RELATIONSHIPS BETWEEN SHORELINE RETREAT RATES, BOTTOM FACIES, AND SUBBOTTOM STRATIGRAPHY, NORTHERN OUTER BANKS, NC

The shoreline retreat rates along the beaches of Dare County, NC are extremely variable. Stratigraphy and bottom types were mapped and analyzed in order to assess the relationship between bottom types, stratigraphy, and shoreline retreat rates. The area of study consists of 40 square miles directly off the beach extending from Duck, NC to Oregon Inlet, NC. Data from high-resolution side scan sonar, subbottom seismic profiling, and a Global Positioning System (GPS) were gathered and entered into a Geographic Information System (GIS) for analysis. By creating a mosaic of the side scan sonar data, surface facies were distinguished and digitized into shape files. In addition, sixty-six vibracores were collected to verify surface bottom types and seismic stratigraphy.

Five bottom types were distinguishable from the side scan sonar data. These bottom types were verified by comparison with the tops of the vibracores. Stratigraphy depicted in the seismic data indicated that the area is composed of stacked sequences of horizontal tabular units cut by multiple channels. Once the bottom types were digitized, mapped, and compared with the subbottom stratigraphy, it was apparent that the surface facies are controlled by the underlying stratigraphy. In the northern portion of the study area, a large fluvial channel (Roanoke Albemarle Channel) dictates the bathymetry and surface facies. In the southern portion of the study area, which overlaps the northern limits of Platt Shoals, Pleistocene sequences cropping out on the inner shelf play a dominant role in controlling the bathymetry and surface facies.

Subbottom stratigraphy and surface facies demonstrate that the lower shore face and inner shelf is erosional in nature. The complex bathymetry patterns are a product of erosional processes regulated by regional flow dynamics. Geotechnical properties of the stratigraphy also play an important role in the evolution of the bathymetry. Therefore, flow dymanics and geotechnical properties of the stratigraphy have an effect on shoreline retreat rates.