Erosion control blankets and turf reinforcement mats (TRMs)-collectively known as rolled erosion control products (RECPs)-are mainstays of erosion control work. They consist of prefabricated blankets or netting formed from natural and/or synthetic materials. RECPs reduce erosion, provide seed protection, and enhance moisture retention, and they typically degrade over time, leaving healthy vegetation in their place.
While EC blankets and TRMs accomplish similar tasks, blankets are generally temporary (typically effective for one growing season or less) and TRMs tend to be more permanent (in some cases lasting for several years). Jill Pack of North American Green notes, “As you go from lightweight single-net blankets to double-net to permanent TRMs, the price will increase.”
For gentle slopes and forgiving environments, erosion control blankets may be the RECP of choice. But, as Stephen Zwilling of Profile Products explains, “Many times, TRMs will be used for more permanent applications-maybe in the bottom of a ditch or a channel where you’re expecting some concentrated flow. Sometimes they will be used on slopes where you’ve got some really severe conditions or you have water cascading over the slope. The idea behind the turf reinforcement mat is that it provides strength for the root base of the plant so that it can handle high-shear values or high-flow values over the surface of the product.”
Both biodegradable and photodegradable products are available. North American Green’s Rob Lawson explains, “While photodegradable blankets are more established and command the largest share of the market, interest and demand in 100% biodegradable blankets has gained much popularity supporting good environmental stewardship. There are no “˜plastics’ residuals left to create hazards, and, more importantly, they eliminate threats to wildlife.”
One recent addition to the biodegradable market is BioGrid from Conwed Global Netting Solutions. When exposed to soil microbes, it eventually breaks down into carbon dioxide and water, leaving no trace.
Pack also notes, “RECPs can be used with a myriad of BMPs [best management practices]. They can be used with structural BMPs, such as earthen berms, vegetated filter strips, and swales. RECPs have also been used with hard armor, such as riprap and concrete; sediment control devices, such as sediment logs; and check dams. It is very common to apply seed to an area using hydroseed or mulch prior to RECP installation. Rock is often used over TRMs as check dams or scour protection. Sediment logs are often used with RECPs as slope interruption devices or for toe protection along streambanks.”
Calista Santha, president of RoLanka International Inc., adds, “In areas where mature vegetation can stand erosive forces, use soft armor products, such as TRMs, to provide resistance until mature vegetation can be established. In areas where vegetation alone cannot provide resistance, hard armor methods should be used.”
While RECPs are often the preferred method in challenging environments, Santha notes that a sometimes-overlooked problem is that wildlife can get caught in the netting. For example, at a US Department of Energy site along the Savannah River in South Carolina, 15 RECPs were installed. They proved effective at erosion control, but an inspection four months later revealed a total of 19 snakes, most dead, caught in the product netting. To avoid this problem, a variety of netless RECPs is now available.
In some instances, RECPs should be avoided, as North American Green’s Lawson explains. “You would not want to use these products in heavily shaded areas or where soil conditions will not support vegetation, or in perennial aquatic areas unless used with vegetation that will survive these conditions.”
Pre-seeded blankets and TRMs are available, but both Zwilling and Santha advise against these, for the following reasons:
- Different seed mixtures are required for different projects. If a blanket is pre-seeded, it will be usable only for that specific seed combination.
- Seed has to be tagged and certified, with a limited shelf life.
- It is important that seeds have good contact with the soil. With some seeded blankets, the seed may remain trapped in the blanket, not contacting the soil.
Most RECPs must be tacked or stapled in place, and for some projects the traditional fasteners may represent an unacceptable hazard. Pack explains, “In an area of high pedestrian or pet traffic, sometimes the wire staples are replaced with biodegradable plastic stakes. In areas where loose soils are a concern, wood stakes are sometimes preferred over staples. If there is concern about wildlife entrapment, a biodegradable net may be selected.”
Following the 2001 Darby Fire in central California’s Stanislaus National Forest, 62 acres were treated with “certified weed-free” rice straw bales for erosion control. The following year, 63 acres of non-native yellow starthistle and tocalote were observed in the treated region.
