Regional Flood Frequency Analysis for Upper Wabi-Shebelle River Basin, Ethiopia

Abstract

Floods are the most common natural disaster for every country in the world and which is the main cause of fatalities. It is one of the major natural hazards in Ethiopia which causes for displacement of people and animals, property and live damages in different parts of the country. Estimation of maximum flood discharge with a specified return period is crucial for the design of hydraulic structures. The estimation of design floods for a site has been a common problem in some regions, and there is always great interest in this estimation process, particularly for ungagged basins or for sites characterized by a short sample length. Therefore, flood management, and especially flood risk assessment, requires the estimation of the relation between flood magnitude and its probability of exceedance. The main objective of this study was to develop Regional Flood Frequency Analysis on Upper Wabi-Shebelle River Basin of Ethiopia. This was addressed by collecting hydrological data and conducting of analysis using ArcGIS to know the location of stations’ distribution and delineation of homogeneous regions, Easy fit software and Matlab2018a for homogeneity testing and selection of flood frequency distribution type and Microsoft excel to rearrange the data, to draw the graphs and tables. Based on gauging stations location and L- moment, the regional homogeneity test was conducted and the study area was regionalized in to three homogeneous regions as region -1, region-2 and region 3. The most suitable flood frequency distributions which were found for the study area were General Extreme Value, General Pareto and Lognormal 3p to region -1, region-2 and region-3 respectively. These flood frequency distributions were selected by using Kolmogorov Smirnov, Anderson-Darling and Chi-Squared goodness-of-tests with easy fit software and linear moment ratio diagram. Based on the identified flood frequency methods the quantile for regions were forecasted for return period of 2-1000 years and flood frequency curve were plotted for each region. The plotted flood frequency curve can help for engineers to design safe, cost wise and serviceable structure in the identified homogenous regions. In addition to this it is helpful for flood management and risk mitigation purposes. Therefore it is advisable for different stakeholders to use the output of this finding in order to increase the life span of the hydraulic structures and to make safe the residents of the basin.