A newly constructed outlet mall, Taubman Prestige Outlets in Chesterfield, MO, was slated to open just in time for the start of the school season in late summer 2013. The mall is located in a floodplain area.
Taubman, the Bloomfield Hills, MI-based mall owner, sought an approach for the highly engineered, landscaped, and maintained stormwater conveyance and bioremediation swale running for half a mile along the frontage road that would provide erosion control while being aesthetically pleasing: a green strip.
The site, at the edge of a parking lot, features a short and steep slope, which varies from 2:1 to 1:1.
“We presented an option to use riprap all the way down the bank, but the mall owner and the city preferred the grass look,” says Mike Pranger, senior project manager for Castle Contracting in St. Louis, MO.
To provide the best solution, Tim Stock of the engineering firm of Stock and Associates Consulting in Chesterfield turned to Guy Litteken of ASP Enterprises, an erosion control product distributor in Fenton, MO.
The two had previously worked together on other projects. From their previous experience, they decided to use Enkamat 7010 from Bonar (formerly Colbond) to meet the site’s requirements.
Enkamat 7010 is a three-dimensional turf reinforcement mat made of continuous monofilaments fused at their intersections. Ninety-five percent of the mat is open space for soil, mulch, and root interaction.
Enkamat is manufactured from nylon so as to avoid the buoyancy factor affiliated with submerged conditions and is designed to provide permanent turf reinforcement mat (TRM) protection on vegetated channels and slopes.
Castle Contracting provided the grading and cut and fill work under Pranger’s direction. Baxter Farms and Nurseries conducted the erosion control installation under the direction of Adam Guthrie, project manager.
In June, in an effort to ensure the grass would root in time for the mall’s opening, the team applied the Enkamat 7010 to a 6,000-square-yard area and made about 400 curb cuts. A significant rain event followed over the weekend. After the project had been completed, Litteken drove by the site on his way to work as he usually does and was dismayed to see the erosion control work had been destroyed by the weekend’s weather event.
“I called the project superintendent for Castle, Scott Beutel, and said, “˜Man, it does not look good. I’m sure you have some ideas and I’d rather be part of the solution than the problem.'”
The rain had turned the site into a testing ground, says Pranger.
“One of the factors that attributed to the problem is that we had a sandy, loamy material along the bank,” says Pranger. “It was down on the bottoms here in St. Louis, so it wasn’t sticky clay. Once we had it graded in we had the bank established. The parking lot was built and there were curb cuts in the parking lot to allow the water to run away.
“We had some rains during construction, so after the biobasins were built, the banks were finished, and the sod was laid, we had to test it out a couple times in June of last year with some heavy rain. What it actually did was wash off of the parking lot underneath the sod into the subgrade and then blew the sod out in certain areas.”
Pranger says the team, including engineers and architects, met to try to figure out a reasonable way to solve the problem.
“The issue was time before the grass can root,” he says. “It was going to have to survive through this type of weather conditions for a couple of months, so that’s when we brought Guy on board. He had used ScourStop drainage mats on other projects.”
ScourStop transition mats, manufactured by Hanes Geo Components, are a biotechnical replacement for riprap and other hard armor. The mats combine natural vegetation with polymer technology.
ScourStop’s mechanical protection is designed to provide impact resistance, tensile strength, and permanent durability against high-erosive stresses.
Some type of cover-such as a turf reinforcement mat, sod, or a combination-is required underneath ScourStop for effective results.
To withstand the shear forces of flowing water, ScourStop transition mats are anchored deep in the soil with bullet anchors, which play a key role in the mat’s strength as well as helping it conform to the native topography.
The construction team and the mall owner looked at the projects in which Litteken had used ScourStop and decided to try the drainage mats. The next day, they set up a mock-up at the site, turning on fire hydrants to recreate the effects of torrential rainfall.
“We took some inlet protectors so we could channelize the water and ran the water off of the parking lot through one of these curb cuts to emulate a storm event,” says Litteken.
The owner’s representative was “very cautious because they were very, very concerned about the appearance of the mall and worried about these panels being seen from the highway or the roadside,” he adds.
Litteken assured the owner’s representative that the grass would cover them.
“They were planning on taking care of this like a golf course,” he says. “It’s all irrigated and manicured. It’s taken care of very well.”
With respect to the Taubman Prestige Outlets, the team elected to go with the combined BMP approach of the Enkamat and ScourStop. Castle crews installed about 1,000 ScourStop transition mats, three per curb cut.
“We were able to salvage the Enkamat,” says Litteken. “We cut the Enkamat away, recompacted the soil, put the Enkamat back with a little soil, sodded it and then put the transition mats over the top of it.”
