INTEGRATED REGIONAL SEA FLOOR MAPPING OF THE SHOREFACE: LINKING THE SHELF AND BEACH SYSTEMS

Integrated high-resolution geophysical surveys of the northern South Carolina shoreface have been linked to similar surveys of the inner shelf and a range of onshore data types defining the structure and behavior of the modern beach. Together they characterize patterns of coastal erosion and evolution on a range of temporal and spatial scales. The imagery provides a linkage between the long-term result of transgression of the shoreline (observed in the relief and patchy deposits on the inner shelf) and shorter termed variability of the eroding coastline (defined by historical shorelines, video, beach profiles, cores and GPR surveys).

The grid of CHIRP, side-scan sonar and interferometric sonar bathymetry document a sediment-starved shoreface where nearshore geometries and behavior are influenced by the architecture of older strata underlying the coastal zone. The modern beach system is a thin and locally patchy veneer of sediment. Cross-shore beach profile geometries are frequently interrupted as underlying units intersect the active surface. Regional sediment dispersal pathways on the inner shelf (>3 km from the coast), defined by trends in surficial sediment texture, backscatter and bedform geometry, are coast-parallel to the southwest except near a few large sediment deposits where interaction between processes and morphology may occur. Redistribution of recent beach nourishments document the importance of cross-shore sediment exchange between the beach and innermost shelf in many areas. Coastal erosion and onshore flux of sediment from the inner shelf are the major sources of sediment to the regions sediment budget.

Beach cam imagery of a 1-km stretch of beach and repetitive regional (30 km) RTK-DGPS surveys of the MHW contour define alongshore patterns in beach variability. DGPS surveys taken immediately before and after the recent passage of Hurricane Isabel offshore of the area showed an influx of sediment onshore during the event. This sediment has been rework, at varying speeds along shore, and the influence of framework on the nearshore has been reasserting itself.

Regional and time series mapping have provided an important basis for the regional coastal erosion study. The seafloor imaging has also been used to assess sand resource potential and distribution of critical biological resources.