Field measurements of swash induced pressures within a sandy beach

This paper describes field measurements of near-surface hydraulic gradients within a sandy beach during individual swash cycles. We show several different types of behaviour which may have implications for swash zone sediment transport: (1) measurements of swash depth; (2) `unloading' events with large upward-acting hydraulic gradients; (3) infiltration events with smaller downward-acting hydraulic gradients; (4) situations where the probes lower in the bed measure significantly lower heads than the probe at the surface, leading to strongly downward-acting hydraulic gradients; and (5) measurements in the capillary fringe. Large hydraulic gradients, which are more than sufficient to induce fluidization, occur in the top 15 mm of the bed under swash and backwash. These large hydraulic gradients, which occur only in the very top section of the bed, may coincide with a significant depth of water (up to 40 mm), which suggests the potential for entrainment of the fluidized bed under backwash