Slippery slopes: Study details Mankato’s landslide risks

Nov. 18--When Phillip Larson moved to Mankato from Phoenix six years ago, he purposefully bought a house on the flattest land he could find.

He wasn't interested in the Minnesota River Valley's beautiful vistas. He wasn't interested in the gorgeous ravines. He wanted to ensure he wouldn't get hit by a mudslide, which is why he chose a home near Rosa Parks Elementary School.

Larson, the head of the earth sciences program at Minnesota State University, knew, like many residents, a landslide or rockfall can happen in the Minnesota River Valley thanks to moisture and erosion. Yet he didn't know how prevalent those slides were. Neither did anyone else in the state of Minnesota or around the world, up until a few months ago.

As it turns out, landslides, mudslides, rockfalls and other geological events are more common here than people know.

Larson and his students have mapped out the Greater Mankato area's ravines, bluffs, nooks and crannies during the past three years as part of a statewide study. During that time, MSU students have found 479 points throughout the Mankato area where some sort of a landslide or rockslide has taken place. The data show there are some areas around town where people could be more at risk of facing landslides or mudslides than normal.

Their study, the first of its kind in the area, shows our river valley is still changing thousands of years after it was first formed. A lot of the ground that makes up ravines and bluffs in the area is getting more prone to eroding and breaking apart, thanks to increasing rainfall and urban development. And though the odds of residents facing property damage thanks to landslides is small, area scientists believe those odds may grow with each passing year.

"Everybody thinks of hillslopes and landslides and these sorts of big erosional issues as something that happens in mountains, right?" Larson said. "This is something that happens in Washington state, or California or Colorado and not here. In reality, it turns out these are far more common here than anybody ever realized."

A history of failure

Larson and several of his students grew curious about the river valley in 2015, after Larson wondered how often landslides took place here.

At the same time, geomorphologists -- scientists who study how land shifts and moves -- across the state were preparing to launch a massive study on landslides.

In 2013, a landslide at a quarry inside St. Paul's Lilydale Park killed two fourth-grade students from a nearby elementary school. The next year, summer rains caused a massive mudslide near the University of Minnesota Fairview Medical Center in Minneapolis, prompting concern from local and state officials who worried the facility would tumble into the Mississippi River.

Carrie Jennings, a longtime field geologist who works as a professor at the U of M's earth sciences department, was asked by Hennepin County to study how often those landslides happen along the Mississippi -- and whether those landslides would increase with more rain.

"People haven't made the connection yet," Jennings said. "There's like 10 years in between the events, but the rainfall pattern is changing. We know that we're going to see more of these."

From there, Jennings was asked by the Legislative-Citizen Commission on Minnesota Resources to submit a statewide proposal to map out landslide activity, known as landscape failures in geological terms, across the state.

It's the first study of its kind, in part because reports of hillslope or landscape failures are so few and far between. When they strike, homeowners are often unprepared for the damage involved in either losing part of their home over a slope or getting a lot of mud and earth in their homes.

That makes it tough for insurance companies to cover landslides or other failure events.

"The flood plain initiative in the '60s took a lot of statistical mapping of what floods have happened or could happen around the state," Jennings said. "We don't have any of that knowledge on landslides currently."

Eight colleges and universities, including MSU, Gustavus Adolphus College and the University of Minnesota, received a $500,000 state grant in 2016 to work for three years on an inventory of geological activity around the state. From the Red River Valley through the Mississippi River, geologists are looking at thousands of examples of landscape failures.

"What we're looking at is trying to understand how these things function, what's actually driving them," Larson said.

In your neighborhood

Residents living near Thompson Ravine are more likely than other Mankatoans to deal with landslides. That's what MSU students have found after examining dozens of signs of hillslope failures throughout the ravine.

"It's my go-to example," said Missy Kohout, a third-year MSU graduate student who has lead the team in cataloging failures around the city.

Thompson Ravine isn't the only place where landslides could occur, however. At pretty much any slope or hilly area, residents can find signs of hillslope failures -- the mudslides, rock falls or landslides that fell at some point.

There are various types of failures to spot: wedge failures, which look like someone tore out a piece of the bluff with an ice cream scoop; plane failures, where a layer of earth comes off the slope (like the top card sliding off a deck of cards); rotational failures, where the mudslide looks like a curled over ice cream shaving, or complex failures, where so many different types of failure occurred over the years it's tough to say how exactly the land eroded.

