BEACH CUSP FORMATION AND SPACINGS AT DUCK, USA

Approximately nine years of daily video images from Duck, NC, USA, wer e analyzed to determine the timing of cusp formation in relation to en vironmental forcing and the distances separating consecutive cusp horn s. 57 independent cusp events (defined as transitions from visibly smo oth to cuspate topography) were observed with most of the cusps formin g after storms. The temporal lag between the peak in storm intensity a nd cusp development was typically 3 days. Approximately half of the cu sp 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 ene rgy conditions become more reflective, as the offshore wave angle appr oaches normal incidence and as the directional spread of the incident wave field becomes narrower. The standard deviation of the observed sp acings relative to the mean spacing for each event was around 15% with the range in spacings for each event being typically less than half t he event's mean cusp width. There were no strong statistical relations hips between mean cusp spacings and environmental parameters (such as swash excursion lengths). Copyright (C) 1998 Elsevier Science Ltd. All rights reserved.