2D hydrodynamic modelling for urban flood hazard assessment: the case of Niamey, Niger

Urban flooding is one of the most critical natural hazards affecting rapidly growing cities in West Africa, where climate variability, land change and limited infrastructure increase vulnerability. This study applies a high-resolution hydrodynamic modelling framework to analyse riverine flood dynamics and spatial flood hazard in Niamey (Niger). A 2D hydraulic model was developed using HEC-RAS 2D to simulate five major flood events between 2010 and 2020. The model was calibrated against observed water levels for the 2020 extreme flood and validated using historical hydrometric data and field observations, achieving very good performance (NSE = 0.982, KGE = 0.970). Spatially explicit hazard indicators (depth, velocity, duration) were derived and combined to produce flood hazard maps. Results suggest an increase in flood impacts over the last decade, with floodplain expanding by about 17% and prolonged flood durations (30–60 days) affecting more than 75% of flooded surfaces during extreme events. Agricultural land and densely populated low-lying neighbourhoods were identified as the most vulnerable zones. The analysis assumes static land use and a diffusion-wave approximation. Therefore, results represent flood hazard rather than full risk. The proposed modelling approach provides a robust and transferable framework for urban flood hazard assessment in data-scarce environments.