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In recent years, West Africa has experienced an increasing number of flood disasters, urging governments and decision makers to take adaptation measures, particularly in cities where the population is growing very fast.The present study focuses on the hydrological hazard of the city of Niamey (capital of Niger) along the Niger River. The regime of the Niger river at Niamey is marked by two successive floods: a first flood (July-August) resulting from the contributions of local Sahelian catchment (Sahelian red flood) and a second flood (February-March) resulting from the contribution of upstream catchments (Guinean black flood). Past studies have shown how the 1st flood, which was almost non-existent in the 1950s and 1960s, has predominated since the 1980s causing the most damaging floods due to a combined effect of climate and land use/cover changes. However, there is no in-depth study on statistical flood modelling, which is an essential step in any flood management strategy. The aim of the study is to evaluate how Niger discharge regime has changed since the 1950s with a specific focus on floods with the aim of deciphering the impact of Guinean and local flows in a statistical model.To do so, an original method is first proposed to isolate local flows from Guinean flows in the Niamey discharge data series. This separation allows us to distinguish three typologies of maximum annual floods according to whether they are generated by Guinean flows only, local flows only or a mix of the two. This observation leads us to propose different models of extreme discharge in Niamey based on the non-stationary extreme value theory able to consider both the typology of the floods and their temporal evolution.

International audience; Niamey, the capital of Niger, is particularly prone to floods, since it is on the banks of the Niger River, which in its middle basin has two flood peaks: one in summer (the red flood) and one in winter (the black flood). In 2020, the Niger River in Niamey reached its all-time highest levels following an abundant rainy season. On the other hand, the floods in Niamey have been particularly frequent in the last decade, a symptom of a change in hydroclimatic behaviour already observed since the end of the great droughts of the 1970s and 1980s and which is identified with the name of Sahelian Paradox. This study, starting from the analysis of the 2020 flood and from the update of the rating curve of the Niamey hydrometric station, analyses the rainfall–runoff relationship on the Sahelian basins of the Medium Niger River Basin (MNRB) that are at the origin of the local flood. The comparative analysis of runoffs, annual maximum flows (AMAX) and runoff coefficients with various rainfall indices calculated on gridded datasets allowed to hydroclimatically characterise the last decade as a different period from the wet one before the drought, the dry one and the post-drought one. Compared to the last one, the current period is characterised by a sustained increase in hydrological indicators (AMAX +27%) consistent with the increase in both the accumulation of precipitation (+11%) and the number (+51%) and magnitude (+54%) of extreme events in the MNRB. Furthermore, a greater concentration of rainfall and extremes (+78%) in August contributes to reinforcing the red flood’s positive anomalies (+2.23 st.dev in 2020). The study indicates that under these conditions the frequency of extreme hydrological events in Niamey will tend to increase further also because of the concurrence of drivers such as river-bed silting and levee effects. Consequently, the study concludes with the need for a comprehensive flood-risk assessment on the Niamey city that considers both recent hydroclimatic trends and urbanisation dynamics in flood zones hence defining the most appropriate risk-reduction strategies.

Highlights ● Hydraulic modelling and satellite imagery enable hazard and exposure mapping. ● Local knowledge enables better understanding of hazards and their impacts. ● Benefit/cost and opportunity analysis support decision-making in risk treatment., In tropical regions, heavy precipitations may lead to catastrophic flooding due to the degradation of catchments and the expansion of settlements in flood prone zones. In the current situation, where information on rainfall and exposed assets is either scant, or requires significant time to be collected, pluvial flood risk assessments are conducted using participatory tools, without any scientific support. Another option is to use satellite precipitation products, digital terrain models and satellite images at high to moderate-resolution. However, these datasets do not reach the required accuracy at the local scale. Consequently, the potential damages and the evaluation component of risk assessment are often missing. Risk evaluation is pivotal for informed decision-making, with regards to the choice of treating or accepting the risk, implementing more effective measures, and for determining the safest areas for development. We proposed an improved method for assessing the risk of pluvial floods, which merges local and scientific knowledge and is consistent with the ISO 31010 standard. The method was successfully applied in five rural settlements in Niger and can be replicated in areas where information is scarce., Graphical abstract Improved method for assessing the risk of pluvial flooding locally. Its application in five rural settlements demonstrates its usefulness in decision-making regarding risk treatment and built-up expansion in safe areas.Image, graphical abstract

Floods have recently become a major hazard in West Africa (WA) in terms of both their magnitude and frequency. They affect livelihoods, infrastructure and production systems, hence impacting on Sustainable Development (SD). Early Warning Systems (EWS) for floods that properly address all four EWS components, while also being community and impact-based, do not yet exist in WA. Existing systems address only the main rivers, are conceived in a top-down manner and are hazard-centered. This study on the Sirba river in Niger aims to demonstrate that an operational community and impact-based EWS for floods can be set up by leveraging the existing tools, local stakeholders and knowledge. The main finding of the study is that bridging the gap between top-down and bottom-up approaches is possible by directly connecting the available technical capabilities with the local level through a participatory approach. This allows the beneficiaries to define the rules that will develop the whole system, strengthening their ability to understand the information and take action. Moreover, the integration of hydrological forecasts and observations with the community monitoring and preparedness system provides a lead time suitable for operational decision-making at national and local levels. The study points out the need for the commitment of governments to the transboundary sharing of flood information for EWS and SD.