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Applied Technology and Management (ATM) has performed various studies in support of the Georgia Ports Authority (GPA) and Savannah District, USACE in the evaluation of the feasibility of proposed Harbor modifications, including deepening a portion of Savannah Harbor for consideration as a Federal Navigation Project. ATM evaluated the effects of deepening the existing 42-foot mean low water authorized navigation channel by a maximum of 8 feet, which would extend a maximum of 35 miles from an inner Harbor port to almost 15 miles offshore. Figure 1–1 shows the study area and the features of Savannah Harbor. The major impacts to estuarine resources were evaluated using a calibrated 3-D, hydrodynamic model that predicted changes to salinity and dissolved oxygen due to the maximum river deepening. These changes were then studied with respect to the estuarine species of concern and to resources of special significance, especially to the Savannah National Wildlife Refuge (SNWR) wetlands.

In order to quantify the effects of the proposed deepening, a three-dimensional hydrodynamic model was applied to the Lower Savannah River Estuary. The model utilized included a boundary fitted hydrodynamic model, BFHYDRO, and water quality model, BFWASP, developed by Applied Science Associates (ASA). The model coverage extended from offshore of Fort Pulaski up to approximately 60 miles upstream of the river mouth. The model was calibrated to an extensive data set that was collected from July through September of 1997. The model showed salinity increases along the navigation channel due to increased levels of stratification and a resultant net increase in salinity intrusion.

Although D.O. levels typically average from 4 to 6 mg/l in the Savannah Harbor, they tend to drop significantly near the bottom for short periods of time during slack tides when mixing is minimal. In 1997 ATM conducted studies, which included the review of available historical information and collection of extensive, continuous field data. These studies were used to identify and model the complex processes affecting salinity and D.O. levels in the Harbor. It was discovered that vertical stratification, the layering of lower density freshwater over higher density saltwater, changed rapidly with the tide cycle. Previous models had limited success with systems that are either stratified or not stratified, but not with systems that switched so dynamically. Using a state-of-theart, 3-D computer model, along with a new formulation for vertical mixing developed specifically for this project; the Savannah Harbor model was able to accurately simulate the saltwater/freshwater dynamics and movement in the system.

These water quality models were applied to determine potential effects to various estuarine resources. To address the effects of the proposed channel deepening on the freshwater tidal marshes of the Savannah National Wildlife Refuge (SNWR), an extensive ecological assessment was performed. To assess the potential impacts of the channel deepening required that a general range in salinity changes be known and these changes then be related to the potential change in the distributions of freshwater, brackish, and saline marshes with the tidally influenced areas of the Savannah River marsh system. The major concern was the displacement and reduction of the areal extent of freshwater tidal marsh. The proposed project also addressed DO decreases that might cause adverse impacts to shortnose sturgeon, an endangered species. The proposed channel deepening will extend up to an area that has been previously identified as a habitat for juvenile fish. Another major concern is that an increase in salinity within the traditional spawning area of the striped bass, or within the habitat that serves as a feeding ground for juvenile species, may adversely impact recovery efforts.

This paper and presentation will outline the approach from the Savannah Harbor Expansion Project, the process used to develop a working team of stakeholders, regulators and scientist who oversee the project, and the results of the project to date. More specifically it presents the use of the hydrodynamic model as an environmental decision-making management tool. The paper discusses how the model is being used to assist resource managers in evaluating (a) impacts to biological resources and (b) assessment of mitigation options. This process is one that provides an excellent example for any group planning a river system study to draw from.
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Document Type: Research Article

Publication date: 2002-01-01

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