A couple of time-proven products used in erosion control are somewhat temporary erosion control blankets (ECBs) and longer-lasting turf reinforcing mats (TRMs)-collectively known as “rolled erosion control products” (RECPs). ECBs are used to help establish the growth of vegetation-the best material for erosion control-before degrading, while TRMs supplement vegetation on a more permanent basis in environments in which it is more difficult or even impossible for vegetation to grow.
The main difference between the two main types of RECPs is the environment in which they are installed. ECBs can be used on slopes, but typically on ones that are not as steep. Vegetation grows more easily on these slopes, and stormwater runoff flow, which can uproot vegetation, is not as fast as on steeper slopes. TRMs are considered “soft armor” for channels and streambanks, one step in permanence below “hard-armor” structures like riprap or retaining walls.
Erosion Pros LLC, an Auburn, AL, general contractor that is licensed to do erosion control work in Alabama, uses ECBs and TRMs alike. Joel Seawell, founding partner, says that choosing among the different classes of RECPs depends on a couple of main variables. “It depends on the region you’re in, and it has to do with the intensity of the rainfall in that area and what kinds of shear stresses you’re up against in channel applications,” he says. The company uses American Excelsior’s RECPs and determines which type is appropriate via the manufacturer’s ErosionWorks software. “It also depends on the region’s soils, topography, and what type of vegetation you’re looking for to hold the soil in place.”
Several industry experts who spoke with Erosion Control say that variables affecting RECP use include slope, runoff flow rate, rate of RECP degradation, eco-friendliness, and RECP accessories.
Blankets: Temporary Vegetation Replacement
ECBs temporarily replace vegetation until that natural soil reinforcement matures to the point where it is functional. ECBs normally are designed to last a few months up to a few years.
Jill Pack, CPESC, manager of technical services for North American Green, argues that knowledge of how long the ECB will last before degrading-and how long it will take the vegetation to become functional erosion control material-is extremely important. Design factors that dictate ECB longevity include the types of fibers used and the type and number of nettings, she adds.
Kurt Kelsey, M.S., CPESC, CPSWQ, director of technical services for American Excelsior, says that choice of ECB fiber type depends on the type of terrain on which the ECB is deployed. Slope steepness, length, and soil type are the primary factors that influence the type of RECP used on slopes, according to Kelsey. Curled and barbed wood excelsior blankets naturally anchor to slopes as the fibers expand and contract with wetting and drying. Conversely, straight, flat straw fibers do not expand and contract when wetted and tend to lie flat on slopes, and the fibers can be moved during hydraulic events. Water depth and channel bed slope are the primary factors that influence the type of RECP used in channels, he adds. If the design shear stress for the channel is less than the permissible shear stress capabilities of the vegetation that will become established in the channel, ECBs can be used in the channel. As with all RECPs, cost increases with the functional design life of ECBs, Kelsey says.
Seawell agrees that RECP costs increase with product longevity and adds that it can be a challenge to sell engineers on the higher cost of the materials. “It all boils down to compliance,” he says. “It’s all going to weigh in on what needs to happen based on four factors: your climate, soils, cover, and the topography that you’re required to achieve. If you’ve got a 40-foot or 50-foot-long 2:1 fill slope, you don’t have a choice-if the engineer chooses to utilize that type of grading and they want to establish vegetation, you’re going to need to utilize some kind of rolled erosion control product. It’s based on critical areas-are you above a wetland, above a stream? That would certainly be a call for maybe a little more reinforcement to make sure you have good cover while establishing vegetation.”
Calista Rohini Santha, Ph.D., president of RoLanka International Inc., places ECBs into three categories based on their longevity:
- Temporary erosion control mats made of 100% biodegradable material such as jute last six to 12 months. A product such as RoLanka’s StrawMat, which has photodegradable synthetic netting, is not recommended for use in streambanks because, she points out, they act like fishing nets and cause harm to fish and other aquatic organisms.
- Extended temporary erosion control mats such as RoLanka’s stitched coir mats, which are available with either synthetic or organic nets, last 12 to 24 months.
- Semi-permanent mats like RoLanka’s fully biodegradable woven coir mats last two to six years. With a “bristle” design, these are said to be safer for wildlife.
“What we’re trying to achieve is water quality, to ensure that we’re not impacting beneficial uses of the waters of the United States,” says Seawell. “Turf reinforcement matting has a lot of synthetic material; that being said, the benefit from a longevity standpoint is that the synthetics will give you the strength you need for quality erosion control over a longer period of time, depending on the situation on the site. There are other ways to utilize erosion control products using natural approaches. We’re preventing erosion, but at the same time, environmentalists might have a problem with the plastic that’s going to be in the ground, which is understandable.
