Flood Inundation Mapping and Hazard Assessment: A Case of Holeta River, Ethiopia.

Abstract

Flood is a natural hazard that causes damage of property and life. Flood hazard assessment is particularly important for policy makers, in order to design mitigation strategies and implement flood risk management planning. During high rainfall depth at summer season of the year the overflow of Holeta River course is occurred and cause flood risk consequence. This study aims to map flood inundation area and hazard assessment, for upstream-Downstream Holeta River by using Geographic Information System, Analytical Hierarch Process, Hydraulic Engineering Center- River Analysis System and Hydraulic Engineering Center – Geometry River Analysis System software. The importance’s of this study is to have detail information about future floods and to serve for flood preparedness. For this study, Log-Pearson Type-III was used to compute peak discharge. However, there was limited recorded data at the downstream interested point, so transposing peak discharge from upstream catchment was employed by using Drainage Area Weighting (DAW) methods. The transposing Coefficient was estimated about 2.1506. Thus, the final peak Discharges for 10, 25, 50 and 100 years was estimated 350.75, 417.445, 465.551 and 511.68 m3 /s respectively. HEC-Geo RAS Software was used to develop river geometry such as: the river centerline, river bank, flow path and cut cross-section for upstream-Downstream Holeta River. HEC-RAS, hydraulic analysis includes the computation of the water surface profiles. The flood extent area of inundation mapping for 10, 25, 50 and 100 years was estimated about 22, 24, 26, 26.7 km2 and 49, 51, 53 and 53.5 km2 for steady and unsteady flood flow simulation respectively. Thus, the inundation mapping was developed for different return period with their extent areas from river course. Based on degree to flooding, the importance of selected parameters were ranked to four flood hazard category, namely very low flooding, low flooding, moderate flooding and high flooding. The weight coefficients were determined for each parameter by Analytical Hierarch Process (AHP) and overlay of ranked spatial information result the final flood hazard map of study areas. The flood hazard maps indicate that 77.9, 627.01, 567.05 and 47.07km2 corresponds with high, moderate, low and very low flood hazard areas respectively for Holeta River. Also, the flood hazard map gives the flood prone areas for upstream-Downstream Holeta River catchment. Hence, most of the catchment areas are under low flooding and moderate flooding. Some parts are very low flooding and high flooding is less in area