Flood inundation mapping and hazard assessment: a case of Upper-middle gilo river, ethiopia

Abstract

Flood is a natural disaster which causes loss of life and property destruction. Flood Hazard Assessment is particularly important for policy makers, in order to design mitigation strategies and implement flood risk management planning. This study aims to map flood inundation area and hazard assessment, for upper-middle Gilo River by using Geographic Information System, Analytical Hierarch Process, Hydraulic Engineering Center- River Analysis System and Hydraulic Engineering Center – Geometry River Analysis System software. Based on degree to flooding, the importance of selected parameter ( Digital Elevation model, slope, rainfall, flow accumulation, drainage density, land use/cover and soil) are ranked to four flood hazard category, namely very low flooding (1), low flooding(2), moderate flooding(3) and high flooding(4). The weight coefficients are 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 76.26, 2410.29, 5817.44 and 598.06 km2 corresponds with high, moderate, low and very low flood hazard respectively for upper-middle Gilo River. For this study, Log-Pearson Type-III was used to compute peak discharge. However, there is limited recorded data at the downstream interested point, so the transposing peak discharge from upstream catchment was employed by using Drainage Area Weighting (DAW) methods. The transposing Coefficient was estimated about 2. 794 (Appendix B). Thus, the final peak Discharges for 10, 25, 50 and 100 years was estimated 255.66, 314.64, 360.60 and 406.53m3/s respectively. HEC-Geo RAS Software used for to develop river geometry such as: the river centerline, river bank, flow path and cut cross-section for upper-middle Gilo 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 is estimated about 54.72, 57.13, 58.88, 60.51 km2 and 71.76, 83.49, 85.98 and 88.12km2 for steady and unsteady flood flow simulation respectively. Thus, the inundation mapping are developed for different return period with their extent areas from river course. Also, the flood hazard map gives the flood prone areas for upper-middle Gilo 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 areas. For further study, the smallest cell size Digital Elevation model, the historical flood existing map or flood record data are better to compare extent areas as well as flood prone areas with simulated flood map.