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

Abstract

A three-dimensional numerical model with a prognostic salinity field is used to investigate the effect of a partial slip bottom boundary condition on lateral flow and sediment distribution in a transect of a tidally dominated channel. The transect has a symmetrical Gaussian cross-channel bottom profile. For a deep, well-mixed, tidally dominated channel, partial slip decreases the relative importance of Coriolis deflection on the generation of cross-channel flow patterns. This has profound implications for the lateral distribution of residual salinity that drives the cross-channel residual circulation pattern. Transverse sediment transport, however, is always found to be governed by a balance between advection of residual sediment concentration by residual lateral flow on the one hand and cross-channel diffusion on the other hand. Hence, the changes in the cross-channel distribution of residual salinity modify the lateral sediment distribution. For no slip, a single turbidity maximum occurs. In contrast, partial slip gives a gradual transition to a symmetrical density distribution with a turbidity maximum near each bank. For a more shallow, partially mixed tidal channel that represents the James River, a single turbidity maximum at the left bank is found irrespective of the near-bed slip condition. In this case, semi-diurnal contributions to sediment distribution and lateral flow play an important role in cross-channel sediment transport. As vertical viscosity and diffusivity are increased, a second maximum at the right bank again exists for partial slip.

Transverse structure of tidal and residual flow and sediment concentration in estuaries

“…These characteristics can be explained Analysis of numerical and analytical model output in the agreement area reveals that the tidally induced transport is relatively small (at most 20%, not shown). These results are consistent with those of Huijts et al (2006) and Schramkowski et al (2010). To understand lateral trapping of sediment, we can therefore focus on transport of the residual sediment concentration by the residual across-channel velocity, T M 0 .…”

“…6d), this leads to saltier water at the right of the cross-section and fresher water at the left. Second, as explained in Schramkowski et al (2010), the correlation between the transverse tidal flow and the tidal component of the density field causes a net density flux towards the right bank. The more symmetrical lateral density profile in Fig.…”

“…The present study used an idealized turbulence closure scheme (constant eddy viscosity and diffusion coefficients) and a no-slip condition at the bed. Schramkowski et al (2010) recently showed that adopting partial slip can have a profound influence on sediment trapping, as it enhances the relative contribution of the tide-induced net lateral sediment transport T M 2 to the total net lateral transport. Another improvement would be to let A z , K z depend on density stratification, as measured by the bulk Richardson number Ri, defined in Eq.…”

“…Most studies focus on the hydrodynamics (Wong 1994;Kasai et al 2000;Lacy and Monismith 2001;Lerczak and Geyer 2004;Winant 2007Winant , 2008Li et al 2008;ValleLevinson 2008;Huijts et al 2009). Several studies examine the lateral trapping of sediment as well (Huijts et al 2006;Schramkowski et al 2010). However, no studies have systematically explored how and why transverse distributions of flow and sediment respond to changes in tidal forcing or water depth.…”

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

“…The third alternative is a combination of a parabolic distribution in a small region just above the bottom and vertically invariant further up in the water column. This latter shape is used to simulate a partial-slip bottom boundary condition in the AM (Schramkowski et al 2010). In all three alternatives, the eddy viscosity scale (its depth-averaged value) may vary in transverse direction.…”