A model for the deposition of cohesive sediment from a range of buoyancy-driven flows is given. The cohesive sediment is made up of small particles that aggregate to form larger flocs, which are in turn broken up by turbulent shear. The equilibrium floc size (and thus the equilibrium fall speed) is a function of the turbulent dissipation rate and the sediment concentration. The flows are modeled 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 a jet-length or so of the source). In addition, two-dimensional and axisymmetric gravity currents and turbidity currents are modeled. The gravity currents are assumed to be steady flows driven by a constant source of dense fluid with the sediment having a negligible effect on the fluid density. In contrast, the turbidity currents modeled are initiated by the release of a finite volume of fluid containing the sediment with the sediment concentration providing the density difference from the ambient fluid. In most of the flow types examined here, the equilibrium floc size tends to reduce with distance from the source, with any reduction in turbulent shear more than compensated for by the reduction in sediment concentration. For all the flows the basic scales are identified and the concentration and deposition distributions are given.
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