Abstract
The increasing hydroclimatic variability observed in the Sahel poses a major challenge to the design and safety of road infrastructure exposed to flooding. This study applies an integrated hydrological and hydraulic approach to the design of the Sorey Bridge in the Niamey Region of Niger. The methodology combines precipitation-frequency analysis, GIS-based morphometric characterisation of the watershed, estimation of the 10-year flood using the ORSTOM and CIEH methods, extrapolation of the 100-year flood using the GRADEX method, and hydraulic analysis based on the Manning–Strickler relationship. The watershed covers 1,359.75 km², with a perimeter of 285.32 km and a main channel length of 50.39 km. Rainfall analysis for 1991–2000 shows marked interannual variability, with mean annual precipitation ranging from 410.7 to 816.3 mm and a maximum daily rainfall of 129 mm. The 10-year discharges estimated by ORSTOM and CIEH are 196.22 and 225.86 m³/s, respectively, giving an adopted mean design discharge of 211.04 m³/s. The GRADEX method gives a 100-year discharge of 896.78 m³/s. Hydraulic analysis yields a flow depth of 3.40 m, a mean velocity of 2.90 m/s, and a backwater rise of 0.208 m. Total pier scour is 3.86 m. The proposed protection comprises 0.74-m-diameter riprap, with a 3.00-m width and 2.22-m layer thickness, together with a 2.00-m freeboard. These parameters support the hydraulic design of the Sorey Bridge under extreme flood conditions.