Deposition of Cohesive Sediment from Turbulent Plumes, Gravity Currents, and Turbidity Currents

Abstract

Models for the deposition of cohesive sediment from turbulent plumes (or “buoyant jets”), gravity currents, and turbidity currents are provided in this paper. The cohesive sediment is made up of small particles that aggregate together to form larger flocs, which are in turn broken up by turbulent shear. The equilibrium mean floc size (and thus the equilibrium mean fall speed) is a function of the turbulent dissipation rate and the sediment concentration. The flows are modeled by using integral and box models, with dissipation related to bulk flow properties. For plumes it is shown that there is a well-defined equilibrium fall speed at the virtual origin and that the fall speed changes relatively slowly in the momentum-dominated part of the flow (within one jet length or so of the source). If the flocs are assumed to adjust instantaneously to their equilibrium size, an integral model for a turbulent plume carrying cohesive sediment can be described in terms of two parameters: the angle between the plume and the horizontal at the virtual origin and the (nondimensional) fall speed there. Next, a typical time scale for flocs to adjust to their equilibrium size is identified, and the model is extended to include an equation for the rate of change of the mean floc size along the plume. The time scale over which the mean floc size changes can be compared with a natural time scale for the plume (the time taken for a particle traveling at the mean plume speed to travel a jet length). Thus, in this nonequilibrium model, a further nondimensional parameter is identified,