Project Profile: Stabilizing Steep Slopes After a Flood

Sept. 4, 2014

Each year thousands of visitors flock to the scenic Jay Cooke State Park near Duluth, MN. Glacial deposits of sandy, silty soils include just enough clay to barely hold them in place, creating some rugged, captivating vistas. Because many of the slopes exceed the angle of repose, there is always some sliding and repositioning of the hillsides. When excessive rains caused massive flooding of the St. Louis River and the Forebay channel on June 20, 2012, the result to the already fragile soil structure was catastrophic.

“It was not unusual that the slopes slid,” says Minnesota Department of Transportation erosion control specialist Dwayne Stenlund, CPESC. “What was unusual was the magnitude, because they all slid.”

Water washed away more than just soil. The historic swinging bridge-originally built in the early 1920s and rebuilt after a May 1950 flood destroyed it-was washed away again. A section of Highway 210 was carried away, along with huge swaths of what used to be riverbank, leaving behind lots of exposed soil and a severe erosion risk. Once the floodwaters receded, crews began the lengthy repair process, including building roads, bridges (spans and boxes), and culverts that had been destroyed. Jim Larson, of J&R Larson Grounds Maintenance, was recently awarded the Industry Leadership Service Award at the Minnesota Erosion Control Association annual conference for his efforts, which stabilized the exposed soil and saved the steep slopes from further erosion.

Because the work centered around Highway 210, the Minnesota Department of Transportation directed the project. Stenlund provided technical support to Duluth District 1 construction personnel, ensured proper application of the provisions, and provided direct support for corrective actions as they occurred. Reuben Johnson & Son (RJS Construction)-a Superior, WI, general contractor with extensive reclamation experience-was hired to begin the long rebuilding process. After securing two bids for the hydromulching portion, RJS awarded the project to Larson.

In some ways it was a “be careful what you wish for” scenario for Larson. While he was happy to be awarded the contract, it was no “plain vanilla” hydromulching effort. With slopes that ranged from 2:1 to 1:1 and stretched to heights of 150 feet or more in spots, this one was going to be a challenge.

“Some of the slopes were 60 degrees,” recalls Stenlund, “at the angle of repose such that trees continued to slide down on root mats. How they stayed upright is interesting.”

“Basically the slopes looked like what is left after a forest fire,” says Larson. “The flood just stripped the slope.”

With the steep slopes heights and heights to 150 feet, hydromulching was really the only viable option available. Larson recalls that crews from RJS would try to grade areas of the slope with an excavator bucket if they could reach them; otherwise, there was not much in the way of site preparation undertaken simply because the slopes were inaccessible.

J&R crews used Profile Products’ Flexterra-a flexible growth medium (FGM) product made from wood-based fiber mulch, which contains proprietary chemistry and locking fibers that provide immediate erosion control when the product dries. This was especially important because the project was undertaken just before the onset of winter. Although the dormant seed would not germinate until the following spring, leaving the soil exposed throughout the winter was not an option, because with unstable conditions any snow might bring further disastrous results. The instantaneous results provided by Flexterra’s interlocking matrix provided security over the snowy winter months and through the spring thaw.

Fifteen hundred pounds of Flexterra was loaded into the 3,000-gallon Bowie Imperial 3000 Hydro-Mulcher, along with MNDOT Special Park Mix grass seed-a combination of native and cover grasses and legumes that was specially designed by Stenlund. Formulated with the goal of preserving the native character of the area, this seed mixture was accepted by the MNDNR as a reasonable compromise on difficult soils, aspects, and steepness factors. The grass seed mixture was loaded at 75 pounds per acre, along with 100 pounds per acre of 3881 fertilizer and 150 pounds per acre of guar tackifier.

Aggressive agitation of the hydromulching solution and straight-through plumbing allows the industry’s heaviest loading rate, which translates directly into more coverage per tank load. At the recommended mulch rate of 4,000 pounds per acre, the expected coverage per tank is approximately 11,000 square feet. These coverage rates allowed Larson and his crews to maximize their daily production for the two days they were on site each week, despite the shortened days due to early onset of darkness.

Larson and his crews used two of the largest model Bowie Hydro-Mulchers on this project. While one was loading, the other was spraying. The pair kept a 9,000-gallon water truck busy when they were onsite.

According to Larson, the biggest advantage of the Bowie model 3000 was “the height and distance that it can throw the mix.”

That sentiment is echoed by Stenlund. “We had one area where the road has a 90-degree bend with a very steep slope. I didn’t think it was possible to shoot mulch that high and get it to stick at the top of the slope. Normally crews would have to drag a hose halfway up the slope to reach the top half, then return to the bottom to finish the bottom half of the hillside. But Jim and his crews were able to stay at the bottom and get it all covered. It was much faster and much safer.”

To ensure that his Bowie was able to reach the top of the seeding zone-up to 150 feet-Larson “tweaked it a little bit,” he explains. “We pulled the nozzles and shot it without the nozzle to reach the top of the hillsides. We also closed the return on the tank. This gave us more momentum out of the pump because it is forcing everything through the turret.”

Bowie does not recommend this, and Larson normally does not take such drastic actions, but, he says, “we had a job to do and this enabled us to do it.”

Another deviation the crews made from standard operating procedures was the addition of more guar per tank load. “Dwayne [Stenlund] spec’d a higher loading of guar tackifier because of the steep slopes. The level exceeded Profile recommendations, and we had to be careful mixing it,” says Larson. “If you dump it in too quickly, it would plug up, especially as it got colder at the end of the day. But if we mixed it slower in the tank, we were fine. We figured that out pretty quickly and kept moving forward.”

The cold also required them to drain the tanks each night because the temperatures were below freezing.

“If it gets below 25 degrees at night, I insulate the back of the cage with 1-inch Styrofoam then cover that with quarter-inch plywood,” explains Larson. “We also put a little RV anti-freeze into the turret so that the pump doesn’t freeze.”

All told, Larson and his crews were on site three times over three weeks for two days each time-a total of six days to hydromulch the 17-acre project.

“Everything worked perfectly,” recalls Larson, “except where the deer were running. They broke through the Flexterra, but otherwise the project went very smoothly.”

So smoothly, and with such outstanding germination and grow-in, that Larson received the award from the Minnesota Erosion Control Association. Looking at the site now it is hard to believe that the land was so scarred just a short time ago. Application of the right erosion control products with the proper equipment can stabilize even steep slopes so that when the spring returns, so can the native grasses and groundcover. More importantly, the people can also return to enjoy this scenic and historic state park.