<p>The formation of beach rhomboid pattern by swash is investigated experimentally. This centimeter-scale structure is classically interpreted as the mark of stationary gravity waves generated by obstacles in supercritical flows. However, thanks to the use of water-based fluids of various viscosity, our experiments show that a rhomboid pattern can develop in subcritical flows. Its angle is primarily a function of the Froude number, as suggested by Woodford (1935), but our data do not support his classical model, nor do they support any of the existing theories. The slowness of the rhombus motion indicates that it is not simply the mark of a hydraulic phenomenon but rather results from the coupling between the water flow and sediment transport.</p>
Devauchelle, O. Malverti, L. Lajeunesse, E. Josserand, C. Lagree, P. -Y. Métivier, F.