“As far as elevations on the site, if we’re down in a wetland stream buffer, more than likely we wouldn’t recommend synthetics in that scenario,” Seawell continues. “The benefits from the synthetics would be at a higher elevation on the site in an area that’s already going to be impacted by construction activity. If you’re dealing with natural buffers and riparian buffer zones and stream restoration work, you’re going to need to stay away from synthetic and metal staples that you’d use on slopes-it all depends on the permitting.”
The experts generally agree that vegetation seeding and ECPs should remain separate. Pack notes that pre-seeded ECBs were developed in the past with the goal of allowing ECB installation and seeding in one step. Since then, she reports, the availability of these products has diminished due to price and low germination and establishment rates. Kelsey points out a limitation of pre-seeded ECBs: their highly regional characteristics based on the seed mixture. According to Santha, a high seed germination rate is essential for erosion control projects, and the percentage of seed germination indicated by testing typically is effective for only six months, meaning that pre-seeded ECBs have a limited effective shelf life.
American Excelsior’s Curlex CL Blankets utilize the company’s Curlex fibers, which are designed to expand and contract when wet and cling to each other when combined with “barbed” fibers. Curlex CL is a lighter version of standard Curlex that is positioned to substitute for straight-lined fiber blankets such as straw. The curled fiber matrix is designed to slow the velocity of water flow and allow moisture to slowly seep into the topsoil. One version has green color-coded plastic netting for applications requiring UV resistance and added strength, and a photodegradable QuickMow netting is color-coded white to identify it as a rapid-breakdown polypropylene netting designed for use in areas to be mowed, such as golf courses and certain roadside projects.
North American Green’s short-term photodegradable ECBs are manufactured with a functional longevity from 45 days up to 12 months and use an evenly distributed layer of 100% agricultural straw stitched with degradable thread to a single or double lightweight polypropylene netting structure. They are intended to be used in moderately sloping areas and low-flow channels. The company’s extended-term and long-term photodegradable ECBs include a long-lasting coconut (coir) fiber component stitched with degradable thread into a heavyweight polypropylene netting structure and have a 36-month design life. The latter types are designed for use in steep slopes, medium- to high-flow channels, and shorelines.
Another product manufactured by North American Green, BioNet 100% biodegradable ECB, is composed of 100% organic materials that biodegrade completely; it is not designed to degrade from sunlight. According to the manufacturer, it is suited for applications where synthetic materials may pose a threat to animals or the environment, such as wetland mitigation and bioengineering projects. Using a dense mulch layer stitched with biodegradable thread to either one or two jute nets, the netting has interwoven strands that move independently of each other to reduce the risk of accidental wildlife entrapment.
A More Permanent Solution
In contrast to ECBs, TRMs are not designed to degrade once installed. Pack notes that TRMs are permanent, non-degradable RECPs that are composed of UV-stabilized components. They offer immediate erosion control in addition to long-term vegetation reinforcement and suit situations where once hard-armor solutions were the only choice. In regard to erosion control in channels, Kelsey points out that when the design shear stress for the channel will exceed the permissible shear stress capabilities of vegetation to become established, TRMs are required. They permanently reinforce the vegetation via stem and/or root reinforcement so that the vegetation does not erode during hydraulic events, he adds.
Santha argues that, due to their synthetic materials, TRMs can threaten wildlife that potentially can get caught in the netting. Eco-friendly alternatives to TRMs are semi-permanent woven bristle coir mats, which are less expensive and completely biodegradable, she adds.
TRMs from North American Green utilize a permanent, non-degradable three-dimensional matting structure and either 100% synthetic components or a combination of synthetic and natural materials. The permanent matting structure is designed to reinforce vegetation against damage and extraction under high-shear-stress water flow, and the matrix filler material is designed to provide immediate to long-term erosion control and mulching to help establish vegetation. The mats do not require soil filling, which typically would necessitate the addition of an ECB over the TRM and soil layer. The company’s Vmax3 Composite Reinforcement series consists of three types of mats. All have a permanent three-dimensional corrugated turf reinforcement matting structure that anchors and reinforces vegetation and is designed to create a shear plane that deflects water flow away from the soil surface. Either organic or UV-stabilized fibers are incorporated into the matting structure to supplement the TRM’s ground-covering and moisture-retention properties.
Recyclex TRMs from American Excelsior are manufactured from recycled post-consumer goods such as green or brown plastic bottles; the company claims that every pound of the TRMs contains 20 recycled bottles. According to the company, 80% or more of the fibers are 5 inches in length or greater. The fibers are crimped to create an interlocking fiber matrix, conform to the terrain, and divert and slow the water flow along the curled fiber matrix. In turn, water flow velocity is reduced. The fibers are stitched together with two layers of UV-resistant polypropylene netting to form a three-dimensional matrix. The mats can be placed over grass seed and topsoil or filled and seeded.
