Spatial Variation of Shoreline Change along An Important Marine Corps Amphibious Training Ground, Onslow Beach, Nc; Part 1: Nearshore Geology and Morphology

Onslow Beach at Marine Corps Base Camp Lejeune, NC is an important amphibious training area located on a barrier island with alongshore-variable erosion rates. The 12-km long barrier island extends from Brown's Inlet in the northeast to New River Inlet in the southwest. High rates of erosion are found in the southern portion of the study region, adjacent to New River Inlet. Erosion rates decrease towards the north, with accretion observed on the beach adjacent to Brown's Inlet. To sustain future amphibious training activities in Onslow Bay, it is important to determine the underlying causes behind these observed alongshore-variable erosion rates. To this end, nearshore geology and morphology were examined using interferometric-swath bathymetry and co-registered side-scan sonar. High-resolution sub-bottom profiles ground-truthed by cores were also collected to examine the influences of sediment volume and framework geology on shoreline stability.

Isopach maps of sediment thickness reveal a 4m-thick sand wedge at Brown's Inlet, in contrast to Oligocene limestone and sandstone units exposed on the seafloor across much of the southern field site. This sand-starved region south of Brown's Inlet is also characterized by shore-oblique bathymetric features which have been related in other locations to shoreline erosional hotspots. We propose that the influence of the irregular bathymetry and shore-oblique features on the local wave field, coupled with the sediment-starved nearshore, leads to the higher erosion rates observed on the southern portion of the island.