Suspended sediment transport and turbidity maximum in a macro-tidal estuary with mountain streams: A case study of the Oujiang Estuary

Abstract

Macro-tidal estuaries with mountain streams are typical estuarine systems in southeastern China. Suspended sediment transport and turbidity maxima are sensitive to changes in the tidal and fluvial regimes of estuaries. To improve our understanding of river-ocean interactions and thus improve estuarine forecasts for biogeochemical processes, we developed a high-resolution hydrodynamic and sediment transport coupled model to investigate the suspended sediment transport and estuarine turbidity maximum (ETM) in the Oujiang Estuary. This estuary is characterized as a macro-tidal estuarine system with a large tidal range and drastic fluctuations in freshwater and sediment discharge. The results led to the following conclusions: (1) the ETM occurred during spring tides due to strong tidal resuspension. In contrast, it was not formed during the neap tide because of weak bottom shear stress. The ETM moves upstream to Yanmen during low river discharge and migrates downstream to the river mouth during high river discharge, causing sediment deposition in the nearshore region. (2) Owing to the characteristics of mountain streams and upstream anthropogenic reservoirs, the ETM is strengthened during low river discharge (∼100 m3 s−1) even during the wet season, which supplements the fact that the ETM is more prevalent during the dry season and withers during the wet season. The suspended sediment forming the ETM mainly originates from shallow offshore shoals owing to tidal resuspension. Owing to the weak vertical exchange flow near the river mouth, the ETM is related to the exchange flow during the low water flow. (3) The ETM excursion shows semi-diurnal variability with different transport distance for high and low river discharges. Tidal pumping dominates the suspended sediment flux in the core of the ETM, which is mainly maintained by two components of the Euler and Stokes fluxes as the tidal energy depletes during the flood tide.