Rapid water table fluctuations within the beach face: Implications for swash zone sediment mobility?

Abstract

To incorporate groundwater infiltration/exfiltration in the description of swash zone sediment transport, it is required that the details of groundwater dynamics within the beach face be clarified. Field measurements of the vertical pore-pressure structure within the bed identify capillarity effects as the primary mechanism driving rapid and relatively large magnitude water table fluctuations within the swash zone. When the upper extent of the fully saturated capillary fringe coincides with the beach face surface, wave runup produces near-instantaneous increase in pore-pressures across the capillary fringe, corresponding to a rapid rise of the phreatic surface (i.e. the surface where pore-pressure = atmospheric pressure) to the sand surface. Therefore, counter to previous conclusions in the literature, the rapid rise and fall of the water table under the swash zone do not equate to regions of the beach face alternating between states that favor sediment deposition (unsaturated) and erosion (saturated). Similar reports that rapid fluctuations of the water table within the beach face correspond to rapid rates of vertical flow and hence bed fluidization, are also a misinterpretation. Field measurements, and a careful consideration of saturation and pore-pressure characteristics within the beach face, demonstrate that rapid water table rise is associated with minute (downwards) swash infiltration, rather than rapid (upwards) groundwater exfiltration. The cause of the pressure fluctuations and phreatic surface oscillations is the alternating appearance and disappearance of meniscuses at the sand surface, which generate pressure head fluctuations of decimeters due to the addition of millimeters of water.