A semi-analytical study on the residual transport of salinity and sediment trapping in well-mixed estuaries

Abstract

Along-channel and cross-channel sediment transport in tidal estuaries is usually driven by tides, density gradients, Coriolis’s force, wind stress, channel curvature and bathymetric variations. Since the water motion is influenced by density-induced gravitational circulation which in turn affects the salinity distribution, the coupled water motion and salinity has a potentially strong effect on the residual sediment transport, and thus the trapping of fine sediment. To better understand the dynamical effects of water motion and salinity on sediment transport, the salinity field has to be computed consistently. In this work, the water density is assumed to be a function of salinity only, thus ignoring the influence of temperature and assuming the sediment concentration to be low. To obtain the coupled water motion and salinity, the three-dimensional shallow water equations and the salinity equation are solved simultaneously using a perturbation method together with an iterative finite element method (Kumar et al., 2016; Wei et al, in preparation), resulting in a consistent water motion and salinity field. This information is then used to calculate the sediment concentrations, so that the influence of the salt dynamics on sediment transport is prognostically calculated. Owing to the adopted perturbation method, the contribution of various physical processes to residual sediment transport can be studied separately, which allows for a full investigation on individual contribution of each process to longitudinal/lateral transport of salinity and sediment. Moreover, as wind is another important forcing of estuarine circulation (Chen et al., 2009, de Jonge and van Beusekom 1995, Ridderinkhof et al., 2000), the influence of wind stress on estuarine sediment transport will be studied. The present work will bring insights into sediment transport and trapping mechanisms in real estuaries, for example, the Delaware estuary.