[1] 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.

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Much recent observational evidence suggests that energy from the barotropic tides may be used for mixing in the deep ocean. Here the process of internal tide generation and dissipation by tidal flow over an isolated Gaussian topography is examined, using 2-dimensional numerical simulations employing the MITgcm. Four different topographies are considered, for five different amplitudes of barotropic forcing, thereby allowing a variety of combinations of key nondimensional parameters.While much recent attention has focused on the role of relative topographic steepness and height in modifying the rate of conversion of energy from barotropic to baroclinic modes, here attention is focused on parameters dependent on the flow amplitude. For narrow topography, large amplitude forcing gives rise to baroclinic responses at higher harmonics of the forcing frequency.Tall narrow topographies are found to be the most conducive to mixing. Dissipation rates in these calculations are most efficient for the narrowest topography.

An analytical and a numerical model are used to understand the response of velocity and sediment distributions over Gaussian-shaped estuarine crosssections to changes in tidal forcing and water depth. The estuaries considered here are characterized by strong mixing and a relatively weak along-channel density gradient. It is also examined under what conditions the fast, two-dimensional analytical flow model yields results that agree with those obtained with the more complex three-dimensional numerical model. The analytical model reproduces and explains the main velocity and sediment characteristics in large parts of the parameter space considered (average tidal velocity amplitude, 0.1-1 m s −1 and maximum water depth,

Two different models for the distribution of flow and sediment over the cross-section of a tidally dominated channel are compared. The first is a state-of-the-art numerical model that solves the three-dimensional shallow water equations with prognostic density field. The second is an idealized model which includes residual and semi-diurnal tidal motions and uses a diagnostic residual density gradient as baroclinic forcing. For both models, an off-line sediment module is used to compute the lateral mean sediment distribution. For fairly high values of vertical diffusivity (∼ 0.01 m 2 s −1 ), a good qualitative agreement is found for residual flow patterns. The agreement of the amplitude of the semi-diurnal velocity components is satisfactory as well, although the phase distributions show deviations. The lateral mean sediment distributions are rather similar, and stem from a balance that is predominantly governed by mean concentration and residual currents. The flow patterns only differ qualitatively for either very low or very high tidal velocities. The sediment distributions only deviate for low tidal flow regimes.