Formation of Sand Bedforms Under Surface Waves

Abstract

Surface waves often generate bedforms at the seabed. Small structures such as ripples with a typical wavelength between ten centimeters and one meter are very common structures in the coastal zone. The formation of these structures under nonlinear surface waves is considered in this chapter. Under regular waves, two modes of pattern formation from a flatbed in a wave flume are reported for well-sorted grains and mixtures of grains. Sand ripples can form uniformly or from isolated ripples spreading on the bed while growing. In this latter case, front propagation speed is measured and a simple model based on the quintic complex Ginzburg-Landau equation can explain features of front propagation on the granular bed. The profile of surface waves propagating in shoaling water approaches the solitary waveform before wave breaking. The main characteristics of solitary waves are presented. The effect of the high nonlinearity of these waves may be very significant on bedforms induced in the nearshore zone. The interaction between solitary waves and a sandy bed is reported. Sandy ripples induce a strong energy dissipation of solitary waves. When solitary waves propagate on the background of a standing harmonic wave, bars are formed with crests located beneath the nodes of the harmonic surface wave. In the case of harmonic standing waves alone, the bar crests are positioned beneath the antinodes of the harmonic surface wave. Grains with different densities may be found on the seabed. The concentration of light sedimenting particles on ripple crests is explained by a simple theoretical model.