Simulating entrainment and particle fluxes in stratified estuaries

Settling and entrainment are the dominant processes governing noncohesive p article concentration throughout the water column of salt-wedge estuaries. Determination of the relative contribution of these transport processes is complicated by vertical gradients in turbulence and fluid density. A differ ential-turbulence column (DTC) was designed to simulate a vertical section of a natural water column. With satisfactory characterization of turbulence dissipation and saltwater entrainment, the DTC facilitates controlled stud ies of suspended particles under estuarine conditions. The vertical decay o f turbulence in the DTC was found to obey standard scaling law relations wh en the characteristic length scale for turbulence in the apparatus was inco rporated. The entrainment rate of a density interface also followed establi shed grid-stirred turbulence scaling laws. These relations were used to mod el the change in concentration of noncohesive particles above a density int erface. Model simulations and experimental data from the DTC were consisten t over the range of conditions encountered in natural salt-wedge estuaries. Results suggest that when the ratio of entrainment rate to particle settli ng velocity is small, sedimentation is the dominant transport process, whil e entrainment becomes significant as the ratio increases.