Effects of flood events on sediment transport and deposition in the waterways of Lingding Bay, Pearl River Delta, China

Abstract

Siltation is of primary concern for the construction and maintenance of the waterways worldwide. Lingding Channel (LC) and Hengmen Channel (HC) are two major seagoing waterways in the Lingding (LD) Bay, Pearl River Delta, South China. TELEMAC-2D numerical modeling was performed to elucidate the characteristics and mechanism of siltation in the two seagoing waterways under usual and extreme flood conditions. The deposition thickness during usual flood conditions was 0.22 m in the LC and 0.16 m in the HC; during extreme flood conditions, the thickness reached 0.32 m in the LC and 0.44 m in the HC. In the LC, the main sediment deposition area was in the lower section of the LC, south of Neilingding Island. In the HC, the main deposition areas were in the Lanshan, Ermao, and Qi'ao Shoals. Under extreme flood conditions, the sediment loads into the LD Bay from the Humen, Jiaomen, Hongqili and Hengmen outlets reach 16.65 × 106 t in total, compared with 4.54 × 106 t under usual flood. The certain amount of sediment in the upper and middle parts of the estuary is resuspended by high bottom shear stress and transported downstream, deposited in the lower sections of the HC and LC. High sediment load combined strong resuspension contribute to the sudden and substantial sedimentation in the navigation channels during extreme floods. Siltation intensity varies with the velocity change along the navigation channel, and the main deposition appears in areas with low velocity. The angle between the current direction and channel axis is favorable to the sediment deposition location and intensity, which results in serious deposition in the lower sections of HC and LC. The stagnation point moves around the middle of estuary under usual and extreme flood conditions, so, the middle and lower parts of the LD Bay is heavily silted. The impacts of extreme floods on the navigation channels can not be overlooked and mitigation measures such as bathymetric monitoring, flood control and dredging should be taken.