Measurements and modeling of swash-induced pressure gradients in the surface layers of a sand beach

Field measurements of swash-induced hydraulic (pressure) gradients in the surface layers of a sand beach are presented and compared to a one-dimensional diffusion model. The model provides a description of flow through a porous, quasi-saturated sediment and is driven by the time-varying swash depth on the beach surface. The field measurements broadly show four different types of pressure propagation behavior, which appear dependent on cross-shore position in the swash zone. The hydraulic gradients within the surface layers of the beach are typically found to be much greater than those likely to be generated by tidally-induced groundwater now. In these instances the diffusion model generally provides a good description of the data. However, very large hydraulic gradients are frequently observed just below the beach surface, and these cannot be adequately described by the diffusion model. A mechanism is proposed through which these large hydraulic gradients could be generated.