures canal: dyke was also included to address the residual flood risk ▪ Elevating the road profile (which implies replacing related to rare and more extreme flood events. the bridge) ▪ A temporary diversion through Ave Kiendrebeogho To compare and rank the measures, the team developed Moryamba a multicriteria-analysis methodology. To this end the following criteria and weights were selected: Maintenance Environmental and socio- Investment cost Flood reduction benefits economic benefits (co- cost (weight: 30) (weight: 30) benefits) (weight: 10) (weight: 30) COSTS BENEFITS Investments costs were marked using a continuous rain gardens. All these solutions require proper routine rating scale designed to allow a good dispersion of and periodic maintenance to maintain their initial effect marks between 0 and 10. For investment costs, unit in the long term. In a few particular cases, it was costs and estimated quantities were used whenever possible to use unit costs to estimate maintenance possible. The cost estimates included in the present costs, but in most cases, other methods had to be report are indicative and a detailed design of the employed. solutions would be required to reduce the uncertainty around those costs. The use of the cost classification in One specific measure (measure 21) proposes to create more qualitative terms (from very low to very high) a team dedicated to the standard maintenance of the reduces the impact of the uncertainty around the costs structures contributing to flood protection of the whole on the results of the multicriteria analysis. bus system. This standard maintenance would consist of cleaning of the structures after rain events and before Maintenance costs were marked using a continuous the beginning of the rainy season. This would be the rating scale designed to allow a good dispersion of best way to ensure that the proposed structures are marks between 0 and 10. Many of the proposed kept in good working conditions. The long-term cost of measures include the construction of new hydraulic this team was estimated by assessing the required staff structures, whether channels, culverts, canals or composition and considering local labor costs. alternative solutions such as swales, porous pavement, 36 Since all proposed measure need to be evaluated ▪ Measures targeting the entire AOI were separately, maintenance costs still needed to be assigned the maximum score of 5. assigned to each measure, including the construction of new structures, although routine maintenance of these The effectiveness of the proposed measure in reducing structures would actually be covered by the the flood impacts on the transportation system was maintenance team proposed in measure 21. assessed using experts’ judgment. In a few occurrences, proposed measures have already been studied in Maintenance operations would typically be divided in previous works, mostly the Ouagadougou Drainage routine maintenance (simple and cheap tasks to be Master Plan, in which case the measures have been performed every year) and periodic maintenance designed in detail to ensure protection against a (heavier work, to be planned every 7 to 10 years). In specific return period storm. In most other cases, the order to assess the maintenance costs applicable to the proposed measures have been pre-dimensioned based proposed structures, the following ratios were used, on experts’ judgement, and more detailed design including for both routine and periodic maintenance: including hydraulic modelling would be necessary to determine the exact required dimensions. The maximum ▪ Maintenance of road surface: 1% of the score of 5 represents a measure allowing guaranteed investment per year full protection against a design storm. Solutions that do ▪ Maintenance of hydraulic structures (channels, not fully prevent flooding of the section but decrease canals): 0.5% of the investment per year the recovery time or reduce the flood height received ▪ Maintenance of green structures (bio swales, lower scores. The minimal score of 1 corresponds to rain gardens, etc.): 2% of the investment per measures that have no impact on flood reduction, but year help minimize the consequences of the floods (diversion scheme to a non-flooded itinerary, for example). These ratios illustrate the fact that a road surface is subject to more tear than hydraulic channels, and that The choice of a relevant formula to combine the two green surfaces require more frequent maintenance than marks into an overall flood impact reduction benefit concrete structures. score required several tests. The flood impact reduction benefits were assessed In the formulas that were considered, the following using two criteria: notation was applied: ▪ The impact of the current flood issues on the I for the Impact of current flood issues, scored from planned transportation system (marked from 0 0 to 5 to 5). E for the effectiveness of the proposed measure to ▪ The effectiveness of the proposed measure to reduce the impacts, scored from 0 to 5 reduce these flood impacts on the transportation system (marked from 0 to 5). FIRB for the Flood Impact Reduction Benefit, marked from 0 to 10 The overall flood impact reduction benefit mark on a scale from 0 to 10 is calculated by combining the two C for the conversion factor, a number allowing to above sub-marks. The rationale behind this evaluation adjust for each calculation method the range of the method is that technical efficiency should not be the results to the 0-10 scale. only parameter considered when evaluating the flood reduction benefits of the proposed measure. A measure The tested formulas were as follows: providing full protection to an area that has a very low importance in the transportation system should not get Simple addition: the maximum grade. On the contrary, a solution that provides limited protection may prove valuable if it allows flood impact reduction on critical sections of the transportation system. This formula has the advantage of being straightforward, and provides good dispersion and The impact of the current flood issues is quantified as reasonable average of the scores. However, this formula follows: had a major inconvenience: a measure that has no impact on flood reduction would still get a mark of 5/10 ▪ For measures targeting a single area, the score if it targeted the most critical areas. Conversely, a is taken from the Table 4.1. in Annex 4 where measure with good hydraulic efficiency would also get this score was computed from 0 to 4. 5/10 even if it targeted an area with no current flooding ▪ Measures targeting several areas were issues. Ideally, these two extreme and theoretical assigned the highest impact score given to situations should lead to a score of 0/10. either of these areas (still from 0 to 4). 37 Simple multiplication: negative impacts related to the resulting extra cost of travel time, fuel consumption and CO2 emissions. Measure 3 (elevation of the road profile of Nongremasson street north and south of Canal Central This formula fixed the above-mentioned issue: by using — future Line 1) would make pedestrian traffic between multiplication instead of a sum a measure with a 0 mark the road and the surroundings difficult. Measure 26 at one of the two sub-criteria would get a 0 score in (enforcement of regulations aiming at limiting water flood impact reduction benefits. However, this came run-off generated by new constructions) that concerns with a serious drawback: the resulting scores were very the whole AOI would increase the cost of construction low, on average, and the dispersion limited (most projects in the city. scores were under 5/10). Thus, the flood impact reduction criteria lost much of its weight in the Implementing a measure that is already recommended evaluation. in the Ouagadougou Drainage Master plan has been considered to have a significant socio-economic benefit, Weighted multiplication: since it would be consistent with the investment plan already targeted by the municipality. 3.4. Results of the multicriteria analysis This formula is a generalization of the previous formula, The multi-criteria analysis ranked 26 different measures, allowing to apply different weighing on the two sub- including gray, green and soft measures that are criteria of flood impact and effectiveness. By changing applied to specific flood-prone areas or to the entire the values of the coefficients, a and b, it was possible layout of the planned transport system (see Table 3). to adjust the dispersion and average of the resulting The map in Figure 18 shows the location of the scores, as well as the relative importance of the two measures in the AOI. sub-criteria. Certain solutions included in the initial long list were After several tests, we selected the weighted not directly included in the list of measures as they multiplication formula, with values of a=0.5 and b= 0.3 appeared ineffective at protecting the planned mass (resulting in C=0.36). This formula gave satisfying transit system or difficult to implement. For example, results: good dispersion of scores, average around the construction of green roofs in areas that are already 5/10, and slight preponderance given to the flood constructed is difficult as it requires complex impact sub-criteria. interactions between the city council and homeowners. This solution is more suitable for new developments. Environmental and socio-economic benefits were However, even if this solution has not been used on the assessed for each measure, and a score from 0 to 10 list of proposed measures, one should note that the was given with the following scale: 0 for very negative enforcement of regulations aiming at limiting water run- impacts, 5 for no impact, 10 for very positive impacts. off generated by new construction projects (measure Since the proposed measure have been designed to 22) would in practice strongly incite developers to take advantage of available space (right of way of the implement such solutions. road, existing canals, public spaces), they do not involve major negative impacts such as demolition of buildings Some of the proposed measures are described in more requiring resettlement of population. Four of the detail in Table 5.3. in Annex 5. proposed measures have negative impacts. Measure 6 (temporary diversion of Line 1 through Avenue de la The multicriteria analysis was used to compare the Liberté and Avenue du Barrage if Nongremasson St. is performance of the different types of measures (green, blocked) and measure 10 (temporary diversion of Line gray, soft). To this end, the 26 measures were classified 5 through the itinerary of future Line 6) have minor into groups of measures applying similar solutions. 38 Table 3: Proposed flood-resilience and adaptation measures Gray measures Green measures Soft measures No. Description No. Description No. Description Rehabilitation of upstream part of the Install a bioswale in the median of the road on Put in place a temporary diversion through Ave de la Liberté and Ave du Barrage if 1 Canal central and development of its 2 Ave Kwame Nkrumah and Ave de l'UMOA (that 4 Nongremasson St. is blocked downstream part are large and have a descending slope) Elevate the road profile of Rain gardens in the open space surrounding the Nongremasson street north and house lots between 14.09 St. and Ave Put in place a temporary diversion of Line 5 through the itinerary of Line 6, which uses an 3 9 10 south of the Canal Central (implies Babanguida (the design should include parking elevated road (Ave Bassawarga) if Ave des Arts is blocked replacement of the existing bridge) spaces for the inhabitants) Build a drainage system to collect run-off water for the neighborhood Water retention solution for the neighborhood Set up a dedicated maintenance plan and team in charge of the periodic and systematic cleaning 5 south-east of 28.257 St. and channel 12 east of Avenue Oumarou Kanazoe: Permeable 21 and maintenance of flood related structures (canals, culverts, rain gardens, pockets parks, etc.) them until the flood control basins of pavements/porous asphalt for the unpaved roads dedicated to the BRT (in particular before the rainy season). Wemtenga canal Water retention solution for the neighborhood Enforce regulations aiming at limiting water run-off generated by new constructions. A simple east of Avenue Oumarou Kanazoe: pocket park way is to impose a limit on the flow a newly developed area can discharge to the drainage Build a drainage system focusing on 6 13 in Mogho Naba Palace and rain garden or 22 network. This engages developers to integrate stormwater management in their design and protection of the 28.257 St. floodable sports field in the 8 Mars garden and implement solutions to reduce flows sent to the network (e.g. green roofs, permeable pavement Lycée Bambata over parking lots, bioswales etc.). Drainage system to protect 28.257 street, combined with water retention solutions for the 7 neighborhood to the south-east: permeable pavements and rain garden in the area in front of Chapelle St Paul Build a drainage system to protect Develop a risk awareness culture in the bus operating company to manage the residual flood 14.09 St. from transverse flows and Build a rain garden on the roundabout Nations risk: Develop a: pre-disaster action plan; a business continuity plan, identifying the sections of 8 ensure longitudinal drainage of Ave 17 23 Unies the lines that should be maintained in case of extreme storms; a communication plan to inform des Arts North of Canal de users based on mobile technologies (SMS) l'université Install stormwater retention trees (street trees Develop a Flood monitoring and disaster prevention system for the city: Pre-disaster action plan combined with an underground structure and (timeline) that aims to prevent damage and allow public transport to resume operation at an Build a drainage system to protect above ground plantings which collect and treat early stage. The actions should be triggered depending on the information provided by a flood 11 Ave Oumarou Kanazoe and Place du 19 stormwater using bioretention) along the roads 24 monitoring tool (considering current and predicted rainfall and water level in the dams) — the rail roundabout of the future transit system that will be paved or system should be managed by the city authorities and the relevant information passed on to the