Panacea to the Catastrophe Through Mapping in Anticipation of Mitigating the Flood Magnitude Along Wabi Shebele River Basin of Somalia

Abstract

Somalia had experienced extreme flash floods in the recent years across the arid regions causing tremendous loss in lives and properties. However, the exact magnitude, depth, extent and frequency of occurrence of flooding is not yet quantified due scarce hydro-meteorological datasets. In this study, integration of observed meteorological and Climate Hazards Infrared Precipitation (CHIRP) satellite rainfall products along with various remote sensing raster data were used to improve hydrological model simulation outputs. The Hydrologic Engineering Center-Hydrological Modeling System (HEC-HMS) and Hydrologic Engineering Center-River Analysis System (HEC-RAS) models were adopted to simulate the rainfall-runoff processes to compute the floods' inundation extent and depth, respectively using land use, soil, and slope map from ALASKA (12.5m x 12.5m). The performance of the rainfall-runoff model was evaluated using Nash-Sutcliffe Efficiency (NSE), Relative Volume Error (RVE), Coefficient of determination (R2), and Percentage Error of Peak Flow (PEPF) performance indices. The HEC-HMS model calibration and validation of simulated flows generated by the model predicted good agreement with the observed flow values of 0.79, 0.74 and 0.78, 0.78 for NSE, RVE, R2, and PEPF, respectively. The HEC-RAS model result indicates the maximum flood depth and velocity were obtained at the floodplain area. The peak flood at 5, 10, 20, 50, 100, and 200-year return period using General Extreme Value (GEV) distribution has been estimated to be 298 m3s-1, 325m3s-1 351m3s-1, 384m3s-1, 409m3s-1, 434m3s-1, respectively. The 100-years peak flood discharge in a specific part of the river revealed a flood depth of up to 7.53m. The provision of levee mitigation measures on the right and left sides of the river banks revealed reduction in high flood extent area. Hence, levee structure was suggested as possible as flood protection measures for the study area.

Keywords: Flood inundation mapping, CHIRP, HEC-RAS, Levee, Mitigation, Beledwayne, Shebelle River basin, Somalia