Inclusion of floc-driven settling velocity in a simple river mouth plume model and the effect on plume concentration and rate of deposition

Abstract:

The flocculation process has the potential to alter the settling velocity of mud in buoyant river mouth discharges as a function of distance from shore due to continual decreases in turbulent shearing and increases in salinity. In this study, we measure the change in floc size in a decaying shear field and compare the results to steady-state experiments conducted on the same batch of sediment. The primary questions we explore are: (1) whether or not flocs are at their equilibrium size throughout the duration of the decay, and (2) whether or not the change in size is all that significant in terms of alterations to settling velocity. These questions are examined in the laboratory using a camera system that allows for flocs to be sized within a turbulent suspension. We also test the ability of current flocculation models to capture data trends, and we examine how inclusion of various floc modeling approaches in a simple river-mouth discharge model impacts plume concentration and deposit location. The data shows that changes in floc size (due only to changes in turbulence) can lead to a 4 or greater fold increase in settling velocity within the first few km of the river mouth. No floc settling velocity model was able to perfectly match the experimental time series; though the basic functionality was captured with the non-equilibrium model of Winterwerp (1998). The plume modeling analysis highlights the strong dependence of the mud deposition rate on the method chosen to account for flocs. In general, inclusion of flocculation shifts the mud depocenter landward. Interestingly, inclusion of flocculation through most current modeling methods often leads to only a marginal change in the local plume concentration due to the strong influence of entrainment on concentration.