Regional Flood Frequency Analysis for Abaya – Chamo Sub Basin, Rift Valley Basin, Ethiopia

Abstract

The purpose of this study is to develop the regional flood frequency (growth) curve and index flood estimation equation (model) essential for computing flood quantiles for ungauged catchments in Abaya-Chamo sub basin by using an index flood method. Annual series model is used for the analysis. Identification of homogeneous region was performed initially on the basis of catchment characteristics like slope, elevation, soil type, soil texture and mean annual rainfall and flood statics are used for test of homogeneity of the proposed region. 17 rainfall stations were selected for analysis based on length and quality of the data they have. Concerning on the above procedure the Abaya-Chamo sub basin was divided in to three regions. Region one is located in the South-West part of the sub basin which covers the area of 4583sq. km, region two is located at the middle of the sub basin with an area of 5701sq. km and region three located at North-east and South-West part of the sub basin also covers an area of 7259sq. km. For the above regions selection of parent distribution was performed on the bases of the three common methods which are probability plot by using SPSS software, statistical method like Chi- square test, Kolmogorov-Smirnov test and moment diagram (both conventional and L- moment). From the candidate distribution from all methods best fit distribution is selected by using the software called Easy Fit test. For the selected distribution parameter estimation technique was selected by performing a Robustness Assessment. Accordingly, GEV with PWM selected for region one, LN (2P) with MOM is selected for region two, finally Gamma (2P) with PWM is selected for region three and the regional growth curve were developed for each regions by using the selected parameter estimation techniques. Accordingly the Abaya-Chamo sub basin was divided in to two regions and the index flood estimation equation were developed with a coefficient of determination (R²⁾ 0.93 for upper region and 0.99 for lower region.

Keywords: Abaya-Chamo sub basin, index flood method, SPSS software, Easy Fit software.