Watershed Characterization for the Construction of Khar Yalla Neighborhood Protection Infrastructure in Saint-Louis

This paper focuses on the design of a flood protection scheme in the Khar Yalla district of Saint-Louis, Senegal. This area, located below sea level, is exposed to severe rainfall and groundwater flooding due to its basin-like topography, the low permeability of its clay soils, and the lack of an effective drainage system. The study is based on a comprehensive hydrological analysis of the 44.4 km² watershed, whose annual useful inflow is estimated at approximately 397,567 m³. The 100-year flood flow, estimated at 164.30 m³/s by the ORSTOM method, served as a basis for the design of the structures. The main dike, designed in homogeneous earth with a predominantly clayey content, has a maximum height of 3.44 m, a crest width of 3 m, slopes inclined at 2 H/1 V and a total length of approximately 615 m. A cyclopean concrete gravity spillway was planned in a frontal position to ensure controlled flood evacuation. It is set at a height of 3.3 m, with a useful length of 13.5 m and a maximum height of 3.25 m, incorporating the rolling effect to optimize hydraulic operation. Downstream, a dissipation basin with a step, reinforced by gabions arranged in a staircase, was designed to control the flow speed and limit the risks of erosion. Despite the presence of these structures, some areas of the neighborhood remain difficult to secure. Thus, partial or temporary relocation of the most exposed residents has been considered. The field survey revealed a moderate but lucid acceptability of this option, provided that it is accompanied by a clear framework of compensation, transparency, and dignified rehousing.