ANALYSIS OF FLOOD MAGNITUDE AND INUNDATION MAPPING: CASE STUDY OF BELEDWAYNE TOWN THE WABI SHEBELE RIVER BASIN, SOMALIA

Abstract

Somalia has been powerful flash floods arid areas with danger frequently when the floods reach the center of the town it causes tremendous losses. This research looked at flood inundation mapping and mitigation steps, the aim of the research is demonstrated in the Wabi Shabelle river basin in Somalia, with a focus on hydrology and meteorology stations, which are used to predict floods and calculate their magnitude, depth, and extent in Beledwayne. The study was floodplain modeling with significant rainfall events on the Shabelle river basin. Meteorological and hydrological time series data based on the time length of 2005 up to 2018 was used. Observed historical ground meteorological and Climate Hazards Infrared Precipitation (CHIRP) satellite rainfall products were used to improve the quality of rainfall data and in turn the model simulation output. The land-use chart, soil map, and slope map from ALASKA (12.5m x 12.5m) were used for research. The HEC-HMS and HEC-RAS models have been used to simulate the rainfall-runoff processes and to compute the floods' inundation extent and depth respectively. The rainfall-runoff model's output was evaluated using Nash-Sutcliffe Efficiency (NSE), Relative Volume Error (RVE), Coefficient of determination (R2 ), and Percentage Error of Peak Flow (PEPF) performance indices parameters, the simulated flows generated the model in calibration and validation showed good agreement with the observed flow with an NSE, RVE, R2 , and PEPF value of 0.785, 0.743 and 0.785, 0.78 respectively. HEC-RAS model result indicates the maximum flood depth and velocity obtained at the floodplain area. The peak flood at 5, 10, 20, 50, 100, and 200year return period using General Extreme Value (GEV) distribution has been estimated to be 298m3 s-1 , 325m3 s-1 351m3 s-1 , 384m3 s-1 , 409m3 s-1 , 434m3 s-1 respectively. Based on the HEC RAS model 100 years of peak flood discharge in a selected section of the river the flood depth of 7.53m, levee mitigation was advised as a flood mitigation technique on the right and left sides of the river banks as a result of the output hydraulic modeling. Communities of Beledwayne have under the condition of flash floods and security, so flood protection should be planned as much as possible with making necessary from flooding area.

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