Bluff failure slump crack along a private property on Lake Superior, Bayfield County shoreline. Photo by Gene Clark
Sea Grant Outreach
Coastal Erosion and Bluff Failures are Issues for People and Property 4/18/2011
By Carolyn Rumery Betz
In the tug-of-war between the force of gravity on soil masses and the resistance against it, gravity will always win.
“Most of Lake Michigan bluffs are marginally stable at best,” said Gene Clark, UW Sea Grant coastal engineer.
The mixture of sandy and clay soils, rain, melting snow and groundwater flow, and the freezing and thawing cycles of ice during the winter months—all create instability.
Additionally, the Wisconsin Initiative on Climate Change Impacts predicts increased storm events, with more precipitation, increased wind velocities, reduced ice cover and increased nearshore wave height—all of which can increase coastal erosion—will occur as part of our changing climate.
“It’s like a teeter-totter where it doesn’t take much to turn from stable to unstable,” explained Clark. “All it takes is a severe storm, someone watering, someone cutting vegetation that was providing some stability, someone putting dead vegetation on the bluff front face. It just makes it worse.”
Property owners should get the advice of a coastal engineer or professional landscape architect to help stabilize coastal shorelines and bluffs, according to Clark, because not every beneficial approach is intuitive. Planting vegetation can stabilize the shore or the front of the bluff, but it can also backfire. Grass or sod requires watering, and adding water to the slope may activate soil movement. Bushes, on the other hand, have a deeper root structure, said Clark. Dogwoods and willows can add greater stability because they send out suckers and put down more roots.
Clark said stability is not acquired by adding things like old Christmas trees, dead leaves or tree branches to the bluff face or an eroding path. These objects can actually make things worse by blocking the sun’s ability to reach the ground and promote natural live vegetation, channelizing flow around the objects when it does rain, or allowing water to pool or pond instead of infiltrating into the ground. Since Mother Nature will always win in the end, coastal experts like Clark recommend placing structures far from the shoreline. Clark warns that even septic systems, especially mound systems, should also be sited far from the shore. Mound septic systems are continually dosing the ground with water, which can make the ground and bluff unstable and prone to erosion.
Most coastal communities use shoreline zoning as a proactive development tool, according to David Hart, a geographic information specialist at Sea Grant.
The delineation of building setbacks is traditionally based on the location of the Ordinary High Water Mark (OHWM), the place where the regular action of water against the bank leaves a distinct mark. That may not be easily seen, particularly on the Great Lakes. Currently, Lake Michigan is at a prolonged low, making the OHWM seem extremely far from the shore, tempting placement of dwellings much closer to the shoreline than would be allowed during a high-water period.
Hart is developing an electronic toolbox full of aids to be used by coastal managers, including historical databases showing erosion over time.
“When you can see how much the shorelines change, and how quickly, you realize how important significant setbacks are,” says Clark. Clark has worked with property owners and/or community leaders in all four of Wisconsin’s Lake Superior shoreline counties and nine of the eleven Lake Michigan shoreline counties, advising on shoreline erosion and bluff instability Best Management Practices.