Inclusion of Floc Growth in a Simple River Mouth Plume Model and Its Effect on Deposition Rate and Deposit Pattern

Abstract:

Rivers are the primary conduits for delivery of sediments and organic matter to the sea. This is visually evident when sediment-laden rivers enter coastal waters, producing sediment plumes. The sediment and organic material from such plumes may deposit and be preserved in estuarine and deltaic zones, or may be carried and mixed by ocean currents to deposit elsewhere on the shelf. Both of these outcomes are governed in large part by depositional mechanics that are dependent, at least in part, on the settling velocity of the sediment. This is especially true in modeling, where the settling velocity has been noted to be the primary controlling parameter for accurate prediction of depositional patters from river plumes. Settling velocity is largely controlled by grain size, shape, and density, which for mud can be quite dynamic due to the process of flocculation. Flocculation yields mud aggregates of variable size and density that may be dependent on the turbulent energy and salt levels under which they were formed. Since turbulent energy and salinity both change in river mouth jet/plumes, the dynamic flocculation process may exert significant control on the eventual distribution of sediment in these zones. In this study, two different approaches to floc modeling are integrated into a steady-state river mouth plume integral model. The two floc models are (1) a version of the Winterwerp (1998) model, and (2) a condition-dependent equilibrium floc size model similar to what is typically used in large-scale 2 and 3D hydraulic and sediment transport simulations. Inclusion of these two models into the buoyant river-mouth plume equations allows for the settling velocity of the mud to be functionally tied to the turbulent shear rate and suspended sediment concentration. The concentration and deposition rates are then compared through the plume both without and with the inclusion of the two different floc treatments. The role that entrainment of ambient fluid plays in the flocculation predictions is explored, and the broad change in deposit patters due to inclusion of flocs is also examined.