Optimization of Hydraulic Flushing Schedule of Patrind Hydropower Project

Abstract

Reservoirs' useful storage capacity dwindles due to sedimentation. On average, worldwide, 1% of the storage capacity of reservoirs is lost due to sedimentation, which has huge socioeconomic impacts. Patrind hydropower project is a run-of-river project located on river Kunhar in Northern Pakistan. It generates 150 MWs and annually adds 1.6 GWh of green energy to the national grid. Patrind HPP has a small reservoir with a 6.14 MCM useable storage capacity; such reservoirs are very sensitive to yearly sediment inflow. Kunhar is a gravel-bed river with a mean annual discharge of 104 m3 /s; in flood season, it carries a very high sediment load. Annual sediment flow varies from less than 1 MT to almost 7 MT, with an average value of 1.8 MT. To address the problem and prolong the reservoir's life, sediments are flushed regularly through the bypass tunnel and low-level outlets, but storage capacity is still reducing. Deposition reduces trapping efficiency, making turbines particularly vulnerable to sand-sized particles, i.e., greater than 0.1 mm, when moving with high velocity, which can cause excessive wear of runners, vanes, propellers, and other associated parts. To devise a sustainable solution to sedimentation, numerical modeling was performed. In this study, HEC-RAS v. 6.3.1, a quasi-unsteady flow, 1- Dimensional sediment transport model, was used to optimize the hydraulic flushing schedule by simulating various scenarios before and after flushing, the model was calibrated, and validated by comparing the observed and simulated bed change. Simulation results indicate that the optimum discharge for flushing is 208 m3 /s. It shall be performed yearly in late July through the bypass tunnel for the main reservoir. The flushing duration shall be five days, and the reservoir water level shall be fully drawdown, followed by flushing of the sand trap with a discharge of 208 m3 /s for one day through low-level outlets of the main weir.