improved to allow the bus to pass (e.g. 28.257 bus operating company St. on future Line 4, which is unpaved) Plant trees along the roads of the future transit Reinforce the solid waste collection system: organize awareness-raising activities, and most Build a longitudinal drainage system system that will be paved or improved to allow 14 20 25 importantly provide an efficient collection system, concrete baskets at the bus stops, garbage on Simon Compaoré street the bus to pass (e.g. 28.257 St. on Line 4, collection points at strategic locations collecting trucks, landfill / incineration sites which is unpaved) Cleaning / rehabilitation of the Moogho Naaba canal downstream of 15 Avenue Ouezzin Coulibaly, as recommended in the Drainage Master Plan Build a new drainage network to 18 protect Ave Houari Boumedienne and Ave de la Grande Mosquée Cleaning of the reservoir lakes north of the area study and modification of the spillway to lower 26 the water level in the lakes and improve outlet conditions of the canals 39 Figure 18: Location of the proposed flood-resilience and adaptation measures Soft measures are associated with the lowest The flood impact reduction benefits of the green and investments and maintenance costs. Gray measures are mixed solutions vary greatly, and it is difficult to identify associated with medium upfront costs and low specific trends. In contrast, soft solutions tend to be maintenance costs. In contrast, the initial costs of green associated with medium to very high benefits as they measures greatly vary (from very high for permeable generally apply to the whole AOI. The flood impact pavements/porous asphalt to relatively low, on average, reduction benefits of the gray solutions are generally for street- and stormwater-retention trees). The average medium to high, as these measures are effective in maintenance costs associated with green measures are reducing flood impacts but apply to limited sections. slightly lower than those of gray and soft measures. Finally, as expected, green and mixed solutions are Finally, the highest upfront and maintenance costs are associated with the highest environmental and socio- associated with the measures that mix green and gray economic benefits (co-benefits). elements. 40 Table 4: Scoring the proposed measures according to the four criteria Mean [min, max0 scores, by criteria (/10) Measures Type of Groups of similar measures no. Flood Environmental measure Investments Maintenance impact & socio- Total score costs costs reduction economic (/100) benefits benefits Dedicated maintenance team 8.1 [7.6, and reinforcement of solid 21; 25 Soft 8.5 [8, 9] 8.5 [8, 9] 7.5 [7, 8] 81 [80, 82] 8.6] waste collection system Enforce regulations aiming at limiting water run-off 22 Soft 10 9 7.6 3 71 generated by new constructions. Risk awareness culture in the bus and City flood 6.2 [6.2, 23; 24 Soft 9 [9, 9] 7 [4, 9] 6 [6, 6] 70 [68, 73] monitoring and disaster 6.2] prevention system Bioswale 2 Green 6 8 7.7 7 70 Temporary diversion of bus 4.3 [3.9, 4 ;10 Soft 10 [10, 10] 10 [10, 10] 3.5 [3, 4] 64 [61, 66] lines 4.8] Cleaning of the reservoir lakes and modification of the 26 Mix of spillway to lower water level gray and 2 0 9.4 10 64 in the lakes and improve green outlet conditions of canals Street trees and stormwater 2.2 [2.0, 19; 20 Green 6.5 [5, 8] 8.5 [8, 9] 9 [9, 9] 62 [57, 66] retention trees 2.4] Rehabilitation and 5.9 [3.4, 1; 15 Gray 5 [3, 7] 8.5 [7,9] 7 [7, 7] 62 [59, 64] development of the canals 8.4] 5; 6; 8; Gray 4.6 [3.0, Build a drainage system 11; 14; 5.3 [4,8] 9 [9, 10] 5.5 [5, 7] 56 [48, 71] 7.2] 18 Permeable pavements/ 12 Green 1 8 6.6 8 55 porous asphalt Rain gardens and pocket 9; 13; Green 4.3 [1, 9] 8 [6, 10] 5 [3.4, 6.8] 6 [6, 6] 54 [37, 75] parks 17 Elevate the road profile of Nongremasson street north and south of the Canal 3 Gray 5 10 6.3 1 47 Central (implies replacement of the existing bridge) Drainage system combined Mix of with green water retention 7 gray and 0 7 3.2 10 46 solutions green Highest ranked individual measures ▪ Measure 25: Reinforce the solid waste collection system: organize awareness-raising activities, and The multicriteria analysis allowed the identification of most importantly provide an efficient collection the top ten measures that should be prioritized (based system, concrete baskets at the bus stops, garbage on the total score): collection points at strategic locations collecting trucks, landfill / incineration sites ▪ Measure 21: Set up a dedicated maintenance plan and team in charge of the periodic and systematic ▪ Measure 17: Build a rain garden on the Nations cleaning and maintenance of flood related Unies roundabout structures (canals, culverts, rain gardens, pockets ▪ Measure 24: Develop a flood monitoring and parks, etc.) dedicated to the mass transit system (in disaster prevention system for the city: Pre-disaster particular before the rainy season). action plan (timeline) that aims to prevent damage 41 and allow public transport to resume operation at awareness culture in the bus management company to an early stage. The actions should be triggered manage the residual flood risk (measure 23), and the depending on the information provided by a flood implementation of a temporary diversion through Ave monitoring tool (considering current and predicted de la Liberté and Ave du Barrage if Nongremasson rainfall and water level in the dams) — the system street is blocked (measure 4). should be managed by the city authorities and the relevant information passed on to the bus The ten highest ranked measures include three green management company measures (17, 2 and 20). Measures 2 and 17 score relatively high on all criteria. Measure 20 (planting trees ▪ Measure 22: Enforce regulations aiming at limiting along the roads of the future transit system that will be water run-off generated by new constructions. A paved or upgraded) presents very high environmental simple way is to impose a limit on the flow a newly and socio-economic benefits as well as very low developed area can discharge to the drainage investments and maintenance costs, which compensate network (e.g., in France it is 3l/s/ha max for the 10- for its modest ability to reduce flooding impacts. year return period rain). This engages developers to integrate stormwater management in their Measure 11 (build a drainage system to protect Ave design and implement solutions to reduce flows Oumarou Kanazoe and Place du Rail roundabout) is the sent to the network (e.g., green roofs, permeable only gray measure ranked in the top ten. This measure pavement over parking lots, bioswales, etc.). scores relatively high on most criteria except in terms ▪ Measure 11: Build a drainage system to protect of co-benefits. Ave Oumarou Kanazoe and Place du rail roundabout Further, the measures ranked 11th and 12th are a mix of gray and mixed structural measures. For example, the ▪ Measure 2: Install a bioswale in the median of the rehabilitation of upstream part of Canal Central and road on Ave Kwame Nkrumah and Ave de l'UMOA development of its downstream part (measure 1) and (that are large and have a descending slope) cleaning of the reservoir lakes north of the area study ▪ Measure 23: Develop a risk awareness culture in and modification of the spillway to lower the water level the bus management company to manage the in the lakes and improve outlet conditions of the canals residual flood risk: develop a pre-disaster action (measure 26) present very high flood impact reduction plan; develop a business continuity plan, benefits because they are both effective and would be identifying the sections of the lines that should be applied to critical areas of the planned future mass maintained in case of extreme storms; and develop transit system. However, the associated upfront costs a communication plan to inform users based on are very high. mobile technologies (SMS) ▪ Measure 4: Put in place a temporary diversion 3.5. Sensitivity analysis through Ave de la Liberté and Ave du Barrage if Nongremasson St. is blocked To evaluate how the choice of weights impacts the rank ▪ Measure 20: Plant trees along the roads of the of the measures, we performed a sensitivity analysis by future transit system that will be paved or improved considering three scenarios, each representing a to allow the bus to pass (e.g., 28.257 St. on future different set of priorities for the decision makers. These Line 4, which is unpaved) different sets of priorities are represented by different sets of weighing of the criteria used in the analysis. The The top two measures (measures 21 and 25) are proposed scenarios are defined by the following solutions related to the maintenance and cleaning of weighing: the flood-related structures and the reinforcement of Scenario 1 is the base scenario, with a balanced the waste collection system. These solutions are highly distribution of priorities between costs, flood impact ranked because they present a very high score for all reduction benefits and environmental and socio- criteria. In other words, they would effectively economic co-benefits. contribute to the flood resilience of the entire future mass transit system, are associated with high long-term Scenario 2 represents a situation where the decision- environmental and socio-economic benefits, and makers consider the reduction of flood impacts as the require limited initial and maintenance investments. absolute priority and the other criteria as secondary. Scenario 3 puts higher priority on the maintenance cost In addition to these, the highest ranked measures and a relatively high priority on co-benefits. This could include four other soft measures: the development of a represent the preoccupations of the Ouagadougou flood monitoring and disaster prevention system for the Municipality, who could more easily obtain external city (measure 24), the enforcement of regulations funding to pay for the investment but will be in charge aiming at limiting water runoff generated by new of the maintenance of the infrastructure and has to constructions (measure 22), the development of a risk consider the acceptability of the measures. 42 Table 5: Weighing of the criteria used in the three scenarios for multicriteria analysis Weighing criteria scenarios Criteria Scenario 1 Scenario 2 Scenario 3 Investment costs 30 10 10 Maintenance costs 10 10 30 Flood impact reduction benefits 30 60 30 Environmental & socio-economic benefits 30 20 30 Total 100 100 100 The results of the sensitivity analysis are shown in Table structures protecting the bus routes) and 25 6, which lists the ten highest ranked measures (reinforcement of the solid waste collection system) are depending on the scenario considered. The top-10 ranked first or second in all 3 scenarios due to their rankings resulting from the three scenarios are close, as high marks in all criteria. they tend to mostly include the same measures although in different order. For example, the first 7 The three measures that were not included in the top measures of the base scenario also appear among the 10 of the base scenario but are included in the top 10 10-highest ranked measures of scenarios 2 and 3. of scenario 2 are measures 1, 26 and 12. These measures clearly see their ranking improve because of Measures 21 (setting up of a dedicated maintenance the lower priority on investment cost (they each cost team in charge of the cleaning of flood related more than US$3 million). Table 6: List of the 10 highest ranked measures depending on the multicriteria analysis Scenario 1 Scenario 2 Scenario 3 Investment cost Investment cost Investment cost Rank Measure Nr. Measure Nr. Measure Nr. (‘000 US$) (‘000 US$) (‘000 US$) 1 21 80 21 80 25 450 2 25 450 25 450 21 80 3 17 84 26 3,300 17 84 4 24 150 1 3,100 1 3,100 5 22 0 2 1,185 11 490 6 11 490 17 84 2 1,185 7 2 1,185 11 490 24 150 8 23 80 22 0 12 4,600 9 4 40 24 150 22 0 10 20 474 12 4,600 8 980 All in all, seven measures rank in the top 10 in all three stops, garbage collection points at strategic scenarios and should therefore be prioritized: locations collecting trucks, landfill / incineration sites 21: Set up a dedicated maintenance plan and team in charge of the periodic and systematic 17: Build a rain garden on the Nations Unies cleaning and maintenance of flood related roundabout structures (canals, culverts, rain gardens, pockets parks, etc.) dedicated to the mass 24: Develop a flood monitoring and disaster transit system, in particular before the rainy prevention system for the city: Pre-disaster season action plan (timeline) that aims to prevent damage and allow public transport to resume 25: Reinforce the solid waste collection operation at an early stage. The actions should system: organize awareness-raising activities, be triggered depending on the information and most importantly provide an efficient provided by a flood monitoring tool collection system, concrete baskets at the bus (considering current and predicted rainfall and 43 water level in the dams) — the system should 11: Build a drainage system to protect Ave be managed by the city authorities and the Oumarou Kanazoe and Place du rail relevant information passed on to the bus roundabout operating company 2: Install a bioswale in the median of the road 22: Enforce regulations aiming at limiting on Ave Kwame Nkrumah and Ave de l'UMOA water run-off generated by new constructions. (that are large and have a descending slope) A simple way is to impose a limit on the flow a newly developed area can discharge to the Table 7 shows a comparison of the cumulative costs of drainage network (e.g. in France it is 3l/s/ha the highest-ranked measures, depending on the max for the 10-year return period rain). This preferred scenario. Scenario 1 privileges measures with engages developers to integrate stormwater lower investment costs compared to scenarios 2 and 3. management in their design and implement As one could expect from its focus on solving flood solutions to reduce flows sent to the network issues, Scenario 2 favors more expensive measures. (e.g. green roofs, permeable pavement over Scenario 3 is only more expensive than Scenario 2 if the parking lots, bioswales etc.). 