MODELING DREDGE-INDUCED SUSPENDED SEDIMENT TRANSPORT AND DEPOSITION IN THE TAUNTON RIVER AND MT. HOPE BAY, MASSACHUSETTS

摘要

A modeling analysis was performed to assess the dredge-induced suspended sediment transport and deposition from a proposed dredging operation in Mt. Hope Bay and the Taunton River, located in southeastern Massachusetts. The dredging is necessary to maintain and deepen an existing nearly 11 km long Federal Channel to allow liquid natural gas tankers to reach a terminal proposed for Fall River. The ultimate goal of the analysis was to determine if the proposed dredging plan that included equipment restrictions and continuous dredging would be protective of aquatic species during periods of particular environmental concern (spawning, fish passage, etc.). The analysis employed a strong science-based approach that included a field program and the use of computer models to determine the environmental effects of the dredging project. SSFATE (Suspended Sediment FATE) is a model jointly developed by the U.S. Army Corps of Engineers Engineer Research and Development Center and Applied Science Associates to estimate the water column suspended sediment and bottom deposition pattern resulting from dredging operations. The model was developed to provide a consistent estimate of the transport and fate of the portion of dredged material lost during dredging. The model requires specification of circulation in the area of interest, either from direct measurements or hydrodynamic model output; the type of dredging technology used, and the loss rate and vertical distribution of initial material release. Using a random walk procedure, the model tracks representative particle classes as they disperse in the water column and settle to the bottom. Model output includes water column suspended sediment concentrations and bottom deposition thicknesses. The SSFATE model was applied to three representative sites along the channel in Mt. Hope Bay and the Taunton River, simulating both maintenance (fine grain) and parent (native, coarse grain) materials for different bucket dredge technologies. Bucket cycle, loss rates, and physical dredged material properties were conservatively modeled assuming continuous dredging at forecasted optimum dredge production rates. Results at the different locations were inter-compared. Using these conservative assumptions for model inputs, it was found that locations in Mt. Hope Bay and the Taunton River did not exhibit any significant levels of either water column concentration or deposition thickness. However, when dredging parent material at the turning basin adjacent to the terminal site; some potential effects to winter flounder eggs due to sediment deposition may be of concern, necessitating operating restrictions during the period of winter flounder spawning.