| Popis: |
Several scenarios were used to evaluate the effectiveness in reducing the retention time for water in Bushy Park Reservoir near Charleston, S.C. Flows were simulated by using the U.S. Geological Survey BRANCH one-dimensional unsteady-flow model on the Cooper River from Pinopolis Dam at Lake Moultrie to Yellow House Creek, 5 miles seaward of Back River, and on the Bushy Park Reservoir. Flushing of Bushy Park Reservoir was simulated by using the particle-tracking function of the BRANCH model, which accounts only for hydrodynamic movement without diffusion, dispersion, or decay of contaminants. The model was calibrated and verified by using data from 15 flow-measurement sites and 17 stage stations. Results are quantified on graphs showing the number of days required to move a particle of water (retention time or days-to-flush) from Durham Canal to the Charleston Commissioners of Public Works (CPW) intake, reflecting changes in the amount of withdrawal by Charleston CPW; the location of the CPW intake at its present location on Foster Creek or at a new location 0.9 mile north of Foster Creek, on the Back River; flow through hypothetical flap-type tide gates at (6-foot concrete pipes) Bushy Park Dam; and whether a thermoelectric power plant is withdrawing water from the reservoir. The withdrawals by the thermoelectric power plant are large enough to improve the quality of water in the reservoir from Durham Canal to its intake to a degree that the water can be economically treated by CPW. Target flushing rates of 3.1 and 5.2 days were established for hypothetical CPW intakes located 0.9 mile north of Foster Creek on the Back River and at the current Foster Creek intake, respectively. This flushing rate uses the same flushing rate achieved by the power plant. Combined maximum CPW withdrawals from the Edisto River and Foster Creek are 50, 118, and 150 million gallons per day (Mgal/d) for current, short-term, and ultimate demand projections, respectively. With 50 Mgal/d withdrawals at Foster Creek, the days-to-flush is about twice the estimated target for eight 6-foot concrete pipes. If the withdrawal rate is increased to 118 or 150 Mgal/d for the same number of pipes, the target rate is exceeded by one day. If the CPW intake is moved to the site on the Back River, the target days-to-flush can be reached by withdrawals of 50 to 150 Mgal/d with six to eight 6-foot concrete pipes. Significant improvement in flushing characteristics could be achieved if the intake was located on the Back River, 0.9 mile north of Foster Creek. A sensitivity analysis showed that flushing rates were insensitive to model roughness estimates, cross-section daturns, or boundary-condition stage daturns. |
