March 13, 2019
By Lydia Salus, Coastal Resilience Project Assistant, Wisconsin Sea Grant
In the last two months, Southeast Wisconsinites have endured temperatures below zero and extolled over temperatures nearing 50°F. With sustained periods of temperatures both above and below freezing, our team decided to learn more about the impacts of freezing and thawing on coastal bluff failure.
We reached out to Luke Zoet, an assistant professor in the University of Wisconsin – Madison’s Department of Geoscience, to enlighten us about the mechanics of freeze-thaw cycling, what makes a bluff susceptible to failure this way, and what we can do about it.
Luke Zoet has his doctorate from the Geosciences Department at Penn State with a focus in geophysics. Though his primary interests are in glacial processes, his research also explores the coastal processes of the Great Lakes region. Recently, he, along with Elmo Rawling of the Wisconsin Geological and Natural History Survey, published an article in the Journal of Great Lakes Research about sudden bluff failure on Lake Michigan.
Here are his answers to our questions about this phenomenon.
What is it about the freeze-thaw cycle that contributes to bluff failure and at what scale can it occur?
We think one of the main factors that drives fractures in the bluff is closer to something called frost heave than the freeze-thaw process. In frost heave, liquid water is continually drawn towards a freezing front in the bluff. This causes an ice lens to continue to grow as long as liquid water is available from deeper within the bluff. As the ice lens grows it wedges cracks in the surrounding bluff material open further and further. If there is already a big pre-existing crack, this extra pressure can be enough to cause it to extend to the point that the bluff fails.
While freeze-thaw events can be large, fortunately they are generally less common than the spring failure events. They can induce (or reactivate) deep-seated slumps, especially if there is already a fracture in place from a prior failure.
What physical properties make a bluff susceptible to erosion from frost heave?
Cracks in the bluff surface are a big factor in erosion. Other important factors include the amount of water in the pore space of the sediment, the temperature change, the hydraulic permeability of the sediment, and the grain size of the sediment.
Is there anything property owners, municipalities, and managers do to mitigate failure from frost heave?
Unlike the spring failures, which are largely driven by increases in pore water pressures from rain and snow melts, freezing events are more difficult to mitigate. One strategy would be prevent the bluff sediment from becoming saturated prior to a freezing event. This could possibly be done by preventing water from pooling on the surface of the bluff in the months prior to winter freeze. In residential areas, homeowners should point drain spouts away from the bluff and avoid watering the grass above bluffs.
As far as being able to predict bluff failures due to frost heave, this is what Luke has to say…
In the one instance that we “caught” a bluff failure from freezing (see this post’s header image for before “A” and after “B” photos), the bluff started moving about 1.5 days in advance of the failure. In the final 2 hours it was really starting to move fast, meaning ~1 cm per hour, which is very vast compared to the rate it was moving beforehand. We were able to use this rate of bluff movement to make a prediction about when the bluff might fail, which worked well in this instance. We might be able to make predictions about buff failure from freezing a few hours up to possibly a day in advance if we have our instruments deployed in the right spots, but in order to verify this we really need to capture a few more to be sure we can apply this technique elsewhere.
This image shows the extensometer records of the failure. Extensometer A was staked to the section of the bluff that failed, and the big peak observed on day 13 is the failure event.
Check out this video of the BADGER instruments used to capture this event!
Images retrieved from: Zoet, Lucas & Rawling, Elmo (2017). Analysis of a sudden bluff failure along the southwest Lake Michigan shoreline. Journal of Great Lakes Research. 43. 10.1016/j.jglr.2017.09.002.