15 highest ranked measures are implemented. This is due to measure 16 (construction of a new flood retention basin for the Mogho Naaba canal), whose cost is estimated at US$ 9 million. Table 7: Cumulated investment cost of the highest-ranked measures Investment cost of the X highest-ranking measures (‘000 US$) Scenario 1 Scenario 2 Scenario 3 X=3 614 3,830 614 X=5 764 8,115 4,204 X = 10 3,033 13,439 11,119 X = 15 10,968 18,039 24,133 X = 26 49,080 44 REFERENCES A Better City (ABC). (2015). Enhancing Resilience in Boston, l’Association Internationale de Climatologie, Liège A Guide for Large Buildings and Institutions. http://www.climato.be/aic/colloques/actes/ACTES_ February. AIC2015/5%20Variabilites%20et%20aleas%20cli matiques/080-HANGNON-497-502.pdf AFD. (2019). Ouagadougou 2050: Embracing the Everyday at the Scale of the Greater Territory. Topic Hanson, S. et al. (2011). A global ranking of port cities with document, International Urban Planning Workshop, high exposure to climate extremes. Climate Change 1–15 March. 104:89–111. AGEIM. (2020). Etude d’actualisation du schéma directeur de Hingray B., Bouvier C., Cappelaere B., Desbordes M. 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Modélisation des flux Wemtenga, ville de Ouagadougou : campagnes inondants dans les voiries d’un secteur urbain de hydrologiques des années 1992 et 1993 : Ouagadougou (BF). NOVATECH 2001 – 25-27 juin chroniques des pluies, des intensités 2001 – Lyon(France), 1011-1014. pluviométriques, des cotes et des débits. ORSTOM, Browder, G., S. Ozment, I. Rehberger Bescos, T. Gartner, and 123 p. multigr. G.-M. Lange. (2019). Integrating Green and Gray: http://horizon.documentation.ird.fr/exl- Creating the Next Generation Infrastructure. doc/pleins_textes/divers16-09/41721.pdf Washington, DC: World Bank and World Resources Malaviya, P. et el. (2019). Rain Gardens as Storm Water Institute. Management Tool. Estimated cost of rain garden Charles River Watershed Association. (2008). Low Impact in Greece to be “less than 50E per m2”. Best Management Practice (BMP) Information Orange. (2019). Le SMS, nouveau Commerciale votre Sheet: Constructed Stormwater Wetlands, entreprise en 2019. 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Comparison of Permeable Pavement Types: Hydrology, Design, Installation, Maintenance and Cost, Prepared by CTC & Associates LLC WisDOT Research & Library Unit, January 13. World Bank. (2010). Evaluation conjointe du 19 Octobre au 6 Novembre 2009. Rapport provisoire Restitué le 14 avril 2010. http://documents1.worldbank.org/curated/en/2 79691468228286575/pdf/568030v10FRENC 1ept0090Rapport0Final.pdf World Bank. (2009). Inondations du 1e Septembre 2009 au Burkina Faso, Evaluation des dommages, pertes et besoins de construction, de reconstruction et de relèvement. Annexe 1: Rapport Sectoriel Logement. World Resources Institute. (2018). Sponge cities promise a wide range of benefits by Lauren Sidner - November 22, 2018. 46 ANNEX 1: DRONE-BASED IMAGERY COLLECTION TO CONSTRUCT A DIGITAL ELEVATION MODEL Flight authorizations: The locally-based drone company, Espace Geomatique, organized a meeting with the HYDROMET Project Coordinator to ask for the support to obtain the flight authorizations. He then informed the National Civil Aviation Agency (ANAC) which is the ultimate authority to give the final authorization after authorizations are received from the Ministries of Defense and Security. Espace Geomatique sent flight authorization requests on June 10, 2020, to the Ministries of Defense and Security. From then on, Espace Geomatique was contacted several times, mostly due to the fact that drone regulations in Burkina Faso are very new. Thus, there is not yet a specified and organized office within the Ministries of Defense and Security that is in charge of giving the requested authorizations. Espace Geomatique received the official authorizations from the Ministries of Defense and of Security mid-July and transmitted them to ANAC for flight organization purposes. A demonstration flight with ANAC took place on July 17, 2020. Ground data collection involved carrying out ground surveys at the level of the crossing structures along the watercourses / canals of the AOI. Espace Geomatique identified around 60 structures to be measured and pictures were taken of each structure. The ground topographic measurements were completed in July 2020. Summary of challenges associated with drone imagery collection: ▪ Espace Geomatique were notified by ANAC that they needed to get authorization from all the ambassies and medical centers located in the AOI. This caused more than 2 months delay in the overall work, as it was not possible to fly without these official authorizations. ▪ Espace Geomatique had to start its drone flights during the rainy season, and this led to delays of several days. Because the rainy season had been exceptional in 2020, it had some consequences on the flights. Indeed, the rains had begun in Ouagadougou by June and terminated by September. Because of this and the fact that it was not safe to fly in rainy and cloudy weather, some flights had to be cancelled and rescheduled. ▪ Flights were organized in coordination with the Ouagadougou International Airport’s control tower, which at times asked Espace Geomatique to change flight plans at the last moment. To avoid collision risks or electro- magnetic influences between the drone and other flying aircrafts, it was imperative to coordinate the flights with the airport control tower. Before each flight, it was obligatory to call the control tower and to give its staff the place and the drone flight conditions (altitude, duration, area to be covered). Moreover, the control tower could call Espace Geomatique and request to wait, to fly back, and to cancel or interrupt the flight depending on the airport traffic and knowing that all other aircraft were prioritized above the drone. For those reasons, several drone flights had to be cancelled at the last minute. Espace Geomatique was also obligated to inform the airport control tower when each drone flight was terminated. ▪ Espace Geomatique discovered that the geodesic requirements needed more densified ground control points. Despite the fact that the drone was carrying a real-time kinematic positioning system which was calibrated with the national geodetic system and the fact that each flight plan was covering an area of less than 2x2 kilometers, it was necessary to measure and put additional ground control points when processing the aerial imagery. ▪ The collected photos were very big in terms of memory size, about 1 To, and the results had to be delivered at a high resolution. Therefore, the data processing operations were very slow even when using two powerful computers of 128 Gb RAM each. Because of the required resolutions, these operations were implemented many times to find the best filter’s parameters and to generate the best results. Individual steps of data processing took 10 continuous days. 47 ANNEX 2: FLOOD RISK MODELING BACKGROUND DATA AVAILABILITY Several studies have already been carried out in urban hydrology in Ouagadougou, establishing the intensity- duration-frequency (IDF) curves. Spatial variability of rainfall can be estimated from radar rainfall measurements made in Ouagadougou in 1992 and 1993, coupled with a network of 17 rain gauges within a 50 km radius of the radar (Kacou, 2014) deployed as part of the MeghaTropiques project. Data are also available from a network of 14 rain gauges in 2016 and 2017 (Bouvier et al., 2017). Rainfall-flow measurements are available on three experimental urban basins monitored from 1977 to 1979 in the Moro-Naba/Airport sector (Le Barbé, 1982), and on two urban basins monitored from 1992 to 1993 in the Wemtemga sector (Lamachère, 1993). These basins, moderately urbanized, nevertheless present high runoff coefficients, due to the low permeability of the unpaved soils, compacted by human activity (Bouvier and Desbordes, 1990). They can still be used as a reference for the calibration of models in urban areas. More recently, three peri- urban basins were monitored from 2016 to 2017 within the framework of AMMA 2050, and made it possible to characterize the contribution of these peripheral zones and, by comparison with measurements on urban basins, the impact of urbanization on runoff. There is no reliable map of the building or sealing coefficients in Ouagadougou. The reason for this is that the notion of waterproofing is ambiguous, insofar as many surfaces are unpaved (roads, concession yards), but correspond to very low permeability compacted soils. What should be considered as sealed or permeable? The various authors do not necessarily speak of the same objects. Bouvier and Desbordes (1990) find low coefficients (10 to 25%) for traditional housing areas in Ouaga, based on an exhaustive count of paved surfaces. Simulated rainfall runoff measurements showed that unpaved soils were very low in permeability, with lower infiltration intensities after a few minutes of intense rain. REFERENCE RAINFALLS Four synthetic reference rainfall events were selected for the study. Three are theoretical double triangle project rainfall events: (i) A frequent rainfall with a 2-year return period, (ii) A rare rainfall with a 10-year return period, and (iii) A very rare rainfall with a 50-year return period. The last rainfall, exceptional, is the historical rainfall of 2009. The theoretical double triangle rainfall was calculated from the times of concentration of the various sub-catchments of the study area. These times of concentration were estimated between 1 and 3 hours for the largest sub-catchment (Gounghin canal), and between 30 minutes and 1h30 for the other sub-catchments of the territory. Thus, the synthetic project rainfall was constructed using the double triangle method, with a total duration of about 3 hours and an intense period of 1 hour. This makes it possible to model the maximum contribution of all the sub-catchments in terms of peak runoff flow without underestimating the volumes of runoff which also condition the levels of flooding reached in the topographical depressions. The major flood of recent years (or even decades) is that of 1 September 2009, when the rainfall reached 263 mm locally in less than 12 hours, which according to the reference statistics gives it a return period well in excess of 100 years over 12 hours (cumulative 100 years over 12 hours = 175 mm). The reconstruction of the hyetogram of the rain between 04h37 and 07h35 according to Traoré 2012 allows to estimate the return period of the rain over different durations: - 5 min: 25 mm return period between 20 and 50 years - 15 min: 52 mm return period between 20 and 50 years - 30 min : 72 mm return period between 20 and 50 years - 1h : 136.5 mm return period >> 100 years - 2h : 176 mm return period >> 100 years - 3h : 178 mm return period >> 100 years Thus, the data of this rainfall was used to model the effect of a real exceptional rainfall. 48 40 Figure 2.1: Hyetograph of project rainfall 35 2 ans 10 ans 30 50 ans Précipitation (mm/6min) 25 20 15 10 5 0 0:00 0:12 0:24 0:36 0:48 1:00 1:12 1:24 1:36 1:48 2:00 2:12 2:24 2:36 2:48 3:00 temps (hh:mm) SPATIAL ABATEMENT AND RUNOFF COEFFICIENTS The various reference rainfall events were applied uniformly to all the catchment areas. Nevertheless, in order to take into account the spatio-temporal distribution of rainfall, a global abatement coefficient was evaluated for each of the four selected rainfall events, allowing for the transformation of point rainfall into surface rainfall. Le Barbé (1982) proposes spatial attenuation coefficients established in Ouagadougou for daily rainfall. Although the rainfall distributions may have changed compared to the period 1954-1977, the relationships between the distributions of point rainfall and surface rainfall probably remain unchanged. Assuming that the spatial attenuation coefficients calculated at the daily time step are also applicable to the sub- daily time steps, the values of these coefficients in Ouagadougou were used to calculate the spatial attenuation coefficient. In the absence of abacuses for catchment areas larger than 5 km², this curve was used and extrapolated to calculate the spatial attenuation coefficients for each rainfall. Figure 2.2: Spatial attenuation coefficient as a function of point rainfall Source: Le Barbé (1982) Given the high and relatively dense urbanization of the study area and the fact that unpaved areas have low infiltration, it was considered best to apply runoff coefficients of the order of 0.7-0.8 for rainfall of up to 50 mm. The four project rainfall events have accumulations above 50 mm. Each raw rainfall is reduced by the spatial reduction coefficient, then by the runoff coefficient. 