Swash‐aquifer interaction in the vicinity of the water table exit point on a sandy beach

Abstract

[1] 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 a simple model describing exit point dynamics which is based only on the force balance on a particle of water at the sand surface and neglects subsurface pressures. A new modified form of the model is shown to significantly improve the model-data comparison through a parameterization of the effects of capillarity into the aquifer storage coefficient. The model enables sufficiently accurate predictions of the exit point to determine when the swash uprush propagates over a saturated or a partially saturated sand surface, potentially an important factor in the morphological evolution of the beach face. 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, behavior 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 location, peak energies are small because of the saturated sand surface within the seepage face acting as a ''rigid lid'' and limiting pressure fluctuations. Landward of the mean exit point the peak energies grow before decreasing landward of the maximum shoreline position.

Estimating hydraulic properties of coastal aquifers using wave setup

“…frictional effects, leading to a stronger penetration into the aquifer. Waves of higher frequencies, such as wave runup, are preferentially damped by the aquifer within tens of meters away from the coast (Cartwright et al, 2006). The characteristic length scale (sT/S) 0.5 describes the distance of diffusive wave propagation (Turcotte and Schubert, 2002).…”

“…The interaction between runup and the groundwater table in the littoral zone is well known (e.g., Hegge and Masselink, 1991). The amplitude becomes increasingly damped inland (Li et al, 1997) and is hardly detectable farther than tens of meters away from the shoreline (Cartwright et al, 2006).…”

Experimental and Modeling Investigation of Shallow Water Table Fluctuations in Relation to Reverse Wieringermeer Effect

“…Such quantification is not easy to handle due to the dynamic nature of these systems having a very fast response time to hydrologic change owing to their closeness to soil surface [1]. In fact, many researchers reported rapid and disproportionate water table rise in response to rainfall events observed in shallow groundwater systems [4][5][6]. They showed that shallow water tables can often rise disproportionately compared to the volume of infiltrated water.…”

A high measurement frequency based assessment of shallow groundwater fluctuations in Metouia Oasis, South Tunisia

“…Such disproportionate rise was observed by many researchers in a variety of shallow groundwater systems (e.g. O'Brien, 1982;Abdul and Gillham, 1984;Cartwright et al, 2006). They showed that shallow water tables can often rise disproportionately compared to the volume of infiltrated water.…”