Flood Modeling of Jemo River Catchement in Addis Ababa City, Ethiopia

Flood affects lives and livelihoods in parts of Ethiopia. The flood from the Jemo river is causing damages to river side houses, infrastructures and displacement of affected population that resulted overflow on the surface following heavy rains and inundated lowland areas in the Nifas Silk Lafto district of Addis Ababa city, Ethiopia. This research involves the integration of Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) and Hydrologic Engineering Center-River Analysis System (HEC-RAS) to develop a twodimensional (2D) river model for flood inundation determination and mapping. The model Nash-Sutcliffe efficiency (ENS) was found to be 0.751 during calibration and 0.79 during validation and the coefficient of determination (R) was found to be 0.786 during calibration and 0.801 during validation. The HEC -RAS model was calibrated by comparing the results of the water level in each selected cross section obtained by the model with the observed historic flood mark levels of the year 2010 and 2013. The peak estimated time series discharge of HEC HMS model result was used to simulate the unsteady state of flow to determine flood extent, water depth and velocity of the study river. Flood hazard vulnerable areas both left and right side of the Jemo river delineated for the return periods 2, 5, 10, 25, 50 and 100 years. The research showed community houses ranging from mud houses to regular story buildings can be affected during each return period maximum flooding event. Jemo river flow capacity also checked at different cross sections and less carrying capacity sections identified for recommended flood protection measures like dyke and retaining masonry walls construction to tackle the flooding impact and avoid possible erosion of the river banks.