MORPHOLOGIC RESPONSE OF THE NORTH CAROLINA INNER SHELF TO HURRICANE ISABEL, SEPTEMBER 2003

We mapped the inner continental shelf between Cape Hatteras and Cape Lookout, North Carolina before (June 2003) and after (September 2003) hurricane Isabel using sidescan sonar and both multibeam and interferometric swath bathymetry systems. The pre-storm survey was part of a regional geologic mapping program and also includes high-resolution seismic data. These data provide continuous coverage of the seafloor in the hurricane landfall area from the west side of Diamond Shoals to 5 km southwest of Ocracoke Inlet, extending from approximately the 7 m isobath near the shoreline to 5 nm offshore (~28 m water depth). Pre-storm imagery shows that the seafloor in this area is characterized by a variety of shore-perpendicular to shore-oblique bedforms and grain-size-sorted surficial features that are morphologically similar to rippled scour depressions. The bedforms have typical wavelengths of 100-300 m and heights up to 1 m, and extend across the entire study area. Post-storm surveys used sidescan sonar and multibeam bathymetry systems to resurvey selected areas within a week of Isabel’s landfall. These data show that on the shoreface, some bedforms migrated over 120 m (a wavelength or more) to the southwest, presumably in response to alongshelf storm flows. On the inner shelf off Ocracoke Inlet, a 63 km^2 field of well-organized, low-amplitude (~0.5 m), ~250 m wavelength grain-size-sorted bedforms was “rearranged” such that coarse and fine sediments were largely dispersed across the inner shelf, in an apparent reorganization towards a larger-scale pattern adjusted to the storm flow regime. These responses are consistent with a numerical model of seafloor bedform evolution treating the transport of coarse and fine sediment fractions as functions of the local bed composition. Large-scale sorted patterns exhibiting the main characteristics of the natural features are a robust prediction of the model and provide insight into physical processes affecting the inner shelf during the storm.