Turf reinforcement mats can change a space into a place.
Residents of a popular housing development near Wichita, KS, recently discovered that serious erosion problems can strike pretty close to home.
With the rising volume and velocity of runoff due to increases in impervious surface, stormwater ponds have become a necessity for many suburban areas. Although they keep home sites and communities safe from flooding, the ponds sometimes take up highly desirable real estate for a civil engineering function. However, some developers have found ways to make the best of the situation by building their new communities on the shores of these manmade lakes, and a trend toward viewing the ponds as an aesthetically pleasing amenity in the homeowner’s backyard has proven attractive to home sellers and homebuyers in many areas of the country.
|Photo: JEFF PEARCE, ASP ENTERPRISES
Turf reinforcement mats are in place along the lake perimeter.
|Native vegetation has been reestablished.
Nonetheless, even with such mutually agreeable plans in hand, unforeseen circumstances can introduce unwanted erosion problems. For example, when the shoreline of a 4- to 5-acre lake in an Andover, KS, subdivision began creeping up homeowners’ lawns, residents started getting a little edgy.
Well-intentioned homeowners attempted to stem the tide of erosion by placing blocks of concrete, stone, and rubble in the eroding channels near their properties; results, however, were unsatisfactory. As time went on the problems made it difficult for the homeowners and builders to finalize their arrangements for management and maintenance of the property. Finally, according to Jeff Pearce of ASP Enterprises, a distributor of erosion control products in the region, “the builder stepped up to the plate” to try and find a permanent solution for the advancing shoreline. The builder contacted Terracon Consultants Inc., and Kim Austin, a natural resources expert with the firm, went out to take a look.
Manicured to Death
“It was in pretty rough shape when we got there,” Austin says. Some areas of the subdivision of single-family homes had lost 10 feet of shoreline in 10 years. “All of the lower components of the sprinkler systems were exposed; they were just dangling in the water.” In addition, Austin says one of the main streams draining the lake had been cut in with a much sharper turn than originally envisioned. The drastic bend exposed the bank to some rather dramatic erosion that was threatening to expand beyond common areas into the abutting lawns. “There was a pretty good cut all the way around the shoreline, and there was a 6- to 8-foot cut in certain areas on the downstream side,” she notes.
But not all of the problems were structural.
Austin says that residents, likely unaware of the stressful effects of rising and falling water levels on traditional lawn grasses and the impact of windblown waves on delicate plants near the shore, had followed standard suburban gardening practices, seeding their lawns with shallow-rooted fescue right up to the water’s edge. And, in accord with the city of Andover’s local ordinances, they kept these grasses closely cropped right up to the shore. “Everything that was trying to grow, every shrub, all of the woody vegetation, everything that was trying to come up was just being hacked down right at the nub all along the shoreline.” Austin says. “There was no chance of holding onto soil; everything was just mowed to as low as it could go.”
Contributing to the problems, Austin says, streams leading into and out of the pond had become clogged with encroaching trees, fallen branches, and debris, causing unpredictable fluctuations in water levels during rainfall events and adding another level of stress for shoreline flora and fauna.
The builders had received a quote for riprap to remedy the situation, which Austin says “for the rock alone came to $150,000. That was just to handle the lake shoreline and did not include the work needed to repair the channel or anything else.” Nor did the figure include the labor for hand-placement of the stone that Austin says would have been essential for a successful riprap installation. “They thought that was extremely expensive, and that it wasn’t going to handle all of the issues they had to deal with,” she says.
Furthermore, Austin says, residents had expressed hopes for a more visually appealing remedy than bare rock.
To address each of these concerns, Terracon devised a solution using an array of erosion control measures that would not only restore and reinforce the shoreline in an ecologically sustainable manner, but also be pleasing to the eye. Rolled erosion control products (RECPs) played a key part in the plan.
Austin’s concept hinged on replacing standard lawn grass with native plantings near the sensitive shoreline to restore structure and hold the soil together naturally. To accomplish this, she would rely on permanent turf reinforcement mats (TRMs) to keep the soil in place while the new breed of plants established a foothold.
