THE ROLE OF GUSTS AND TURBULENT EDDIES IN TRANSPORTING SAND ON THE LEE SLOPE OF MASSIVE PARABOLIC DUNES

Turbulent structures are increasingly understood to have an important impact on aeolian sediment transport rates. Turbulent eddies, which rotate in the horizontal plane and resemble dust devils, have frequently been observed transporting sand on the lee slope of large parabolic dunes during wind storms. These eddies often correspond with gusts at the crest and take different paths down and across the lee slope under different wind conditions. The eddies disperse sand grains falling on the lee slope from the crest, and they redistribute sand that is already deposited on the lee slope. Under some conditions, non-rotating, laterally moving gusts also occur on the lee slope. Observations suggest that these gusts are also important for redistributing sand along the lee slope.

Characterizing these turbulent structures and quantifying their impacts on aeolian transport rates requires high-frequency, coincident (in space and time) measurements of wind velocity and sediment flux. During a strong wind event in November, 2017 we measured sediment flux and wind velocity at two locations on the lee slope of a large parabolic dune (Green Mountain) on the southeastern shore of Lake Michigan near Holland, Michigan. Sediment flux was measured with modified miniphone sensors, which record individual grain impacts using a small electret microphone placed in the sediment stream and a digital audio recorder (sample rate 44.1 kHz). Wind velocity was measured with three-dimensional ultrasonic anemometers (sample rate 25 Hz). Simultaneously, horizontal wind velocity was measured at the brink and lower on the stoss slope using cup anemometers and wind vanes (sample rate 1 Hz).

We investigated patterns of high frequency fluctuations in measured wind speed and direction as gusts and turbulent eddies pass through measurement sites with the objective of using the patterns to automatically identify and distinguish between these structures in high-frequency wind data. Pulses of sediment transport are also apparent in the corresponding miniphone data. Consistent with field observations, these patterns changed as regional winds rotated throughout the storm. Our results suggest that this is a promising approach for understanding the impacts of these structures on aeolian transport rates on the lee slope of dunes.