Students also can spot failures based on how much grass and trees are nearby. If the hillslope or the bluff has a bare patch of land with no nearby vegetation, it's a good bet there was some sort of a landslide or geological failure that recently took place.

It's tough to tell when those failures occurred. Kohout and others use a variety of ways to date those failures, including a method to tell when the last time soil was exposed to sunlight. But some of those failures are quite high off the ground, and the project doesn't involve trying to rappel down potentially shaky slopes.

Yet they can always find new failures whenever it rains.

Climatologists have found Minnesota is getting more annual rainfall during storms that pour more water on the region.

While Minnesota rainfalls have risen by about a quarter of an inch each year, those totals in south-central Minnesota have increased by three-tenths to four-tenths of an inch. That means the area is dealing with more runoff each year.

In the 1940s, '50s and '60s, an unusually large storm would have topped out at 4 to 5 inches of rain. Today, storms bringing 5 to 7 inches of rainfall are becoming more common, and it's not surprising to see some storms bring 8, 9 or 10 inches of rain.

Those rainfall totals, combined with more urban development, cause water to move at a faster pace through area ravines and stormwater systems. And MSU students have found each major storm brings a new landslide or mudslide.

After the Sept. 20 storms that produced up to 18 tornadoes across southern Minnesota, Kohout and Mankato Area Public Schools officials found a mudslide brought down a sizable chunk of the hillslope behind Jefferson Elementary School.

The school, which was built in the 1960s, already has stormwater drainage near its baseball field, close to where the mudslide took place.

"This happens maybe once every four or five years," said Scott Hogan, the school district's director of facilities.

Hogan said the district installed holding bins about three years ago for silt that falls down the hillside. In addition, the district put up some barriers near Jefferson after the 2014 rains caused several mudslides throughout the Mankato area.

Yet the mudslide came as no surprise to Kohout, given how steep the hillside is near the school.

"That has to do a lot with how the landscape is right now," she said. "(Failures) typically but not always happens with steeper slopes."

What happens next

Kohout and other students are busy compiling data to submit to Jennings and other state officials. They're also preparing to present their findings at the American Association of Geographers conference next spring, an international geography gathering.

As it turns out, the Minnesota River Valley is a unique area in the U.S. Whereas other valleys were cut by rivers and glaciers over a long period of time, the Minnesota River Valley was likely created due to a series of massive, catastrophic floods over a 600-year period more than 13,000 years ago, shortly after glaciers moved through the area.

Those massive floods brought plenty of dirt and silt with them that helped form the valley surrounding the Greater Mankato area. But that same earth is moving around at a faster rate than similar river valleys in the region.

Granted, watching dirt move takes far longer than watching paint dry -- it will take hundreds, if not thousands, of years for the landscape around the Mankato area to noticeably change. But the valley is definitively changing, at a rate geologists estimate is similar to still-evolving mountain ranges.

"It's very energetic landscape," Larson said. "It's very active right now."

Residents can find smaller evidence of that change in their own backyards. Homeowners on area bluffs or near area ravines have likely found warped foundations, fences leaning out of alignment, and even sheds that appear to be tilting out of place.

There are reports of geological failures in the area in almost every year over the past decade. The heavy rains in 2014 prompted several mudslides in the area, including one that overtook Highway 66 near Mankato. A similar mudslide closed Highway 68 between Mankato and New Ulm earlier this summer, which took Minnesota Department of Transportation workers almost three days to clear out.

A large rock fell from a bluff near North Mankato City Hall just last year. A year before, mudslides crept into at least one basement of a home off of Lake Street in North Mankato. And area residents near the Le Sueur River are concerned about losing their homes to river erosion.

Geological experts hope the landslide inventory will lead to predictive landslide regulations similar to the ones found in Washington, Oregon and other parts of the country with lots of hills and mountains. Several states have developed rainfall thresholds to warn residents to take shelter from potential landslides.

"If we can just tighten that up a little bit and warn people during a potential landslide event, it could save lives," Jennings said. "It's more than just waiting to hear the noise in the middle of the night."

The data also could be used to predict whether certain ravines are more prone to landslides, which could be used to calculate insurance costs and construction risks.

"We could help make sure homeowners don't lose their homes in the future," Kohout said.

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