But it was more than a matter of spreading some wildflower seeds and walking away. “Being sensitive to the issues the parties were facing, we knew we needed to get some kind of instant gratification, and with native vegetation it could possibly take three to five years before people would be able to see the benefits. So we decided to use a combination of seeds, but also plugs. The plugs provide that instant gratification, so that people are seeing color—that attracts the butterflies and songbirds fairly immediately while we’re waiting on the rest of the seeds to mature.”
After reviewing a variety of RECPs in the marketplace, Austin selected a TRM from North American Green, a standalone subsidiary of Tensar International Corporation, as the cornerstone of the project. She said Tensar’s Erosion Control Materials Design Software (ECMDS) was a big help in the planning process. “I really liked that they have a desktop engineering program that you can put everything into and look at your problems and your controls—and they’ll give you a value for which blanket needs to go in each specific location.”
In addition, she says, it appeared that North American Green’s mats “were going to meet the needs of the timeframe for the project, reduce our labor cost, and really work well with the native vegetation we were going to be installing within the mats.”
According to Tensar, the turf reinforcement mat, fabricated from long-lasting polypropylene, is designed to remain permanent, protecting young plants, seedlings, and root systems virtually forever.
And the mats can be installed with ease, notes Pearce. “A guy can lay 2,000 to 3,000 or 4,000 square yards a day under the right conditions,” he says.
While hard-armor erosion control measures such as rock riprap or concrete cost in the range of $45 to $65 per square yard, Pearce says, “the costs for turf reinforcement mats can be as little as five to twelve dollars for the same coverage.”
And they save labor and transportation costs. “Eighty yards of P550 TRM will weigh only 56 pounds, but a similar coverage in stone would weigh tons,” he says. Unlike stone riprap or concrete channel linings that often require major excavation or grading to prepare the site, Pearce says installing RECP is simple. “Basically you roll them out and staple them into the ground.”
He says the TRMs provide a very cost effective alternative for erosion control whether installed “over sod, over existing vegetation, or on bare soil.” Wherever they’re installed, though, Pearce advises not skimping on the staples. As with any TRM, he says, “You want intimate contact between the mat and the ground.”
At the start of the project to repair the lake in Andover the developer invested $8,000 in fuel pumping the pond down to further ease installation.
|Photo: JEFF PEARCE, ASP ENTERPRISES
In Andover, homeowners have lakefront views of the restored stormwater pond.
The Brookfield Landfill on Staten Island, closed for 20 years, has been transformed into an urban park.
A Gentler Curve
Workers graded the banks with fresh fill material to restore a stable contour and then placed C350 mats two blankets wide, with a 6-inch overlap along the perimeter of the lake.
Austin noticed that as a consequence of the high-velocity flows, the rocks on the streambed at the stormwater inlets “were having a hard time staying put. We went in with a geotextile fabric, put that down, and then put the rock back in to stabilize the soil.” She says the combination provides a barrier to help ward off unwanted vegetation in the channel in a way that would be aesthetically pleasing while reducing the need to apply herbicides.
Although they were not able to completely smooth out the tight bend in the stream that had precipitated some of the deeper cuts in the channel, crews moderated the angles, forming a gentler curve, and installed HydroPave Conlock open-cell articulated concrete blocks to protect the toe. Austin says the blocks allow sediment flowing through the channel to drop in and fill the cells, reinforcing the integrity of the system. The banks of the channels were lined with P550 TRM for extra strength, and that combination was tied in with the permanent C350 mat protecting the perimeter of the lake.
To ensure protection of the shoreline in dry years and wet years, Austin installed the toe 3 or 4 feet below the normal pool level and placed the blanket all the way up the shoreline “at least another 3 or 4 feet toward the top of the embankment” to protect shore up to the high-water mark during a typical storm event.
A few years have passed, and with the seeds and plantings now established on the site, Austin says the entire project has been “an educational experience in every way.”
Before the project began, she notes, “The city of Andover had a policy in place that would not allow lawn vegetation taller than 18 inches within residential or commons areas. We had to work with the city, the developer, and the homeowners association to educate them on native vegetation. We helped them work through rewriting their city ordinances so that native vegetation could be used in these types of projects to prevent erosion in an aesthetically pleasing and cost-beneficial way.”
