South Carolina Coastal Erosion Study

The Issue

The region of interest extends from Bull Island to Seabrook Island; a shoreline of approximately 60 km centered near Charleston, SC characterized by multiple barrier islands and tidal inlets with a wide range of spatial scales. An extensive foundation of numerous site-specific databases and published studies already exist on portions of the reach. Inlet-beach interactions are known to play a dominant role in short-term changes, and significant historical changes in shoreline and inlet positions have been documented (Sexton and Hayes, 1983). Stratigraphic foundations of the modern barrier islands, inlet systems, and shoreface in the region are believed to strongly influence coastal processes; however, there have been no comprehensive framework studies to verify this hypothesis.

How the USGS is addressing this issue

Four program themes have been identified and developed to accomplish the objectives of the study:

1) Geological Framework Studies

Establishing a detailed geologic framework of the barrier island/inlet systems and the adjacent shoreface/inner shelf is central to the study's purpose of understanding coastal erosion and process response on the ranges of spatial and temporal scale that control the evolution of the coastal zone. To understand the geologic framework of the region the following approaches will be used:

• Conduct sea-floor mapping in the shoreface and inner shelf using high resolution seismic sub-bottom profiling and bathymetry
• Vibracoring on the shoreface/inner shelf to ground-truth seismic stratigraphy
• Onshore vibracoring and truck-mounted auger rig drilling will be conducted to determine the barrier island stratigraphy and evolution of the barrier island/inlet systems
• Selected sections of the sea-floor in the study region will be mapped using high-resolution side-scan imagery techniques

2) Sediment Inventory

The large-scale regional sediment inventory, or sediment budget, will identify and quantify the volumes and movements of sediment accumulating or eroding along beaches, inlets, and within offshore sand bodies.

3) Sediment Transport

An accurate description of sediment transport rates, both longshore and cross-shore, and the associated temporal and spatial variation in each, and the ability to apply these rates to predict gradients in sediment transport (hence, zones of accretion and erosion) will be extremely valuable for both scientific and coastal management purposes.

4) Coastal Inventory and Change

A GIS bases inventory of coastal features will be developed in order to assess past and currents rates of shoreline change and to provide a scientific data base that includes those variables known to contribute to coastal change. The focus will be to acquire and archive existing data sets which have been compiled by project researchers (historic and recent shoreline position and bathymetry, beach profiles, aerial photography, sedimentology) in order to produce a user friendly CD-ROM product. LIDAR, through a cooperative effort with NOAA, will be the primary tool for mapping coastal features.

1998 Activities

• Complete seismic reflection surveys north of Charleston Harbor. (Kindinger, Flocks, Coop)
• Complete vibracoring on the inner shelf north of Charleston. (Kindinger, Flocks, Coop)
• Conduct radiocarbon and amino acid racemization dating on select cores to aid in age determination of units within the Quaternary. (Coop)
• Expand side scan sonar surveys to the north of the Charleston jetties, between Breach and Dewees Inlet. (Schwab)
• Drill deep core holes on Kiawah and Seabrook Islands. (Coop)
• Analyze borings and determine the subsurface strata extending to the northern boundary of the study area. (Coop)
• Conduct micropaleontological analyses of core samples to substantiate age relationships. (Coop)
• Conduct amino acid analyses to aid in chronostratigraphic correlation of Quaternary units. (Coop)

Sediment History

• Assemble all dredge and fill records related to changes in the littoral zone, and compute annualized and unitized volumetric change rate for relevant periods of interest. (Coop)
• Analyze historical aerial photos and delineate and scale washovers. (Coop)
• Develop empirical relations for South Carolina delta volumes and shoal bypass volumes in relation to the size of inlets in the study area (size defined mainly by cross-sectional area and tidal prism). (Hansen)
• Develop statistical relationships (temporal and spatial) which relate to beach width, slope, unit volume, and sediment grain size on an island-by-island basis. (Coop)
• Develop sediment budgets for selected time periods and island groups within the study area. (Hansen,Coop)

Sediment Transport Studies

A large but relative thin (10-100cm) lens of sand has been identified offshore of Folly Island beach in depths of 3-10m, extending beyond typical profile closure depth. It has been suggested that this sand is derived from the "wash-out" area of Folly beach and the sand appears to be mobile. Study investigators propose a detailed look at this area is warranted and that sediment transport investigators divert part of their effort to understand the origin and dynamics of this feature.

• Measure nearshore wave field and currents at the Folly Beach sediment " conduit. (Coop)
• Collect wave data offshore for description of wave conditions incident to Dewees and Price Inlets. (Coop)
• Investigate ability of existing wave transformation models to predict nearshore wave conditions measured in the field. (Coop)

Coastal Change and GIS

• Produce a CD-ROM of aerophotography. (Coop)
• Continue the development and the maintenance of the GIS database for the central coast. (Coop, Barton)
• CD-ROM of quantitative coastal change with interpretation for the South Carolina coast. (Barton, Coop)
• Begin production of the Coastal Change Atlas for the central coast barrier islands of South Carolina. (Coop)

The recent availability of coastal LIDAR data in South Carolina provides an opportunity to expand the project with a detailed inventory (at 1:25000 scale) of coastal features. This effort has the potential to provide a quantitative basis for making real time forecasts of coastal changes in response to storm events. A GIS based inventory will be developed to include historical shoreline position, coastal topography/bathymetry (using LIDAR), land use, sedimentology, geology, storm history, and sea level change. Data from the existing GIS data base developed by our cooperators will be incorporated into this inventory data base.