Is It Weed-Free or Not?
Along Highway 36 in Humboldt County, CA, straw was used for roadside erosion control. Canada thistle appeared shortly thereafter.
Nearly half a million acres of national forests and grasslands in the Pacific Northwest region alone have been degraded by infestations of invasive, non-native plants.
According to the North American Weed Management Association (NAWMA): “Harmful non-indigenous plant species cost the agricultural industry over $20 billion annually, a cost ultimately passed on to all consumers. Harmful non-indigenous plant species invade our natural ecosystems from coast to coast, displacing native species, destroying wildlife habitat and recreational areas, and increasing soil erosion and stream degradation. These effects will be felt for generations. Prevention, early detection, and effective and efficient management are the most cost-effective means of preventing or reducing the loss caused by invasive plant species.”
There are other problems with invasive non-native weeds. They can reduce crop yields, costing farmers time, labor, and money. They may out-compete native plants, reducing biodiversity, and threatening rare plants. Noxious weeds can choke waterways, reducing habitat for fish and reducing opportunities for boaters, fishing enthusiasts, and swimmers.
As a result, in recent years, there has been a concerted effort on the part of local, state, and federal agencies to reduce the spread of harmful weeds by certifying various forms of hay, straw, and mulch as weed-free. Because prevention of the spread of noxious weeds is much easier than their eradication, the hope is that universally accepted use of weed-free products will significantly reduce the impact of these noxious weeds over time.
In the erosion control industry, this is especially pertinent to the use of straw wattles and erosion control blankets. Manufacturers who utilize certified weed-free material in the RECPs they produce provide a valuable service to contractors who may be required to prevent the spread of noxious weeds in their work. For example, effective in 2009, the USDA Forest Service requires that only certified weed-free hay and straw is to be used for projects within its jurisdiction.
Alison Halpern, executive secretary of the Washington State Noxious Weed Control Board, explains the general certification process: “Let’s say there’s a straw producer in Spokane County. A certified inspector who has been through the NAWMA training will go out and inspect the fields before they’re cut. This is going to be within 10 days of harvest. The inspector will go through the field, do a walk-through around the perimeter of the field, and look at where the product is going to be stored while it’s on that property. If the crop passes inspection, the grower will go ahead and harvest. At harvesting time, he will be issued a certain proprietary twine or tag so that each bale from the certified crop can be marked.”
In Washington State, inspections are typically provided by local county noxious weed control boards, whose weed coordinators go out to the fields. It is Halpern’s State Noxious Weed Control Board that oversees the inspection process throughout Washington state. “We issue the twine and the tags and the inspection certificates,” she says.
Each state handles weed control issues as it deems most appropriate; there is no federal mandate except for agencies such as the Forest Service or the Bureau of Land Management that handle activities within their specific jurisdictions.
Bill Scott, program manager at the Kansas Department of Agriculture, Plant Protection and Weed Control in Topeka, explains that his inspectors use the same NAWMA-designed protocol as described above for Washington state. However, instead of being administered on a county level, Scott says, “In Kansas, it’s our state staff that does all the inspections. Some other states handle it differently.” He adds, “We have five inspectors for the state of Kansas, and it gets pretty busy at times, but we’ve been able to handle it.”
Because weed-free certification is still in its infancy, with each state determining how, if at all, to handle this issue, there is the potential for growers and for erosion control manufacturers to abuse the process. At least one company that sells certified weed-free RECPs has hinted that some competitors may play fast and loose with the certification rules, claiming their products as being certified weed-free when in fact they may contain uncertified straw or mulch.
“I’m not saying it couldn’t happen or hasn’t happened,” says Scott from his Topeka office. “But we’ve not been notified of any problems. No one has questioned the certifications where we’ve been involved.”
Halpern, in Washington state, echoes Scott’s comments. “I could see there’s room for people to say that they’re selling certified weed-free straw without actually being NAWMA-certified,” she says. “I just haven’t received any calls about it. We do get calls about people advertising weed-free hay and weed-free straw, but if they’re not advertising it as state certified, we don’t get involved. We don’t regulate the term “˜weed free.’ We just handle NAWMA certification. There can be a bit of confusion there.”
