EOLIAN EROSION OF WET DUNE SURFACE AND SAND TRANSPORT IN SUSPENSION AT MOUNT BALDY DUNE ON THE SOUTHERN SHORE OF LAKE MICHIGAN

Moisture increases sand grains cohesion and greatly diminishes their removal and transport by wind. However, strong winds (> 10 m/s) under certain conditions, are capable of eroding wet sand from beaches or dunes, and carry it away by saltation and suspension. First half of October 2011 was very dry at Mount Baldy dune in the IDNL, located at the southern shore of Lake Michigan. On October 13-14 a low pressure system moved through this region and added 40 mm rain, which was accompanied by strong WSW to W winds. A second low pressure system moved through this area on October 19-21, with steady rain (60 mm) and strong northerly winds with gusts of 24.7 m/s (88.9 km/h) and sustained speed above 10 m/s for 45 hours. Very high Drift Potential (DP = 89.7) and Resultant Drift Potential (RDP = 83) of this storm, as well as a high RDP/DP ratio of 0.93, suggest very strong and narrow unimodal wind regime. Monitoring stations at the base of the Mount Baldy slip face indicated up to 13.5 cm of sand deposition during this storm. However, much more dramatic changes were observed on the dunes' stoss side, crest, upper slip face, and area downwind from the dune. The lower stoss side was stripped of loose sand and revealed eastward and soutward dipping grainflows and pin stripe laminations. The upper stoss side contained deeply eroded hollows, small yardangs, and rat tails, carved into very smooth dune surface that was coated with thin (1-3 mm) crust. By the end of the storm the dune brink extended inland for 2.83 m. A bulge of wet sand about 0.5-0.7 m thick extended from the brink to 5 m down the upper slip face. Trees in a forest southward from dune were plastered with wet sand to their tops. There was more than a 2 cm thick layer of wet sand plastered on tree barks and piles of sand up to 36 cm high on N to NE facing sides of trees after this storm. Presence of pebbles and granules at the dune crest suggest that strong winds accompanied by steady rain winnowed some sand and made a smooth crust at dune surface that was later pitted by pebbles and granules picked up from the beach that was upwind from the dune. Irregularities at the dune surface served as a foci of wind abrasion by granules and pebbles. After breaching through the wet crust, pebbles and granules impacted sand and sent it in saltation but also in suspension off the brink, mixing it with rain and plastering it on trees as far as 100 m from dune.