ANALYSIS OF OFFSHORE SAND RESOURCES FOR THE NORTH CAROLINA OUTER BANKS WEST OF CAPE HATTERAS

Two potential zones of island breaching were identified in an area of the North Carolina (N.C.) Outer Banks barrier island chain west of Cape Hatteras. These areas are located near Hatteras Village (site of inlet formation during Hurricane Isabel in 2003) and along the eastern half of Ocracoke Island. Breaches and significant overwash of this barrier system are major concerns for N.C. due to the reliance of barrier island villages on Highway 12 for their economic survival. In order to prepare for a breach, or to nourish the beach/dune system in erosional “hot spots”, the N.C. Dept. of Transportation contracted the N.C. Geological Survey (NCGS) to identify viable offshore sand resources. The project is using high-resolution CHIRP sonar data, a side-scan sonar mosaic, and swath bathymetry collected by the USGS in combination with textural and stratigraphic data from 33 vibracores (~10 cm diameter by up to 6 m long) collected by the NCGS.

Very little modern sand is present on the shelf in this area. It occurs mainly as a thin (10-20 cm) sheet or as small (<50 cm) bedforms where present. Tabular Pleistocene units make up the majority of the shelf surface. Shallow, muddy or fine-sand filled channels of indeterminate age locally cut into the Pleistocene surface.

For this study, “viable” sand resources are defined as containing medium- or coarser-grained sand (coarser than 2 phi) and <10 percent mud in a weighted average of sieved samples for each core. Cores were characterized as being of poor (1 core), fair (15 cores), or good (17 cores) quality based on their textural and lithologic characteristics. Eight lithofacies were defined from the cores. The more favorable lithofacies are gravelly sand or sandy gravel. In the seismic profiles, finer grained sediment tends to be acoustically transparent while coarser grained deposits exhibit stronger internal reflections. Good correlations exist between the vibracore lithostratigraphy and the seismic data where cores are in close proximity to seismic tracklines. GIS tools are being used to delineate areas of highest resource potential and to estimate the volume of the resource. Additional coring will be needed to assess specific volumes and sand quality for mitigation activities. This study, using existing data, should significantly expedite and reduce the cost of a final resource assessment.