MULTIPLE SHALLOW CHANNEL STRUCTURES ON GPR PROFILES PROVIDE CLUES TO RECENT COASTAL DRAINAGE ALONG SOUTH CAROLINA'S GRAND STRAND

As part of the South Carolina Coastal Erosion Study (SCCES), over 10 km of ground-penetrating-radar (GPR) profiles were acquired between Myrtle Beach and North Myrtle Beach. These data assist in providing a cross-shore link between nearshore geophysical surveys and onshore boring and vibracore activities, also associated with the SCCES. Profiles were collected along the upper portion of the beach and along coastal roads. Surveying in these areas made it possible to avoid attenuation of the GPR system’s electromagnetic signal, which is often a problem when operating near seawater. The system was typically able to penetrate between 2-4 m beneath the surface, reaching as much as 6 m in some areas. Preliminary interpretations indicate that the surveys were successful in imaging a variety of features within the sub-surface stratigraphy, including lithified Pleistocene-Holocene beachrock at depth, overlying Holocene beach and dune deposits, artificial beach fill placed during recent beach nourishment projects, and numerous buried channels. These paleochannels have been interpreted to represent tidal inlets and smaller ephemeral swashes, which were likely active in the recent past. In general, these features can be spatially correlated with adjacent low-lying upland areas and marshes, suggesting that their function was to provide local drainage. Widths of the channels measure between 10’s and 100’s of meters with average depths of 2-4 m. Horizontal spacing between channels is variable, but within one surveyed section along North Myrtle Beach, a concentration of channels was identified within an approximately 8 km stretch of the barrier. Single-channel incisions, characterized by conformable or prograded fill, are observed, and represent typical examples of non-migrating channels. Nesting of multiple channels are also observed, indicating lateral migration, as well as abandonment and subsequent reoccupation of larger channel complexes. Further work will include the collection of sediment cores to groundtruth the reflections identified on the geophysical records. The data will also be integrated into the overall SCCES program to improve the correlation between the onshore and nearshore datasets.