Swash-aquifer interaction in the vicinity of the water table exit point on a sandy beach (No. R863)

Abstract

The coupling of sandy beach aquifers with the swash zone in the vicinity of the water table exit point is investigated through simultaneous measurements of the instantaneous shoreline (swash front) location, pore pressures and the water table exit point. The field observations reveal new 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 is correlated 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 existing models describing exit point dynamics which is based only on the force balance on a particle of water at the sand surface and neglects both sub-surface pressures and shoreline proximity. A new modified form of the model is shown to significantly improve model-data comparison through a parameterisation of the effects of capillarity into the aquifer storage coefficient. Observations of the shoreward propagation of the swash-induced pore pressure waves ahead of the runup limit shows that the magnitude of the pressure fluctuation decays exponentially and that there is a linear increase in time lags, behaviour similar to that of tidally induced water table waves. The location of the exit point and the intermittency of wave runup events is also shown to be significant in terms of the shore-normal energy distribution. Seaward of the mean exit point, peak energies are small due to the saturated sand surface within the seepage face limiting pressure fluctuations. Landward of the mean exit point the peak energies grow before decreasing landward of the maximum shoreline position.