Impact of Reduced Fluvial Sediment Supply on Saltwater Intrusion in the Yangtze Estuary.

A decline of the fluvial sediment supply leads to coastal erosion and land loss. However, the fluvial sediment load may influence not only coastal morphodynamics but also estuarine hydrodynamics and associated saltwater intrusion. Previous studies revealed that suspended sediments influence estuarine hydrodynamics through various flow–sediment interactions. In this contribution, we systematically investigate how changes in fluvial sediment load and other climate‐change‐induced environmental change influence estuarine hydrodynamics and sediment dynamics. For this purpose, we utilize a well‐calibrated fully coupled model in which hydrodynamics, saltwater intrusion, and sediment transport interact with each other, to explore saltwater intrusion in the Yangtze Estuary in response to a decline in the sediment load, modified discharge, and sea‐level rise. Model results suggest that a 70% decline in the suspended sediment load weakens the impact of sediments on salinity‐induced stratification and thereby reducing saltwater intrusion. Sea‐level rise or discharge peak reduction increases saltwater intrusion. However, a fully coupled model accounting for sediment effects predicts a much larger increase in saltwater intrusion compared to noncoupled models. Whether this effect is important depends on estuarine sediment concentrations and therefore the potential role of sediments should be carefully investigated before applying a noncoupled model. This work highlights not only the relevance of a suspended sediment decline but also the use of fully coupled models for predicting saltwater intrusion in turbid estuaries and has broad implications for freshwater resource management in turbid estuarine systems influenced by human interventions and climate change. Plain Language Summary: Saltwater intrusion is strongly influenced by river flow, tides, and morphology in estuaries. In highly turbid estuaries, high suspended sediment concentrations interact with salinity, which also plays an important role in saltwater intrusion and freshwater resources. However, many of these estuaries are suffering from a reduction in sediment supply, leading to reduced sediment concentration in recent decades. In this study, we employ a fully coupled three‐dimensional numerical model to investigate the effect of reduced sediment supply on saltwater intrusion. The fully coupled model predicts a much larger increase in saltwater intrusion in response to sea‐level rise or modulation of the river discharge compared to noncoupled models. In contrast, a decline in riverine sediment supply weakens sediment–turbulence interaction, thereby decreasing saltwater intrusion. These findings are important for the management of sediment and freshwater resources in turbid estuaries. Key Points: A decline in riverine sediment supply weakens sediment–fluid interactions, thereby decreasing saltwater intrusionSaltwater intrusion may be as much influenced by a sediment load reduction as by changes in river discharge distribution or sea‐level risePredicting saltwater intrusion in highly turbid estuaries requires evaluation of the role of sediments before applying a noncoupled model [ABSTRACT FROM AUTHOR]