Both Scott and Halpern comment that purchasers of certified weed-free products do have some control. “I think a lot of it is making sure that the purchaser of the product understands what to look for. If it is NAWMA-certified weed-free, then the purchaser should know to look for the proprietary twine or tags to make sure that it is valid and dated,” says Halpern.
“The buyers-they’re really the ones in the driver’s seat to ask about the documentation,” says Scott. “There’s the certificate of inspection or, if it’s baled hay, we use a tag on each bale. For erosion control mats, they will use our bale tag on a pallet of mats or wattles or whatever they have.”
North Creek Lake Erosion
Situated between Washington, DC, and Baltimore, the town of Montgomery Village, MD, is home to 40,000 people, lots of beavers, and 18 recreation and park areas. Among those recreational sites is the 80-acre North Creek Lake Park, which offers walking, jogging, fishing, and attractive picnic spots to local residents. It also contains the 6.5-acre North Creek Lake and its three small islands.
Over time, an eroding shoreline threatened to destroy portions of a popular walkway around the lake. Erosion was progressing to the point where the gravelly loam shoreline was eroding inland at a rate of about 1 foot per year. Vegetation was being destroyed as park visitors walked along the edge of the lake and as geese and ducks fed along the shore. The exposed soil was subject to accelerated erosion from rainfall, stormwater runoff, and the lake’s wind-driven waves.
It was important to park administrators to control the erosion, but they wanted to retain a natural appearance and continue to provide areas where both people and wildlife could access the lake without disturbing the soil.
The erosion control project was designed by Chesapeake Environmental Management of Bel Air, MD. It included four major components to protect the eroding banks along the perimeter of the lake, as well as the shorelines of the three small islands. According to the plan:
- Coir rolls would provide initial bank stability until roots of woody species could establish a network to hold the soil in place. The coir fiber cores of the rolls support the growth of trees and shrubs that are planted next to them. In time, sediment deposited by wave action around the coir rolls provides a growth medium for the vegetation.
- Coir blocks would be used to construct fabric-wrapped layers of compacted soil to stabilize the higher banks and areas subject to stronger wave forces. The blocks offer a faster, easier approach to constructing encapsulated soil lifts than conventional geotextile fabrics. Structures of imbricated stones would protect water-access sites.
- The areas between the stabilized shoreline and the asphalt walkway would be seeded with tall fescue mixture to control erosion from stormwater runoff.
These elements were installed over a three-month period. For the coir rolls, Chesapeake Environmental Management selected BioD-Roll 40, made by RoLanka International Inc. The 16-inch-diameter, 10-foot-long roll consists of coir fiber densely packed inside a coir twine netting. The rolls are designed to resist erosion and support establishment of vegetation for at least five years. A natural coir twine netting was chosen over a polyethylene netting for two reasons: The lake has a relatively low wave velocity, and coir twine is more environmentally friendly.
Before the rolls were installed, the slopes were reshaped to gradients ranging from 3H:1V to 4H:1V for improved long-term bank stability and easier installation of the rolls. Two adjacent rolls were installed along 1,775 linear feet of the lake’s perimeter, and three rolls were placed side by side along 1,125 linear feet of the higher banks of the islands. They were held in place using 3-foot-long, 2-inch-by-2-inch wood stakes.
In addition, such aquatic plants as duck potato, blue flag, and arrow arum were planted in the water several inches deep below the lowest row of coir rolls along the outer shoreline of the lake. Three- to 4-foot-long dormant stakes of wetland tree species were also planted in between the rolls on the lake perimeter. These included red osier dogwood, silky dogwood, and streamco willow.
Areas of the shore used for public access to the water were protected from foot traffic and wave action with imbricated stone. Weighing up to 2,000 pounds apiece, the square-edged quarry stones were installed in two or three rows in stairstep fashion along the shore to stabilize the bank toe.
The surface of the top row of stones is about 1 foot above the average water level of the lake. A total of 1,040 linear feet of shoreline was treated in this manner. Throughout the project, turbidity curtains were placed in the lake around each construction site to contain any sediment disturbed by the site preparation and material installation.
