Extreme along-beach variations in morphodynamics attributed to nearshore bathymetry and underlying geology over the short-term, Onslow Beach, North Carolina

Variations in underlying geology can have a significant influence on short-term (event to annual) beach response to hydrodynamic forcing. Understanding the linkage between beach morphodynamics and underlying geology is vital to predicting response to storm events and adequately managing the coastal zone. New methods of quantifying changes in areal extent and volume of the beach were developed in this study, allowing for a more detailed analysis of the morphodynamic response of the beach. Onslow Beach, in Eastern North Carolina, was selected as the study site because of distinct along-beach variations in the underlying geologic framework, determined from 42 cores collected across the barrier and previous studies, and a high-resolution bathymetry dataset. We measured beach topography biannually (autumn and spring) since November 2007 at 15 focus sites and before and after storms at three focus sites using a terrestrial laser scanner. Digital elevation models, based on over 4 million data points, were created for the focus sites and subdivided into three beach zones: foreshore, backshore, and dune. The surface models were used to quantify beach volume and elevation change along the beach zones and translation of the contacts between the beach zones. The Digital Shoreline Analysis System (DSAS) was used to analyze movement of the contacts between the beach zones. These data were compared with variations in nearshore bathymetry, underlying geology, and hydrodynamic processes. Analyses of the changes in volume, elevation, and position of the contacts between zones suggest significant variations in the amount and direction of change from the New River Inlet, in the south, to Browns Inlet, in the north. At the New River Inlet, the foreshore is accreting, the backshore and dune is eroding, and the backshore/dune contact is moving landward, which is likely in response to recent shifting of the main ebb channel. Just north of the New River Inlet, ripple-scour depressions and peat exposure in the nearshore spatially correlate with areas of increased foreshore and backshore erosion and landward movement of the backshore/dune contact. In the middle of Onslow Beach, we observed the highest accretion, but landward movement of the backshore-dune contact. This corresponds to bathymetric highs (exposed rock) in the nearshore that might dampen currents. The northern area of Onslow Beach shows little accretion in the beach zones which corresponds with a smooth nearshore bathymetry. The morphologic response of the beach to hydrodynamic forcing appears to be strongly related to spatial variations in the nearshore bathymetry and sediment supply, which is controlled, in part, by variations in underlying geology. Onslow Beach is an end-member type barrier in terms of the degree of along-beach variations in nearshore bathymetry, underlying lithologic units, and beach-zone morphological change over short distances.