Tide-induced coastal and estuarine suspended sediment transport and equilibrium morphology formation

Abstract

The tide acts on coastal and estuarine systems, amongst others by inducing (1) sediment transport and, as a consequence, (2) morphological change. The tide-induced behavior of suspended sediment transport has been studied by individually solving the 1D vertical suspended sediment advection-diffusivity equation and the 1D depth-averaged suspended sediment transport equation. The solutions provide a good understanding of two aspects of the problem, namely that (1) the phase lag increases linearly with height above the bed, whereas the amplitude of the SSC variation decreases exponentially with height; and that (2) the relative contributions of local resuspension and advection to depth-averaged suspended sediment concentration can be determined and that the empirical decomposition terms of horizontal residual fluxes of suspended sediment have clear physical explanations. The morphological modeling of back-barrier tidal basins in the Wadden Sea shows that, in equilibrium states, (1) the dimensional parameters of channel area and volume are proportional to the 1.5 power of the basin area and tidal prism, respectively, whereas the dimensionless parameters of relative channel area and the ratio between channel volume and tidal prism are both proportional to the square root of basin area. The values of the coefficients before the power in these relations are of the order of 10-5, and (2) large basin areas and low tidal ranges favor strong concave-up hypsometries, whereas small basin areas and high tidal ranges favor less concave-up hypsometries, which have been interpreted as a response to the relative area of intertidal areas or channels in the tidal basins. The long-term (6,000 years) modeling results of the large funnel-shaped tide-dominated estuarine morphology shows the development of a sand bar that reached equilibrium within about 3,000 years as the leading order characteristics of the estuarine bed. Its general shape, size and position is consistent with historical observations in the Qiangtangjiang Estuary, China. A series of sensitivity analyses suggests that the estuarine convergence rate, sediment supply, and river discharge are the main controlling factors of sand bar formation.