OVERWASH AND DECADAL BARRIER-ISLAND CHANGE, OCRACOKE ISLAND AND SHACKLEFORD BANKS, NC

Storm-generated overwash is a critical process in barrier island dynamics causing nearshore erosion, vertical island accretion and long-term landward migration. Storm processes significantly influence coastal morphology and can cause considerable dramatic changes. Considering the enormous value of barrier islands, it is important to monitor short-term geomorphic change and stratigraphic development and identify the responsible processes. In this study a time series of aerial photos (1940, 1954, 1998, 2011) was used to calculate rates of shoreline change through GIS analysis. Areas of notable change or recent deposition were investigated by excavating trenches and with the collection of vibracores. Trenches and vibracores were photographed and logged, and subsamples were extracted and analyzed to determine sedimentological character via grainsize analysis and loss on ignition. Thus, depositional events visible in aerial photography were stratigraphically validated through field sampling.

The deposit thickness of the Hurricane Isabel overwash ranged from 3 to 48 cm at >20 sites on three transects across Ocracoke Island, NC. Overwash layers were prominent and evidenced by a sharp basal contact between organic-rich, muddy sand and the well-sorted, 2–3 phi overwash sand. After characterization of overwash layers from recent known events (e.g., Isabel) other event layers were correlated to historic storm events, indicating recent stacked overwash deposition at some locations. AMBUR analysis on Shackleford Banks, NC showed that 90% of the island eroded from 1998 to 2011, with a mean erosion rate of 2.5 m/yr. However, overwash at both sites was very localized and limited over the last half century. This research provides an insight into the recent geologic evolution of NC barrier islands which reflect the influence of acute (e.g., tropical cyclones) and chronic (e.g., sea level rise) processes.