Numerical model for sediment flushing at the Nakdong River estuary barrage

Abstract

The Nakdong River is located in the southeastern region of South Korea and flows 521.5 km from the Taebaek Mountains to the East Sea. The Nakdong River is the second largest river in Korea and flows through major cities, including Daegu and Busan. 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 near Busan 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 about 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 the need for dredging operations at the NREB. Two numerical models, a steady state model and a quasi-steady state model with variable discharge, were developed. The upstream model simulation spans 40 km upstream of the NREB. This model simulates sediment transport capacity and bed elevation changes. Sediment flushing curves have been developed with respect to upstream discharge and downstream flow depth using the 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, with the maximum amount of flushing being 80% in the 2003 simulation. The total flushed amount of sediment without redeposition should be in excess of the annual dredging volume. Therefore, sediment flushing controlled by water level operations including tidal effects should be effective at NREB. Optimization and generalization of the sediment flushing procedure can be accomplished by comparing steady-state sediment flushing curves, flow duration curves from 1998 to 2003, and quasi-steady state sediment flushing simulations based on a numerical model. Simulations of sediment transport and water level variations with and without dredging operations are conducted. Quasi-steady state simulations indicate that at high flow, the water level differences with and without dredging are very small. However, water level changes can be significant at low flow because of tidal effects. Numerical simulations indicate that the sediment deposits can be effectively sluiced during the early flood season without sediment dredging. It is also found that the absence of dredging operations at the NREB would not cause significant water level changes against the levees during major floods. The effects of the sediment flushing technique on the sediment concentration changes are examined. Higher sediment concentrations generally occur during the early flood season. The average increase in sediment concentration by the flushing technique is not significant. The differences between flushing and non-flushing simulations are 58.8 ppm in 2002 and 49.5 ppm in 2003. However, flushing will increase peak sediment concentration. For example, the maximum sediment concentration difference between flushing and non-flushing simulations at a discharge of 1,924 cms in 2002 is 911.3 ppm.