Evaluation of Predictive Accuracy of Regional Flood Frequency Estimations: A Case Study of Tekeze River Basin, Ethiopia

Abstract

Floods are among the most recurring and devastating natural disasters and are responsible for significant loss of life and property throughout the world. It causes physical suffering, economic losses, limit the efficiency of drainage, and disturb existence of life in the study area. An evaluation of predictive accuracy of regional flood frequency estimation methods has been the backbone of water resources project planning, design of any structures and the economic analysis of flood control projects. It is due to the fact that floods represent the most disastrous natural event causing several damages to enormous economic and life losses in the study area. The aim of this study was to evaluate the predictive fit of probability distributions to annual maximum flood data, and in particular to evaluate which combination of distribution and estimation method gives the best fit of Tekeze River Basin. Subsequently, the probability distribution fits were evaluated according to several goodness-of-fit measures and to the variability of the predicted flood quantiles. To achieve this, based on data from eleven stream gauged sites, three hydrological homogeneous sub regions were defined and delineated based on L-moment homogeneity tests, namely Region-A, Region-B and Region-C. Delineation of homogeneous regions were accomplished using ArcGIS10.4.1. Discordancy of regional data of the L-moment statistics was identified using Matlab2018a. All regions have shown satisfactory results for discordance measures and homogeneity tests. For the regions, best-fit distributions were selected. L-moment ratio diagrams and Easy Fit statistical software was used to select best-fit probability distributions. The performances of the distributions were evaluated using Kolmogorov Smirnov, Anderson-Darling and Chi-Squared goodness-of-tests. After three goodness of fit tests was carried out, generalized extreme value (GEV) with MOM for Region-A and generalized Pareto (GPA) with PWM for Region-B and C were identified as suitable distributions for analyzing accurate annual maximum flows in the basin. Based on best-fit distributions for the three regions, regional flood frequency curves were constructed. In this study, the flood magnitude is estimated for 2, 5, 10, 15, 20, 25, 50, 75, 100, 200, 500 and 1000 years return period. The derived flood frequency curves at a given return period suggested that how important engineering decisions and actions such as design and operation of the water resources project have to be undertaken. Consequently, statistical analysis of gauged sites was revealed an acceptable method of regionalization. Finally, the study can be further extended into flood hazard, risk and inundation mapping of identified regions of the study area.