Swash-aquifer interactions in sandy beaches

Abstract

In this paper, simultaneous measurements of the instantaneous shoreline (swash front) and the response of both the pore pressure and the water table exit point are presented and discussed. The new datasets reveal additional insights into swash-aquifer coupling not previously gleaned from measurements of pore pressure only. In particular, for the case where the exit point is seaward of the observation point, the pore pressure response correlates well with the distance between the exit point and the shoreline in that when the distance is large the rate of pressure drop is fast and when the distance is small the rate decreases. The observations expose limitations in the only existing theory describing the motion of the water table exit point. The theory is based only on the force balance on a particle of water at the sand surface and therefore neglects the effects of both sub-surface pressures and shoreline proximity. The model performance is greatly improved through a parameterisation of the effects of both capillarity and truncation of the capillary fringe in the vicinity of the exit point. These effects are parameterised as a reduction in the aquifer storage coefficient by three orders of magnitude. Observations of the propagation of the swash-induced pore pressure wave reveals behaviour similar to that found with tidally induced water table waves. That is, the magnitude of the pressure fluctuation decays exponentially and there is a linear increase in the time lag. The energy transfer of high frequency swash zone forcing to a low frequency response in the water table exit point and pore pressure is shown to be as a result of wave runup exceedance probabilities relative to the location of the water table exit point.