Improved nearshore mapping techniques reveal locations along relatively straight, uniform beaches that have significantly different rates of erosion and accretion than surrounding areas and pose problems for beach nourishment. Such "hotspots" are usually associated with anomalous nearshore bar behavior and often recur year after year. Hotspots are thought to be correlated with variations in underlying coastal geology but the physical mechanism(s) responsible for coupling shoreline changes with shoreface geology remains speculative. Because hotspots occur in the shallow, energetic surf zone and evolve most rapidly during and immediately after storms when access to the surf zone is difficult, traditional methods of observation poorly document hotspots and give little insight into mechanisms for their creation and evolution. Recognizing the need for observing frequent, rapid and highly three-dimensional changes in sediment type, thickness, and bar morphology in the surf zone, we collected swath bathymetry, sidescan and Chirp seismic sub-bottom profiles from an amphibious vessel.
Our findings provide strong circumstantial evidence that a non-sandy underlying substrate present near the seafloor surface influences bar behavior and ultimately beach erosion and accretion. Eight surveys of a hotspot at Duck, North Carolina carried out over the last two years reveal a highly variable surface sand layer thickness and outcropping muddy substrates associated with anomalous nearshore bar behavior. Surface sediment samples and vibracores collected in conjunction with the surveys confirm that isolated and ephemeral spots of mud and gravel are exposed in the surf zone. Hourly ARGUS video imagery shows the shoreline closest to the outcrops and irregular nearshore bars has a much higher variance in erosion and accretion than the adjacent shoreline. We hypothesize that exposure of a non-sandy substrate across the surf zone inhibits the development and perturbs the usual migration patterns of nearshore bars which, in turn, changes the wave energy affecting the adjacent beach.
Supported by the Army Research Office and the National Research Council.
