Lateral entrapment of sediment in tidal estuaries: an idealized model study

Two physical mechanisms leading to lateral accumulation of sediment in tidally dominated estuaries are investigated, involving Coriolis forcing and lateral density gradients. An idealized model is used that consists of the three-dimensional shallow water equations and sediment mass balance. Conditions are assumed to be uniform in the along-estuary direction. A semidiurnal tidal discharge and tidally averaged density gradients are prescribed. The erosional sediment flux at the bed depends both on the bed shear stress and on the amount of sediment available in mud reaches for resuspension. The distribution of mud reaches over the bed is selected such that sediment transport is in morphodynamic equilibrium, that is, tidally averaged erosion and deposition of sediment at the bed balance. Analytical solutions are obtained by using perturbation analysis. Results suggest that in most estuaries lateral density gradients induce more sediment transport than Coriolis forcing. When frictional forces are small (Ekman number E <0.02), the Coriolis mechanism dominates and accumulates sediment on the right bank (looking up-estuary in the Northern Hemisphere). On the other hand, when frictional forces are moderate to high (E > 0.02), the lateral density gradient mechanism dominates and entraps sediment in areas with fresher water. Results also show that the lateral sediment transport induced by the semidiurnal tidal flow is significant when frictional forces are small (E 0.02). Model predictions are in good agreement with observations from the James River estuary.