49 Table 2.1: Description of the four reference rainfalls used in modeling 2-year return 10-year return 50-year return Rainfall event 2009 period period period Spatial abatement coefficients 0.95 0.87 0.78 0.7 Runoff coefficients 0.7 0.75 0.8 0.8 Total gross accumulation (mm) 70 106 142 173 Total net accumulation (3h) 47 69 89 97 Gross accumulation over the 54 70 97 / intense period (1h) Net accumulation over the 36 46 61 / intense period (1h) CONSTRUCTION OF 2D HYDRAULIC MODEL The modelling approach chosen for this study is a two-dimensional (2D) modelling of flows over the entire study area. The US Army Corps of Engineers' HECRAS 2D software was used to construct the 2D model. This modelling approach allows constructing a single model allowing the representation of all the flooding mechanisms of the sector: urban runoff, runoff in the undeveloped areas, overflow of the canals, blocking of the flows behind the embankments, etc. The results of the model thus allow a detailed understanding of the genesis of the flows, the areas drained by the canals, etc. The Digital Terrain Model (DTM) was further refined to represent the cross-sectional profile of the canals and thus realistically reproduce their respective real hydraulic capacities. Certain canals are in particular in water all year round, these particularities were also taken into account during the construction of the model in the calculation of the hydraulic capacities. The calculation mesh of the hydraulic model is based on the topography of the terrain from this DTM. COMPUTATIONAL MESHING The model was meshed in several stages: - Producing a first regular mesh (5 to 10 m) of the entire study area. Within each of these meshes, the software reproduces a height-volume law from the sub-mesh topography, calculated on the basis of the DTM with a resolution of 50 cm (there are therefore 400 calculation points per mesh); - Refining this grid by forcing the boundaries of the grids on the routes of canals, roads, built-up areas and on the route of road and rail infrastructures in embankments. This forcing thus makes it possible to take into account with greater precision these characteristic elements of the territory. In total, the study perimeter is represented by 680,000 2D calculation meshes structured by 6,000 forcing lines. CONSIDERATION OF BUILDINGS AND LAND USE The different land uses in the study area were taken into account in the form of differentiated roughness in the model in order to distinguish between dense and loose built-up areas, vegetation, roads and canals. Table 2.2: Roughness according to land use Roughness Strickler (m1/3 /s) Manning Dense building 10 0.1 Loose building 20 0.05 Vegetation and isolated buildings 7.5 0.133 Watercourses and canals 30 0.033 50 CONSIDERATION OF PASSAGE STRUCTURES The main structures present along the canals (scuppers, bridges, nozzles) were identified (42 in total) and were the subject of a land survey. These structures were integrated at full capacity in the model of the reference scenario in order to ensure the hydraulic continuity of the flows on both sides of the crossed infrastructure embankments (road, railroad). However, as some canals are partially filled with water throughout the year, the hydraulic capacity of these structures was adapted locally in order to simulate the effective capacity of these structures in the event of a rainfall event in the study area. To take into account the risk of jams and floatation entrapment, and the current state of obstruction and deterioration of some of the structures, a specific scenario was also simulated (scenario 2). This scenario assumes that jams completely obstruct the flow section of the structures. The safety spillway of dam no. 3 is located at the 287 m level. In order to take into account in the models a normal downstream condition at the outlet of the sub-catchment areas, a water level 50 cm below the level of the safety spillway, i.e. 286.5 m, was applied at the three dams for the reference scenario with and without jams. At the level of Bangr Weogo Park in the north-east of the study area (outside the influence of dam n°3), a normal level was retained as the downstream condition allowing the evaluation of flows outside the model In order to measure the impact of a high level of the dams (at the end of the rainy season for example) on the drainage of runoff and the overflowing of the canals in the event of a concomitant rainy episode, a specific scenario was developed (scenario 3), with a high downstream condition of the dams. In this scenario, the elevation of the three dams is set at 289.5 m. This level corresponds to the average level of the N3 road overhanging the safety spillway of dam n°3. POST-PROCESSING OF RESULTS The raw results of the simulations have been filtered and simplified: (i) Precipitation is applied to the whole territory. In order to improve the visibility of the results, and to see the preferential paths of runoff, a minimum threshold of 3 cm of water height has been set up for the display of the results; and (ii) a filtering of the isolated hazard polygons with a surface of less than 500 m2 (average size of a plot in urban area) is applied to concentrate the analysis on the main risk areas. Thus, all polygons smaller than this size are filtered. CAVEATS Simplification of the runoff coefficients: The run-off coefficients used in the model have been considered identical over the whole project area, varying only with the rain intensities. In reality, this coefficient is higher in densely urbanized areas than in mildly densified neighborhoods. With the growing urbanization, run-off coefficients would remain high in already developed areas, and would increase in newly developed parts. This impact of urbanization cannot be simulated with the assumption of homogenous coefficients. This simplification was imposed by the absence of reliable data on soils occupation at small scale. The impact of the simplification is also limited by the fact that most of the AOI is already densely occupied, and due to the type of soils, even the non-paved surfaces show a high run-off coefficient. We thus consider that the selected coefficients (0.7-0.8) are on the safe side, and that this simplification does not significatively impact the model’s ability to simulate future floods. Impact of climate change: The design storms used as inputs in the simulations are based on intensity-duration- frequency curves built from past hydrological records, covering a period ranging from 1950 to 2014. The length of the period where meteorological data are available is satisfying with regards to the return periods the model aim at simulating. However, the design storms do not take into consideration the potential impact of climate change, which, as noted earlier, will have an impact on rainfall intensity that is quite difficult to predict in Burkina Faso. Modeling of hydraulic structures: The hydraulic model is based on a DTM created from data collected during an aerial survey. By definition, such a model includes all hydraulic structures visible from the sky (such as channels) but cannot reflect the presence of underground hydraulic structures (such as pipes and culverts). The main structures observed on site have been manually added to the models, but small-scale culverts, mostly in the upstream parts of the watersheds, are not simulated in the model. This means that in some cases, the maps resulting from the model show flooding where part or all the flow would actually transit under the street through a culvert that is not simulated in the model. However, the impact of this simplification is limited because the culverts that are not implemented in the model are small and quickly overflowed anyway. 51 Moreover, the DTM produced by the local firm contains several unexpected low points on road surface which appear to be due to error rather than reflect the reality on the ground. Therefore, the flood maps resulting from the hydraulic model based on this DTM indicate several individual small flooded areas (“false positives”). However, despite these localized limitations of the flood model, it is considered able to represent the hydraulic system as a whole (in particular flooding areas caused by an overflow of canals). Figure 2.3: Origin of a DTM anomaly: local low point in the Digital Surface Model in the area next to Sainte Camille hospital Figure 2.4: DTM anomaly caused by the incorrect low point: 7 m Figure 2.5 Consequence of the DTM anomaly in the hydraulic deep “hole” in the road surface model: very localized “false positive” flood result The above points highlight some limitations of the model resulting from low availability of input data. These limits do not fundamentally challenge the results of the hydraulic model. 52 ANNEX 3: COMPARISON OF FLOOD RISK Figure 3.1: 2-year return period, with low water level in dams (Scenario 1) Figure 3.2: 2-year return period, with high water level in dams (Scenario 3) Source: SEPIA Conseils 53 ANNEX 4: IMPACT OF FLOOD HAZARD ON TRAFFIC FLOWS To better understand the flood events affecting the planned layout of the bus system, it was necessary to analyze the results of the models with more detailed information than the information on peak flows and heights. A set of points was thus selected in the area of interest, representing different flood types, for which hydrographs were generated showing the evolution of water height over time during the 2-year RP flood and the 2009 flood. The analysis of the ramping up and down of water heights associated with rainfall intensities allowed us to characterize the type of flooding associated with various situations and causes. Point of interest 1 This point of interest (POI) is located on the United Nations roundabout (“rond -point des nations unies”), which is crossed by most bus lines. A few meters east of the roundabout there is a 130m-longsection of the canal central, which is buried. The entire area undergoes flooding caused even by 2-year return period storms. Figure 4.1: Areas flooded by the 2-year return period storm around POI 1 Source: Results from the SEPIA hydraulic model The profile of the hydrograph, too, confirms what could be inferred from the map: the floods in this point are caused by an overflow of the canal central. Floods appear almost one hour after the peak of rainfall intensities. This duration is in the order of magnitude of the watershed’s concentration time. We observe a sudden rise in water level, corresponding to the moment when the canal overflows to the neighboring areas, and then a slow decrease (more than 4 hours to return to normal conditions). It is very likely that this slow decrease is the consequence of the canal central’s capacity being reduced in its downstream part, due to obstru ction and to the high-water level at the outlet, which both affect the canal’s capacity to evacuate water. Solving floods in this area will involve solutions focusing on improving the hydraulic capacity of the canal central’s downstream part, or increasing storage capacities of the upstream areas. 54 Figure 4.2: Hydrograph in POI 1 under the 2-year RP storm Source: Results from the SEPIA hydraulic model Point of interest 2 This point is located on rue Nongremasson, a few meters south of a bridge over the downstream part of Canal Central. The 2-year RP storm causes the canal to overflow in all its downstream part, where its section is reduced by silt, aquatic plants and market gardening activities. The overflow is important enough for the water to flood perpendicular streets, including Nongremasson street used by line 1, which is flooded on more than 200 meters to the south. The profile of the hydrograph is very similar to the one in POI 1. This is not surprising since both points are located next to the same canal central, POI 2 being approximately 1500 meters downstream compared to POI 1. The solutions adapted for these areas are the same as POI 1, improving the hydraulic c apacity of the canal central’s downstream part, or increasing storage capacities of the upstream areas. Figure 4.3: Areas flooded by the 2-year return period storm around POI 2 Source: Results from the SEPIA hydraulic model 55 Figure 4.4: Hydrograph in POI 2 under the 2-year RP storm Source: Results from the SEPIA hydraulic model Point of interest 3 This point is located on Avenue Ouezzin Coulibaly, 60 meters east of the bridge over the Mogho Naaba Canal. This bridge is located 300 meters downstream a reduction of the canal’s section, transitioning from a newly constructed portion of 25 m width to an older reach of only 11 meters width. The capacity of this ancient section of canal is clearly insufficient, which causes overflows on the east bank, which in this part is lower than the left side. Figure 4.5: Areas flooded by the 2-year return period storm around POI 3 Source: Results from the SEPIA hydraulic model 56 Figure 4.6: Terrain elevation and cross section of the Mogho Naaba canal and its surroundings near POI 3 Source: Results from the SEPIA DEM/DTM Figure 4.7: Hydrograph in POI 3 under the 2-year RP storm Source: Results from the SEPIA hydraulic model The hydrograph of the 2-year return period storm in POI 3 shows that the water level rises quite slowly, reaching its peak in more than one hour, and takes even longer to get back return to its normal level (more than 4 hours). This hydrograph can be interpreted as follows: ▪ The ramp up of the flood is relatively slow because the flat area on the east side before the bridge (including René Monory stadium) acts as a flood expansion zone. ▪ The very slow ramp down is due to the limited capacity of the downstream part of the Mogho Naaba canal, whose section is reduced and outlet in the lake is partially obstructed by silt and market garden activities. This causes large volumes to overflow all along the canal during the peak of the storm, and these volumes take time to be evacuated by the canal when the rain stops. Solving the flood issues around the Mogho Naaba canal has been studied in detail in Ouagadougou’s Drainage Master Plan, which proposed two variants: ▪ Enlarging the 3.5km downstream section of the canal from until the lake. ▪ Building a new flood control basin south of avenue Ouezzin Coulibaly This second option has been selected by the municipality and is under study. The identified area for the basin being just upstream POI 3, it would fix the flood issue there and thus protect this section of bus line 7. 