Description:
Through the use of a Neutron Probe thermal neutron technique, shore-normal moisture profiles over a half tide cycle are presented from two contrasting Australian beaches. The theoretical notion of a saturated capillary fringe zone above the watertable is confirmed by the presence of a significant body of saturated sand above the watertable. This zone does not remain stable and simply rise with the rising watertable. When the rate of runup infiltration from above exceeds the rate of interstitial air expulsion, the striking phenomenon is observed of the watertable rising through a zone of unsaturated sand. Net changes in moisture profiles from low to high tide reveal that regarding any region in a beach as reverting from 'dry' to 'wet' is imprecise, rather the change is a more subtle variation in the 'degree of wetness'. The coincidence of the top of the saturated capillary fringe zone with the beachface results in the phenomenon known within hydrolgical texts as the 'reverse Wieringermeer effect'. This phenomenon accounts for the much misunderstood ability of the watertable to respond almost instantaneously to individual swash events. In contrast to previously proposed large upward flow at velocities sufficient to fluidize the beachface, the minute addition of water to the sand surface induces an increase in pore-water pressure and by definition a rise of the watertable through the capillary fringe.