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THREE-DIMENSIONAL MODELING OF COHESIVE SEDIMENT TRANSPORT IN A PARTIALLY STRATIFIED MICRO-TIDAL ESTUARY TO ASSESS EFFECTIVENESS OF SEDIMENT TRAPS
Citation:
Hayter, E J., V. Paramygin, AND C. John. THREE-DIMENSIONAL MODELING OF COHESIVE SEDIMENT TRANSPORT IN A PARTIALLY STRATIFIED MICRO-TIDAL ESTUARY TO ASSESS EFFECTIVENESS OF SEDIMENT TRAPS. Presented at 7th International Conference on Nearshore and Estuarine Cohesive Sediment Transport Processes, Williamsburg, VA, October 1-4, 2003.
Impact/Purpose:
This research task will develop modeling approaches and protocols for developing sediment TMDLs for in-stream and watershed (i.e., land surface) processes, and will eventually produce a model of stream and watershed geomorphology. General goals to accomplish this objective are the following:
1. Develop watershed-based modeling systems to forecast the effectiveness of alternative management plans in meeting sediment-related criteria and standards based on appropriate biological endpoints; and
2. Develop and maintain a comprehensive technical support capability that directly links sediment TMDL exposure research activities and products for the EPA Office of Water, EPA Regional Offices, and the States to be used for implementation of policy, regulatory development, remediation, and enforcement needs.
Description:
The three-dimensional (3D) finite difference model Environmental Fluid Dynamics Code (EFDC) was used to simulate the hydrodynamics and sediment transport in a partially stratified micro-tidal estuary. The estuary modeled consisted of a 16-km reach of the St. Johns River, Florida, and two tributaries - the Ortega and Cedar Rivers. The Cedar River flows into the Ortega a few kilometers from the confluence of the Ortega and St. Johns Rivers. The objective of this study was to investigate the effect of sediment traps (with varying sediment removal efficiencies) placed along the Cedar River on the net sediment flux from the Cedar River. Sediment traps have been proposed as a remedial measure to minimize the transport of contaminated sediments from the Cedar River into the Ortega and St. Johns Rivers. In addition to the Cedar and Ortega Rivers, there are three secondary tributaries of the system - Fishing Creek, Butcher Pen Creek and Williamson Creek. At the mouth of the Ortega River, the semi-diurnal tidal range varies from 0.14m (neap tide) to 0.28m (spring tide), with a mean of 0.19m. The bottom and suspended sediments are mostly mixture of clays, silts and organic matter. Suspended sediment concentrations are normally in the 10 - 20 mg/L range. However, during runoff events it usually exceeds 100 mg/l. The dominant range of organic content in the bottom sediment is 20-30%.
An orthogonal-curvilinear grid was developed to represent the modeling domain. The model consisted of 2,856 active cells. Six vertical layers were used. Measured water surface elevations and salinities at three levels over the water column were used at the open water boundaries in the St Johns River. In the six tributaries, the required discharge time-series were obtained from a watershed model (SWMM). The boundary values for suspended sediment concentrations were obtained using a sediment concentration-discharge-rating curve. The model was cold-started for a 30-day spin-up period. The resulting restart file was used to hot-start the runs made during the subsequent 121-day calibration period and 180-day confirmation period. Satisfactory agreements were obtained between measured and predicted water surface elevations, current velocities, and sediment fluxes (at three transects during the calibration period) during both the calibration and confirmation periods.