57 Point of interest 4 This section is at the very end of future bus line 4, in the upper part of the Wemtenga watershed (the most eastern watershed of our area of interest). The DTM shows that this section of street 28.257 used by future line 4 is perpendicular to the general slopes of the terrain. Two flood control basins exist in the area, but they are located west (downstream) of the 28.257 stream, and thus does not protect this street. Besides, the neighborhood located south-east of the street lacks a drainage network. Therefore, the perpendicular streets act as channels carrying run- of water straight through the surface of 28.257 street. Figure 4.8: Terrain elevation of the area around POI 4 Source: Results from the SEPIA DEM/DTM Figure 4.9: Areas flooded by the 2-year return period storm around POI 4 Source: Results from the SEPIA hydraulic model 58 Figure 4.10: Hydrograph in POI 4 under the 2-year RP storm Source: Results from the SEPIA hydraulic model The hydrograph shows that the water height in POI 4 during the 2-year storm follows the same profile as the rainfall intensities, with a short response time and limited flood height. This is due to the fact that the area is very close to the upper part of the watershed: 28.257 street receives the run-off water from a small (a few ha) and short (400 meters) area, thus run-off gets to the street soon after the beginning of the rain and stops soon after its ending. The obvious solution to fix this type of flooding caused by lack of drainage of lateral streets is to create a proper drainage network in the area to intercept run-off flows from the perpendicular streets and convey them (under the axis to protect) a until a natural outlet. In this case, it would mean intercept water from south-east streets and direct them to the downstream basins. An alternate solution (which would not remove the need for a drainage network, but allows for smaller dimensions works), would be to retain as much as possible run-off water in the upstream neighborhood, thus flattening the hydrograph. Here one could imagine implementing permeable pavement in the south-east streets, pocket parks or even a rain garden in the area next to the Saint-Paul church (if available). A newly developed street parallel to the 28.257 street, 700m to the south-east, is not affected by flood risk, and could thus constitute an alternate path for the bus line. Point of interest 5 This point is located on a critical axis on the Nationale 4 road to the East of Ouagadougou. It is not affected by 2 and 10 years return period events, but this area was severely flooded during the 2009 flood when the lake overflowed the dam. In 2009, the peak of rainfall intensities lasted about 10 minutes. Flooding over POI 5 area started 40 minutes after the beginning of that peak, and lasted one hour, with a maximal height of flooding of 17cm. Figure 4.11: Simulation of the maximum water heights caused by the 2009 storm event Source: Results from the SEPIA hydraulic model 59 Figure 4.12: Hydrograph in POI 5 under the 2-year RP storm Source: Results from the SEPIA hydraulic model From the hydrograph and the location map, flooding in POI 5 is caused by overflow from the Zogona canal, and only during the highest intensities (higher than 140mm/hr). When intensities remain lower, the Zogona canal’s capacity seems sufficient to evacuate the flow. In this area, floods can be prevented with solutions focused on the Zogonal canal: increasing its capacity, improving the outlet conditions, or creating retention areas upstream. Since the flooded section is only 300m long, and does not cross populated areas, it would also be feasible to elevate the road in this section to protect it from floods. However, since a flyover bridge has recently been built just west of the flood-prone area, it seems unlikely that new earthworks will be executed here soon, since it would require to modify the profile of the access ramp to the overpass. 60 Table 4.1: Exposure of the planned mass transit lines to flood risk: Flood criticality score and Impact of the current flood issues on the planned mass transit system 61 62 63 64 65 66 ANNEX 5: IDENTIFYING SOLUTIONS TO ADDRESS FLOOD RISK Figure 5.1. Canals crossing the area of interest Source: AGEIM (2020) Table 5.1. Reference of the unit costs considered to estimate the investments and maintenance costs of the proposed flood resilience and adaptation measures Construction: A Better City (ABC). (2015). Enhancing Resilience in Boston, A Guide for Large Buildings and Institutions. February. Bioswales Maintenance: Osouli, Abdolreza & Akhavan Bloorchian, Azadeh & Grinter, Mark & Alborzi, Aneseh & Marlow, Scott & Ahiablame, Laurent & Zhou, Jianpeng. (2017). Performance and Cost Perspective in Selecting BMPs for Linear Projects. Water. 9. 302. 10.3390/w9050302. Cleaning of the reservoir SCET Tunisie and AGEIM. (2019). Drainage Master Plan of Ouagadougou. lakes north of the area study Communication plan based on mobile Orange. (2019). Le SMS, nouveau Commerciale votre entreprise en 2019. technologies Construction: Ellwood, N. (2012). “A Detailed Look at Costs Associated with Green Stormwater Controls.” Proceedings from Water Environment Federa on Stormwater Symposium. Permeable pavements Maintenance: WisDOT Southeast Region. (2012). Comparison of Permeable Pavement Types: Hydrology, Design, Installation, Maintenance and Cost, Prepared by CTC & Associates LLC WisDOT Research & Library Unit, January 13. Construction: Malaviya, P. et el. (2019). Rain Gardens as Storm Water Management Tool. Estimated cost Rain Garden of rain garden in Greece to be “less than 50E per m2”. Reinforcement of the Purchase and installation of concrete baskets; and construction of concrete garbage collection points: solid waste collection Drainage Project of Louis Berger in Douala, Cameroun system Organization of awareness raising campaigns: Projects of RESALLIENCE and Louis Berger 67 Installation & maintenance: The IGNITION Project. (2020). Nature-based Solutions to the Climate Retention basins Emergency: The benefits to businesses and societies. June. Installation: SCET Tunisie and AGEIM. (2019). Drainage Master Plan of Ouagadougou. o Street trees: Installation: Project of Louis Berger in Cameroon Installation & maintenance: The IGNITION Project. (2020). Nature-based Solutions to the Climate Emergency: The benefits to businesses and societies. June. Tree planting o Stormwater retention trees (or SuDS-enabled street trees): Installation & maintenance: Charles River Watershed Association, Low Impact Best Management Practice (BMP) Information Sheet www.charlesriver.org Urban parks and green Maintenance: The IGNITION Project. (2020). Nature-based Solutions to the Climate Emergency: The spaces benefits to businesses and societies. June. Wetland creation or Construction & maintenance: Charles River Watershed Association. (2008). Low Impact Best restoration Management Practice (BMP) Information Sheet: Constructed Stormwater Wetlands, August. Box 1: Illustrations of terminology The following terms are used to characterize hydraulic structures, whether existing or projected: Culverts: buried structures, either circular (pipe culverts – “buses” in French) or rectangular (box culverts – “dalots” in French), whose function is to ensure hydraulic transparency of a road by channeling water under the road surface. They are also referred to as transverse/cross-section structures. Canals: large open conduits conveying water in the surface. They form the primary drainage network, collecting water from secondary structures and channeling it to the natural outlet, which in the area of interest are the reservoir lakes and the Kadiogo river north of the AOI. In Ouagadougou, existing canals are generally formed by large trapezoidal section concrete structures, but some undeveloped parts still exist with earth bottom and sides. Their dimensions range from a few meters wide to 20m wide in the area of interest, which includes 4 main canals, one per watershed. Channels: they form the secondary and tertiary drainage network. They are typically rectangular section concrete structures, with dimensions ranging from 50 cm to a few meters wide. Channels are generally used to collect rainwater along the road surface. When playing this function of longitudinal drainage, they are generally called “caniveaux” in French (open rectangular street gutters). Larger channels collecting water from several smaller one and leading to a canal may be called “collecteurs”. In this report, both types will be referred to as channels, since they are represented by similar rectangular concrete structures. In Ouagadougou, longitudinal channels are often partially covered with concrete slabs. Box culverts in the AOI A typical canal in the AOI A typical open channel in the AOI 68 Table 5.2. Long list of solutions Implemen Solutio Adaptability to Situations for which the Nr. Name Short description Advantages Disadvantages tation n type Ouagadougou solution is adapted scale Enforce interdictions to build in the right of way of the primary network (rivers, main • Reinforce the urban channels, reservoirs, lakes) and within a rules and land uses plan • Might take time Medium - there This solution has similarities few meters along the channels. Preserve • Allow the densification to implement • are not many with the construction of Preserving non-urbanized areas along the channels Soft, along the BRT outline, Entailed Street available areas water retention basins. flood that can be used as flood expansion areas. Grey & and create spaces • Do demolition of 1 level suitable for However, it offers less expansion Since unoccupied spaces will quickly be Green no create empty spaces existing houses flood expansion impact on flood control, but areas filled with informal constructions, these or corridors • Protect or informal in the area of is easier to implement and areas should be used to build public inhabitants and users settlements interest has more co-benefits; spaces such as floodable sports facilities, from floods and the lack pedestrian paths/health trails, socialization of facilities accesses spaces. This solution should be • Disruption of reserved for very particular the urban fabric Construction cases where no other and cross-cutting High - standard of an Construction of an embankment allowing solution is practical and movements • civil embankment the road to be placed higher than the • Strong protection of Street where the impact on the Grey Makes access engineering, 2 for the road water level of floods, with a longitudinal the bus lanes level surroundings will be limited. difficult • Protects applicable platform for and transversal drainage system to ensure Example: in a low point / bus lanes but not everywhere. the bus the hydraulic transparency of the system. basin where drainage is not their routes practical, and over a limited surroundings • length to avoid the "barrier High cost effect". Bioswales are vegetated, shallow, landscaped depressions designed to • They may be a Bioswales can be widely Medium - capture, treat, and infiltrate stormwater potential source applied and are particularly • Ease of design• Cost requires runoff convey stormwater at a slow, of odor and useful along streets or in effective, sustainable, available space. controlled rate, and the flood-tolerant mosquitoes if parking lots where they can Bio-swales and environment • Street Best suited in vegetation and soil act as a filter medium, water is left redirect water from the (vegetated friendly solution • level • regions were cleaning runoff and allowing infiltration. Green stagnant. • May curbside. Bioswales have 3 swales, Reduces flow velocities Neighbor rainfall is Bioswales are an effective type of green require irrigation flexible siting requirements, bioretention and filters and cleanses hood regular over the infrastructure facility in slowing runoff to maintain allowing them to be swales) water naturally • level year. Can be velocity and cleansing water while vegetation • They integrated with medians, Beautifies surrounding adapted by recharging the underlying groundwater cannot be used cul-de-sacs, bulb-outs, and landscape using local table. Bioswales are generally installed on in areas with other public space or traffic species. within or near paved areas (e.g. parking steep slopes. calming strategies. lots, roads and sidewalks). • High cost : long Construction linear length of Adapted in flood-prone • Solving local flooding • Street High - standard of new new works may areas (neighborhoods with Development of natural primary drains in problems • Securing the level • civil drainage be necessary to no drainage system or in an 4 flood-prone areas. Construction of Grey infrastructure • Neighbor engineering, works in connect the new embryonic stage). In secondary network when missing. Improving accessibility hood applicable poorly networks to an particular, the outlying in all seasons level everywhere. drained areas acceptable outlet districts. • Disturbances to 69 road traffic during construction works Medium – • Ease of design• Can be used in most Porous Precipitation falling on the surface of the Effective to improve situations. At small scale (on pavement is structure (and possibly other surfaces) is drainage of the run-off a street, portion of street), more sensitive directly infiltrated by a permeable from road surfaces • effective to improve to lack of pavement and percolates to the Once installation costs drainage of the run-off from routine Permeable underlying granular structure. These are factored, it can cost • Street road surfaces, but offers maintenance pavements/ structures increase the time delay of run- as much as 50 % less level • little protection against than standard 5 porous off, and favor infiltration capacities. Porous Green than conventional • Cost Neighbor flows coming from outside pavement. asphalt concrete, asphalt, or interlocking pavers pavement systems, and hood the right-of-way. At large Might be an system allow water to percolate through their can be cheaper in the level scale, a large number of issue in surfaces to be treated and stored in soils long run to maintain • streets equipped with developing and rock beds below. Some applications Permeable pavements porous pavement will act as countries where have demonstrated a 90% reduction in can help reduce the a water retention basin and maintenance is run-off volumes. concentration of some contribute to reducing peak often pollutants flows. overlooked. • They may be a potential source Medium - grass Rain gardens can be • Flexible layout to fit of odor and areas are best installed in various scales Vegetated land depressions designed to into landscape • Cost mosquitoes if suited in and shapes: in planter detain and treat stormwater runoff. The effective, sustainable, • Street water is left regions were boxes or integrated with runoff is filtered through densely planted and environment level • stagnant. • May rainfall is streetscapes. They can also 6 Rain gardens surface vegetation and then percolated Green friendly solution • Neighbor require irrigation regular over the act as ‘standalone’ soil through a prescribed filter media (soil Beautifies surrounding hood to maintain year. Can be filtration systems within layers). Unlike bioretention swales, they do landscape • Detains and level vegetation • They adapted by residential areas, parklands, not convey stormwater runoff. cleans stormwater cannot be used in using local schools, car parks and other runoff areas with steep species. developments. slopes. Trees provide a natural stormwater management system by intercepting Trees can be planted High – no issue precipitation in their leaves and branches. strategically throughout the • Reduce slow • Street to grow local Many cities have set tree canopy goals to city. Homeowners, stormwater runoff • level • tree species, restore some of the benefits of trees that businesses, and community 7 Planting trees Green Beautifies the city • None Neighbor which are were lost when the areas were developed. groups can participate in Purifies the air • Create hood already A mature deciduous tree can capture as planting and maintaining shade level numerous in the much as 700 gallons of rain a year, while trees throughout the urban city. a mature evergreen can absorb up to environment. 