Austin says she came away from the project with some new perspectives on how the various stakeholders, be they homeowners, developers, or local officials, view erosion control priorities, which she says will be helpful on future projects.
But for the pond in Andover, Austin says it’s just a matter of waiting and letting nature do the work. “Roots of the native plantings can be 6 to 8 feet deep.” And they’ll finally get the chance to grow up, thanks to the new policies. “We’re not going to be mowing every week,” she says.
Although she now works for Atwell, a separate consulting firm with offices in Andover, Austin visits the shore of the lake periodically, making careful note of new native species that have made an appearance among the mats. She says discovering new species there broadens her list of options for future erosion control projects. “I’ve been trying to come up with a list of species that would provide different aesthetics for homeowners.” She says a lot can be learned by “looking at natural settings, native species, and Mother Nature and what species she has been using for erosion control.”
Pearce says the water quality at the site is now remarkable. He recently fished the lake with his 10-year-old son, casting in tandem while walking along the banks. At their first count, Pearce says, “I had caught one and he had caught eight. When we got up to the other side, I took the lead, and by the time we got to the west end of the north side, I had him out-fished 10 to nine. One of the fish that I caught was probably a four-and-a-half pound largemouth bass.”
Top of the Heap
Sullivan County, NY, is contemplating plans for development on and around the former Monticello Landfill. The county entertained proposals for reuse that include such possibilities as greenhouses heated by methane collected from the site, aquaculture installations, and commercial centers or retail establishments, any of which will rely upon a successful rejuvenation of the landscape.
Bob Arello’s firm, Hydrograss Technologies, contributed to the restoration effort at Monticello and numerous other landfill projects employing a wide range of erosion control products including RECPs.
As a practical matter, he says each product has its appropriate uses. For instance, he says, “Riprap has its uses on slopes in areas where TRM might not be appropriate.” But, he notes, “In a lot of situations you can use a TRM in lieu of riprap.”
For the capping and closure project at Monticello Landfill, he turned to Enkamat 7020, which he used along with GeoMatrix SS hydraulic infill spray to protect 6,000 linear feet of benches and slopes.
Although engineers had initially recommended mats from a different manufacturer, Arello selected an Enkamat product and a hydroseeding technique that allowed the grass to add to the mat’s strength.
Arello says engineers had the impression that a permanent non-photodegradable mat would provide the strongest long-term defense against erosion on the landfill. However, in discussions with the engineers, he explained that UV resistance is not always a direct measure of durability. He says he learned from experience using Enkamats on landfills that “you get better germination and fuller, faster, better long-term turf density using the Enkamat because of its open weave manufacturing process, because it allows air and water to penetrate.” He says the open weave allows the plant material itself to create “a mass, or thatch layer, into the weave that will actually hold better for the long term.”
To combat UV degradation, he says, flourishing plant growth can itself translate into increased durability for an erosion control mat. “With an open weave like the Enkamat, the grass will germinate and a lot of the rhizomes for the grass will grow laterally,” which he says forms a protective layer of grass shading the weave from UV light, extending the life of the material.
He contrasts his approach to erosion control on the landfill to a recent project he has been involved with reinforcing the banks of canals for the US Army Corps of Engineers in south Florida.
|Downstream from the Andover stormwater pond
|Photos: JEFF PEARCE, ASP ENTERPRISES
The revegetated shoreline
Strength in the Numbers
“On the canals, they’re going to have equipment driving up and down the slopes mowing and doing maintenance. In order to protect them, you’re going to need something with 3,000 psi or greater—enough to prevent tearing up the soil if they actually do get mowers on it.”
He said the canals would see “a couple of additional scenarios,” including wave action and rising and falling water levels, that wouldn’t be expected on a landfill. “When water levels in the canals go up, any of the sod that was planted initially will die out, and when the water levels recede in the wintertime, that grass is going to be dead, so you need something that is going to grow in once you’ve installed the mat to prevent any erosion or leaching under the mat.”
