WIND AND DEPOSITION PATTERNS ON A LARGE MIGRATING PARABOLIC DUNE, SOUTHEASTERN SHORE LAKE MICHIGAN

Studies which combine information on sand transport and deposition with wind patterns on parabolic dunes have great potential in helping us understand the development of these landforms. Green Mountain Dune is a parabolic dune 500 m long, 200 m wide, and 40 m high actively migrating inland from the shore of Lake Michigan, 9 km southwest of Holland, Michigan. Erosion pins on the lee slope of the depositional lobe, supplemented by sand traps at the crest, have been used to monitor aeolian deposition since September, 2001. Wind velocity and direction are measured at least hourly at Tulip City Airport 7 km due east of the dune. On July 2, 2002 wind velocity and direction were measured simultaneously on the beach and at 6 places within the dune. Wind shear velocities were calculated from wind speeds measured at three heights (0.5, 1.0 and 1.8 m) on a tower placed first at the crest of the dune and then on the backslope. The field measurements will be repeated several times from July to October. The amount of deposition on the lee slope can be modeled (r2=0.90) by the potential sand transport equation of Fryberger (1979 Geol. Sur. Prof. Pap. 1052 137-169) using the airport wind data and a threshold velocity of 16 knots. For all wind directions, maximum deposition occurred along the parabolic axis suggesting that sand transport is modified by topographic steering of winds. Steering was evident on July 2: average wind direction on the beach was 197^o while along the dune axis it was parallel to the axis (250^o). A north-south array of stations indicated higher velocities (average 4.7m/s) and less steering (average wind direction 235^o) at the north side of the dune compared to the center (4.4 m/s, 250^o) or south side (4.2 m/s, 258^o). Wind roses since September, 2001 show a prevalence of southwest winds consistent with our observations that more sand was transported to the north side of the depositional lobe. Numerous eddies have been observed picking up sand on the upper lee slope and depositing it as they dissipate on the middle of the slope, indicating that turbulent flow in the zone of separation plays an important part in the transport of sand.