Bedform dimensions and migration rates under shoaling and breaking waves

Abstract

Ripple wave lengths, heights and migration rates have been measured during mainly swell wave conditions on a macrotidal beach in the U.K. Three high frequency acoustic backscatter sensors were used to follow the changes of the bed position at three locations while a fourth was used in a side-scan mode to examine the wave lengths and movements of the bedforms over a 1 m shore-normal section. Two scales of bedforms are clearly visible; low amplitude (∼0.5–2cm) ripples with wavelengths of 7–20 cm and larger bedforms, usually correlating with lunate megaripples (LMR) observed by divers and discussed by O sborne and V incent, 1992, Proceedings of the 23rd Conference on Coastal Engineering ASCE, pp. 2321–2331, which are 3–8 cm in height and 0.3–0.8 m wavelength. Both forms coexist outside the breakpoint under shoaling waves but in the surf zone the smaller ripples can be destroyed by plunging breakers and high orbital current speeds. The larger bedforms occurred in the surf zone during all the tidal cycles for which measurements were made and appear to be significant surf zone features. No significant correlation was observed between the wave lengths of the small ripples and the orbital excursion A 0 nor with the wave Reynolds number, Shields number or Mobility number, but the migration rate was significantly correlated (at 99%) with all these parameters. Conversely, for the larger bedforms, significant correlation was found for the wave lengths but not the migration rates. We conclude that deployment of high frequency ABS in a side-scan mode is a simple and relative effective method for obtaining bedform measurements in the surf zone.