FLOOD RISK ANALYSIS FOR LOWER GUDER RIVER FLOODPLAIN IN RIFT VALLEY LAKE BASIN

Abstract

River flooding is a natural process and part of the hydrological cycle of rainfall, surface and groundwater flow and storage. Floods occur whenever the capacity of the natural or manmade drainage system is unable to handle the volume of water generated by rainfall. Guder river catchment is found in South Nation and Nationality region in the upper rift valley. Guder flood plain has been frequently affected by flood events during rainy season. The main objective of the study is to assess flood risk in Gudar floodplain of lower Gudar River basin. The data used for this study was annual maximum flow data, DEM, land use land cover map, and soil map and surveyed cross sectional data flood frequency analysis of annual maximum flow data was analyze. Homogeneity and stationary tests at different significance levels were done using Wald-Wolfowitz and Mann-Whitney tests. Guder station was found to be independent, homogenous, stationary and no outlier at 5% significance level Using the method of goodness of test technique, the time series of the flow data was fitted to Log Pearson Type III. The flood frequency analysis method, HEC-RAS, HEC-GeoRAS and ArcGIS are used to determine the peak flood for different return periods and flood hazard map. The simulation result for return period of 5, 25, 50,100 and 200 year floods magnitude are 34.17 m3 /s , 67.92 m3 /s , 83.18 m3 /s , 102.82 m 3 /s and 122.32 m3 /s respectively. The maximum inundation depth (in m) the corresponding area coverage (in ha), and potential loss of crops in quintal are estimated to be 1.25 m/90.9ha/2,199.78 quintal; 1.733m/102.1ha/2,470.82quintal; 1.97m/ 114.6ha/2,773.32 quintal; 2.13m/ 122.8ha/, 2,971.76 quintal; and 2.37m /127.4ha/, 3083.08 quintal respectively with a maximum velocity of 5.95 m/s. Lastly the study shows that the lower part the study area is more inundated than the middle and upper parts specially those areas closer to the Guder River. Appropriate land use planning and soil conservation practice should be carried out in the upper catchment in order to reduce the magnitude of flood and siltation plus for regulated discharge of water at downstream area.