FLOOD MAPPING ON GREAT AKAKI RIVER IN ADDIS ABABA, AKAKI KALITY SUB-CITY

Abstract

In recent years, the patterns of flood across all continents have been changing and becoming more frequent, intense and leading the people to face risks. Therefore, the risk should be investigated and quantified properly. The objective of this thesis was to develop flood inundation maps of Great Akaki river in Addis Ababa, Akaki Kality sub-city. Streamflow, digital Elevation model (12.5*12.5 m) resolution and land use data were used as an input for the RAS mapper in HECRAS model. The 2D-HECRAS and ArcGIS models were used for mapping the flood extent, depth and velocity for various return periods. The Mann-Whitney and Wald-Wolfowitz statistical tests were used to hydrological data test. The basic assumption in statistical flood frequency analysis for its homogeneity, independency and stationarity of the time series at 5% significance level. The Easy fit 5.0 software was used to fit and identify the parent probability distribution for the streamflow data. The frequency analysis result depicts that Log-Normal probability distribution with 3 parameters best fitted the flood time series. The estimated quintiles for 2, 5, 10, 25, 50 and 100 years return periods were found to be 210.29, 333.04, 453.16, 626.19, 769.75, and 925.41 m3 /s respectively. The 2D-HECRAS model output indicated that about 86,123,156,228,285, and 350 ha land has been inundated for an event of 2, 5, 10, 25, 50 and 100 years of return period respectively. The 100 years flood magnitude inundated about 78, 272 and 34 ha of irrigated command area, swamp area and population settlement respectively. Flood risk map was developed based on hazard and vulnerability indicators. A preliminary alignment of structural flood protection dike was identified with length of 1.5km at downstream and 0.5 km at upstream of bridge which would make 95% of the settlement area on the left bank safe. The study has shown that the middle and lower part the study area is more inundated than upper parts. Therefore, the affected areas were whether to be free of infrastructure development, investment and residence of people or construct flood protection structure in order to avoid the risk of flooding in the area especially closer to the Great Akaki River.