Modeling the effect of suspended sediment stratification on bottom exchange processes

Abstract

A vertical gradient of suspended sediment concentration often exists in estuaries, particularly within the bottom boundary layer where sediment erosion and deposition take place. This results in a vertical density gradient and hence modification of the flow. However, this important effect has often been ignored in past sediment studies. Because of this and because of other empirical assumptions, exisiting erosion models cannot be used as predictive tools. This paper employs a simplified second-order closure model to simulate the effect of sediment-induced stratification on bottom boundary layer dynamics, and particularly the erosion process. Numerical models, which employ the concept of Richardson number dependent eddy viscosity, have been developed in the past for stratified flows. These models require a large number of data for tuning parameters of the eddy viscosity formula. The simplified second-order closure model used here consists of the dynamic equations of motion for mean variables (velocity, temperature, salinity, density, and suspended sediment concentration) and turbulent quantities of turbulent kinetic energy and turbulence macroscale. Model constants are thus invariant owing to the added physics. Model simulations of laboratory sediment-laden boundary layers indicate that, as sediments are eroded or resuspended from the bottom, a vertical gradient of suspended sediment concentration can lead to a significant reduction of the turbulent shear stresses and a slowdown of the erosion-resuspension process. Hence significant error may be contained in some empirical sediment erosion rate formulae, which were derived by ignoring flow-sediment interactions. Simulations of a wave boundary layer and a thermally stratified boundary layer, obtained with the same numerical model, are also presented. There is good agreement between measured and simulated mean and turbulent quantities in both cases. column, which can significantly affect the vertical transfer of momentum and materials (e.g., oxygen and sediments). Near the bottom of estuaries and •Now at Woods Hole Oceanographic Institution, Woods Hole, Massachusetts. lakes, strong currents can cause erosion or resuspension of bottom sediments, producing a vertical stratification in suspended sediment concentration, which can in turn affect the flow. Although the effect of sediment stratification on flow is qualitatively understood, a predictive model is lacking. In addition to the stratification, many mechanisms such as wave-current interaction and large bottom features often exist which affect the bottom boundary layer (BBL) dynamics. Thus both for the sake of fundamental understanding and for practical prediction, there is a need for a rather comprehensive boundary layer model which incorporates such effects as stratification, waves, and wave-current interaction. While relatively simple eddy viscosity models have been developed and calibrated to simulate 14,429 14,430 SHENG AND VILLARET: SUSPENDED SEDIMENT STRATIFICATION AND EXCHANGE bottom boundary layers where ...

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