THE ROLE OF STORM WINDS IN SHAPING DUNES ALONG SOUTHERN AND SOUTHEASTERN LAKE MICHIGAN

It has been hypothesized that rates of coastal dune growth and migration depend largely on the frequency and intensity of storms. We are pooling observations from three separate research sites to study effects of storms along Lake Michigan. At our southern site, Indiana Dunes National Lakeshore, we use marked trees and erosion pins to monitor sand movement on two large blowout dunes with north-facing troughs. From Oct. 2010 to May 2011 over 80% of the erosion on the stoss slopes occurred during four storms with strong northwest winds. The greatest erosion occurred during a March storm after a thaw had removed ice between surface sand grains. During the winter, sand eroded from the stoss slope tends to freeze in place near the dune crest. Thus deposition on the middle and lower lee slopes does not immediately follow storms but is delayed until the spring thaw. At the Saugatuck Harbor Natural Area (southeastern shore) we use an array of 211 pins and six anemometers to monitor sand movement and wind patterns in a complex blowout with a large northwest-facing trough and smaller troughs facing west and northwest. Topographic steering of winds in the dune is accompanied by a loss of energy. Sand transportation is most effective when storm winds approach the opening of a trough at a relatively low angle. The storm with the strongest northwest winds during the 8-month measuring period (10/26/10–10/28/10) accounted for 25% of the period’s measured sand transport. Farther north, at Hoffmaster State Park, we use 135 pins and an anemometer tower to monitor an active foredune and a west-facing blowout on an established foredune ridge. The average change at pins along the foredune during the 10/26–10/28 storm was 24% of the average measured seasonal change. Wetting of the beach by waves appears to have inhibited sand transportation to the foredune during the early part of this storm. Although high-energy wind events appear to be responsible for a significant part of sand transport at all three sites, the amount of transport also depends on the angle of the wind, the wetting of sand by waves and precipitation, and the presence of ice between sand grains.