4,000 gallons annually. Green roofs are covered with growing • Underlying Green roofs can be • Reduce slow media and vegetation that enable rainfall structure may Low - Low retrofitted onto existing stormwater runoff • • Street infiltration and evapotranspiration of have to be compatibility buildings and can be Beautifies the city• level • stored water. Rooftop vegetation enables strengthened to with the located strategically in 8 Green roofs Green Purifies the air • Neighbor rainfall infiltration and evapotranspiration cope with the standard buildings located in highly Insulates the building• hood of stored water, which helps slow extra load • architecture in exposed flooding sections Increases lifespan of the level stormwater runoff by reducing the rate at Requires extra Ouagadougou. of the road as well as bus roofing which water reaches the drainage system. maintenance • stations and bus stops (e.g. 70 Green roofs can retain on average 75 % Higher cost than this was done in Holland in of the stormwater they receive. traditional roof the cities of Wageningen and Utrecht) Medium - grass • Combine grey and areas are best green approaches to suited in Adapted to slightly increase Hybrid embankments or small levees • Street maximize water • Requires lands regions were flood protection along Hybrid covered by vegetation (plants, communal level • Green absorption and and organization rainfall is primary channels, with 9 infrastrructur gardens) along the canals, sidewalks and Neighbor & Grey infiltration in the long of uses of public regular over the limited costs, and e public spaces (rows of seats), with porous hood term • Ease of design • spaces year. Can be environmental and pavement along the embankments level Improve quality of life• adapted by wellbeing co-benefits Limited costs using local species. To maximize the positive impact on flood reduction, this action should target Awareness areas generating significant raising on quantities of solid waste waste Development of an awareness campaign • Only effective in likely to end up in the management (e.g. awareness day) tackling waste • Easy to implement • the long term • drainage system: markets, (education in management and participative clean-up of Contribute to the Requires the areas missing formal waste local canals in informal settlements (94% of the High – no maintenance of coordination of Neighbor collection systems, and neighborhoo city), and neighborhoods along the BRT Soft specific 10 infrastructures • Include several actors hood informal settlements. Should ds, in line will help recover the full drainage constraints the civil society and and the level be combined with conjunction capacity of the infrastructures. To include makes people more organization of improvements of the with the city civil society and local stakeholders in the accountable • Low cost local committees collection system, as council and a management of the resilient /neighborhoods awareness raising will have crisis unit per infrastructures. a limited impact on littering neighborhoo if the inhabitants are not d) provided with more suitable alternatives to evacuate waste. The urban dams of Ouagadougou are no This solution is relevant to longer essential for the water supply of • Reduce the risk of the • change of High — The Changes in improve the protection of the city's inhabitants. It is thus possible to lakes overflowing to the approach in the dams are not the use of the areas located next to consider reorienting their function towards area of interest• management of Areas necessary dams Soft & the reservoir lakes against 11 flood prevention:- Preventive emptying improve the outlet the lakes• close to anymore for downstream Grey rare to exceptional storms before heavy rainfall, so that it does not condition of the primary requires the lakes drinking water of the study (similar to the 2009 storm coincide with a high lake filling level;- channels which emerge institutional supply. The only area which caused the lakes to Integration of sluice gates;- Lowering of into the lakes. arrangements issue is cost. overflow). overflow weirs; The basins of dams No. 1, 2 and 3 have • High one-time This solution is relevant if Cleaning of silted up as a result of solid deposits and cost for the combined with the previous the reservoir the height filled by silt has been • Significantly increase cleaning works• Areas one. It would allow further High — Only 12 lakes north of determined at 1.5m, 2m and 1.5m Green the efficiency of the Requires close to reduction of water height in cost is an issue. the area respectively. This situation considerably previous solution. complete the lakes the reservoir lakes, and thus study reduces the retention capacity of these emptying of the improve flow conditions in basins (estimate of 3,9M of m3 dams the downstream parts of the 71 corresponding to the quantities of silt). primary channels. To Cleaning this silt would thus increase the improve its efficiency, it storage capacity of the lakes and reduce should be combined with the risk of overflow towards upstream long-term efforts to reduce areas. the quantities of silt and waste getting to the lakes. Suited along primary drains • Cost (higher Improvement already developed into than maintenance of existing lined canals: Mogho Naaba or repairs) • hydraulic Canal, Zogona Canal, Creation of elevated lanes on both sides Waterproofing of tracks Technical structures & High — already Wemtenga Canal.For of the main collectors; Widening of certain and securing of complexity • Watershe 13 construction Grey implemented in example, some sections of sections of collectors; Widening of surrounding Requires d level of dykes to Ouagadougou the Mogho-Naaba canal are crossing structures on the main collectors infrastructure acquisition of prevent smaller than upstream new land, and overflow onto sections, causing a potential the road constriction which increases expropriations flooding risks. • Positive impact on flood protection in all When land is available downstream areas • upstream of a catchment Flood control areas are filled during Reduces the need for • Requires basin, this solution offers Construction storms and progressively emptied after new works downstream significant land • great opportunities to of new flood the storm, thus reducing the peak flows Grey & • Different types High – already Significant Watershe reduce floods in all the 14 retention and limiting floods downstream. They can Green available: from concrete exists in construction and d level downstream areas. A basins / flood take the form of concrete retention basins, retention basin for Ouagadougou maintenance suitable 10 hectares area control areas natural flood expansion areas or multiple- maximum flood costs was identified along the uses flood control areas. reduction, to full natural Mogho Naaba canal in the area or shared uses 2020 Drainage Master Plan. area to foster co- benefits. Restoring the hydraulic Less capacity of existing water disadvantages retention areas has one of Maintenance than creating a • same advantages as the best cost/efficiency / new basin: does new basins• possibility ratios. Of course, it is improvement Cleaning of existing retention basins; not require new High – already Grey & to modify existing Watershe limited to watersheds where 15 of existing Equipping existing basins with water level land (but may exists in Green basins to allow for d level flood control area already water control devices require some Ouagadougou shared uses (park, exist. Two such basins exist retention evictions if areas urban agriculture) upstream of the Wemtenga basins of the retentions canal. They are partially basins are filled with silt, and should illegally occupied) be cleaned and refurbished. Repairs of • Sometimes Well suited in areas where existing Rehabilitation or reconstruction of requires evictions Rapid solution for High – already flooding of the road is canals and degraded sections to restore their Grey & of activities Watershe 16 potentially severe flood exists in caused by damages to the clearing of hydraulic capacities, including restoration Green illegally d level issues in localized areas Ouagadougou neighboring hydraulic invaded of natural systems of flow and vegetation. established in the structures. structures right of way of 72 the hydraulic structures • Cost High — the current solid Improvement in flood The most effective way to reduce the Needs actions on waste system is protection is a secondary filling of hydraulic structures by solid • Co-benefits on Reinforcemen a large scale, insufficient, but effect of an efficient waste waste is to act on the source of waste, pollution reduction, • t of the solid since a structure access to proper collection system, which either through awareness-raising activities, water quality and Watershe 17 waste Soft can be affected waste collection should be implemented for but mostly by ensuring an efficient quality of life • Reduces d level • collection by waste coming system is a health reasons first. Should collection system is in place: street litter the need for cleaning City level system from upstream growing be combined with bins, collecting trucks, landfill / actions areas. concerning awareness raising measures. incineration sites. African major cities. In Ouagadougou there is potential to restore the ecological integrity and Park spaces offer a wealth of pervious retention capacity of the surface that can be used to absorb urban Parc Bangr, rainwater and runoff from adjacent • Opportunistic solution, experiencing increased developed landscapes. Green urban parks often sited on whatever Medium - in environmental pressures. Restoration can act as a sponge to absorb excess land is available • Ouagadougou • Street Linear parks are and creation stormwater. Green spaces design that may Increases interception such green level • strategically built along the of green also be considered includes: linear parks and storage capacity of urban spaces Neighbor river/canal banks to assist 18 urban spaces to create green areas along the Green stormwater in urban Requires land will be relevant hood in the preservation and (e.g. linear watercourses through riverbank spaces, reducing only in areas level • conservation of stream parks, pocket naturalization to dampen flood peaks and pressures in the urban where water is City level beds. Pocket parks are parks) bring green space; and pocket parks drainage system • available opportunistic, often sited on which represent small public parks Contribute to social and nearby. whatever land is available, (generally occupying less than one acre of human wellbeing and might be constructed to land) and can be sited strategically to revitalize unused or revitalize unused land. underused land (e.g. decommissioned railroad tracks). Pre-disaster action plan (timeline) that Requires the aims to prevent damage and allow public Flood collaboration and This measures does not transport to resume operation at an early monitoring coordination of High - can be reduce the flooding risk, but stage. The actions should be triggered • Low cost • Can be 19 and disaster Soft public authorities, City level applied aims at being better depending on the information provided by rapidly implemented prevention private anywhere. prepared when extreme a flood monitoring tool (considering system companies, and floods occur. current and predicted rainfall and water inhabitants level in the rivers) The use of wetlands for stormwater In Ouagadougou, the Low - the areas management is widely adopted in many • Filters and cleanses Complexe du Parc Urbain where this urban areas. The wetland needs to be water naturally • Bãngr – Weoogo et du lac solution would restored or constructed such that system Encourages habitat Requires lands des trois barrages has been Wetland be applicable hydraulic efficiency is optimized, healthy creation and promotes and organization designated as Wetland of 20 restoration or Green City level (such as the vegetation is sustained and a balance of biodiversity • Beautifies of uses of public International Importance creation Bangr Weogo ecosystem maintained. In Ouagadougou, landscapes • Detains spaces (Ramsar Site no. 2367) is park) are the Complexe du Parc Urbain Bãngr – and cleans stormwater suffering from increased outside of the Weoogo et du lac des trois barrages has runoff degradation and restoration Area of Interest. been designated as Wetland of efforts combined with 73 International Importance (Ramsar Site no. integrated nature-based 2367). solutions could greatly enhance the stormwater infiltration capacities of this important urban wetland. The site, located in the heart of Ouagadougou, covers 945 hectares including two linked areas: the Bãngr – Weoogo Urban Park, and the three reservoirs and dams of the city. Suppressing all natural hazards would require unlimited resources. Any infrastructure is designed to be protected against a given level of risk, • Does not associated with a return reduce flood Although protection measures will be Medium — period, and is not protected risks• May take Develop a implemented, the residual flood risk mobile against more severe events. time to risk should be taken into account in the technologies are The idea of this solution is implement a awareness management of the bus operating • Cost-effective solution well adopted in to accept that this residual functional culture in the company. Risk awareness includes to reduce flooding Burkina Faso risk will cause disruptions system• Requires bus system measures such as: impacts on bus (90% of the to the bus service and plan coordination 21 management - Develop a pre-disaster action plan operation• Allows users City level population and measures to limit the impact between company - Develop a business continuity plan, to adapt • Can be used 75% of women) of these disruptions. One institutional (SOTRACO, identifying the sections of the lines that to signal other — similar can imagine that network stakeholders, bus future mass should be maintained in case of extreme Soft disruptions solutions were operators would send such operating