According to Arello, because the main erosion concerns on the slopes and benches of the landfill would be sheet flow rather than wave action or the wear and tear of maintenance equipment, the landfill mats could be a bit lighter.
For the landfill application, he says, “When we put the mat down we actually applied the mulch into the mat, unlike most geosynthetic mats where you seed first and put the mat on top and hope the grass grows through it.”
Although he says Enkamat can be seeded the other way around, when applying the mulch on top, he likes to lay it on a little heavier. “You normally apply it a little thicker than you would if you were to spray it first, put the mat on, and then walk away. This way, it goes into the weave, so you have you put on around 2,500 pounds of mulch per acre; you’re going to be spraying it to kind of force it into the mat to be sure to have that contact with the soil.”
Arello, like Pearce, advocates generous use of staples for securing the mats in place, recommending one-and-a-half 8-inch staples per square yard of TRM to secure the blanket to the soil. “Eventually the staples will decompose and the grass will stabilize the site,” he notes.
The landfill installation, he says, was “pretty simple. The swales were about 22 feet wide, with a 6-inch overlap on each blanket. The only challenge, if it’s a good size landfill, is getting the blanket into the site.” Monticello, however, at 13 to 14 acres, “was not very large” from Arello’s perspective.
He notes that on a project such as a landfill, “Many times the engineer will use multiple varieties of grass.” Ultimately the slopes and benches of Monticello were populated with deep-rooted grasses, “so that during periods when it’s dry your roots are going to be a foot and a half deep, along with perennial horizontal creeping grasses.”
He adds, “Once the bonded fiber matrix is set up and dry, it will be very strong and the seed grows right into it, and you have yourself a nice established mature turf.”
From Trash to Treasure
John McLaughlin, project manager for Environmental Planning for the New York City Department of Environmental Protection (NYCDEP), says it’s not often that you see a landfill metamorphose into a city park. But it’s happening right now in New York, at Staten Island’s Brookfield Landfill. The landfill ceased operations 20 years ago after a history that included controversy over illegal dumping of hazardous material, and even criminal penalties for some of the people involved.
Situated practically at the center of Staten Island, the site lay dormant for two decades—a source of frustration for the surrounding community. But all of that is over now. In April 2010, New York City broke ground on a project that would transform the former dumping ground into a priceless resource, replacing the landfill with a 132-acre, ecologically healthy, 21st century urban park, with opportunities for both passive and active enjoyment of the outdoors.
McLaughlin says the Brookfield renovation represents somewhat of a new idea for a former landfill. “The traditional thinking was, primarily, you put a lining on it and lawn grass just to have a vegetative cover. But there was a shift in thinking; many of these sites are in urban areas, and they are fairly large, so they can serve as an ecological value as well.” He says in the case of Brookfield, the idea was “not to just make this a grassy mound with lawn grasses, but to convert it into valuable habitat.”
He adds, “Trees and native plants and grasses can serve the same engineering value to hold the soil in place and protect the investment with far greater ecological benefits than if you just used a lawn grass.”
According to McLaughlin, the new approach can save money in the long run. “If you know anybody who has a lawn, they have to mow every now and then, and during dry periods they need to provide supplemental watering—and also fertilization.” By contrast, he says, “These native plant communities thrive in low-organic, low-fertility soils that won’t need supplemental help once they’re established.
“We’re not just planting individual species; there has been a lot of work concerning plant communities. We know the species composition of the various plant communities, so we try to replicate that as best we can—looking at existing literature and actually going out to these plant communities, whether eastern Long Island or New Jersey or other locations, to get a better understanding of their composition.”
As a part of that research, technicians took soil samples from the sites of various native and naturalized plant communities throughout parts of the Mid-Atlantic region “to get a sense of the physical and chemical properties of the soils that would be needed” to support each community, he says. Using this information, NYCDEP developed specifications for specially formulated topsoil for the landfill to mimic the natural habitat as closely as possible.
The $266 million Brookfield remediation project will cover the 132-acre solid waste disposal portion of the site with 2 million tons of soil and an impermeable landfill cap and turn the site into parkland. The remaining acres, a mixture of undisturbed land, forest, wetlands, and streams, will require no remediation. The total cost to the project for erosion control will be $9,736,110. The completion of the project is slated for early 2017.
