Physical drivers and parameter sensitivities of pearl river-derived sediment dispersal on the Northern South China Sea Shelf: a modeling study

Abstract

This study employs the Coupled-Ocean-Atmosphere-Wave-Sediment-Transport (COAWST) modeling system to quantitatively assess the seasonal suspension, transport, and annual fate of Pearl River-derived sediment (riverine slow-settling single fine grains and high-settling flocs) on the northern continental shelf of the South China Sea (SCS). Following careful model validation, a series of sensitivity experiments were conducted to investigate the effects of tides, waves, background circulation, sediment settling velocity, critical shear stress, and sediment spin-up durations. The results reveal strong seasonal variations in sediment dynamics driven by the East Asian monsoon. During the wet summer, weaker hydrodynamic conditions promote the initial deposition of riverine sediment via the surface buoyant plume. In contrast, stronger winds and waves during winter enhance sediment resuspension and southwestward transport, particularly toward the Beibu Gulf. Spatially, approximately two-thirds of the annual Pearl River-derived sediment load is retained near the Pearl River Estuary. About 9 % reaches the continental shelf east of the estuary, while similar proportions accumulate in the Beibu Gulf and south of Hainan Island. Sensitivity experiments highlight the distinct and significant roles of different physical forcings in sediment dispersal. Tidal dynamics strongly enhance sediment mobilization and transport within the estuary by increasing bottom shear stress, which promotes offshore sediment export and limits local deposition. Wave forcing plays a dominant role in sediment resuspension near the river mouth and along the coast, especially during winter, facilitating sediment redistribution across the shelf. The remotely forced (large-scale, non-local forcing) ambient shelf circulation in summer drives eastward sediment transport, enabling sediment to spread widely across the shelf. Model outcomes are also sensitive to sediment parameterization. The natural seasonal increase in critical shear stress for erosion during winter counteracts part of the wave-enhanced resuspension capacity, thereby reducing resuspension and erosion on the continental shelf east of the Leizhou Peninsula. Higher settling velocities decrease suspended sediment concentrations and promote near-source retention, limiting long-distance transport. Spin-up duration experiments indicate that Pearl River–derived sediment, which enters and accumulates in various regions of the model domain during the first year, continues to migrate southwestward in the second year under the influence of the mean annual flow field. In contrast, the spin-up duration of seabed sediment has little impact on the retentions of Pearl River–derived sediment on the shelf. Overall, this study reveals the transport pathway and fate of the Pearl River-derived sediment and provides a model-based assessment of its seasonal behavior and the sensitivity of suspended sediment dispersal to physical drivers and sediment parameters or conditions on the northern SCS shelf. It identifies key physical drivers regulating sediment transport and deposition patterns, offering new insight into sediment fate in a monsoon-dominated shelf system.