Tide-induced suspended sediment transport: Depth-averaged concentrations and horizontal residual fluxes

In coastal and estuarine waters, depth-averaged suspended sediment concentration (DASSC) at a fixed observation site has two sources: local resuspension and advection along a horizontal gradient. The empirical decomposition method can separate the horizontal residual suspended sediment flux (HRF) into several terms, e.g., Eulerian flux, Stokes' drift and tidal pumping. A simple depth-averaged 1D model is solved analytically for an observational site over one semi-diurnal tidal cycle to explore the determinants of the forcing factors comprising residual, M2 and M4 current velocities, mean water depth, M2 tidal amplitude and the DASSC gradient. The analytical solutions provide clear physical explanations of tidal DASSC variations and HRF. The solutions are applied to fit and explain the observations at a fixed station in a macro-tidal channel located along the Jiangsu Coast, China. The results suggest a dependence of the M4 DASSC variation on resuspension induced by the M2 velocity, and the dependence of the M2 DASSC variation on the advection and resuspension induced by the interaction between the M2 and M4 velocities. The horizontal DASSC gradient results in large seaward tidal pumping, which is offset by the landward Lagrangian flux to produce a reduced landward HRF.