Lake Michigan Coastal Dunes
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Environment - Winter and spring thaws


Thaws--periods of warming that interrupt winter processes--occur on Lake Michigan coastal dunes during the winter and early spring. Above-freezing temperatures change dune conditions during and after the thaw.

During the thaw:

  • Frozen ground thaws from the surface down, most often resulting in a layer of damp sand on top of a frozen sand layer.
  • Slope instability in thawed or wet sand produces small mass movements such as flows and slumps.
  • Melting snow and ice produce meltwater which ponds at the surface if drainage is prevented by frozen ground.
  • Previously-dry sand becomes wet as liquid water drains into sand or moves laterally through sand above frozen layers.
  • Surfaces dry as water evaporates. Cool temperatures and large amounts of water from melting snow and ice keep drying rates slow.

Soggy beach during February thaw in Hoffmaster State Park
Mid-February thaw melts snow and ice and thaws top layers of frozen ground. (Hoffmaster State Park in February 2002.)

...After the thaw:

  • Water ponded on the surface freezes to become surface ice.
  • The ground freezes from the surface down. Frozen ground may have higher moisture contents than before the thaw because of the movement of water into the sand.

 

 

 

 



During thaws, sand movement by wind occurs by warm-season processes: exposed dry sand will move if wind speeds exceed threshold levels. The amount of moisture present from melting snow and ice restricts sand movement from many dune surfaces.

After thaws, sand movement by wind occurs by cold-season processes: exposed sand moves if it is released from frozen ground by sublimation or impact. Increased water in the frozen ground (from meltwater during the thaw) strengthens the cementation of frozen sand and slows down the rate of sand grain release by sublimation.


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Last updated 03/23/10.