BEDFORM DIMENSIONS AND MIGRATION RATES UNDER SHOALING AND BREAKING WAVES

Ripple wave lengths, heights and migration rates have been measured du ring mainly swell wave conditions on a macrotidal beach in the U.K. Th ree high frequency acoustic backscatter sensors were used to follow th e changes of the bed position at three locations while a fourth was us ed in a side-scan mode to examine the wave lengths and movements of th e bedforms over a 1 m shore-normal section. Two scales of bedforms are clearly visible; low amplitude (approximately 0.5-2 cm) ripples with wavelengths of 7-20 cm and larger bedforms, usually correlating with l unate megaripples (LMR) observed by divers and discussed by OSBORNE an d VINCENT, 1992, Proceedings of the 23rd Conference on Coastal Enginee ring ASCE, pp. 2321-2331, which are 3-8 cm in height and 0.3-0.8 m wav elength. Both forms coexist outside the breakpoint under shoaling wave s but in the surf zone the smaller ripples can be destroyed by plungin g breakers and high orbital current speeds. The larger bedforms occurr ed in the surf zone during all the tidal cycles for which measurements were made and appear to be significant surf zone features. No signifi cant correlation was observed between the wave lengths of the small ri pples and the orbital excursion A0 nor with the wave Reynolds number, Shields number or Mobility number, but the migration rate was signific antly correlated (at 99%) with all these parameters. Conversely, for t he larger bedforms, significant correlation was found for the wave len gths but not the migration rates, We conclude that deployment of high frequency ABS in a side-scan mode is a simple and relative effective m ethod for obtaining bedform measurements in the surf zone.