Application of HEC-RAS and HEC-GeoRAS model for Flood Inundation Mapping, the case of Awash Bello Flood Plain, Upper Awash River Basin, Oromiya Regional State, Ethiopia

Abstract

Flood is the devastating natural events as it causes massive destructive of life, economy and infrastructure. The main objective of this study was flood inundation mapping using HEC-RAS model: the case of Awash Bello flood plain, Upper Awash River basin, Ethiopia. River geometry, annual peak flood, and boundary conditions are the most important input parameters for RAS preprocessing. The river cross-sections along the flood plain were extracted using HEC-Geo RAS. Annual peak flood frequency analysis for different recurrence intervals (i.e., 2, 5, 10, 25, 50, and 100 years) was computed using the calibrated and validated HEC-HMS model based on a 25 years (1990–2015) hydro-meteorological data collected from Ministry of Water Resources Irrigation and Electricity and National Meteorological Agency, respectively. The model result depicted the flood extreme value for the respective recurrence interval was 526, 610, 828.8, 1072.8, 1263.6, and 1461.3 m3/s, respectively. For more reliability, the HEC-HMS model outputs were compared with the flood extreme value obtained from Log Pearson type-III, General Pareto and Gumbel extreme value distribution function and it was found the HEC-HMS model result for every recurrence interval was higher than the extreme value distribution function outputs. After verifying the acceptability of the HEC-HMS model result, the verified peak flood was inserted into HEC-RAS model and flood inundation mapping for different recurrence intervals were executed. The HEC-RAS model outcome indicated that the flood inundation mapping area for 2, 5, 10, 25, 50, and 100-year recurrence intervals, respectively, was 71.475, 76.630, 89.150, 100.290, 105.160, and 109.462 km2. Finally, it was realized that the whole Awash Bello flood plain is under the influence of flood inundation due to the intensive rainfall event.