Riverine Flood Inundation Mapping, Hazard, And Vulnerability Assessment, in the Lower Omo-Gibe River Sub-Basin, Ethiopia

Abstract

Flooding is a significant natural hazard in Ethiopia, frequently resulting in property damage, loss of life, and disruption of livelihoods. With increasing frequency and severity, floods pose a serious threat to sustainable development and demand effective management strategies. This thesis focuses on the Lower Omo-Gibe River Basin, where flood inundation, hazard, and vulnerability were analyzed using the HECRAS hydrodynamic model integrated with geospatial tools. The main objectives were to simulate flood extents and water surface profiles, delineate flood hazard zones, evaluate the applicability of HEC-RAS for flood mapping, and assess the vulnerability of communities and assets in flood-prone areas. Regional flood frequency analysis was conducted using L-moment-based methods, with streamflow data from three gauging stations. Generalized Extreme Value and Gumbel distributions were selected as the best fits for estimating flood quantiles for return periods ranging from 2 to 200 years. These quantiles were input into the HEC-RAS model to determine flood depth, extent, and velocity. The model was calibrated at Omorate station and showed strong performance (NSE = 0.87, PBIAS = -3.79, R² = 0.83) with an adjusted Manning’s n value of 0.03. Simulation results revealed an increasing trend in inundation extent with longer return periods, with the 200-year event causing the highest flood levels. Kuraz, Hamer Bena, and Dizi woredas were identified as the most vulnerable areas. Land use and land cover analysis indicated that croplands, rangelands, and built-up zones are highly susceptible to flooding. The study highlights the importance of integrating hydrodynamic modeling and geospatial analysis in flood hazard assessment. Recommendations include improving DEM accuracy, enhancing data accessibility, promoting ecosystem based flood management, and using model outputs for planning, mitigation, and long-term disaster risk reduction.