Back-Barrier Sediment and Hydrodynamic Processes: Insights from Rodanthe, NC

Barrier islands are found around the world, and their geomorphic evolution is related to ocean and estuarine processes. Processes including sediment mobilization and shoreline evolution on both the ocean and estuarine side of barrier islands, control long-term evolution through many short-term (days) events (e.g., hurricanes, nor'easters). The Outer Banks of North Carolina are bounded by the Atlantic Ocean and the Albemarle and Pamlico Sound Estuarine System, the second largest estuary in the U.S. Back-barrier environments in the system are extensive with over 1500 km of estuarine shoreline in Dare County (McVerry, 2012). The back-barrier coast of Rodanthe, a small town on the Outer Banks, consists of an undulating shoreline, adjacent to a broad (~4 km) shallow shoal ([less than 2] covered with submerged aquatic vegetation (SAV). Shoreline and volumetric change rates, bathymetry, surface sediment grain properties, and hydrodynamic conditions were measured to evaluate sediment processes, SAV coverage and change over time. Understanding these back-barrier sediment processes is important for navigation, shoreline, and ecosystem management. Shoreline change rates (SCR) were evaluated using aerial photographs from five time steps (i.e., 1949, 1974, 2007, 2012, 2015). The average long-term SCR across the study region was -0.41 m y-1, but there was much variability. Data show the southern shoreline dominated by erosion and marsh loss with an increase in sediment banks and modification. Single-beam bathymetric confirmed the presence of a broad (~4 km) back-barrier shoal. Surface sediment grab samples displayed a dominance of fine sands with modest variation in grain size across the region and very low mud percent and organic content. Bathymetric change of the emergency navigational channel showed large deposition, and the timing suggests the importance of storm-related transport (e.g. Hurricanes Isabel, Ophelia, Irene). Calculated bed shear stresses based on the measured waves and currents indicate that bed shear stress during storms can exceed threshold of motion conditions (i.e., 0.18 N/m2) for the mean basin grain size (199 [micro]m). Aerial photography revealed the area was largely covered by SAV. Occurrence of SAV over 10 years showed little variation with consistent coverage. An optimal depth range of SAV (0.5-2.2 m) was determined based on bathymetric mapping. Three conclusions were derived from data: (1) Erosion and shoreline hardening are both important shoreline change process along on the back-barrier. High shoreline erosion rates and marsh shoreline loss lead to an increase in sediment banks or anthropogenically modified shorelines. (2) Critical shear stresses of motion are exceeded episodically with fresh breezes or stronger winds ([greater than] 10 m s-1), and ferry channel bathymetry suggest considerable sediment transport and deposition during high-wind events (e.g., hurricanes). The dominant back-barrier shoal sediments were clean, medium sands (i.e., low mud %, low loss on ignition). The low mud percent and low loss on ignition are likely maintained by regular wave reworking. (3) SAV in the study area has been persistent through time at water depths between 0.5-2.2 m. This depth range is consistent with other SAV studies and is attributable to water-level and light limitations