Spatial and temporal surface moisture dynamics on a coastal beach

Yvonne Smit, Gerben Ruessink
Department of Physical Geography, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands

Surface moisture is a major factor limiting aeolian transport from sandy beaches to the dunes. It increases the shear velocity threshold required to entrain sediment and is a primary control on the development of the fetch effect that is particularly relevant on narrow beaches. Therefore, a better understanding of the spatial and temporal dynamics in surface moisture will contribute to a better prediction model for aeolian sand transport. Here, we study the spatial and temporal moisture variability on the narrow (~ 100 m during spring-tide ebb) beach of Egmond aan Zee, The Netherlands using measurements with an infrared terrestrial laser scanner (TLS; RIEGL VZ-400). Our earlier research has shown a robust negative relation between TLS reflection output and the full range of gravimetric soil moisture contents (0% - 25%). This relation holds to about 80 m from the TLS. The TLS was deployed during several days in autumn 2015 for 6 hours during falling and rising tide with a 15- to 30-minute interval between individual surveys. Each survey produced O(106 - 107) reflection values, which we converted and averaged into surface moisture maps with a 0.25x0.25 m resolution. Preliminary analysis of the maps reveal that surface moisture varies highly in the intertidal zone (5% - 25%), but only little near the dune foot (2% - 5%). A striking result is the rapid desiccation of the intertidal sandbar when its crest is just below the high-tide level. Then, the surface-moisture content can decrease with a rate of ~2.5% - 4% per hour, eventually reaching the same moisture content as the dry back beach. In contrary to the sandbar, the trough is constantly saturated as the groundwater table intersects the trough surface even during low tide. Moreover, surface moisture on the sandbar keeps on decreasing during rising tide until it inundates. Thus, there are no processes visible by capillary forces in advance of the rising tide. Concluding, the TLS can create surface moisture maps that clearly show the spatial and temporal moisture dynamics on a beach. The next steps in our research will be to investigate how meteorological conditions and tide-driven groundwater fluctuations, which were both measured during the TLS surveys, caused the observed surface-moisture variability.