Reservoir Sedimentation Study for the Bunakha Hydro-electric Project
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This thesis presents a study on the sedimentation of the reservoir of the Bunakha Hydro-electric Project in Bhutan. Although the Detailed Project Report for this peaking reservoir scheme has already been completed, it has been used for further and additional investigations related to reservoir sedimentation. The sediment inflow into the reservoir was determined through two independent studies: i) Application of Pacific Southwest Interagency Committee (PSIAC) and Revised Universal Soil Loss Equation (RUSLE) approach integrated with Geographic Information System (GIS), ii) Data analysis of available suspended sediment data of Tamchu gauging station. The application of the PSIAC approach taking into account three different boundary conditions related to catchment properties resulted in estimates of the average annual sedimentation rate for the Bunakha catchment/watershed at the location of gauging station as 0.4 mm/year, 0.18 mm/year, and 0.63 mm/year, respectively. The determination of sedimentation rates based on the RUSLE approach was implemented in a GIS and estimated in two stages. First, the annual soil loss was determined using the RUSLE Model and in the second stage, the Sediment Delivery Ratio (SDR) was applied to estimate sediment yield. Using three different empirical SDR-equations, the estimated average annual sedimentation rate at the gauging station location using this approach was 0.303 mm/year, 0.203 mm/year, and 0.226 mm/year, respectively. The sediment load to the reservoir was also estimated from available data for the period 2009-2015 employing two approaches. In the first approach, the measured suspended sediment load was combined with bed load transport rates estimated from various computational approaches. In the second approach, both the suspended and bed load were computed. The average annual sedimentation rate resulted from these approaches ranged from 0.13-1.22 mm/year and that of bed load calculated ranged from 10-25% of the suspended sediment load. The comparison of the results from the different approaches showed that the estimated sedimentation rate ranged from 0.13-0.63 mm/year. For the subsequent prediction of the reservoir sedimentation, a mean value of 0.50 mm/year was used taking into consideration a safety factor of 1.3 to account for potential extreme events resulting from landslides due to glacier lake outbursts. New Zero-capacity Elevation (NZE) determined with the application of Empirical Area Reduction method for 30, 50 and 70 years are 1908 m, 1926 m and 1944 m respectively indicating that even after 70 years of reservoir operation, the sediment deposit level will be well below the Minimum Draw Down Level (MDDL) if there is no flushing of the sediments. However, the sediment will be flushed during monsoon season with low level spillway sluice that will be installed at 1915 m, whereby sediment will never rise beyond this level (DHPS, 2013). The perspective downstream impact due to reservoir sedimentation were studied through literature reviews.