Regional Flood Frequency Analysis And Flood Inundation Mapping: The Case Of Middle Awash River Basin, Ethiopia

Abstract

As flood is a natural disaster and causes loss of life and property destruction, its estimation with a certain frequency and mapping of its inundation area are one of the most important factors for the planning, design and management of flood risk controlling hydraulic structures. The intent of this study is to carry out the regional flood frequency analysis using L-moments and analyze flood inundation mapping using HecGeo-RAS and HEC-RAS models in combination with ArcGIS on middle Awash River basin. The annual maximum flow records of six river gauging stations of middle Awash River Basin from U/S to D/S: Awash below Koka dam, Awash @Wanji, Awash @Nura Hera, Awash @Metehara, Awash @Melka Sadi and Awash @Melka Werer, were examined based on L-moments to investigate and delineate the hydrologically homogeneity of the region using regional heterogeneity measure, identify the discordancy measure of the sites using L-moment based discordancy measure (Di), select best fit regional probability distribution using goodness-of-fit tests by Easy Fit software and L-moment ratio diagram, estimate parameters for the best fit distribution, estimate extreme flow quantiles under return period of 2, 5, 10, 20, 25, 50, 100, 200, 500, 1000, 2000, 5000 and 1000-year (QT) at each gauging station, develop regional frequency and growth curves, mapping flood inundation and estimating the extent of flood inundation area for the selected Awash river reach using HEC-RAS MApper. The result of the analysis shows that the region is homogeneous, all sites are not discordant, i.e. data from all river gauging stations are suitable for flood frequency analysis, the Generalized Pareto (GPA) was selected as best fit distribution for Awash below Koka, Awash @Wanji and Metehara gauging stations and Generalized Extreme Value (GEV)was selected as best fit distribution for Awash @Nura Hera, Awash Melka Sadi and Awash Melka Werer gauging stations. The estimated extreme flow quantiles for each gauging station from U/S to D/S respectively under a 100-year return period are 6077.64, 991.88, 564.31, 14324, 365.24 and 698.91 m3/s. The result of flood inundation area for the river reaches from U/S to D/S respectively under a 100-year return period are 15.19, 50.97, 61.73, 126.54 and 55.58 Km2. The mapping result indicated that all parts of Awash below Koka dam up to Wanji, some parts of Awash Nura Hera up to Metehara and some parts of Awash @Metehara up to Melka Werer River reaches were under the impact of 25, 50 100, 200, 500 and 1000-year flood problem