An interesting challenge faced by the workers was that, for work around the three islands, the only access was by boat. It took nearly a full day to transport all necessary material. Site preparation was all done by hand using picks, shovels, and a plate compactor.
Significant rain fell while the project was under way, but the newly built areas remained very stable, except for some minor erosion where the coir rolls butted up against the imbricated stones. Repairs were accomplished by placing riprap at the junctions, covering it with topsoil, and reseeding.
Revegetation Near Yellowstone
Wyoming’s Highway 14 runs adjacent to the North Fork Shoshone River and forms the east entrance to Yellowstone National Park near Cody Peak. Construction activities along 10 miles of Highway 14 just outside the park’s boundary, in Shoshone National Forest, left a significant amount of the area’s characteristically dry, rocky volcanic soils unvegetated. It was important to quickly revegetate these bare roadside slopes and channels before serious soil erosion occurred. In addition, excessive sediment deposition in waterways would endanger the area’s abundant freshwater fish population.
Two major issues needed attention:
- As part of the US Forest Service’s erosion control plan, only 100% biodegradable erosion control products are allowed inside or near the park’s boundaries. An important reason for specifying only biodegradable products is for the protection of wildlife. History has shown that animals of all sizes are prone to entanglement in plastic net structures found in some erosion control products. Thus, the reconstruction plans for Highway 14 excluded the use of all plastic netting materials.
- The area involved receives only about 15 inches of rain annually, and the soil along the highway is relatively infertile. Therefore, any solution implemented must do more than provide reliable erosion control and be 100% biodegradable. It must also possess excellent mulching capabilities to regulate soil surface temperatures and effectively retain moisture at the seedbed to quickly promote seed germination.
After reviewing several proposals, the Wyoming Department of Transportation selected two BioNet erosion control blankets from North American Green. The SC150BN blanket was used on roadside slopes, and the C125BN blanket was specified for use in the drainage swales alongside the road.
The SC150BN features a mixture of 70% straw and 30% coconut fiber, resulting in significantly greater functional longevity (about 18 months) than most 100% straw blankets. This is crucial in this dry Western climate where revegetation often requires two growing seasons to achieve full maturity.
For the small drainage swales, the C125BN, with its 100% coconut-fiber matrix, provided increased shear stress resistance and a functional longevity of 24 months. This ability to handle shear stress is important when spring snowmelt from the high mountain slopes accumulates in the narrow roadside channels.
Both of these blankets employ two woven jute nettings that are 100% biodegradable. In addition, the BioNet products are constructed in such a way that the strands of the netting can move independently of each other while still retaining structural integrity. This minimizes the risk of wildlife entanglement.
More than 175,000 square yards of erosion control blankets were installed along Highway 14 to re-establish vegetation and prevent soil and sediment from washing into North Fork Shoshone River. One year later, the native seed that was planted under the blankets has sprouted into a thick mat of brown, green, and red grasses, and there have been no visible signs of erosion occurring on the site.
Minnesota High School Construction Site
When the city of Savage, MN, a suburb of Minneapolis, built a new high school, the surrounding land needed a lot of care. Installation of both temporary and permanent erosion control protection was required on slopes and drainage swales.
Anderson-Johnson Associates Inc. from Minneapolis developed the erosion control plan. Initially, the project was designed with a TRM that combined a long-term biodegradable component with permanent components to satisfy the short- and long-term requirements of the project. However, after closely considering the final grades, the site soils, the desired permanent vegetation, the time of year for seed application, and the probability of overseeding in the spring, an alternative erosion control system was chosen. The final hybrid erosion control system utilized consisted of a combination of Enkamat R2M, a root reinforcement matrix manufactured by Colbond Inc., and Soil Guard, a bonded fiber matrix (BFM) from Mat Inc.
