Model for Evaluating Material Transport from Ocean Dredged Material Disposal Sites

An initiation of sediment transport model (ISTM) is described for predicting the initiation of dredged material transport by ocean currents at Ocean Dredged Material Disposal Sites (ODMDS), and it is applied to two ODMDS sites offshore of Texas and Louisiana coasts where previously measured nearby bottom current data are available. The model incorporates the hydrodynamics of current boundary layer flow, fluid-sediment interaction near the bottom, and sediment transport mechanics. The fundamental approach to predicting sediment transport is to relate the friction factor and bed shear stress to the sediment transport rate. The approach is based on comparing the calculated bottom shear stress created by an ocean current to the critical shear velocity for sediment movement. Two model limitations are that the hydrodynamic environment is limited to near-bottom unidirectional steady current and the sediment is cohesionless. The model predicts the critical bottom current speed that is expected to initiate sediment movement based on the Shields parameter for a particular grain size. The model is further used to estimate the current velocity at a depth where existing current meter data are available. Near bottom current meter data collected and processed during the period 1982 through 1984 in the vicinity ODMDS sites of the Freeport ship channel and the Calcasieu ship channel are used to demonstrate the application of the ISTM model. The results indicate the percent time for each month of the year when the bottom currents are sufficient to move the sediment. The current direction is divided into eight quadrants and is used to show the direction of sediment movement toward or away from the Freeport and Calcasieu ship channels. The model can be used to assist in the timing of the dredging operations and indicating the percentage of time when the ocean currents could possibly redeposit the dredged material back into the ship channels. The results show that during the dredging season the currents were shown to be away from the Freeport and Calcasieu ship channels.