Abstract
AbstractThe moisture content ws of a beach surface strongly controls the availability of sand for aeolian transport. Our predictive capability of the spatiotemporal variability in ws, 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 (365μm) 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 (ws≈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 (ws < 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 (ws < 10%) for sand to become available for aeolian transport. ©2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
Highlights
Wind-blown sand transport from a beach constitutes the main sediment source into dunes, allowing them to grow and to recover after the erosive effects of marine processes during severe storm surges
We adopted the water retention curve proposed by Van Genuchten (1980); the surface moisture content is given by ws0.x, t/ D wres C
Observations on an approximately planar (1:30), medium-grained (365 m) beach show water table depth to be the main driver of spatiotemporal surface moisture content
Summary
Wind-blown sand transport from a beach constitutes the main sediment source into dunes, allowing them to grow and to recover after the erosive effects of marine processes during severe storm surges. A sufficiently strong wind is needed to initiate aeolian transport, model predictions of sand supply based on wind characteristics alone often grossly overpredict measured sand deposition in the dunes (e.g., Sarre, 1989; Davidson-Arnott and Law, 1996; Delgado-Fernandez, 2011; De Vries et al, 2012; Keijsers et al, 2014). While considerable progress has recently been made in measuring the temporal and spatial variability in surface moisture (e.g., Yang and Davidson-Arnott, 2005; Nield et al, 2011; Smit et al, 2018), practical models to predict this variability (e.g., Hoonhout and De Vries, 2016) are still in their infancy and must be improved and validated further as a precursor to quantitatively realistic aeolian sand-supply models
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