Modeling the transport of flocculating suspended sediments resulting from dredging operations

Dredges and dredging operations release sediment into the water column during normal operation. These sediments are transported away from the dredging site by ambient and induced currents. If undesirable chemical species are attached to the sediments, these contaminants are also transported away from the dredging operation. The ability to predict the transport of these sediments and associated contaminants is necessary to assess the water quality impact of a dredging operation. Previous models of turbidity plumes from dredging operations assumed the sediment particles in suspension settle with a constant velocity at all times and locations within the plume. However, larger particles that settle according to Stokes' law with a constant settling velocity settle quickly to the bottom in the immediate vicinity of the dredging operations leaving a suspension of only fine sediment particles and colloids. Dilute colloidal suspensions flocculate causing settling velocities to vary continuously in time and space.; This research develops empirical models for instantaneous and depth average settling velocity in flocculating suspensions. The models can be fit to any sediment by conducting flocculent settling tests at the anticipated initial TSS concentration near the dredging operation. The depth average settling velocity model is incorporated in two-dimensional (horizontal plane) transport equations to develop suspended sediment transport models for bucket and hydraulic cutterhead dredges. An analytical solution was derived for the bucket dredge model by assuming the dredge is a continuous point source of TSS. Hydraulic cutterhead dredges are best simulated as a horizontal line source normal to the direction of the dredge. This source geometry required a numerical solution for the hydraulic cutterhead dredge.; The bucket dredge model is compared to a previously developed bucket dredge model using a constant settling velocity term. The results show that a constant settling velocity significantly overestimates TSS concentrations near the dredging operation (settling velocity is too low) and underestimates TSS concentrations away from the dredging operation (settling velocity is too high).; Field observations of TSS concentrations and laboratory column settling data from hydraulic dredging projects near Savannah, GA and New Bedford, MA are used to test the veracity of the hydraulic dredge model. The results agree well with the field data indicating the flocculent settling velocity model is a useful tool for modeling flocculating dilute colloidal suspensions.