Browsing by Author "Ermias, Lemma"

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    Sedimentation Problem and Mitigation Measure of Koga Reservoir
    (AAU, 2018-03) Ermias, Lemma; Bayou, Chane (PhD)
    Reservoir sedimentation originates from the catchment area and the river system then gradually, settles in reservoir. It is a major problem that endangers the performance and the life of the reservoirs by reducing water holding capacity. Inappropriate use of land, rapid population growth, and poor ecosystem management are the main causes of reservoir sedimentation. Since Koga dam is constructed on the outlet of degraded watershed, measurements of sediment load during the study have confirmed high rate of sedimentation. The aim of this study was to estimate sediment yield inflow in to the reservoir, to investigate the spatial variation of sediment yield and to recommend the appropriate sediment reduction measures. The ArcGIS interface Soil and Water Assessment Tool (ArcSWAT) was used with sediment data from rating curves to model the amount of sediment yield inflow in to Koga reservoir from catchment with an area of 155.75 km², located at the upper Blue Nile Basin, Amhara Region, Ethiopia. Primary data were used to identify the distribution, the grain size variation of sediment load of the reservoir. Secondary data were used as input for SWAT and SWAT CUP. For stream flow calibration, the most sensitive parameters were Base flow alpha factor for bank of storage (ALPHA_BNK.rte); and for sediment calibration, were SPEXP (Exponent parameter for calculating sediment restrained in channel sediment routing). The results indicated that stream flow calibration and validation period indicated that R²=0.81, NS=0.76 and R²=0.77, NS=0.74 respectively and sediment yield calibration and validation period indicated that R²=0.66, NS=0.61 and R²=0.76, NS=0.70 respectively. Both the calibration and validation result indicated good agreement between measured and simulated stream flow and sediment yield values. The simulated result shows that the average annual sediment yield is 158 ton/km²/year at the dam site. Subcatchment 25 and 26 are the most erodible areas. The spatial variation of sediment ranges from 1.79 ton/ha to 62.53 ton/ha. The laboratory result indicated that relatively more clay soils are eroded from the upper catchment and enter in to the reservoir. These clay soils can be retained by suitable sediment detention mechanism to be used as nutrient input. For the identified erosion prone areas, Scenarios were developed for reduction of sediment yield in the Koga reservoir and S5 (changing 50% of agricultural land to forest mixed land) was the best scenario.