On the Formation of Bars by the Action of Waves on an Erodible Bed: A Laboratory Study

Abstract

Sand bars having spacing equal to half the surface wavelength may be formed by the action of partially standing waves on an erodible bed. Here the stages of development of a patch of bars has been investigated in a wave tank, using rapid, accurate, ultra-sonic detection systems to monitor both the changing bed profile and the evolving surface wavefield. Initially, small-scale ripples formed on a (flattened) bed surface. These ripples varied in size according to position in the wave envelope, and had asymmetrical profiles due to asymmetry in the near-bed oscillatory motion. Vortex shedding from the ripples gave rise to regions of net sediment accumulation (bar crests) and erosion (troughs) linked to position in the wave envelope. The bars formed with wavelength satisfying the Bragg condition and with bar crests positioned downwave of the antinodes of elevation. As expected, this produced an increase in the overall reflection coefficient of the bar patch as the bars grew in height. The presence of the small-scale ripples had only a minor effect on the reflection coefficient, the flow being essentially inviscid away from the boundaries of the motion and the ripple wavelength being far smaller than the surface wavelength. However, the ripples played an important role in dissipating wave energy in the tank, wave dissipation factors being 3 or 4 times larger than equivalent values for laminar flow above a smooth bed. The present observations of the resonant interaction between surface waves and an erodible bed are believed to be the most detailed and accurate to have been reported to date.