WIND STEERING AND SEDIMENT TRANSFER ASSOCIATED WITH STORMS IN A BLOWOUT DUNE AT SAUGATUCK HARBOR NATURAL AREA, MICHIGAN

The effects of 8 extratropical cyclones on wind directions, wind speeds, sand erosion, and sand deposition were studied in a blowout dune in the Saugatuck Harbor Natural Area on the southeastern shore of Lake Michigan during Fall, 2010 and Spring and Fall, 2011. The blowout is on the limb of a parabolic dune isolated from the beach by an established foredune ridge. It is 53m long, 8 m high, SE trending and opens to the northwest with two notches on the western limb. Wind speeds and directions were measured with anemometers and wind vanes at 6 different localities within the dune while erosion and deposition were measured with an array of 211 pins. Regional wind directions at a high angle (E, NE, S, SSW) to the opening of the dune, led to bifurcated flow of relatively low velocity winds within the trough, which can sweep sand out of the trough and deposit it in the northwest notch. Regional winds at a lower angle to the dune opening (WSW, W, NW, N), were steered up the axis of the trough with the loss of wind energy increasing with increasing steering angle. These winds are associated with erosion along the trough axis and deposition on the southern outer slope. Winds with a strong westerly component are also funneled through the notches leading to scoring along the inner limbs of the dune and deposition within the trough and eastern outer limb. The exact response of a dune to a given storm is the result of a complex interplay between wind direction, which depends on cyclone track, and local topography.