Lakebed Erosion
Great Lakes communities experience a wide range of natural hazards that include flooding from upland runoff, high and low lake levels, severe storms, storm waves and storm surge, shoreline erosion, ice heaves, landslides, and seiches usually associated with low-pressure systems or cold fronts. All these coastal hazards threaten lives and property, a problem that is becoming more critical as coastal development pressures increase and potential impacts of climate change manifest themselves. Climate change and natural disasters are challenging the recovery of at-risk species and enhancing manmade disaster including harmful algae blooms. Michigan has around 3,000 miles of coastline comprised of a variety of shore types from high rock cliffs to low-lying marshes. Most of the coast, however, is made of highly corrodable glacially deposited sand, clay, till, and gravel.
A number of natural processes are at work around the Great Lakes, lakebed erosion and retreating shorelines are but one of these processes. Waves erode shorelines at different rates, depending on the makeup of the materials. Harder studier materials tend to create headlands which jut out into water and create bays with sheltered areas. Wave attacks createe differential erosion, and the bays and beaches that result from this process tend to be protected from the power of the waves. Shorelines as a result retreat at various rates—sometimes slowly and sometimes rapidly. Shores that have cohesive materials (clay, till, and bedrock) have strong binding forces. Shores that have non-cohesive materials (sand and gravel) have weak or no binding forces. Natural shorelines tend to experience slower erosion than those whose trees and shrubs have been replaced by lawn grass. The removal not only destroys wildlife habitats but also prevents the root structures of natural plants from hindering erosion. The amount of erosion that happens along any particular coastline depends on a number of factors including rock type, weather changes, and fetch (distance over which waves build up). The bigger the fetch, the bigger the waves. Waves that build up across the Great Lakes are large and powerful especially when they hit the coastline during stroms.
There are several ways in which the lakes erode the land:
- Wave pounding: The weight of the water hitting the shoreline repeatedly erodes the material away. Large waves can exert pressures of up to 30 tons per square meter.
- Hydraulic action: The sheer power of the waves crashing against the shore. Air is compressed into the bedrock and splits the until pieces break off.
- Corrasion/abrasion: When the sand or sediment the waves are carrying is thrown against the cliff/shore lines which causes it to break off.
Changes in beaches on the lakes are caused by "sand starvation." Lakebed erosion is a common occurrence along the shoreline, appearing where sand and gravel wear away under the constant motion of the waves. Since waves approach beaches ar an angle, sediment brought in by the waves pushes in an alongshore direction. The sideways movement of the sand or sediment is called littoral drift. Along Lake Michigan, this generally happens in a southernly direction. The littoral drift system is what causes the changes in stability of sand bluffs. Landslides and slumps along the bluffs can be unpredictable. A bluff can remain relatively unchanged for decades, and in the space of a week, lose fifty feet of shore. National parks in Michigan have faced these problems. A letter form the MI Department of Natural Resources to George Weeks in 1971 confirms a "massive earth slide" in Sleeping Bear Sand Dunes National Park. The letter also confirmed that the hydrographic mapping of the area confirmed a similar incident at Sleeping Bear Point in 1913, despite no photographic evidence. The DNR went on to explain that Sleeping Bear Point was at risk of similar slides in the future because of the dymanics of the land and water.
Sleeping Bear Point's position and the northwesterly storm patterns show buildup of materials and the processes of shoreline headlands and littoral drift at work. Because of the size of the slide in 1971, the DNR predicted that any additional slides would not occur again until a similar buildup of material and instability of sand dunes created the conditions possible for slippage.
The erosion of a coast often occurs in response to storm waves, rising groundwater, and instability in slope soils, surface water runoff, and other factors. Contributing factors include soil composition, weathering of the slope face by freezing and thawing, steep slopes, lake levels, nearshore shoals and lakebed slope, storm wave energy and duration, precipitation, ice cover, and the types of shore protection used. Residents in the Great Lakes State face significant problems with erosion. Storms and shoreline erosion can wash away houses, destroy roads, and threaten critical infrastructure around the state. Climate change and the rising waters of the lakes are forcing officials in the Great Lakes Basin to search for more long-term solutions. Water levels in the lakes have always fluctuated, but the changing seasonal patterns and dwindling of ice cover during the winter has had detrimental effects on the evaporation cycles. Towns along the coast, including Ludington, MI, Sheboygan WI, South Haven, MI, and Duluth, MN are facing million dollar price tags to fix water treatment and energy plants.
