Flood Inundation Mapping In Webishabele River Using Google Earth Engine And Hec-Ras Model: Case Study Of Beletwayne City, Somalia

Abstract

Flooding from the Shebelle River is a persistent and severe hazard in Beletweyne City, Somalia, causing significant loss of life and infrastructure damage. This study modeled these dynamics by integrating the HEC-RAS hydraulic model with the Google Earth Engine (GEE) geospatial platform. A 1D/2D HEC-RAS model was developed using a 30m Digital Elevation Model (DEM) and Manning’s roughness coefficients derived from land use and land cover (LULC) data. The 2D model was calibrated against the October 2019 flood event, yielding a coefficient of determination R2=0.89. Spatial validation against Sentinel-1 satellite imagery via GEE showed 54.106% overlap with the observed flood extent. Simulation results for 25, 50, and 100-year return periods reveal escalating risk to the urban area: the 25-year event produced a maximum flood depth of 3.80 m and an inundated area of 4.87 km², while the 50-year event produced a maximum depth of 5.97 m and an inundated area of 8.56 km². The 100-year event reached a depth of 8.58 m and an inundated area of 11.65 km², aligning closely with the 8.30 m bank-full level observed during the 2019 extreme floods. Unsteady flow simulations further generated critical risk maps for flood velocity, arrival time, and inundation duration. These high-resolution outputs provide a scientifically validated framework essential for risk-based zoning and the development of early warning systems in Beletweyne.