Removal of the tide gate structure from operation and closing of New Cut is predicted to result in a $2.0-2.9 million reduction in the savings the Savannah Harbor sediment control works provides to the Savannah District Operation and Maintenance dredging program. This is based on a prediction that the sediment basin will become less efficient once the tide gate structure is removed from operation. Means of reducing this increase in dredging cost are being investigated, such as dredging the sediment basin more often to keep the basin in its most efficient state. It is expected that dredging the basin twice each year will decrease the impacts of the project modification on maintenance dredging cost, since the sediment basin is most efficient when it is dredged to project depth. WITHOUT PROJECT CONDITIONS The 'Without Project' conditions for this study of Savannah Harbor deepening include implementation of the Section 1135 project modification, which includes closure of New Cut and ceasing operation of the tide gate structure. The sediment basin will continue to function at reduced efficiency as a sediment trap. Description of Project Area Savannah Harbor is a deep draft harbor on the South Atlantic coast 75 statute miles south of Charleston Harbor, South Carolina, and 120 miles north of Jacksonville Harbor, Florida. The harbor comprises the lower 21.3 miles of the Savannah River and 11.4 miles of channel across the bar to the Atlantic Ocean. The Savannah River, with certain of its tributaries, forms the boundary between Georgia and South Carolina along its entire length of 313 miles. The Savannah River within the harbor limits is generally divided into two channels by a series of islands. From the Atlantic Ocean to river mile 10, where the river converges, the harbor is separated into the South and North Channels. Within this area, the navigation channel is maintained in the North Channel. After divergence of the river into the Front and Back Rivers at River Mile 11, the navigation channel is maintained in the Front River and passes by the business district of the City of Savannah. The navigation channel is maintained in the Front River to the upper limits of the harbor at River Mile 21.3. The Atlantic Intracoastal Waterway crosses the navigation channel approximately 6 miles upstream of the entrance to the harbor. A navigation channel 9 feet deep and 90 feet wide is authorized in the Savannah River from the upper limits of the harbor to River Mile 202.6 at Augusta, Georgia. Geography The mainland on the south side of the harbor is dominated by the city of Savannah, Georgia. The city's historic downtown area is located on a south bluff approximately 18 miles above the river's mouth. Heavy industry and shipping facilities are along the south side of the harbor upstream from the city's historic downtown area to the head of the harbor. More heavy industries and a few shipping facilities line the harbor downstream from the historic downtown area of the city to the Atlantic Intracoastal Waterway. From the Intracoastal Waterway to the river's mouth, both sides of the harbor are predominantly undeveloped areas consisting of islands, marshes, dredged material disposal areas, and other natural sites. The areas along the South Carolina side of the harbor are characterized by dredged material disposal areas, former rice fields constructed in the 18th and 19th centuries, and marshes. Land use on the South Carolina side of the Savannah River is basically agricultural, silvicultural, with some recreation. Wetland habitat types found along Savannah Harbor include saltwater aquatic, saltwater coastal flats, saltwater marshes, freshwater aquatic, freshwater flats, and freshwater marsh. Climatology Temperature. The Savannah, Georgia, area has a temperate climate characterized by warm, humid summers and mild winters. The seasonal mean temperatures are 51 degrees F in winter, 64 degrees F in spring, 80 degrees F in summer, and 66 degrees F in autumn. Precipitation averages 48.9 inches per year, about half falling during summer thundershowers. Snow is rare. The frost-free season averages approximately 270 days. Hurricanes pose an occasional threat, principally in September or October. Delays to shipping activities in the Savannah area due to fog or inclement weather are relatively infrequent. Wind. The prevailing winds are from the southwest during May to August and from the northeast during September to December. Sustained winds (1 minute or more) vary from 29 to 46 miles per hour (mph), with gusts of 38 to 68 mph. There is about a 10 percent chance in any 1 year (return frequency period of 10 years) of storm winds of 74 to 95 mph (64-83 knots). Detailed information on storm winds and corresponding water level elevation is presented in the Engineering Appendix. Fog. Heavy fog can be expected about 40 days per year. Fog occurs in all months of the year from about 1 to 5 days per month. The heaviest fog is generally found in November and January. Heavy fog has caused shipping delays for both inbound and outbound vessels. Geology and Soils