FACTORS THAT CONTROL COASTAL DUNE ACTIVITY ALONG THE SOUTHERN SHORE OF LAKE MICHIGAN

The activity of coastal blowouts is controlled by wind (strength, direction, and frequency) and sand supply. Along the southern shores of Lake Michigan winds are very strong, frequent, and with high energy (average DP ~ 750 VU), especially from October through April, when about 85% (~DP 603 VU) of annual DP, and 85% of annual dune migration occurs. Sand is plentiful along the beaches and on stoss sides of blowouts and its availability for wind transport depends on air temperatures, precipitation (amount, and frequency), duration of snow cover on ground, lake level, and duration of shelf ice. The six years (2007-13) of monitoring Mount Baldy dune in the Indiana Dunes National Lakeshore reveals that this is the most active coastal dune in the Great Lakes region. Despite lower than average (18-65 cm) Lake Michigan levels throughout this period, and generally wetter and cooler seasons (October to April) than long term averages, the annual rates of dune migration remained relatively high. In its most active season (June 1, 2011 through May 31, 2012) Mount Baldy dune moved inland on average 4.34 m, with a maximum of 6.52 m along the axis of blowout. This unusual season had: 1) six storms with sustained gale force (> 17.5 m/s) winds; 2) shelf ice that lasted only one day; 3) air temperatures (December through March) that were 2.5 ^oC warmer than long term average; 4) only 93 days with air temperatures below freezing; 5) snow cover for only 19 days; 6) precipitation that was 25 mm below long term seasonal average. The dune was least active during the 2007-08 season, when its average inland advance was 1.83 m with a maximum of 3.52 m along the blowout axis. Even though DP in this season was higher than in two other seasons, reduced dune activity was due to: 1) only two storms with gale force winds; 2) unfavorable direction (SW) of gale force winds; 3) more frequent precipitation (122 days); 4) longer than average snow cover (66 days); 5) wetter than average season (95 mm more than average). The annual rates of Mount Baldy dune migration inland are controlled, in order of their significance, by the number of storms with sustained gale force northerly winds, duration of gale force winds during a single storm, air temperatures, extent and duration of shelf ice, frequency of precipitation, total DP and RDP, duration of snow cover, and the total amount of precipitation.