Spatiotemporal Analysis of Cuspate Features within the Shoreface-To-Shelf Transition Zone, Grand Strand, South Carolina

A multiyear time-series of high resolution multibeam bathymetry and backscatter analysis has documented the behavior of the shoreface-to-shelf transition along the Grand Strand of northeastern South Carolina during a period of unusually frequent large storm events. These surveys have repeatedly highlighted rhythmic cuspate features with varying wavelengths of 50-200 meters found almost continuously throughout the 9 km study area. These features are also found to be aligned on stratigraphic boundaries within the geologic framework based on onshore borehole data. GIS analysis has found that these features maintain their overall cusp shape and textural pattern throughout time yet have changing depths with increasing depths in the center of the feature and decreasing depths on the sides of the features. The textural pattern suggests that these features may be formed and maintained by a feedback system similar to that of transverse bedforms and possibly contributing a significant amount of sediment to the modern beach environment. This sediment source would challenge some definition of the “depth of closure” for the interannual to decadal scale which is often not considered in short term engineering projects. In addition to large storm events, this area has been subject to successive large-scale beach renourishments, which may provide insight into cross-shore exchange of sediment through the shoreface that may only be found on decadal scales.