Stabilizing Hillsides and Creek Bottoms
Turf reinforcement mats for a variety of soil types
Editor’s note: This article first appeared in the March/April 2016 edition of Erosion Control.
The rolling landscape of Scott County, MN, is rural but not particularly remote. “That area is farm country, and the Minnesota River goes through the entire area. On the top of the bluffs it’s farmland, but at the river, the elevation drops about 200 feet in 800 feet,” says Paul Nelson, environmental services program manager for Scott County. The scene is typical for the Minnesota River Valley; as the river winds through the 30-square-mile Blakeley Township, he says, “Parts of it are up on old glacial deposits, and some of it in ravines where glacial rivers cut through drop 200 to 300 feet in a mile or less. There are lots of eroding ravines.”
According to Jake Balk, Scott County’s Highway Division program manager, “There’s a 15% grade on the roads—the ravines are steeper than that. Water flows through these ravines at 20 to 30 feet per second during storms.” And the local geology primes the soil for massive erosion. “There’s 10 feet of clay on top of everything, and under that is clean sand. So once the water breaks through the clay it gets into that sand and really starts to wash,” says Balk. Highlighting the pernicious erosive potential in the area, says Nelson, there is essentially no bedrock substrate present to tie into to enhance stability of the surface soil.
Contemplating a Threat
Erosion at Blakeley Trail ravine was threatening infrastructure along County Highway 61, while sediments released during the erosive process contributed to degradation of the Minnesota River. Scott County’s Environmental Services Department, along with the Scott County Highway Division, set out to find a remedy and undertook a stabilization project to shore up the slope of the ravine.
The project had multiple goals, ranging from protecting County Highway 61 running along the ravine, as well as other infrastructure from slides, to reducing the volume of sediments flowing into the Minnesota River where the creek emptied. The river had already been listed as impaired for high turbidity.
With plans for a retention basin at the top of the ravine to slow the water down, stabilization of the slopes, and a series of check dams at the bottom, construction began on the Blakeley Trail Ravine Stabilization project. In the spring of 2014, while work was underway, disaster struck in the form of unprecedented rainstorms. News accounts report flooding and damage across the entire midwestern United States during the mid-June storm, but Nelson says the Twin Cities region, including Scott County, seem to have borne the brunt of the storm’s fury.
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Balk agrees: “We got 14 inches of rain in 16 hours.” Runoff from agricultural lands up on the bluffs, he says, carried huge volumes of sediments and dumped them on the lower-lying landscapes, including the roadways and communities like Blakeley Township.
Credit: BARR ENGINEERING
Rebuilt slope in Scott County
Ravines all over the county began to collapse, including the project at Blakeley Ravine Trail. That project, at the time stripped of vegetative cover for anticipated construction, was nearly wiped out. But worse than that, slides had taken out all three road access routes to Blakeley Township itself, potentially leaving the farm community of fewer than 100 residents marooned in the face of rising waters.
“We had to move quick,” says Balk. A hurried evacuation in the rain, along a single lane of what was left of the collapsing road, shuttled the residents out of harm’s way. Balk says that in all, the storm resulted in at least 110 different landslides. In its aftermath, he notes, “We moved 80,000 cubic yards of clay, mud, sand, and gravel from roadways.” Attention quickly veered from the multiple potential benefits of the Blakeley Ravine Trail repair project to the singular goal of getting emergency repairs done to restore the lifeline and livelihoods of Blakeley Township residents cut off from their homes by landslides on County Road 1.
Barr Engineering was tasked with the daunting repair job. Steve Klein, vice president and senior civil engineer with Barr, had a long track record of success with Profile Products’ Futerra turf reinforcement mats (TRMs). When Profile introduced the GreenArmor System in 2007, Klein began specifying it whenever a situation needed quick germination and extra holding power. With “vertical fill planned for the site to a depth of 50 to 100 feet,” says Klein, the mat selected would need to be able to withstand any intrinsic settlement and movement that might occur on the newly reconstructed slope. In addition, the initial problem that resulted in the landslides remained. “The landscape was steep and we had the potential for high-velocity flows due to concentrated runoff coming off of the bluff into the tributaries.”
Klein says he considers Futerra TRMs to be “the most stout” of the TRM class, but at the same time, he says, “They still allow good growth and offer the advantage that tackifier can be applied directly.” The GreenArmor System consists of a Futerra TRM infilled with hydraulic mulch, Flexterra High Performance-Flexible Growth Medium (HP-FGM). The combination offers a technologically advanced solution with quick installation to protect high-discharge waterways.
However, Klein notes that one of the keys to success in any TRM application is careful installation. He visited the Blakeley Township site during construction to confirm that crews and contractors had used the recommended techniques. “When protecting an area for concentrated flows, where you have a swale going down a hill, the first row of that material needs to go right up the flow line of that swale, and then subsequent rows of that material are overlapped in a shingled fashion on top of that.” He says he has on occasion been called in on projects where TRMs have failed to help the owners figure out what went wrong. He observes that in these cases, “Inevitably, the contractor started from the uphill side and started laying the TRM material down shingled in the reverse order of the way it would normally be shingled, so it didn’t act as shingled product. As a result, water got under it and allowed it to scour beneath it, and it ultimately failed.”
Credit: BARR ENGINEERING
TRMs and Flexterra hold the slope in place.
Klein also recommends a double-seeding technique, with one application of seed going down before the mats are run out and a second seeding on top of the mat, applied with the hydromulch. “Seed is a very inexpensive component” of a project, he says, and therefore a little extra seeding is an economical way to obtain extra holding power for a TRM application. “Double seeding works extremely well,” he says.
According to Balk, repairs to County Road 1 required 9,000 tons of riprap at the bottom of the channel along with Profile Products’ TRM, sheet piling, and subsurface drains to keep the water moving away from the slope. “We used a turf reinforcement mat with Flexterra, which has worked fantastic to date. We got five inches of rain last week and we haven’t seen any damage. The water’s staying on top of the Flexterra, and the vegetation looks great,” Balk said several months after the installation.
After completing the road repairs, allowing township residents to return to their regular lives, crews returned to revive the stabilization project on Blakeley Ravine Trail.
To continue reading the full article, which includes additional case studies and in-depth reporting, check out the May edition of Erosion Control. Please click here. You may need to log-in or subscribe to our magazine.