Fishing for Landslides

The highest water levels in over 30 years on Lake Michigan have caused erosion that threatens people’s homes and property. Wisconsin Sea Grant-funded researchers responded to the problem by developing an instrument that can measure bluff failures and help predict when they might occur.

The unique instrument is a Bluff Assessment Data Generating Experiment Recorder, or BADGER. It is the brainchild of Lucas Zoet and J. Elmo Rawling with the University of Wisconsin.

“Bluffs don’t retreat at a stable rate,” explained Zoet, an associate professor of geoscience at the University of Wisconsin-Madison. “What’s happening is that the bluff will sit in the same configuration for days, weeks or months. Then, over a span of a few seconds or minutes, all of the material will landslide its way down from a higher elevation to a lower elevation.”

To capture what happens during this crucial time, the researchers deployed three BADGERs — two in Racine County and one in Ozaukee County. Zoet describes the BADGER in terms of a fishing pole. “There’s a spool like the reel on a fishing pole, and then there’s a line that comes out of it. At the other end of the line where you would hook a lure, we stake that into the part of the bluff that we think is going to fall apart. The reel box is back on the stable part of the bluff. When the bluff fails and the reel unspools the line, we can measure accurately how fast the bluff is moving by how fast the reel unspools.”

The reel box also contains instruments that, when stuck in the ground, measure soil humidity and temperature. Other instruments inside the box measure air humidity and temperature.

The BADGERs were developed by the researchers with help from UW-Madison Department of Geoscience engineers in partnership with the Wisconsin Geological and Natural History Survey and with support from Wisconsin Sea Grant. “We’re the first to invent something like this,” Zoet said. “Nobody’s ever put one out on a bluff, and they haven’t coupled it with soil moisture measurements.”

Zoet wasn’t sure they’d be able to catch a landslide in action. “It’s kind of like fishing,” he said. However, they were successful. On December 13, 2016, a bluff in Racine County slumped, dropping about 500 cubic feet of material onto the beach below.

Catching a landslide during December was unexpected. “Most landslides occur during spring when there’s a lot of water moving around, not winter,” Zoet said. The BADGER recorded the ground moving for a day beforehand at “a pretty high rate, which is about one millimeter or two per hour,” Zoet said. During the hours before the landslide, the bluff started to move at an ever higher rate.

The researchers suspect that freezing water inside the soil expanded and wedged open cracks in the bluff, causing the landslide.

Zoet said that better understanding the physical processes that cause bluff failures will help researchers make more educated predictions about what will happen in the future with respect to erosion at the bottom of bluffs and how that can lead to bluff failures. The information will be useful for people who live in areas with bluff erosion. City planners and beach managers will also find the information useful in determining building locations and beach stability.

“We learned that bluffs fail in a couple of different ways that we didn’t anticipate before,” Zoet said. “This project also sets the ground for us to deploy BADGERs in other parts of the coast to learn how different types of geology and different types of bluffs might fail compared to what we’ve already observed in Racine County.”