Impact of high/low flows on sediment-yield and hydropower potential

Bunji Hydropower Project is a run-of-river hydropower project, with peaking reservoir, on Indus River near Gilgit. The study aimed at the impact of high/low flows on sediment-yield and hydropower potential of Bunji Hydropower project. The flows were divided into high-low flows, on the basis of sediment-yield. High-flow months include June to September, whereas low flows from October to May. Suspended sediment-yield for high/low flows was determined at Kachura sediment gauging station by sediment-rating curve method whereas bed load for high/low flows was determined with different bed load formulas. By adding both the loads, total sediment-yield for high/low flows was determined. Then this sediment-yield was transposed to Bunji Dam site. The bed material load is determined by three equations, England Hansen (1967) equation, Ackers- White (1973) equation and Yang's (1972) unit stream power. None of the equations satisfy the observed values of load. According to the location of gauging stations i.e. Kachura and Partab Bridge on Indus River and Alam Bridge on Gilgit River, the flows for Bunji dam site were determined. From these flows, power, peak/off-peak energy was computed. Benefits for power and energy were computed with specially designed worksheet (M.S. Excel). The river cross-sections and longitudinal profile of Indus River in the project-area, surveyed for pre-feasibility study, have been used for estimating trapped sediments, and amount of flushed material. The reservoir life was determined by M.A. Churchill (1948) method. The unsluiced Bunji reservoir life by actual trap-efficiency (40 percent) is 6 year, whereas by 50 percent trap efficiency (percentage of sand is 50 percent in suspended, sediment) it is 5 years. The sluiced reservoir life on the basis of 20 percent bed-load trap efficiency is 83 years. Delta profile in reservoir is provided on monthly basis for the two years of data i.e. year 1973 and 1994 by estimating the transport slope for zero bed-load transport rate by Meyer-Petter and Muller bed-load equation. As the results are small, the top set slope is fixed as half of the bed slope. The movement of delta pivot point was 6 and 9 Km for the years 1973 and 1994, respectively. (author)