The project will use more than 602,000 square yards ECS-2 biodegradable erosion control blankets from East Coast Erosion Blankets LLC to provide a secure foundation for the ecological renewal of the site. Supplied in the Mega Roll format, the mats will represent one pillar of a comprehensive erosion control strategy for the former landfill.
Aligning Engineering With Ecology
McLaughlin finds that East Coast Erosion Blankets’ products have the “right balance” for the requirements of the job. “They’re not so heavy that they prevent the grass form coming up, but not too light so that they encourage soil erosion. They strike the right balance between the two disciplines of engineering and ecology.”
For making decisions in the planning stages of a project, Diane Hitt, general manager of East Coast Erosion Blankets, says the company offers a free online designer for slopes and channels that assists in selecting the correct product for each project. She says the company takes pride in its customer-friendly approach; “East Coast’s director of technical services can provide free educational seminars on the company’s products.”
Nayan Shah, project engineer with the NYCDEP, says, “Basically, this product is easy to install, economical, plus it’s photodegradable and biodegradable so in the first six to eight months we are going to see it disappear and the grass growing up. You won’t see the erosion mat, you’ll only see grass.”
Shah says several additional measures to control erosion are taking shape at the landfill as well. “We have silt fences; we will use riprap. We’re providing drainage channels. We’ll provide proper slopes. All of these things are required to make sure the erosion control mats can stay in place” to perform their proper function.
He cautions that without planning for proper drainage prior to installing the mats, “No matter what you do, it will become mud and they will slide down.”
Calm During the Storm
Because of its location in New York City, Shah says, the project faced two big logistical challenges. “This landfill was very close the residential community, so we have to be very careful with all the material that we bring in, to make sure nothing interferes with the community. We built a separate road to come in so it won’t impact the community by air, noise, or traffic.” In addition, he says, the project includes a significant community information program to keep residents up to date on activities and progress at the site.
The second challenge, a little more difficult to control, was New York City weather. “It drove us crazy,” says Shah. “In the month of September we got 19 inches of rain. So every day is a challenge to control erosion on the 125 acres of the landfill. We built swales, we built dams, we built channels.”
And in spite of all the effort, the going was still tough. “The reason we were so mad was we cannot do the job in bad weather,” Shah says. “We have to have a week of dry weather, otherwise the moisture level is too much; the machines cannot even move, and people cannot even walk on the muddy areas.”
Although he can’t control the weather, Gary Canuso, an East Coast Erosion Blankets distributor with Ferguson Group, notes that when it comes to managing logistics, having a manufacturer in close proximity to the job site can help keep things flowing. The fact that East Coast Erosion Blankets has a manufacturing facility in the Philadelphia/New York metro area in addition to its facility in South Carolina provided some logistical advantages. “On a day when it’s raining and everything stops and they say ‘We are not going to be able to work for two days, but I’ll need everything here on Friday,’ well, it will be there Friday,” Canuso says. “When it comes to filling orders and onsite issues, we can be onsite within two hours.”
Making Sure Things Mesh
McLaughlin says that from an engineering and ecological perspective, mesh size can make an important difference when selecting an erosion control mat. “It needs to be dense enough to hold the soil in place, but open enough to allow the grasses to emerge.” And just the right mesh size would be critical to giving seedlings a good start on the Brookfield project. He explains, “Native plantings such as grasses and wildflowers have a fairly wide blade; if the mesh is too dense, they won’t pop through so that they can begin to photosynthesize,” resulting the failure of the revegetation.
Rob Caughey, quality control manager for Brookfield Construction Associates, the general contractor for the Brookfield remediation, says that just 10 days after watching workers laying the first of the mats down to top off a 20-acre cell, he could “see green grass already” coming through.
McLaughlin notes that it is exciting to watch “the transformation of what was once considered a wasteland into a very valuable environmental resource, both for wildlife and for people.”
Writer David C. Richardson is a frequent contributor to Forester publications.