The Effect of Marine and Aeolian Processes on Time Dependent Vertical Grain Size Variability in the Intertidal Area

Predictions of coastal dune development require estimates of aeolian sediment transport. However, commonly used sediment transport equations do not fully take into account the variability in supply-limiting factors that reduce the uptake of the sediment by the wind (Davidson-Arnott and Law, 1990). The time-varying grain size distribution in the intertidal area is expected to be an important factor that determines sediment availability for aeolian transport (Hoonhout and De Vries, 2016). However, the grain size dynamics in the intertidal area are largely unknown. This study investigates the effects of hydraulic mixing and aeolian sediment transport on grain size variability on multiple beaches in the Netherlands and the United States. Cross-shore distributed sand samples were collected from the intertidal area on three occasions during the tidal cycle. Vertical grain size distributions were determined by sieving samples from 2-6 mm layers of the upper 5 cm of surface sediment collected using a custom-made sand scraper. Results from Noordwijk (NL) (Fig. 1) display the vertical grain size variability at three locations before high tide (3:00), between high and low water (12:00) and after low water (17:00) (Fig. 1). Topographic profiles (dark lines in Fig. 1) show the elevation of the intertidal area at each of the three sampling occasions. There is a remarkable differentiation of grain sizes in the top 5 cm of the sediment bed. The grain size in the uppermost layer (2 mm thickness) of all measurement locations decreased during the high tide. This indicates that the sediment availability for aeolian transport increased at these locations compared to the end of the last low tide. Also, the most seaward sampling location shows an interesting decrease of fine grains after an aeolian transport event that occurred during the low tide, which could indicate grain size selective transport. Further results from this unique dataset will include the effects of deposition and erosion, and the measurements conducted at other field locations.