Tide-induced variability in beach surface moisture: Observations and modelling

The moisture content w(s) of a beach surface strongly controls the availability of sand for aeolian transport. Our predictive capability of the spatiotemporal variability in w(s), which depends to a large extent on water table depth, is, however, limited. Here we show that water table fluctuations and surface moisture content observed during a 10-day period on a medium-grained (365 mu m) planar (1:30) beach can be predicted well with the nonlinear Boussinesq equation extended to include run-up infiltration and a soil-water retention curve under the assumption of hydrostatic equilibrium. On the intertidal part of the beach the water table is observed and predicted to continuously fall from the moment the beach surface emerges from the falling tide to just before it is submerged by the incoming tide. We find that on the lower 30% of the intertidal beach the water table remains within 0.1-0.2 m from the surface and that the sand is always saturated (w(s)approximate to 20%, by mass). Higher up on the intertidal beach, the surface can dry to about 5% when the water table has fallen to 0.4-0.5 m beneath the surface. Above the high-tide level the water table is always too deep (>0.5 m) to affect surface moisture and, without precipitation, the sand is dry (w(s) < 5 - 8%). Because the water table depth on the emerged part of the intertidal beach increases with time irrespective of whether the (ocean) tide falls or rises, we find no need to include hysteresis (wetting and drying) effects in the surface-moisture modelling. Model simulations suggest that at the present planar beach only the part well above mean sea level can dry sufficiently (w(s) < 10%) for sand to become available for aeolian transport. (c) 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.