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 s urface layers of a sand beach are presented and compared to a one-dimension al diffusion model. The model provides a description of flow through a poro us, quasi-saturated sediment and is driven by the time-varying swash depth on the beach surface. The field measurements broadly show four different ty pes of pressure propagation behavior, which appear dependent on cross-shore position in the swash zone. The hydraulic gradients within the surface lay ers of the beach are typically found to be much greater than those likely t o be generated by tidally-induced groundwater now. In these instances the d iffusion 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.