Video‐based observations of nearshore sand ripples and ripple migration

Abstract

Observations of O(1 m) length and O(0.1 m) height sand ripples at Waimea Bay, a steep (slope ∼ 0.05) pocket beach on the north shore of Oahu, Hawaii, are presented. Time series of ripple patterns over a 900 m2 section of the nearshore in depths of 1–2 m are obtained using time‐averaged video images from an Argus station overlooking the bay. Ripples are detected during weak wave conditions in the summer or between winter swell events. The ripple field exhibits narrow‐band wave number structure, with ripple crests oriented parallel to the shoreline. The ripple wavelengths vary with wave orbital displacements, but they are shorter than predicted by orbital vortex ripple scaling relationships. A new suspension‐limited model (Smith and Wiberg, 2006; Traykovski, 2006) in which the suborbital ripple wavelengths vary with wave period appears to describe well the observed ripple wavelength scaling. Lagged correlations between sequential Argus images reveal area‐average ripple migration rates ranging from −3.3 m day−1 (offshore) to 4.5 m day−1. The corresponding estimated sediment transport ranges from −0.3 m3 m−1 day−1 to 0.5 m3 m−1 day−1, similar to previous estimates of cross‐shore transport based on observed volume changes at the subaerial beach. This suggests that the recovery of beach sand following major swell‐driven erosion events may be accomplished in part by the shoreward migration of seabed ripples.