MONITORING DECADAL-SCALE EVOLUTION OF THE INNER SHELF AT WRIGHTSVILLE BEACH, NORTH CAROLINA USING REPETITIVE GEOPHYSICAL SURVEYS

A series of seven sidescan sonar surveys were collected on the shoreface and inner continental shelf off Wrightsville Beach, North Carolina. This dataset provides a unique opportunity to characterize the storm response and decadal-scale movement of a previously-identified set of sorted bedforms. The surveys span eleven years from 1994-2005 and map the position of seafloor features annually (1994 and 1995) as well as after four separate hurricane events (Bertha, July 1996; Fran, September 1996; Bonnie, September 1998; and Ophelia, September 2005). Sonar data were collected to provide continuous, overlapping coverage of approximately 30 km² of the sea floor. More recent surveys also include interferometric or multibeam bathymetry, as well as high-resolution CHIRP seismic data. The sorted bedforms begin on the shoreface at 3-4 m water depths and extend out to depths of 15 m on the inner shelf. They are linear, shore-normal features comprised of coarse shell hash and gravel, with typical widths of 50-100 m. The bedforms are bounded by areas of fine sand producing visually distinct boundaries that can be digitized within a Geographic Information System. Bedform movement through the sequence of surveys was quantified by measuring changes in feature boundaries, as well as the position of the centroid for each digitized polygon. Between 1994 and 2005, the bedforms moved an average of 100 m to the southwest. Sorted bedform movement does not appear to occur on an annual basis, but rather as a response to storm events. For example, movement averaged ~50 m for each of the four hurricanes impacting the area. The direction and magnitude of sorted bedform movement varied for each storm. Some features also experienced variations in width but no net movement. Wrightsville Beach is an intensively-managed shoreline and has experienced over 35 years of beach nourishment. The spatial and temporal scales of movement and position of these large-scale bedforms provides insight into the sediment budget of this coastal system over time scales relevant to coastal management.