Wave ripples in mixtures of cohesive clay and cohesionless sand: Preliminary results

Novel large-scale flume experiments show that regular water surface waves require longer to reshape a flat bed into wave ripples if the bed consists of mixed cohesive clay and cohesionless sand than if the bed consists of pure sand. Yet, the equilibrium height and length of these bedforms are not affected by bed clay fraction. The delay in the growth rate of the wave ripples, which increases exponentially with increasing initial bed clay fraction, can be explained by the ability of the cohesive clay to bind sand grains together and thus increase the critical shear stress for bed erosion and decrease the erosion rate of the bed sediment. The constancy of equilibrium ripple height and length is related to the highly efficient entrainment (i.e., winnowing) of clay particles from the clayey sand through wave shear, which eventually produces wave ripples that comprise 100% clean sand and therefore have dynamic similarity to equilibrium ripples in sand that never contained clay. These findings imply that empirical equations for the prediction of the size of wave ripples, which are essential for sediment transport modelling in the marine environment, are valid for mixed sand-clay beds only if it can be proven that these bedforms are at equilibrium with the wave conditions. This requirement needs more awareness amongst scientists and practitioners, given that long delays in the formation of equilibrium wave ripples are associated with sand that contains only 4.6% clay. The highly efficient wave-induced winnowing of clay from the bed may adversely affect water quality, but it may also suspend nutrients to the benefit of marine organisms.