FLOOD HAZARD AND RISK ASSESSMENT USING GIS AND REMOTE SENSING: A CASE OF WESTERN WABE SHEBELLE RIVER BASIN, ETHIOPIA

Abstract

Floods are among the most common natural disasters around the world, particularly occurs in the vicinity of rivers. Ethiopia has continuous topographical land arrangement that ranges from high altitude of highlands to plain lands extending to lowlands, experiences high floods during rainy season. Curbing the recurrent flooding through addressing the problem and identifying flood-prone areas; mapping flood hazard, risk and locating the inundated area and stating some recommendable alternative mitigation measures. The main objective of this study was to execute flood hazard and risk assessment using GIS and Remote Sensing in Western Wabe Shebelle River basin. The DEM with high-resolution 30m was used to delineate the basin watershed in the catchment area 5,040.58 . Multi Criteria Evaluation procedure was evaluated to meet a specific objective technique to assess flood hazard in the Agarfa catchment using GIS environment for the selected basin and Eigenvector criteria weight was calculated for each factor in IDRISI and Excel software. Flood hazard Weighted Overlay with development factors such as elevation, slope, land cover, rainfall, soil and drainage density. Flood risk were also Weighted Overlay using the elements of population density, land-use and the flood hazard layer within the river basin. The study results showed that the downstream catchment area were more vulnerable to high flood hazard and risk. The best fitted probability distribution was GEV which calculates the maximum peak discharge of 1,215.46 ⁄ , 2,595.62 ⁄ , 3,179.53 ⁄ and 4,978.06 ⁄ for a return period of 10, 100, 200 and 1000 years respectively. The inundation map was developed in RAS Mapping tool of HEC GeoRAS; mapping water surface generation and floodplain along the Western Wabe Shebelle River basin with inundated area of 105.2ha, 143.6ha, 153.9ha, and 178.4ha for 10, 100, 200 and 1000 years of return period, respectively. The flooded area in the return period 1000 years was the highest compared to other return period tested due to the increased stream flow and the flood depth was excessive that ranges from 0.0015m to 92.6m and hence, the consequence of the flood on life and property was very high. The mitigation measures must be taken shall include: taking the application of structural determinations that focuses on reducing the impacts of flooding on communities and taking the non-structural measures like land-use control, sampling signal measures, relocations and early flood warnings.