With the test area performing successfully, the subsequent mitigation efforts were wrapped up in June and July.
“They’re performing beautifully,” points out Litteken.
He says the experience provided lessons learned for future jobs. In this case, it was the dredged borrow soil. “No matter how good a spec you have, how good a design, and how high-quality a product, you’re picking to accomplish what you want to do by reinforcing the turf. What they didn’t count on in this project was the poor performance of the very high-quality borrow soil infill,” he says.
“Even while it was compacted it became liquid, and that was something they didn’t foresee, that in these rains it was going to come down and saturate the sod,” Litteken adds. “It oozed out in between the openings of the TRM and the sod seams. It just got destroyed.”
Litteken says that although everything was designed properly, and even if the latest software was used for the mitigation efforts, the results still would have shown to use one transition mat per curb cut.
“And it wouldn’t work,” he points out. “So we used three. That was mainly due to our field test.”
During the test, the team sandbagged certain areas, opened a fire hydrant, and allowed the water to run down the face of the bank to see how it would hold up, Pranger says.
“When we emulated the rain event in our mock-up, we started with one [transition mat], and we had our doubts,” says Litteken. “When it was coming off of the apron onto the transition mat, you could see the sod starting to bubble up, so we went with two mats. And then we ultimately decided on three mats.”
Litteken points out that designing to manage water is difficult at best. “It’s not an exact science. As much as everybody tries to make it exact, it’s spontaneous and it’s hard to manage.” That means the design community needs to consult with as many sources of information as possible, he says.
Pranger says he had never seen a bank undermined underneath sod such as this one.
“This is the first time I used ScourStop and saw its effectiveness,” he says. “We’ve definitely put it in our arsenal of possibilities for owners as we move forward and make suggestions for design on early parts of the job. That was a particular subgrade down there on the bottom, so it was good to have an understanding of how to possibly take care of that issue on future projects.”
The property owner could have picked a different approach, says Litteken.
“They could have used gravel, but the owner really wanted that green,” he says. “They had a lot of natives planted in the bioswale. They wanted to soften up the look.
“Obviously, there’s a gigantic parking lot, but they did a wonderful job landscaping. The owners were very meticulous on all of the curbs, the pole bases were all hand-rubbed, hand-finished concrete. The owner obviously paid a premium for the design and still had some issues, but with some help and throwing a little more money into it, they did get what they wanted. They are a responsible developer.”
Pranger adds that if one were to drive by the site now, “unless you know that material was installed, there’s a faint outline, but it looks pretty natural. All of the grass has come up through it and you can’t even tell there’s any erosion control there. We never had another issue with another rain event.”
Minnehaha Falls Park
In fall 2011, a section of Minnehaha Falls Park in Minneapolis, MN, needed a facelift. Officials sought to vegetate a riprap-lined channel for a more aesthetically pleasing appearance.
The contracted company for the job, Minnesota Native Landscapes, was using a native seed mix and opted against importing soils, choosing instead to use Verdyol Biotic Earth Black. Verdyol Plant Research is a wholly owned subsidiary of ErosionControlBlanket.com.
Biotic Earth Black combines specially processed straw, flexible flax fibers infused with Canadian sphagnum peat moss, mycorrhizae, Triacontonal growth stimulant, soil conditioners, and micronutrients.
Used in areas where the soil lacks organics or has poor tilth, Biotic Earth Black is designed to improve moisture holding capacity, water infiltration, soil structure, and soil microbial activity; optimize seed germination; and improve plant photosynthesis and cell growth.
In the Minneapolis project, Verdyol Biotic Earth Black also was used to fill the voids in the stone and was installed at a heavier rate than typical to fill the voids to the proper depth. The contractors then covered the material with a wood fiber mulch.
“The work was done in an area on a very steep slope that had been eroding for a number of years,” says Jeff Renier, Minnesota Native Landscape’s project manager, adding that it was a 1:1 slope.
“The city wanted to put riprap on the eroded slope, but from an aesthetic standpoint, they wanted it to be vegetated,” he says. “They wanted strong erosion control protection like riprap on the bank, but they wanted it to somewhat disappear,” says Renier. “So they wanted to inject it with this enhanced soil product and get some fast growth.
The project took two people and half a day to complete. The tasks included installing riprap on the steep bank-the most challenging part of the job-and filling it in with the Biotic Earth from the top of the slopes, injecting it into the rocky area.
Eventually, vegetation began to fill the voids and stabilize the area. “We had some fast establishment of the seed,” he says.
Renier says the choice was to use a product that would “green up” fast, provide erosion control, and look good, especially because of the site’s location.
“You always hope you have the right conditions long enough to let the seed germinate and become established with the ton of heavy rainfall and flow from the top of the slope that it can get.”
