Sediment Problem of Gefersa Reservoirs and The Appropriate Mitigation

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Addis Ababa University


In the world concerning dam construction the big problem and issue are sedimentation of reservoir because it consumes the reservoir volume which is designed to carry water and requires high amount of cost to handle from this problems. The Gefersa reservoir which is used as the main water supply of 30,000m3 per day to Addis Ababa is affected by sediment load from the watershed. Soil and Water Assessment Tool model is used to analyze the daily data to calibrate and validate the hydrologic component and sediment of Gefersa catchment. According to the model output the high percentage of sediment of the catchment is from subbasin two and one respectively. To check the performance of the model Sensitivity analysis, model calibration and validation of the area is done. According to global sensitivity analysis of the model the most sensitive parameters were curve number for flow calibration and Biological mixing efficiency for sediment calibration. The coefficient of determination and the Nash-Sutcliffe efficiency for the daily runoff are obtained to be 0.57 and 0.52, respectively. The coefficient of determination and the Nash-Sutcliffe efficiency for the daily runoff at the outlet of the watershed for validation is obtained to be 0.65 and 0.61 respectively. The calibration of the model for the yearly sediment observed at Gefersa gives model efficiency of -4 which means 4% over estimation. To manage the watershed area, in the future scenario, using Vegetation land change to 40% open broadleaved forest, 35% Rain-fed croplands and 30% Grass-land result in the reduction of sediment volume by 15.75%, 10.91% and 14.54% respectively. Even if both structural and non-structural measures can reduce sediment load of an area, the non-structural measures are the best methods because they need small capital and can be applied easily according to the model scenario. Key words: Gefersa Reservoirs, Sedimentation, SWAT model, Catchment



Gefersa Reservoirs, Sedimentation, SWAT model, Catchment