A preliminary appraisal of sediment sources and transport in Kings Bay and vicinity, Georgia and Florida

Water-quality, bottom-material, suspended-sediment, and current velocity data were collected in Kings Bay and vicinity to provide information on the sources and transport of estuarine sediments. Kings Bay and Cumberland Sound, the site of the Poseidon Submarine Base in southeast Georgia, are experiencing high rates of sediment deposition and accumulation, which are causing serious navigational and operational problems. Data were collected between November 10-18, 1981, at cross sections in upper and lower Kings Bay, Cumberland Sound, and St. Marys Entrance. Additional water-quality data were collected at one consecutive low and high tide at 29 sites on November 15, 1981, to assess the potential suspended-sediment sources and to define salinity variation throughout the study area. No appreciable vertical or lateral variation in salinity and temperature was detected at the measurement cross sections or at the 29 lowand high-tide measurement sites. With the exception of the upper St. Marys River sites, the waterways measured in Kings Bay and vicinity would be classified as vertically and laterally homogenous; Sediments in bottom-material samples collected at the cross sections ranged from coarse-gravel size shell fragments to fine silt and clay-size inorganic particles. Silt and clay-size particles and organic detrital material, however, were dominant only in bottom materials at the lower Kings Bay cross section. Approximately 50 percent of the silt and clay-size particles in the bottom material at lower Kings Bay consisted of planktonic and benthic diatom remains. Most diatom remains probably originated outside Kings Bay proper. At the other three cross sections, the percentage of remains in the .silt and clay-sized fraction of the bottom sediments was 15 percent or less. Velocity, bathymetry, turbidity, and bottom material data suggest that the area in the vicinity of lower Kings Bay is accumulating deposits of suspended sediment transported from Cumberland Sound on the floodtide and from upper Kings Bay and the tidal marsh drained by Marianna Creek on the ebbtide. Suspended-sediment discharges computed for consecutive 13-hour ebbtides and floodtides showed that a net quantity of 62 x 10^ kilograms of suspended sediment was transported seaward from upper Kings Bay and Marianna Creek. A net landward transport of suspended materials did not occur at the lower Kings Bay cross section, even though velocity and turbidity data suggested that suspended material may have been lost landward of this cross section. A net landward transport of 1,260 x 10^ kilograms was computed for the St. Marys Entrance cross section. Areas seaward of St. Marys Entrance may be supplying sediment to the shoaling areas of the estuary, including lower Kings Bay. The St. Marys River is the single major source of freshwater inflow to the estuary; however, the upland drainage of the St. Marys River does not supply significant quantities of suspended sediment to the estuary. INTRODUCTION High rates of sediment deposition and accumulation are causing serious navigational and operational problems in Kings Bay and Cumberland Sound, southeast Georgia. Kings Bay, formerly the site of the Kings Bay Army Terminal, is now the site of a Poseidon Submarine Base. The existing base will soon be enlarged to accommodate the larger Trident submarine. Of particular concern to the U.S. Navy is the impact of sediment shoaling on naval operations in the area. Continued dredging is required to maintain navigational depths in the Kings Bay wharf area and the access channel to the open sea. Sediment deposition rates have been estimated to be 3.8 x 105 m3/yr (5 x 105 cubic yards per year) in Kings Bay (Environmental Science and Engineering, Inc., 1977, p. C-210) and &. 83 x 105 m 3/yr (1.08 x 105 cubic yards per year) in the Cumberland Sound access channel (Jenkins and Skelly, 1981, p. 2). To accommodate the Trident submarine, Kings Bay and the access channel will be made deeper and wider. The impact of shoaling on the Trident Support Base is uncertain. One prediction is that channel alterations will cause current shoaling rates to increase slightly in the access channel and about 6-fold in the quiet water facilities around Kings Bay (Jenkins and Skelly, 1981, p. 2). Even at the current shoaling rates, expenditures of millions of dollars will be required to maintain navigational depths. Alternative systems for the control of sediment are being pursued by the Navy. However, important information needed to design and to evaluate the systems is lacking. Needed information includes determination of shoaling rates for specific reaches, identification of the major sediment sources, and determination of rates and characteristics of sediment transport. In November 1981, the U.S. Geological Survey conducted a preliminary investigation of the nature and magnitude of sediment transport in Kings Bay and vicinity for the U.S. Navy, OICC (Officer in Charge of Construction), Trident. The purpose of the investigation was to collect and to evaluate basic hydrologic data that are relevant to the determination of the sources and transport characteristics of sediments in the Kings Bay area. This report reviews descriptive background information of the Kings Bay area and presents data on currents, salinity, temperature, turbidity, suspended sediment, phytoplankton, and on the chemical and physical characteristics of bottom sediments. Water, salt, and suspended-sediment discharges are computed for consecutive ebbtides and floodtides that occurred during the November investigation. The information presented in this report is based primarily on data collected over a short period of time during extremely high tidal conditions that were influenced by local weather. Therefore, the interpretations of the data relevant to the sedimentation problems are limited by the fact that the data represent only a short time period. Nevertheless, the data provide important information that is needed to appraise sediment sources, to understand sediment transport characteristics, and to design meaningful datacollection programs. Previous Studies Review of the literature reveals that numerous studies of water and sediment movement have been conducted in estuaries and tidal embayments of the Atlantic Coastal Plain, including the Georgia coast. However, only a few studies have been conducted in the vicinity of Kings Bay and St. Marys estuary. Oertel and Howard (1972) considered the associated water circulation and sediment movement patterns in all major estuary inlets of the Georgia coast, including the St. Marys inlet. Howard and Frey (1975) reported on the characteristics of bottom materials collected from Cumberland Sound and St. Marys River. Olsen (1977) studied the effects of inlet stabilization at St. Marys Entrance. The most comprehensive investigation in Kings Bay and vicinity was the environmental impact assessment for the Poseidon Submarine Base conducted in 1976-77 by ES & E (Environmental Science & Engineering, 1977) for the U.S. Navy. As part of that investigation, water-quality and tidal-flow data were collected periodically for a year to assess the integrated characteristics of water circulation and patterns of sediment erosion, deposition, and accumulation. Acknowledgment s We wish to acknowledge Lt. Commander Anderson, U.S. Navy, OICC, for his assistance with the logistics of this project and U.S. Navy personnel at the Kings Bay docking facility for recording wind data. Our special thanks go to Wayne York of Tradewinds Charter who was extremely cooperative by providing boat support. We thank Bill Harris, the Superintendent of Cumberland Island National Seashore, for allowing a tidestage recorder to be installed at a dock on Cumberland Island. The authors also acknowledge the following U.S. Geological Survey personnel for their assistance: J. L. Glenn for providing helpful suggestions regarding the investigation'and the report; Howard A. Perlman and David W. Parker for their outstanding effort in developing computer graphics software for this report; Myron H. Brooks who provided invaluable assistance in the field operation; Janet Groseclose who edited and typed the manuscript; and lastly, the field personnel who, during the process of data collection, had to frequently endure long working hours and severe weather conditions. DESCRIPTION OF THE STUDY AREA The project study area (fig. 1) was Kings Bay, Cumberland Sound, St. Marys River, Crooked River, Cumberland River, Amelia River, and several smaller tributaries. It did not include any area seaward of the St. Marys Entrance cross section (D) or northward of Cumberland Dividings. Physiography and Topography The estuarine system of Kings Bay and vicinity is a bar-built system in the sea island section of the lower Atlantic Coastal Plain physiographic province of Georgia. Bar-built estuaries are defined as shallow basins,