Estimation of soil erosion rate and sedimentation in Koga reservoir, upper Blue Nile basin, Ethiopia

The construction of a dam for water storage can significantly affect the hydrodynamics of a river, directly affecting habitat and river morphology. The primary objectives of this study were to identify erosion rate and sediment deposition in Koga reservoir. The RUSLE model was employed to identify erosion rate and risk zones, while bathymetric analysis was used to determine the depth from the water surface to the reservoir; to quantify the amounts of sediment deposition in the study period and estimating useful time of the reservoir. From the RUSLE, Koga watershed annual soil loss ranges from 1.148 to o516.743 t/ha/year, with a mean rate of 53.19 t/ha/year. The soil loss classification indicated that about 51.24% was classified as slight, 30.04% as moderate, 17.11% as high, and 1.61% as extremely high. While the bathymetry findings indicate a reduction in storage capacity from 83.1 million cubic meters in 2006 to 82.7 million cubic meters in 2012, and to 78.91 million cubic meters in 2020. This illustrates that the last 11 years of operation, the reservoir storage capacity decreased by 4,190,018.94 m³, or 5.04% of its total volume, mean an average loss of 0.38% per year. From the result, yearly average sediment yield obtained from bathymetric analysis and the RUSLE model are 20.24 and 16.52 tons/year/hectare, respectivel. Therefore, in the Koga watershed, sheet and rill erosion was responsible for sediment deposition and counts 81.62% of the total yield. Based on this, the estimated useful life of the Koga reservoir is only 23.5 years. Consequently, the upstream watershed contributes a significant amount of soil loss, making it extreme erosion in the Koga watershed and inevitable for sedimentation in the reservoir. To extend the lifespan of the reservoir, water conservation and management strategies should be implemented.