Numerical model for sediment flushing at the Nakdong River estuary barrage

The Nakdong River is located in the southeastern region of South Korea and flows 521.5 km. The Nakdong River Estuary Barrage (NREB), a hydraulic structure, was built in 1983-87 at the river mouth to prevent salt-water intrusion. The upstream channel of the NREB has experienced sedimentation problems requiring annual dredging operation after the construction. The main purpose of sediment dredging is to prevent flooding during late summer. According to the past records, the annual average dredging volume is 665,000 m3 in the upstream channel of the NREB.; This dissertation documents the evaluation and development of sediment control and flushing methods that reduce and possibly eliminate dredging operations at the NREB.; Sediment flushing curves have been developed with respect to upstream discharge and downstream flow depth using the developed steady state model. The analysis of flushing curves and past records of annual dredging sediments (665,000 m3) indicate that sediment flushing is possible at the NREB.; Annual simulation scenarios of sediment flushing are developed and analyzed based on flow, stage, and tide level data to evaluate the feasibility of the flushing technique. Annual simulations for the period from 1998 to 2003 were performed using the quasi-steady state model. Flushing simulation results indicate that an average 54% of the annual dredging volume with redeposition in the upstream bed can be eliminated by flushing. The total flushed amount of sediment without redeposition should be in excess of the annual dredging volume. Optimization and generalization of the sediment flushing procedure can be accomplished by comparing steady-state sediment flushing curves, flow duration curves, and quasi-steady state sediment flushing simulations.; Quasi-steady state simulations of sediment transport and water level variations with and without dredging operations indicate that at high flow, the water level differences with and without dredging are very small and the sediment deposits can be effectively sluiced during the early flood season without sediment dredging.; The effects of the sediment flushing technique on the sediment concentration changes are also examined. The average increase in sediment concentration by the flushing technique is not significant with 58.8 ppm in 2002 and 49.5 ppm in 2003.