Impact of Sedimentation on Hydropower Energy Generation (The Case of Gilgel Gibe I Reservoir)
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Date
2021-03
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Addis Ababa University
Abstract
The most important practical and critical problem related to the performance of reservoirs is the estimation of storage capacity loss due to sedimentation process. The problem to be addressed is the rate of sediment deposition and the period of time at which the sediment would interfere with the useful functioning of a reservoir. Modeling of hydrology and sediment yield at watershed level is important for better understanding of the processes that are used for identifying appropriate measure to reduce soil erosion.
This study was conducted to simulate sediment yield in the watershed using Soil and Water Assessment Tool (SWAT) was applied to Gilgel Gibe River, which comprises an area of 402181.775 hectares and distribution of sediment deposit in the reservoir using area-reduction methods. The model was calibrated and validation on Gilgel Gibe River at gauging station. The coefficient of determination (R2) and Nash-Sutcliffe efficiency (ENS) was used in the model evaluation and validation. Sensitivity analysis, was also performed to assess the model performance.
Fifteen highly sensitive parameters were identified for stream discharge flow and twelve sensitive parameters of stream sediment for calibration and validation. The results found for stream discharge flow (R2 = 0.83, ENS = 0.83 for calibration and R2 = 0.74, ENS = 0.68 for validation) and for sediment yield (R2 = 0.79, ENS = 0.75 for calibration and R2 = 0.75, ENS = 0.73 for validation) were satisfactory for the gauging station. The model prediction estimated 27.56 ton/ha/year.
The storage capacity of reservoir is gradually depleted due to sediment accumulation. In this research, sediment distribution concerning reservoir dam of Gilgel Gibe I was estimated by area reduction method. The experimental area reduction method is a technique for predicting sediment distribution in dam’s reservoir and its parameters (c, m, n) have been obtained by Borland and Miller on the information from a limited number of dams in America. The reservoir will lose its capacity due to sediment deposition. After a period of 41 years, it will be completely filled and the estimated sediment load is 4.179*106 m3 per year with the trap efficiency of 96%.
The dam was originally expected to serve at least for 50 - 70 years. The reservoir storage capacity will be lost at an average rate of 0.58 % per year. The total storage lost due to sediment deposit within 41 years from its operation, be 170.97*106 m3 million m3 or 28 years was left from 2017 till data used. The total annual economic loss due to the live storage loss found to be 2.289 million kwh/year.
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Keywords
SWAT, Gilgel Gibe I dam, sedimentation, Empirical method, sediment distribution