WINDFLOW PATTERNS AND SEDIMENT TRANSPORT ON THE LEE SLOPE OF A LARGE PARABOLIC DUNE, SOUTHEASTERN SHORE OF LAKE MICHIGAN

Measurements of wind velocity, wind direction and instantaneous sand transport were performed for six distinct storm periods, for an aggregate of 250 hours, from November 2017–March 2019 on the lee slope of a 40 m high parabolic dune 9.5 km southwest of Holland, Michigan. The typical experiment consisted of cup anemometers with vanes on the stoss slope and brink and two 3-D ultrasonic anemometers on the lee slope of the dune. Instantaneous sand transport was measured with modified miniphones adjacent to one ultrasound. Video cameras were used to record visible sand transport for short ( ~1 hr) periods. Grain fall on the lee slope was measured during one storm by a set of traps set 1 m high on poles. From November 2017–April 2019, grain fall was measured under the forest canopy beyond the dune with a series of traps set at a height of 1.4 m on poles and extending 215 m out from the base of the lee slope. Preliminary analysis of the data shows at least two patterns. When winds at the brink are at relatively low angles to the dune axis, wind directions on the lee slope are highly variable and average wind speeds are lower than wind speeds at the brink. Rotating vortices of suspended sand migrating along the lee slope were recorded on video. Instantaneous sand transport recorded at the miniphones occurs in isolated bursts lasting about 2 s. The evidence suggests sand transport by turbulent eddies. When winds at the brink are at a relatively high angle to the dune axis, wind directions on the lee slope are much less variable and both wind speeds and direction are similar to those at the brink. Sand transport at the miniphones occurs in burst of 8 to 30 s. This evidence may indicate sand transport due to gusts in winds blowing across the lee slope. The amount of grain fall on the lee slope decreased exponentially from just below the brink to the base. The amount of grain fall in the forest canopy beyond the lee slope also decreased exponentially. Grain size distributions of grain fall on both the lee slope and forest are well sorted and unimodal. The size at peak abundance along the lee slope decreased from 340 to 295 microns from the brink to the lower slope. At the base of the slope under the forest canopy, there is a discontinuous size increase at the peak to 330–340 microns followed by a decrease to 280 microns at 215 m from the dune base.