Wetlands and Landfills
For 30 years, Green Start Inc. (GSI) in Gilbertsville, PA, has been engaged in commercial erosion control and landscape projects, using products from East Coast Erosion Blankets in its work. Many of the company’s projects are in wetlands and landfills.
The most common product GSI uses is the ECS-1, a single-net straw blanket primarily used on slopes of 3:1. Sometimes the company also uses ECS-2, a double-net straw blanket used on slopes of 2:1. East Coast Erosion Blankets also offers the ECSC-2, a blanket composed of 70% straw and 30% coconut fiber. It is designed for functional longevity of approximately 24 months.
The products feature a rolled edge designed to keep the material in the product and to assist the contractor with ease of unrolling.
“I think matting is a good thing, because the matting holds the moisture in,” says GSI owner Tim Bortz.
In one project during winter 2013, Green Start applied matting to a 70,000 square-foot slope on a day when the high temperature was 15 degrees Fahrenheit.
“It was freezing as we were spreading it. We matted it and we had 100% germination off of that seeding, which occurred on the coldest day of the year,” Bortz says. “Matting has a lot of advantages when it comes to holding the heat in and helping the germination.”
In contrast, straw used with a tackifier may be good until winds hit about 30 miles an hour, and then the straw can blow about, he adds.
“If you use a matting, it may cost a little bit to put it down, but the end result is a lot better,” Bortz says.
Bortz says he finds that using a heavy polyester blanket can present problems in the heat. It’s a lesson learned that changed his company’s approach when working at a 120-acre Superfund site.
“We previously found that when we used the poly mat, the heavy TRM, that we got germination, but then through the summer it burned right out because of the heat of the plastic,” he says. “We found on that job that it was cheaper to bring in sod. We brought in six tractor-trailer loads of sod and used it in place of the TRM and found that worked a lot better because we had the germination that would hold the sod in place.
“A lot depends on any heavy rain event,” he adds. “I don’t know that it’s an exact science. You’re using BMPs and you try to do the best you can, and you don’t always win.”
Weather events, especially rain, can present a challenge, says Bortz.
“Each job has its own specific set of problems,” he says. “We’ve been at this long enough to know that the approach you use on one project may not necessarily work with another project because of the amount of rain.”
While most of Green Start’s jobs are in the rolling terrain of Pennsylvania, the company also works in Delaware and New Jersey, where the land is generally flatter.
Slope considerations aside, one of factors in determining which erosion control products to use is what is specified by the engineer on the job.
“We have no choice,” Bortz adds. “Usually, if it’s a slope that severe, then the engineer is involved and you have to do what he wants you do to. Most of what we do is commercial work, so they’ve got an engineer that’s telling you what to do. Even on privately owned landfills, there are engineers that are involved.
“Often, an engineer is a P.E. and he’s charging the client $120 to $200 an hour and he’s not always receptive to listening to the opinions of the landscape guy,” notes Bortz. “That’s a fine line you walk, because you’re trying not to go over the head of the engineer, but you’re also trying to find a solution to the problem. At the end of the day, you still own the problem, because you’re the guy hired by the client for the owner.”
There are times when Green Start has had to go back and maintain or redo a job.
“That’s just the way the construction industry is,” he says. “We’ve done landfills where four days after we did the work, they’ve ripped it all up because they’ve needed to do something and we had to go back in a week later and redo everything we did before.”
Regulations play a key role on the choice of erosion control. The higher up the government agency chain a job goes, he notes, the more regulations there are to which an entity must adhere.
“Most of the work has to be done on the county level. It gets a little bit bigger and it goes to the state level, which is the Department of Environmental Protection. Then if it gets really big, it’s an NPDES permit,” says Bortz.
“A lot of times with landfills, they’re trying to comply and keep the public happy, especially a lot of the municipal landfills, and for the most part, private landfills are the same way. They don’t really want to mess around. They want to get the germination and do what it take to attain that. Usually spending money isn’t a factor,” Bortz says.
Erosion control budgets can be significant, Bortz adds. “We do jobs where just the erosion control and the silt fence is close to $1 million. It’s a big part of the budget,” he says. “It’s a high percentage just to maintain the regulations that are required. It didn’t use to be that way. You might have just had silt fence.”
Making the appropriate choice is “not perfect, and the weather isn’t predictable,” Bortz notes. “But for the most part, the engineers do a pretty good job.”
He notes that over the years, as regulations have increased, the number of developers has decreased, given the money that’s needed to develop projects.“The small developer is getting squeezed out, and it’s mainly the big players, people with deep pockets, who are able to comply with everything,” he says. “A lot of the smaller operations don’t have the money to do that anymore.”