OPERATION ANALYSIS OF KARADOBI RESERVOIR IN BLUE NILE RIVER BASIN AND IT'S IMPACT ON DOWNSTREAM WATER USES

Abstract

This study is carried out using the HEC-ResSim model to simulate the operation of Karadobi reservoir and analyze the modifications in the flow pattern, power generation and sediment transportation that may occur on the down stream areas due to regulation of Blue Nile at Karadobi in Ethiopia. Thirty-one years (1972-2002) of extended inflow records to Karadobi reservoir (dam site) were used to simulate the .. model.The main input data to the model were the reservoir physical characteristics (Elevation-Capacity-Area curve), net monthly evaporation from the reservoirs (mm), daily inflow (m3 /s) and Elevation maximum capacity relation of dam appurtenant structures such as spillway and penstocks. The outputs from the model were the inflow to the down stream reservoir, out flow and power generated at Karadobi and at the immediate downstream reservoir (Roseires in Sudan), storage and level pool of reservoirs. After running the model the changes in flow magnitude and power generation for the existing down stream water uses(pre regulation at Karadobi) and the regulated condition (down stream water uses with Karadobi) has been analyzed. From the result of the analysis, it is observed that the mean monthly inflow to Roseires reservoir has increased by about 397m3 /sec(1.12Mm3 /day) during dry period from December to June and the inflow to Roseires has decreased on average, by about 938 m3 /sec during the flood season (July to October). Because of regulating Abbay at Karadobi with FSL of 1164m a.m.s. l and MOL of 1088m a.m.s.l the gain in discharge at Roseires will improve the power generated at Roseires by about 68.75% during dry season on monthly basis with only 2.0% reduction during flood seasons than the existing condition with out any additional investment on the existing down stream reservoir. The sediment inflow rate to Karadobi reservoir was estimated to be 104. 1 Mm3 /year with an out flow of only 6.3Mm3 /year. Thus, the sediment contribution of Karadobi watershed to the down stream areas will be reduced from 104.1 to 6.3Mm3 /year. More over, the operational analysis result shows that the regulation of Abbay at Karadobi will reduce the mean reservoir level at Roseires by about 1 m; as a result, regulation of Blue Nile at Karadobi will help to save the water lost through high rate evaporation at the downstream low elevation areas.