Morphological variability and beach-dune-nearshore interactions on a barrier island: Shackleford Banks, North Carolina

Field intensive in-depth investigation of the beach-dune-nearshore system of Shackleford Banks yielded insights about complex interrelationships between these components. The downdrift barrier island of the Cape Lookout, North Carolina cuspate foreland, Shackleford's stratigraphy is dominated by inlet fill deposits and its morphology is transitional between wave-dominated and mixed energy barriers.; A robust and highly accurate RTK-GPS methodology was developed to map beach and dune topography in unprecedented detail during monthly surveying. Other data included 1997-2000 LIDAR, current meter measurements, sediment grain size and composition analysis and CHIRP/sidescan/bathymetry.; Four primary morphologies were identified. The western island is wide with extensive dune fields, the near-eastern island is narrow and dominated by overwash features, the far-eastern island is low-lying and wide and the ends of the island are undergoing scarping by inlet-associated processes. Morphology was in many ways a reflection of past conditions and not necessarily predictive of modern change, with most areas experiencing significant erosion during the time period of the study. Mechanisms of change were variable. Sections with continuous unbroken foredunes experienced repeated scarping of the beach and/or dunes. Western areas with wide berms and continuous but variable foredunes underwent backbeach accretion and landward movement of the berm with sediment pulled offshore. Near-eastern areas experienced overwash and dune scarping but beach profile shape was maintained. The far-eastern area prograded by foredune development and the welding of shore-oblique bars. Hurricane Isabel (September 18, 2003) was the major agent of change during the study.; Sediment availability and transport pathways explain why the near-eastern island is transgressing in spite of having less wave energy than other areas. Material is removed toward either end of the island by alongshore processes. Sidescan and CHIRP data, as well as the exposure of relict peat and shell material on the beach, indicate that the near-eastern nearshore is reworked and sediment-starved, while the western island has numerous channel fill deposits and greater sediment availability. In contrast, sheltering by Core Banks, Cape Lookout and Power Squadron Spit on the far-eastern island create calm conditions in this area, allowing sediment to accumulate.