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

Abstract

[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.

Effect of bottom stress formulation on modelled flow and turbidity maxima in cross-sections of tide-dominated estuaries

“…Such processes have been shown to be important for along-channel distribution of sediment (Postma 1961). Whilst the use of the current sediment module gives results that agree qualitatively with field data for a no-slip boundary condition (Huijts et al 2006), this is not yet established for the partial-slip case. Besides, for situations where advective contributions are not negligible, a full 2DV sediment equation should be solved in order to analyse the effect of spatial settling lag.…”

“…As is shown by Huijts et al (2006), their model is able to simulate the zone of lateral entrapment in an estuarine section of the James River estuary. Moreover, the model is fast and allows for systematic analysis in terms of physical mechanisms.…”

“…Geyer et al (1998) demonstrated that an important mechanism of trapping is that the baroclinic pressure gradient force and Coriolis force act together during one phase of the tide, whereas they oppose each other during the other phase of the tide. Huijts et al (2006) developed an idealised, analytical model in which the flow and sediment distribution are explicitly computed in a cross-section of a tide-dominated estuary. Their solution is obtained by imposing that the total, net lateral sediment transport vanishes at each location.…”

“…In this respect, a major assumption of the analytical model of Huijts et al (2006) is that the vertical eddy viscosity A v and eddy diffusion coefficient K v are constants and a no-slip condition for the flow is imposed at the bed. However, it is known that, near the bed, A v and K v become very small and that the flow has a logarithmic profile in the near-bed layer.…”

Importance of cross-channel bathymetry and eddy viscosity parameterisation in modelling estuarine flow

“…Wong 1994;Valle-Levinson et al 2003;Cheng and ValleLevinson 2009) or residual and tidal flow (e.g. Friedrichs and Hamrick 1996;Huijts et al 2006Huijts et al , 2009Huijts et al , 2011 in estuarine cross sections. Recently, Burchard et al (2011) adapted a numerical code that partly bridges the gap between complex numerical models on the one hand and analytical models on the other hand.…”