Soil Moisture Dynamics and The Effects on Initiation of Aeolian Sand Transport

Abstract

Aeolian sediment transport is the most important source of sediment supply for the growth of foredunes, which provides an important societal function by protecting the hinterland from flooding. Furthermore, delivery of aeolian sand to the hinterland creates new habitat which pioneer species can colonize, enhancing biodiversity in coastal dune areas. Soil moisture is recognized as important aeolian sediment transport controlling factor by increasing the shear velocity threshold needed to mobilize sediment and by reducing the aeolian transport rate over a wet surface under certain wind (<12 m/s) conditions. In this study, results of detailed measurements on spatial and temporal soil moisture dynamics conducted with a Delta-T Theta probe on a wave-dominated beach in Egmond aan Zee, the Netherlands are presented. Moisture measurements along a well transect over the course of one month focus on cross-shore spatial (x = 110 m) variability in relation to the groundwater level below the surface of the beach. Furthermore, the cross-shore distribution of moisture content is discussed in terms of tidal and morphological induced variation and as potential aeolian transport reducing factor. Piezometric head measurements reveal tidal induced oscillations of the phreatic surface and showed a correlation with the soil moisture variation at the surface of the beach by a fitted retention curve. Observations of a seepage face during and after a storm surge reveal that the potential area available for aeolian transport is strongly reduced, because shoreward of the seepage face exit point the aeolian saltation system is closed and wet moisture conditions remain after the surge has passed. Furthermore, field based observations of soil moisture and wind speed near the threshold of aeolian transport revealed an increasing threshold of wind speed with moisture content. These observations do not match with predictions using a moisture corrected threshold value based on wind shear. The shifting of a moisture dependent threshold region could be used to fine-tune process based models that use the fetch-effect to calculate a downwind increase in transport. Incorporation of this threshold region implies defining a spatial and temporal varying contour of equal (threshold) moisture content, landward of this zone the critical fetch distance is reached and aeolian transport starts.