Sediment-Transport Pathways in Estuarine Channels With Lateral Bathymetric Variation

The longitudinal and lateral transport of suspended sediment in estuarine channels have been thought to be fundamentally linked. The convergence of longitudinal sediment transport is the key to understand sediment sorting, accumulation, and ETM formation. In this idealized numerical study, the sediment-transport pathways in periodically stratified estuaries are investigated by means of a two-dimensional cross-sectional mode of the General Estuarine Transport Model (GETM). A Gaussian-shaped depth profile is prescribed, assuming longitudinal uniformity. It is found that the along-channel residual sediment flux shows nonmonotonic dependencies on the channel width-to-depth ratio, the sediment grain size, and the Simpson number. The tidal pumping flux (i.e. the covariance between sediment concentration and current velocity) dominates the total along-channel sediment transport in response to changes in bathymetry and forcing. Based on temporal decomposition of the tidal pumping flux, longitudinal and lateral hydrodynamic processes are found to jointly result in tidal asymmetries in sediment resuspension and settling, which determine in what direction and how much the sediment is transported at the residual timescale. It is pointed out for the first time that even without the mean river discharge, internal tidal asymmetry alone can result in seaward sediment flux that opposes the classical baroclinically-induced landward sediment flux. Lateral-vertical circulations within the estuarine cross section influence tidal pumping of sediment not only through advection of salinity that creates tidal stratification asymmetry, but also through the direct advection of suspended sediment that modifies the concentration-velocity covariance. The relative importance of stratification-driven and advectively-driven tidal pumping flux is analyzed across a wide range of model parameters.