transit storms, implemented in notification messages to all company and system) - Develop a communication plan to inform Nairobi, for mobile phones connected to network users based on mobile technologies (SMS) example the antennas of the city, or operators at the of specific areas. This city level solution doesn't solve any problem, but avoid users waiting in vain for a bus that will not come due to a flood in another part of the city. Maintenance of existing This measure provides an structures Efforts need to immediate improvement of Develop institutional arrangements to plan (canals, be maintained in High - can be flood protection in the areas periodic and systematic cleaning and • Low cost • Ease of 22 culverts) Soft the long term in City level applied where floods are caused or maintenance works (before the rainy execution whose order to keep anywhere. aggravated by obstruction season). current their efficiency of the hydraulic structures capacity is with silt and waste. reduced 74 Medium - Enforce regulations aiming at limiting • Additional efficiency of this As a first step, such water run-off generated by new constraints and measure would regulations could be constructions. A simple way is to impose a costs for project depend of the applied only to construction • No cost for the city • Implement limit on the flow a newly developed area developers → ability of the projects by companies and Reduction of peak flows regulations can discharge to the drainage network acceptability municipality to real estate developers. It arriving to the drainage 23 on (e.g. in France it is 3l/s/ha max for the 10 Soft issues• Applies City level enforce it and would limit aggravation of network → reduces the construction year return period rain). This engages only to new make sure that soils impermeabilization, by need for grey projects developers to integrate stormwater constructions → new imposing alternative infrastructure management in their design and takes time to development techniques such as porous implement solutions to reduce flows sent have a significant projects meet pavement in parking lots or to the network. impact the new bio-swales. requirements. Table 5.4: Explanations and sketches of some of the proposed measures Technical Relevance Description Depiction solution Rain gardens are proposed as water retention solutions in several measures (i.e. 7, 9, 13 and 17). The open space situated in the land of the Mogho Naaba Palace would be suitable to Included in install a rain garden. However, the Mogho measures 7 Naaba being the monarch of Wogodogo (protecting future (Ouagadougou), one of the Mossi Kingdoms Rain gardens Line 4), 9 (Line 5), located in present-day Burkina Faso, there and pockets 13 (Line 7) and 17 may be some cultural barriers and land parks (protecting the ownership issues related to the installation of road section in city a rain garden in this area. Therefore, we center shared by propose instead to install a pocket park to most lines) green this space, which will require less design and construction works. It is worth noting that the rain garden and pocket parks should be designed to include resilient and endemic species. Sources: Top sketches: Public Utilities Board (PUB) Singapore (2018); Bottom left sketch: © Hoerr Schaudt. https://nl.urbangreenbluegrids.com/projects/normals-uptown-water-circle-waterrotonde-in- normal-illinois-us/ Bottom right sketch: Sfbetterstreets.org. www.sfbetterstreets.org/find-project- types/greening-and-stormwater-management/stormwater-overview/bioretention-rain-gardens/ 75 Install a bioswale in the median of the road on Measure 2 Ave Kwame (protecting the Nkrumah and section in the city Ave de centre shared by l'UMOA (that most future mass are large and transit lines) have a descending slope) Source: Public Utilities Board (PUB) Singapore (2018) Put in place a temporary The itinerary of the proposed temporary diversion diversion is shown in the sketch on the right. through Ave de la Liberté and Avenue du Barrage Measure 4 Avenue de la appears unimpacted by the 2-year RP storm (protecting future Liberté and event (scenario 1). Although, it seems that Line 1) Avenue du Avenue du Barrage could be flooded (the area Barrage if next to the lake), the road is elevated and Nongremasson therefore unimpacted. St. is blocked The first 1.8km of the canal are built in concrete and in good structural condition, but needs cleaning since its bed is partially obstructed with waste and vegetation. The downstream part of the canal (after the crossing with line 1 of the bus on Rehabilitation Nongremasson street) is in earth, with variable Measure 1 of upstream sizes, and is largely filled with silt, with the (protecting future part of the presence of vegetation and solid waste of all Line 1 in Canal central kinds. On this downstream section, the canal Nongremasson St and behaves like a "dustbin" due to the discharges and the section development of the riparian populations directly into the shared my most of its canal and market gardening in the vicinity of lines in the city downstream the canal. centre) part The obstruction of the downstream part of the canal causes water level to be permanently Canal central (Avenue de la liberté bridge) Canal central (Nongremasson street bridge) high in this section of the canal. This water is used for gardening activities on the shores of the canal. It should be noted that the canal 76 receives wastewater from nearby buildings, thus posing health concerns. The current condition of the canal central, especially its downstream part, causes frequent floods during the rainy seasons, including in the upper part of the canal, up to the bridge of Nongremasson street used by line 1 of the planned bus system. This situation is clearly identified in Ouagadougou’s Drainage Master Plan, which recommends the following measure (page 150): “To remedy this situation, we propose to convert the existing earthen section (Reach No. 3) until the RN3 road crossing structure into a trapezoidal concrete channel. A canal 14m wide at the base and 19.4m wide at the top and 1.8m high is proposed. The development of the downstream section of the central canal will make it possible to eliminate Downstream section of Canal Central Culvert at the end of canal central – fully obstructed the flooding of the 10-year return period. In addition, it is necessary to plan cleaning and rehabilitation work on the existing concrete The cost of cleaning / rehabilitation of existing concrete sections has been estimated at US$100,000 reaches.” in the Drainage Master Plan. The construction cost for developing the downstream part into a concrete canal (19.4 m wide and 1.8 m deep) is estimated at US$2,031 per meter for a length of 1,480 meters, resulting in a total construction cost of US$3,005,319. The upper part of the Mogho Naaba canal (upstream from its crossing with avenue Ouezzin Coulibaly, and until its beginning next to Lycée Universalis) has been recently developed, with the construction of a flood control basin near its beginning, and a new trapezoidal concrete canal 10 to 25m wide. This newly developed part is connected to an older section of the canal which is only 10.5m Construction wide. This very significant narrowing of the of a new flood section causes a rise in the water line and Measure 16 retention overflows in the vicinity of the canal and the (protecting future basin along districts drained towards Mogho Naaba. Line 7) the Mogho Flooded areas include a section of the Naaba canal Ouezzin Coulibaly Avenue, where line 7 of the bus system crosses the canal. Two possible solutions have been proposed in the Ouagadougou Drainage Master Plan to solve these floods caused by overflow of the Moogho Naaba canal: - Rebuild and enlarge the downstream part of the canal - Create a new flood control basin Limit between the newly constructed canal (upstream), and the old existing part (downstream) 77 The second option has been selected by the municipality of Ouagadougou. It consists in the construction of a new flood control basin of 10ha on the east bank south of Avenue Ouezzin Coulibaly. With a depth of 4.7m, this basin would offer a 410 000m3 storage capacity, enough to prevent overflow of the downstream part of the canal against a 10- year return period storm. This solution also includes construction of 120m of new concrete canal in the downstream part of the canal, to reach the lake, and rehabilitation of the damaged section of canal between Avenue Ouezzin Coulibaly and Avenue Kadiogo. Being located just upstream of the flood- prone area on future bus line 7, this flood control basin designed to avoid floods during Damaged section of the canal between Avenue Ouezzin Coulibaly and Avenue Kadiogo. The Drainage storms up to the 10-year return period would Master Plan reports these damages as being caused by recent overflows. Inlet photo: SCET-Tunisie thus be enough to protect the targeted area and AGEIM (2019) of the mass infrastructure system on line 7. This is a minor part of the benefits that this basin would bring since its main impact would be to reduce floods in the downstream part of the canal. Area identified for the construction of a new flood control basin (Source – Actualisation du schéma directeur de drainage des eaux pluviales de la ville de Ouagadougou, p 155) 78 Two reservoir dams are located north of the AOI. They were initially designed for water storage in order to supply drinking water to the city. This role is now largely obsolete since much larger dams were built further north, to face growing needs in the water supply. While there was initially a need to store as much water as possible in these dams, their current situation has adverse impacts on flood control: - The high altitude of the spillways (elevation 287.35m for lake 3) results in a high water level in the lakes, which creates a downstream condition for the canals that flow into the lake, thus limiting their capacity and causing water level to rise in the canals (particularly the Mogho Naaba canal, whose downstream Cleaning of region is continually underwater). the reservoir - These dams have been partially filled by the lakes north of accumulation of silt, thus reducing their the area study storage capacity. and Location of Lake 2, Lake 3, the SOTRACO bust depot and road N3 modification of These issues are well described in the the spillway to Measure 26 Drainage Master Plan: “The basins of dams N° lower water 1, 2, and 3 have silted up as a result of solid The Drainage Master Plan recommends lowering the water level in the dams through the construction level in the deposits and the excavation heights of new spillways for dams 2 and 3, with the flowing characteristics: lakes and determined are 1.5m, 2m and 2m - New 60m long spillway at elevation 286.2m for the Ouaga 3 dam improve outlet respectively. This situation considerably - New 70m long spillway at elevation 286.2m for the Ouaga 2 dam conditions of reduces the retention capacity of these basins the canals and 3,801 418 m3 of water, corresponding According to the Drainage Master Plan, these new spillways would ensure the water level in the lakes to the amount of silt, threaten the banks every does not go above 288m, even during 50-year return period floods. This would avoid overflow of the year.” lake on the Nationale 3, and on the south bank which is protected by a levee reaching the 289m The Master Plan indicates the Mogho Naaba elevation). canal is the most affected by these issues: “The canal on its section between the Kadiogo This lowering of the lakes level would also improve the flow in the Mogho Naaba canal, thus solving bridge and the N°2 dam is constantly under some of the flooding issues of the surrounding areas of the canal as well as stagnant water issues. water over almost its entire section, due to the Although these impacts do not affect the bus system, they are significant co-benefits. The significant silting up of the dam downstream construction of the new spillways has been estimated in the Drainage Master Plan at 1.8 billion FCFA of the canal”. (US$3.3 million). “For the proper hydraulic functioning of the Mogho Naaba canal, it is imperative to find a To be fully effective, the construction of the spillways should be completed by the cleaning of the basins mechanism to lower the water level in the dam of dams 2 and 3. Due to the volume of silt accumulated over the years (more than 2 billion m3), the (by adjusting the spillway's level) and also to cost of fully cleaning the basins of all silt appears prohibitive (over 40 million $). For this reason, it provide for a local cleaning of the dam in the seems more reasonable to consider at least yearly cleanings of a smaller scale, aiming at least at basin area adjacent to the canal. Total removing more silt than accumulated over the past year. cleaning of the dam is necessary in the long term.” Construction of drainage systems comprises Hydraulic structures Unit price Construction Small concrete channel (60x60cm), covered with concrete slabs $240/m (130 000 FCFA/meter) Relevant for several longitudinal channels and transversal of secondary measures hydraulic structures (box culvert or pipe Medium concrete channel (80x100), covered with concrete slabs $510/m (280 000 FCFA/meter) and tertiary culvert). The construction cost of these 79 drainage structures has been assessed by drawing a Large concrete channel (150x120), covered with concrete slabs $880/m (480 000 FCFA/meter systems scheme of the necessary networks and using Pipe culvert (concrete, diameter 800mm) $730/m (400 000 FCFA/meter) average unit prices provided by the local member of our consortium or taken from Pipe culvert (concrete, diameter 1200mm) $1,100/m (600 000 FCFA/meter) quotations of construction projects in Africa supervised by Louis Berger. Box culvert (concrete, 2mx2m) $4,760/m (2 600 000 FCFA/meter) Build a drainage system to There is currently no existing drainage system collect run-off in this flood prone area. This measure includes water for the small channels and pipe culverts to ensure neighborhood Measure 5 hydraulic transparency of the 28.257 St used south-east of (protecting future by line 7: run-off water in the streets to the 28.257 street Line 4) south east is collected in these streets, passed and channel under the street 28.257 with several culverts, them until the and carried until the retention basins to the flood control west with several channels. basins of the Wemtenga canal This measure is an alternative to measure 5, to solve the same flooding issue in this area which has no existing drainage system. This measure includes a simple drainage system: Build a run-off water coming from perpendicular drainage Measure 6 streets is collected by a channel running all system (protecting future along the 28.257 street, and carried away to focusing on Line 4) the flood control basins through a single large protection of channel following Naaga Loada street. This 28.257 street measure focuses on protecting against flood the 28.257 street used by line 7. It does not improve the situation of the upstream streets to the south-east. 80 This measure is an alternative to measures 5 & 6, to solve the same flooding issue in this Drainage area which has no existing drainage system. It system to follows the same principle as the previous protect measure, with a channel on the right side of 28.257 street, the 28.257 street to intercept run-off water combined with coming from the south. In this option, the water channels are smaller than in the previous one, retention because the streets to the south east are solutions for equipped with porous pavement used to slow the Measure 7 down run-off and thus reduce the peak flows. neighborhood (protecting future A rain garden in the area in front of Saint Paul to the south- Line 4) Proposed drainage system to collect run-off water on the side of street 28.257 apply permeable church reinforces the upstream storage of east: pavement (orange) on the main streets to the east and build a rain garden (green) in the water. permeable administrative reserve in front of Saint-Paul church pavements and rain This solution can be implemented in two garden in the steps: area in front of - Build the main channels Chapelle St - When the streets to the south-east will be Paul paved, use porous pavement. 