Flood Risk Mapping (Case Study of Ketar Watershed Ziway-Dugda Woreda, Ethiopia)

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Addis Ababa University


Flood is probably the most devastating, widespread, and frequent natural hazard of the world that producing many socioeconomic and environmental consequences within the affected floodplains. In Ethiopia, there is a lot of areas under flood problem and Ketar river floodplain located at the downstream of Abura gauging stations is among the most frequently flood-affected area for which flood risk mapping is so important. The objectives of this study were to model rainfall-runoff using HEC-HMS for Ketar watershed and flood risk mapping by using HEC-RAS and HEC-GeoRAS for Ketar river floodplain, located downstream of Abura gauging station in Ziway-dugda woreda, Ethiopia. The HEC-HMS software requires daily hydrological data of 16 years Collected from a National Meteorological Agency of Ethiopia for rainfall-runoff modeling. A normal-ratio method was used for filling missing values of precipitation data and data consistency was checked bya double mass curve. The initial and constant loss for precipitation loss model, Clark unit hydrograph for excess precipitation transformation to direct run-off, monthly constant for base flow modeling, and Muskingum for flood routing modeling were chosen.Gage weight meteorological method was selected to assign the weighted precipitation to each sub-basins using the Thiessen polygon method. Among the collected 16 years of hydrological data for rainfall-runoff modeling, 11 years (1988-1998) were used in model-calibration and 5 years (1999-2003) were used in model validation. The model-performances were evaluated using performance measuring techniques including Nash Sutcliff Efficiency (NSE) and Coefficient of Determination (R2). NSE during calibration and validation was 0.72 and 0.67 respectively whereas R2 during these two processes was 0.87 and 0.81 respectively Flood frequency analysis was conducted using HEC-HMS’ frequency storm method for 2, 5, 10, 25, 50,and 100 year return periods. The peak flood for each respective return periods were (198.5, 224.1, 242.9, 251.8, 267.5, and 283.5 m3/s). Flood risk mapping was modeled for a peak flood of each return period using HEC-RAS, HEC-GeoRAS,and Arc GIS software; and resulted in the inundated areas of 2.86, 2.91, 2.94, 2.95, 2.98, and 3.01 km2 respectively.



Flood risk mapping, HEC-HMS, HEC-RAS, Arc GIS, Rainfall-Runoff Modeling