Evolution of small scale regular patterns generated by waves propagating over a sandy bottom

In this paper the evolution of a sandy bottom subject to a wave generated f low has been analyzed using an image acquisition technique. In particular, ripple formation was observed starting from a flat bed till a stable config uration was attained. The experimental findings showed that rolling grain r ipples never appeared as a stable configuration but only as a transition to ward the equilibrium, represented by vortex ripples. The latter stage was r eached after about 100 cycles if lightweight sediments were used, or about 400 cycles if quartz sediments were adopted. It was also observed that when the bedforms appear their wavelength is much smaller (about one half) than that at the equilibrium stage, this result being in contrast with most of the analytical models on bedform evolution. Finally, it was observed that r ipples migrate as soon as they appear. The measured velocity of migration s eems to confirm, from a qualitative point of view, the theoretical findings obtained for rolling grain ripples by Blondeaux [Eur. J. Mech. B/Fluids 19 , 285 (2000)]. (C) 2001 American Institute of Physics.