81 Build a drainage Small channels already exist along street system to 14.09, but they seem to collect only run-off protect 14.09 from the 14.09 street surface, and they do not St. from intercept water from the perpendicular streets. transverse Measure 8 Field data confirms that this area suffers from flows and (protecting future floods coming from the streets to the east. We ensure Line 5) thus propose to keep the existing channels, longitudinal add portion of new channels to intercept run- drainage of off from the east, and create several outlets to Ave des Arts the Zogona canal. North of Canal de l'université 82 Some existing channels are visible on avenue Kanazoe, between its intersections with avenue ME/ Pacere and with Ave Coulibaly, draining part of the avenue Kanazoe to the Mogho Naaba canal to the west. No such channel is visible in the northern part of the street, but 2 culverts have been observed under the Place de la bataille du Rail intersection (light blue in the map). Only the Build a outlet of the existing culvert could be drainage Sketch of proposed drainage network to drain the northern part of Ave Kanazoe observed. Further inspections would be system to necessary to make sure the whole culvert is in protect Ave Measure 11 good structural condition. Oumarou (protecting future The proposed measure includes a new Kanazoe and Line 7) network in the north of Avenue Kanazoe, Place de la including a branch of channel collecting run-of bataille du rail east of the Place du Rail roundabout. The roundabout existing culvert would be kept in place to conveys the flow under avenue Kadiogo, to a new large channel towards the Mogho Naaba canal. In the southern part of the street, the proposed measure uses the existing channels and outlets to the canal, simply adding missing sections of channel on both sides of the street. Sketch of the drainage network proposed to drain the southern part of Ave Kanazoe. Existing channels are shown in purple 83 As an alternative or supplement to the previous measure, one can consider improving water storage in the upper areas upstream of Avenue Kanazoe, using permeable pavement on the currently unpaved streets and creating water storage areas on the open space Water Bambata high school and the palace of the retention Mogho Naaba. These water retention areas solutions for Measures 12 and could either take the form of rain gardens, the 13 (both pocket parks or floodable sports fields. neighborhood protecting future However, access to the grounds of the Mogho east of Avenue Line 7) Naaba palace might prove difficult. The Mogho Oumarou Naaba is the traditional chief of the Mossi Kanazoe people. Although he has no legal power, he represents a highly respected moral authority in the country. Expropriation of his property to build a water storage facility is thus unthinkable, and such development would Sketch of the proposed retention solutions to reduce peak flows towards Ave Kanazoe. channels are require prior consultation to build it by mutual shown in purple, permeable pavement in orange, pocket parks / rain gardens in green agreement. 84 Build a longitudinal drainage Measure 14 There are no drainage system along this axis. system on (protecting future We propose a simple longitudinal system to Simon Line 7) protect Simon Compaoré street. Compaoré Sketch of the network proposed to drain Simon Compaore street towards Mogho Naaba canal street Some channels have been observed in this area, but they do not seem to form a coherent and sufficient drainage system, as field data reports frequent floods. Avenue Boumedienne Build a new crosses the valley of the canal central drainage watershed. It has a low point around its network to intersections with Avenue de l’UEMOA and protect Ave Measure 18 Avenue du PNUD. We thus propose to Houari (protecting future intercept waters coming from the south with a Boumedienne Line 5) channel on the side of Ave Boumedienne, and and Ave de la to convey them to the canal central which Grande starts 300 meters to the north, using a Mosquée channel through avenue de l’UEMOA. Another channel along Avenue de la Grande Mosquée would collect run-off water from the west, and channel it to the canal central through Patrice Lumumba street. Sketch of the proposed drainage network to avoid floods on Avenue Boumedienne and Avenue de la Grande Mosquée 85 In several instances, the extent of the flood simulated by the hydraulic model initially led to the conclusion that a street / area was completely lacking drainage system, and that the obvious measure to implement to solve flood in this area was to build gutters, or bioswales when possible. In situation where a drainage system exists but is not included in the hydraulic model, the simulated floods are thus exaggerated, and the construction of new channels is not relevant. However, also the existing channels are often partially filled with Set up a solid waste, silt, or even vegetation, which dedicated hampers their drainage capacity. This led to maintenance proposing a measure consisting of the Measure 21 plan and team maintenance of all flood-protection structures. (protecting the in charge of The best way to estimate the cost of this road section the periodic measure was considered to be not to use Example of channels partially filled with waste, silt or vegetation shared my most and systematic linear costs for the cleaning of a channel or a future mass transit cleaning and canal, but to propose the dimensioning of a lines in the city maintenance team that would be specifically assigned to center) of flood the task of maintaining all the hydraulic related structures related to the flood protection of structures the bus system layout. It is proposed that this team could be tasked with the routine maintenance of all hydraulic structures, including channels but also culverts, that are regularly obstructed by solid waste, as well as canals and flood retention basins which get progressively filled with silt when left without proper maintenance. Green solutions such as bioswales or rain gardens, if selected and built, will also require frequent maintenance to remove waste and invasive Example of partially obstructed box culvert Example of canal to be cleaned plants and also cut excess grass. 86 The activities of the team would vary along the year: ▪ During the rainy season, they would focus on daily tours after significant rain events to identify and remove newly formed obstructions. ▪ During the dry season, they would follow a pre-established plan focusing on larger scale activities, such as clearing the banks and bottoms of the canals, removing silt accumulated in flood retention basins, cleaning the surface of permeable pavements or maintaining green areas. Considering the number of structures involved in protecting the bus routes against flooding, we have estimated the required size for this dedicated team to be around a dozen workers, split into two teams, with two trucks and small equipment (shovels, gloves, protecting equipment, etc). A manager would be added to plan and organize the team’s activities, based on regular inspections of the structures included in the scope of the maintenance team, in order to detect sections requiring cleaning. It is to be noted that cleaning and maintenance of all hydraulic structures in cities is part of the standard responsibilities of the municipalities. However, in Ouagadougou as in many other large African cities, maintenance is often insufficient, leading to decreases in the efficiency of the structures. The analysis and solving of the reasons for this recurrent issue of insufficient maintenance is out of scope of the present study. However, one should consider this factor when proposing the development of a dedicated maintenance team. Without proper planning, it is to be feared that lack of maintenance would also rapidly affect the newly developed rapid bus system. The idea of having a team dedicated to the structures protecting the bus routes partially reduces that risk: instead of relying on the ability of the municipality to ensure maintenance of the whole city (which is not very likely due to resources shortage) the development of a small team dedicated to specific sections increases the chances of having maintenance correctly performed in this limited scope. One should still define the institutional arrangements that would be put in place to manage this team. We have identified two options: ▪ The maintenance of urban infrastructures belonging to the city being a standard responsibility of the municipality, the most obvious measure would be to include this team among the municipality staff, as a sub-team among the municipal employees in charge of urban cleaning. ▪ The proposed team being dedicated to the maintenance of structures protecting the bus network, one could also imagine transferring this responsibility by contract to the bus system operator. Since the municipality of Ouagadougou currently struggles to ensure maintenance of the existing network, it seems appealing to entrust maintenance of the structures contributing to the protection of the bus system to its operator. However, this solution also shows some serious drawbacks: ▪ While cleaning of channels along the bus routes and culverts under it seems accessible for a team employed by the bus operator, it is worth noting that many structures contributing to the protection of the bus routes are actually far from these routes: the proposed measures include upstream retention basins, portions of canals and bridges downstream the bus route, bioswales, rain gardens and permeable pavement that are located outside of the right-of-way of the roads used by the buses. ▪ Putting the bus system operator in charge of the maintenance of some hydraulic structures, and the municipality in charge of the same type of structures just a few meters away may generate coordination issues. ▪ While the main function of longitudinal drainage is to prevent flooding of the road surface, other structures like the proposed flood retention basin or canals are designed to protect larger areas of the city. Asking the bus system operator to maintain these structures is very far from the typical activities of such entities. ▪ Adding maintenance activities to the bus system operator’s scope of work would increase its exploitation costs. Since the fin ancial analyses included in the OPTIS report show that the financial balance of the operation is already fragile, adding cost to its operation would involve an increase in the tariffication, which is not desirable. For the above reasons, we consider the “standard” solution of having the municipality manage the maintenance of the structure s involved in the prevention of floods on the bus route to be better suited. However, a mechanism should be established to make sure that the municipality takes ownership of the need for such maintenance, otherwise it will likely be diluted within its many other urgent needs. One of the key issues of a similar initiative in Cameroon was to help the urban communities improve their own resources (human, technical and financial resources) to make sure that they are able to properly manage and maintain the new infrastructures built under the program and transferred to them. Sustainability of the investments also relied on “City contracts”, which are written agreements between the Ministry in charge of Urban Development and the cities benefiti ng from the investment program. These contracts covered the following aspects: ▪ an investment framework program aimed at meeting the economic and social development objectives of the cities, specifying the sources of financing; ▪ a road maintenance framework program, aimed at providing a road maintenance account with sufficient funds to guarantee the sustainability of the works carried out; ▪ a management improvement program, allowing the settlement of cross-debts between the State and the urban communities. This program also includes measures to strengthen the financial services of the cities in order to improve the transparency and legibility of the preparation and execution of urban communities' budgets. Similar arrangements could be implemented in Ouagadougou, and a contract or convention could be signed with the municipality, specifying what investments the municipality will receive, and what efforts it is committed to making, including in terms of maintenance of the existing or new structures. 87