Savannah, Georgia is located in the Lower Atlantic Coastal Plain Physiographic Province. The majority of soils primarily have a sandy surface layer over a loamy or sandy subsoil or underlying layers. These soils are nearly level or gently sloping and occur as broad, smooth areas drained by wet depressions. They generally are seasonally wet or almost always wet, except for the better drained soils on the slight ridges and dunelike relief. A band of marshland parallels the coastline and extends inland along the major streams. Limestones of tertiary and quaternary age underlying the Coastal Plain form one of the most productive aquifer systems in the country. Hydrology and Tides Tidal fluctuations within Savannah Harbor are semidiurnal, averaging 6.8 feet at the mouth of the harbor and 7.9 feet at the upstream limit of the harbor, with tidal influences extending upriver approximately 45 miles to Ebenezer Landing, Georgia. Maximum velocities encountered in the navigation channel are approximately 4 feet per second on the flood tide and 5 feet per second on ebb tide. Freshwater discharges near Clyo, Georgia (River Mile 65) average 11,600 cubic feet per second (cfs), with maximum and minimum annual mean discharges of 20,900 cfs and 9,820 cfs, respectively, since 1962. The average tidal range, independent of msl, is shown in Table 13. TABLE 13 Location Tidal Entrance (mouth), Sta. 0+000 (mile 0.0) Ft. Pulaski Gage 6.95' Bull Street Gage, Sta. 76+000 (mile 14.38) 7.87' Kings Island Turning Basin, Sta. 99+000 (mile 18.75) 8.00' U.S. Highway 17 Bridge, Sta. 114+000 (mile 21.59) 7.97' Local mean high water above mean sea level varies from +4.02 feet at the mouth to +4.58 feet at the upper end of the harbor. Local mlw below mean sea level varies from 2.93 feet at the mouth to -3.48 feet at the Kings Island Turning Basin. The elevation of mlw has continued to rise since 1935. This rise amounted to about 0.63 feet in 1986. This indicates that the average water level rise in the harbor is at a rate of about 0.012 feet per year. It has not been determined whether this phenomenon is geologic or astrologic. This is further discussed in the Engineering and Design Appendix. During storms, the water level in the harbor will rise about 0.5 feet or more about once a year, resulting in a tidal range of about 7.5 feet. More infrequent storms magnify this effect, i.e., once in 5 years a tidal range of 9.1 feet can be expected, and once in 10 years a 10.0-foot range can be expected. The Engineering and Design Appendix contains probabilities of storm events and their effect on tidal range. Water Quality South Carolina has classified the portion of Savannah Harbor within its boundaries upstream from Fort Pulaski as Class B and the portion oceanward as Class SA waters. Class B is defined as fresh waters suitable for secondary contact recreation and as a source for drinking water supply after conventional treatment in accordance with the requirements of the South Carolina Department of Health and Environmental Control. These waters are suitable for fishing, industrial, and agricultural uses, and for the survival and preparation of a balanced indigenous aquatic community of fauna and flora. Class SA is defined as tidal salt waters suitable for primary contact recreation and also suitable for uses listed in Class B. The State of Georgia has classified the Savannah River from mile 0 at Fort Pulaski to mile 5 at Field's Cut as recreation waters. From Field's Cut to the U.S. Highway 17 Bridge at mile 22, the river is classified as Coastal Fishing. The latter stretch of the Savannah River was classified as Industrial/Navigation. However, studies were conducted by the Georgia Department of Natural Resources during the fall of 1985 which resulted in the recent reclassification of that stretch of the river to Coastal Fishing. During the Savannah Harbor Environmental study, it was discovered that the sediment basin in the Back River often exhibited anoxic conditions during operation of the tide gate structure. Taking the tide gate out of operation as part of the Section 1135 project modification will reduce stratification in the basin and thereby reduce the potential for depletion of dissolved oxygen in the basin. Dissolved oxygen levels would still be lower near the bottom in the basin due to the nature of the sediments which accumulate there. However, anoxic conditions would seldom, if ever, occur. Salinity Past harbor construction projects, mainly construction and operation of the Back River tide gate, resulted in substantial increases in salinity in the Savannah estuary. The immediate result of the Section 1135 action will be the lowering of these salinity levels. Based on predictions from the Laterally Averaged Estuary Model with sediment module (LAEMSED) model, closure of New Cut would, in almost all cases, lower salinity in the estuary. Table 14 and Figures 10 through 12 indicate the LAEMSED model predictions for the maximum salinity levels which would occur in the Front, Middle, and Back Rivers after implementation of the Section 1135 project modification. |