Spatiotemporal variability in saltation intensity on a narrow sandy beach

Winnie de Winter, Gerben Ruessink, Geert Sterk
Department of Physical Geography, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands

Commonly used aeolian sand transport models overestimate sediment transport rates measured in the field. These models are based on the time-averaged shear velocity, which point to the assumption of a continuous flow of sediment. However, due to turbulence in the wind, the saltation in the field is intermittent, which results in a non-continuous flow of sand. This intermittency causes the transport to be restricted to a smaller amount of time and therefore affects the time-averaged sediment transport rate. Here, we examine the relation between wind turbulence and the spatiotemporal variability of saltation with field observations at Egmond Beach in The Netherlands. Three-dimensional wind velocities were measured with a downwind array of 3 ultrasonic anemometers from an intertidal sandbar to the dune foot, with each sensor measuring at a height of 0.90 m above the bed. Additionally, we installed a home-built Saltation Detection System (SalDecS) with every ultrasonic anemometer which measured small-scale saltation intensity variability at a frequency of 10 Hz and a spatial resolution of 0.10 m on a wind-normal oriented horizontal array of 3.10 m. The measurements lasted for two periods of 1 and 6 hours and were performed during an approximately 6-week field campaign in autumn 2015. The 10-Hz velocity data were processed into 5-minute mean < u> velocity and turbulent kinetic energy (TKE). Preliminary analysis shows a positive dependence of the TKE on < u>, which appears to increase from the sea towards the dune. On a 5-minute temporal scale, the spanwise averaged standard deviation < s> in saltation intensity was found to increase with an increasing spanwise averaged mean < x> saltation intensity. TKE values below 0.55 m2/s2 did not result in any saltation, whereas TKE values above 0.55 m2/s2 show a large range in < s> and < x> without any clear dependence. Apparently, a sufficient large TKE is needed to initiate saltation, but other factors, such as soil moisture or fetch length, affect the actual saltation intensity and variability therein. Future work will focus on elucidating the influence of these transport limiting factors on the spanwise averaged saltation intensity.