On the role of flocculation, hindered settling and sediment-induced damping of turbulence in trapping sediment in estuaries, with focus on the North Passage, Yangtze Estuary

Many estuaries are characterized by one or more locations where the concentration of fine sediment attains a maximum. The locations and intensities of these estuarine turbidity maxima (ETM) are sensitive to river discharge, tides, depth and sediment properties. In this contribution, results are presented of a width-averaged process-based model that describes tides, residual currents and sediment transport in an estuarine channel. The aim is to quantify the sensitivity of location and intensity of ETM to 1) flocculation and hindered settling of fine sediment and 2) sediment-induced damping of turbulence. The model is applied to the North Passage of the Yangtze Estuary, which is a prototype estuary that undergoes strong variations in environmental conditions. The sediment settling velocity is allowed to vary along the channel due to the effects of flocculation and hindered settling, by parametrizing settling velocity as the function of the subtidal near-bed sediment concentration according to results obtained from laboratory experiments. Sediment-induced turbulence damping is taken into account by parametrizing eddy viscosity and eddy diffusivity coefficients as functions of bulk Richardson number.

In the flocculation (low concentration) regime, where the settling velocity increases with sediment concentration, the rapid settling of flocs induces larger landward sediment transport due to upstream flow in the lower layer of density-driven flow, leading to a landward shift and intensification of the ETM (with respect to the case of a constant settling velocity). In the hindered settling (high concentration) regime, the settling velocity decreases with bottom concentration. This induces a decrease in upstream sediment transport due to density-driven flow and an increase in seaward sediment transport due to river flow, leading to seaward migration and attenuation of the ETM. In both regimes, sediment-induced damping of turbulence results in stronger upstream flow in the bottom layer of density-driven flow and more vertically stratified sediment distribution, which significantly intensifies the landward sediment transport due to density driven flow, and hence causes a landward shift and intensification of the ETM.