SPATIAL AND TEMPORAL EVOLUTION OF CUSPATE FEATURES OF THE MID TO LOWER SHOREFACE, GRAND STRAND REGION, SOUTH CAROLINA

A four-year time-series of high resolution multibeam survey has documented the behavior of the shoreface-to-shelf transition in northeastern South Carolina during a period of unusually frequent large storm events. The character of modification of the lower shoreface varies along the coast and rhythmic cuspate features, with heights of 10-20 cm and along shore wavelengths of 100-200 meters, were identified. In many locations, position of these cusps appear to correlate with stratigraphic boundaries within the shallow geologic framework being exhumed by the eroding beach and shoreface and serves as a significant source of sediment to the modern beach system in an otherwise sediment-starved system. This onshore transport may challenge some definitions of “closure” within the beach system at least on interannual to decadal scales. Based on backscatter imagery, these features display a variation in sediment texture along the innermost shelf which may provide a mechanism to initiate feedback with shelf processes and contribute to formation of growth transverse bedforms common on the adjacent shelf (Murrary and Thieler, 2004) during transgression periods. In other locations, similar morphologic structures exhibit depositional characteristics and may reflect local offshore transport and potential loss of sediment from the beach system.

The study area has received a series of successive large-scale beach nourishment projects over the last three decades. While the area exhibits relatively low long-term erosion rates the relatively stability of the beaches in the region does vary alongshore. Impact of several tropical storm systems in recent years and imaged by the study may provide insight into cross-shore exchanges along the coast over a few years that might typically only be active on a decadal scale and highlight the potential influence of the lower shoreface on modern annual to interannual variation in beach behavior; particularly as storm frequency and tracking may differ in the future.