Swash Hydrodynamics on Sand and Shingle Beaches

Abstract

Measurements of uprush and backwash velocities, the shape of the swash lens, pore water pressures in the beach and hydraulic gradients were obtained on a sand beach and a gravel beach on the south coast of the UK. Detailed time histories of a single swash event on both the sand beach and the gravel beach are shown. On the sand beach, large upward-acting hydraulic gradients which exceed the potential for sediment fluidisation are frequently observed. In some cases, these large upwardacting hydraulic gradients occurred under a falling head of water with offshoredirected velocities of 0.7 to 1.4 ms -1. In other cases, the upward-acting hydraulic gradients occurred when the head of water at the surface was zero. The phasing between these destabilising hydraulic gradients and uprush/backwash velocity is critical to the potential for sediment transport. On the gravel beach, downward-acting hydraulic gradients (infiltration) existed for most of the uprush and backwash cycle. The hydraulic gradients for the infiltration events on the gravel beach were an order of magnitude greater than infiltration events measured on sand beaches.