Hydraulic Thresholds to Mitigate Sedimentation Problems at Sangju Weir, South Korea

Abstract

The construction of eight consecutive weirs along the Nakdong River changed the erosion and sedimentation patterns after the Four Major River Restoration Project. This study examines the sedimentation problems upstream of Sangju Weir with a new procedure combining: (1) the flow-duration; (2) the sediment rating curve; (3) the series expansion of the modified Einstein point procedure; and (4) the trap efficiency. For weirs or low-head dams, trap efficiency is not only a function of sediment size ds, but also varies with discharge Q and water stage of the reservoir H. At Sangju Weir, trap efficiency varies from 24 to 99%, the incoming sediment load is about 425, t/year and the annual reservoir sedimentation rate is estimated at 332, t/year, which corresponds to a .76% annual reduction in reservoir storage capacity. Two hydraulic threshold values for discharge and weir operation stage are obtained from a 2-year daily sediment transport simulation. The two threshold values balancing the hydropower production revenue and the sediment excavation cost are (1) a flow discharge threshold of 6 m3/s and (2) a stage threshold of EL 43.6 m. The stage should be kept low during floods to minimize sedimentation.