A Laboratory Experiment of Beach Cusps

Beach cusps with a longshore spacing of 20 to 150 cm have been built by the continuous action of incident waves on a steep laboratory beach. In the formation stage of beach cusps, all bed materials on the beach moved shoreward. The backwash vortex, which was found first by Matsunaga and Honji (16,18) on a laboratory beach, gave a good explanation of the shoreward movement of bed materials. Beach cusps formed when the value of a dimensionless parameter $H_b{}^2{L}_b/h_b{}^{2}L,$ , which controls swash motion on a steep beach, became larger than 1.12; H_b, is the height of the breaking wave, L_b its wavelength, h_b the water depth at the breaking point and L the horizontal distance from the shoreline for still water level to the breaking point. The observational spacing of beach cusps formed regularly were in quite good agreement with half a wavelength of the zero-mode subharmonic edge wave generated on a sloping flat beach. As a result of this study, the contribution of edge waves on cusp formation becomes more undoubted.