Beach cusp formation and spacings at Duck, USA

Aproximately nine years of daily video images from Duck, NC, USA, were analyzed to determine the timing of cusp formation in relation to environmental forcing and the distances separating consecutive cusp horns. 57 independent cusp events (defined as transitions from visibly smooth to cuspate topography) were observed with most of the cusps forming after storms. The temporal lag between the peak in storm intensity and cusp development was typically 3 days. Approximately half of the cusp events had formations predicted by an empirical threshold relating storm presence, breaker angle, and beach reflectivity. This threshold and other statistical observations suggest that Duck cusps form as energy conditions become more reflective, as the offshore wave angle approaches normal incidence and as the directional spread of the incident wave field becomes narrower. The standard deviation of the observed spacings relative to the mean spacing for each event was around 15% with the range in spacings for each event being typically less than half the event’s mean cusp width. There were no strong statistical relationships between mean cusp spacings and environmental parameters (such as swash excursion lengths).