ABSTRACT

Nine experiments with 2-inch waves breaking for as much as 48 hours on a beach of pebbles and sand produced cusps in 8 cases. Seven runs produced cusps which changed continuously and one run became stable after two hours and unchanged for 24 hours. The stable beach was analysed for size and shape sorting in area and depth. Large particles were concentrated on the surface at the upper and lower limits of the beach, and near the waterline the modal size increased vertically downwards. There was no significant difference in the size sorting between cusps and bays. The most spherical particles concentrated at the foot of the beach, the plate-like ones at the back of the bays, and the cusp points were a mixture of plates and tetrahedra. This is explained in terms of transport up the beach y suspension, and down-beach by dragging and rolling. Stability was increased by the high permeability of the mixture on the cusp points.