Subsurface processes generated by bore-driven swash on coarse-grained beaches

Large-scale laboratory experiments presented in this paper involved bore-driven swash on permeable immobile coarse-grained beaches. Two different sediments were used (d(50) = 1.5 and 8.5 mm) resulting in different beach permeability and surface roughness. The experiments yielded detailed measurements of swash depth and velocities, wetting front, pressure, and groundwater levels across the swash zone. This paper is focused on the processes occurring within the beach. The measurements provide the shape of the wetting front and the groundwater table and reveal the behavior of air in the unsaturated region of the beach. Air is initially at atmospheric pressure, but the pressure builds up when air becomes encapsulated between the saturated region formed below the swash and the groundwater table. For the 1.5 mm beach, entrapped air significantly affected the water exchange between the swash and the subsurface. The considerable buildup of interstitial air pressure reduced vertical hydraulic gradients and thus infiltration rates. At the lower end of the beach the hydraulic gradients even became negative, indicating flow reversal and exfiltration. In contrast, for the 8.5 mm beach the rate of infiltration was only slightly affected by the buildup of pore-air pressure. The vertical hydraulic gradients were more than twice the magnitude of those within the 1.5 mm beach. The results presented in the paper clarify the mechanisms that drive and impede the water exchange between the surface and subsurface flow. In particular, infiltration into the initially unsaturated part of the beach and the resulting air entrapment play a significant role in swash and similar flows.