Formation of Turbidity Maximum in the Modaomen Estuary of the Pearl River, China: The Roles of Mouth Bar

An estuarine turbidity maximum (ETM) functions as a filter and plays a key role in transporting suspended sediment. The features of ETM and their formation mechanisms vary with different types of estuaries. Their responses to morphological factors require further study. In this study, the Modaomen Estuary of the Pearl River was selected to examine the formation mechanisms of the ETM and roles of the mouth bar. Hydrodynamics, suspended sediment concentration, and sediment size distribution were measured via cruise shipboard in the mouth bar zone during the dry season of 2020, which indicated that the ETM occurred in the central and western bars. Furthermore, numerical modeling of an idealized estuary based on Delft3D was performed to explore the effects of the existence of a mouth bar on the dynamic structures causing the ETM, including saltwater intrusion, residual longitudinal circulation, and bed sediment resuspension. The results indicated that the existence of a mouth bar contributed to the occurrence of double residual circulations and enhanced bed shear stress on the mouth bar, which favor the ETM formation. In general, two different dynamic regimes were responsible for the ETM formation, that is, sediment trapping by saltwater intrusion and sediment resuspension induced by tidal currents. The mouth bar played a key role in these two mechanisms, including altering the estuarine circulation and bed shear stress. The results contribute to a better elucidation of the ETM formation in estuaries and provide scientific guidelines for estuarine management and engineering. Plain Language Summary In estuaries, sediment can accumulate forming a sediment-rich zone with high suspended sediment concentration. The occurrence and distribution of sediment-rich zones affect water quality and ecological systems in estuary, as well as dredging and shipping needs, which has attracted significant interests of scientists and engineers. We have conducted a field survey to investigate the locations of the sediment-rich zone and their causes. And we find that high sediment concentration mainly occurs on a shallow depth zone in estuary, which is caused by saltwater intrusion and sediment resuspension. Furthermore, we use a computer model to explore the roles of shallow topography on the accumulation of massive sediment. The results indicate that the shallow topography enhances the impacts of saltwater intrusion and sediment resuspension on sediment accumulation, favoring the occurrence of sediment-rich zones. These findings have important scientific implications for estuarine management and engineering efforts.