Beach Monitoring

• The collection of wading depth beach profile surveys at 400 survey monuments along the SC coast will continue (funded through the South Carolina Coastal Council).
• The collection of deep water beach profiles at 100 survey monuments along the SC coast will continue (funded through the South Carolina Coastal Council).

Accomplishments to Date

• A total of 500 Km of digital Boomer seismic data has been collected since 1994 in the shoreface and on the inner shelf in the region of Charleston Harbor, Folly Beach, and Sullivan's Island.
• Fifty 10m vibracores have been collected offshore of Folly, Kiawah, and Sullivan's Island. Cores have been described, photographed, and sub-sampled for sediment analysis.
• A total of 24 deep holes were drilled on Folly Beach and Isle of Palms and over 200 sediment samples were collected from Quaternary and Tertiary Stratigraphic units.
• A sea-floor map was produced using high-resolution digital side-scan imagery techniques for the region offshore of Folly Beach and Morris Island. Approximately 200 surface sediment samples were collected at selected locations within the imaged area for ground truthing purposes.
• Historical shoreline data and recent shoreline data (interpreted from aerial photography) were archived into an ARC/Info database
• NOS historical bathymetry from Charleston Harbor to Bull's Bay have been digitized and entered into an ARC/Info database. Preliminary maps for three time periods have been produced.
• Surveys of the ebb-tidal shoal complexes and inlet throat sections at Price and Breach Inlets have been completed using the high-resolution SANDS bathymetric system
• Tidal current measurements were collected at Price and Breach Inlets over several tidal cycles using an Acoustic Doppler Current Profiler

USGS Cooperators

• South Carolina Sea Grant Consortium

• South Carolina Office of Ocean and Coastal Resource Management

Products

• Annotated Bibliography of South Carolina Mesotidal Shorelines and Processes Technical Reports. This report references all the major technical (published) papers concerning coastal and geological investigations within the study area.

• Department of Health and Environmental Control, 1997, South Carolina's Annual; State of the Beach Report, Office of Ocean and Coastal Resource Management, 30 pp.

• Donovan-Ealy, P. and Gayes, P.T., 1996, Beach Renourishment Resources on the inner Shelf off Folly Beach, South Carolina: Final Report of the SC INTERMAR Task Force on Sand Resources and Critical Habitats on the SC Continental Shelf.

• Harris, M. S.; Gayes, P. T.; Donovan-Ealy, P.; Nelson, D. D.; York, L. L.; Gielstra, N.; Kindinger, J.; Flocks,J. and Krantz, D. 1996. Geologic Framework Influences on Modern Coastal Morphology and Processes along the South Carolina Coast. Geological Society of America, Annual Meeting, New Orleans, Louisiana. Abstracts with Programs, p. A-78.

• Harris, M. S.; Krantz, D. E.; Wehmiller, J. F.; Gayes, P.T.; Kindinger, J. L.; and Flocks, J. 1996. High Resolution Stratigraphy and Paleogeography of the Upper Cenozoic System beneath the Coastal Plain and Continental Shelf, Charleston, South Carolina. Geological Society of American, Annual Meeting, Denver, Colorado. Abstracts with Programs, p. A-118.

• Harris, M. S.; Kindinger, J.; Schwab, W. C.; and Gayes, P. T., 1996. Geologic and Bathymetric variability, Folly Island, South Carolina: An example of a Geologic Framework Influencing Near-Shore Processes. Southeastern Section, Geological Society of America, Jackson, Mississippi, Abstracts with Program.

• Harris, M.S., Gayes, P.T. and Krantz, D., 1996, Influence of shallow Cenozoic subcrop stratigraphy on modern coastal morphology and processes: Central South Carolina: Journal of Sedimentary Geology.

• Kana, T. W., , 1995, Signatures of Coastal Change at Mesoscales, Proc. Coastal Dynamics '95, Gdansk, Poland, American Society of Civil Engineers, New York, NY.

• Katuna, M.P., 1995, Study of Shoreline Migration Rates and Sediment Budgets for Seabrook, Kiawah and Folly Islands, South Carolina Final Report, SC Task Force on Offshore Resources, M.M.S., 25 p. + appendices.

• Katuna, M.P., Gayes, P. T., and Harris, S., Geology Framework of the SC Coastline - USGS Open File Report (in preparation).

• Kriebel, D. L., 1995, Evaluation of Ideal Present Profile Method for Beachfront Management, Office of Ocean and Coastal Resource Management, 110 pp.

• Work, P.A., Rogers, W.E., and Hayter, E.J., 1995 South Carolina Coastal Erosion Study: Inlet Morphodynamics and Sediment Transport, Proc. Coastal Dynamics '95, Gdansk, Poland, American Society of Civil Engineers, New York, NY.