The site initially was rough-graded, then topsoil was spread for the finish grade. Next, the Enkamat TRM was installed, but instead of the traditional deployment parallel down the slope face, the contractor opted to install the Enkamat perpendicular to the slope face, from the shoreline of the pond, and proceed toward the top with a layered installation. In the contractor’s experience, this installation procedure was the most effective for keeping runoff on top of the TRM to prevent rills from occurring under the mat-a common result with poorly installed erosion protection. Trenches were dug along the top of the slope, but instead of acting as anchor trenches, they were simply a means of moving the runoff on top of the Enkamat.
Six-inch-long U-shaped staples were applied to keep the TRM in place, but a few extra staples were used in specific spots to ensure intimate contact with the ground. About 11,000 square yards of Enkamat were applied to the retention pond area on the school grounds.
Afterward, the contractor hydraulically applied seed and Soil Guard, together with fertilizer, directly into the Enkamat. The purpose of the Soil Guard was to enhance seed germination by retaining moisture and temperature, while the Enkamat TRM provided the permanent root reinforcement necessary to ensure mature plant growth long after the emergence of the initial vegetation. The three-dimensional open structure of the Enkamat allowed the hydroseed, the fertilizer, and the mulch of the Soil Guard BFM to penetrate and contact the soil, a vital element in achieving the desired seed germination and protection. Establishing vegetation with the use of a BFM reduces the likelihood of tenting, which can occur when vegetation germinates, but then is inhibited by the closely entangled (natural or synthetic) fibers of some TRM products, causing the mat to be lifted from contact with the surface.
The seeding and installation work took place in the fall, and by spring, the vegetation that had been established was sufficiently healthy that, contrary to the initial expectations, no overseeding was required.
Central Texas Turnpike Project
A high-profile, $1.3 billion road construction project known as the Central Texas Turnpike was under way. Along two segments of the privately financed roadway, however, it was proving extremely difficult to establish vegetation. Drought conditions, poor soils, and sporadic, damaging rainstorms resulted in slope revegetation being far behind schedule.
By late summer 2006, it was clear that aggressive measures were needed to establish sufficient cover to ensure that these two segments-totaling about 20 miles-could be opened for traffic before the end of the year. There had been heavy runoff, and the soil environment ranged from rocks and shale to sandy clay. These conditions made it difficult and time consuming for the reclamation contractors, ABC Erosion Control and C-3 Environmental Services, to achieve the optimum grade conditions needed to use rolled blanket-type materials for growth establishment.
A recommendation was made to use Flexterra flexible growth medium from Profile Products, based on experience showing the product had the ability to establish growth under the existing challenge of drought conditions while protecting against the heavy rainstorms common to the area.
With greater than 99% effectiveness in controlling soil loss, in some instances Flexterra may outperform erosion control blankets and BFM products. It creates a bond with the soil surface to form a continuous, porous, and erosion-resistant blanket that allows for rapid germination and accelerated plant growth.
Flexterra also absorbs and holds 15 times its weight in water, delivering moisture to the seedbed for fast vegetation establishment.
In August 2006, Flexterra was applied to two test segments. It was sprayed on a Thursday, and millet was coming up by the following Monday. Dan Corrigan of C-3 Environmental Services reported at the time, “The demonstration test we did was on a steep (2H:1V) grade. We shot about 5,000 square feet with the Flexterra, and we had vegetation within three days. I think the speed is due to the fact that the Flexterra bonds to the ground.”
After the success on the two test segments, the contractor applied Flexterra to a total of approximately 80 acres, using it for those areas where it would be most difficult to hold the topsoil and establish vegetation.
For the ditches, the contractor chose Profile Products’ GreenArmor System, which combines an Enkamat TRM with Flexterra to create a system that protects against elevated levels of hydraulic lift and shear forces on slopes and in channels. The Enkamat component is a three-dimensional TRM made of polyamide (nylon) filaments. Because the matrix consists of 95% open space, it readily accepts the hydraulically applied Flexterra infill. As the roots grow, they become entwined within the Enkamat matrix, creating an extremely stable cover. The GreenArmor System is often viewed as a replacement for hard armor solutions, and at a fraction of the cost.
The Flexterra and GreenArmor System applications proved successful. One segment of the new highway was able to open on October 1, while the second segment opened December 10, meeting the goal of a year-end completion.