Variety of BMPs, Accessories Used
Several best management practices (BMPs) used along with the installation of RECPs are intended to accelerate vegetation growth and give it more staying power.
Pack lists several examples. Hydroseeding often is used as a means to apply seed quickly and uniformly prior to RECP installation. Seeded hydromulch can be applied prior to installation as a means to apply seed and enhance vegetation establishment. High-performance hydromulches may use tackifiers that bind soil fines.
RECP accessory products are closely related and help preserve vegetation and keep RECPs in place to enhance their effectiveness.
According to Kelsey, sediment retention fiber rolls such as American Excelsior’s sediment logs and straw wattles are commonly used in conjunction with RECPs. The fiber rolls can help filter stormwater runoff or dissipate runoff velocity. Porous excelsior sediment logs work better at filtering stormwater because they actually allow water to flow through them, whereas denser fiber rolls only dissipate runoff velocity. Another function that fiber rolls may serve in an erosion and sediment control plan is to break up long slopes.
Occasionally, designers may call for hydroseeding before or after the installation of RECPs. Examples of natural accessories are plugs, which are commonly used in conjunction with RECPs for streambank stabilization projects, Kelsey notes. For example, willow trees are commonly plugged into RECPs along streambanks because they naturally grow well in wet conditions and typically assist effectively in stabilizing streambanks.
Pack adds that other types of accessories-sediment-control devices-are installed over the top of the RECP to capture sediment that may flow onto the project site, or to act as slope interruption devices on long slopes. Hard armor is another type of accessory. Examples are rock that can be placed as toe protection for RECPs installed along streambanks and shorelines, and rock check dams that are sometimes installed over RECPs in vegetated swales. Santha points out that natural vegetated methods are the best choices for erosion control. But when these systems alone cannot provide erosion resistance, she adds, all hard-armor applications can be altered to be environmentally friendly using suitable coir products and vegetation.
“When you’re above a critical area, [RECPs] can be utilized as a temporary measure, especially the lower-cost blankets,” says Seawell. “Down here in Alabama, with a road or a bridge, you’re going to be required to maintain the vegetative cover. There are other issues-seed bed prep, soil amendments-and on steep or long slopes, slope interrupter mechanisms or wattles can be used to break the length of the slope. Diverting flow away from steep slopes is another good practice, capturing water and diverting it to a stabilized outlet rather than allowing a significant area of drainage to run over a slope.”
E-Staples from American Excelsior are a new biodegradable anchoring staple system. They can be used with the Curlex NetFree system for a completely biodegradable project. The staples biodegrade with exposure to bacteria in accordance with ASTM 5338 and ASTM 5271 and have barbed shoulders and heads for holding capability.
SedimentStop is a biodegradable sediment filtration system from North American Green that consists of straw and coir fiber reinforced with a completely biodegradable netting that is rolled from edge to edge to create a temporary, water-permeable sediment filtration structure. The system is designed to reduce soil loss caused by stormwater runoff as it traps soil particles while allowing water to pass through. Net-reinforced layers prevent failures if the outer netting wrap is damaged after installation. The system is configured for flexibility and conformity to the ground surface to minimize undercutting. A Splash Apron attachment is designed to improve sediment filtration while reducing potential downhill erosion.
North American Green also has several fasteners for blankets and mats. The company’s rigid, biodegradable BioStakes, available in 4- and 6-inch lengths, are an alternative to metal staples. The company also provides wooden EcoStakes in 6-, 12-, 18-, and 24-inch lengths.
BioD-Roll coir rolls from RoLanka are made from coir fiber densely packed into a tubular 2-inch-by-2-inch outer netting and provide initial structural stability for shorelines and streambanks by resisting wave action and flow velocity. The coconut-fiber core is engineered to provide an environment for plant growth. Over time, sediment is deposited around the coir rolls, providing an environment for riparian vegetation. The company says that the product lasts for more than five years. It is available in 12-, 16-, and 20-inch diameters and in densities of 9 or 7 pounds per cubic foot. The manufacturer also offers BioD-Block, a fabric-attached coir fiber block system for slope stabilization, streambank, and shoreline restoration and stream relocations. It utilizes a densely packed, elongated mattress, coir fiber block attached to a bristle coir woven fabric. Coir fabric is tightly wrapped around two block sizes: 12 inches tall and 5 inches thick, or 16 inches tall and 9 inches thick. The fabric is connected to the coir block on three sides, leaving one side open to fill with dirt.
Maintenance Concerns
The potential for maintenance to disrupt the effectiveness of RECPs is a concern in regard to these products.
Mark Deitering, partner with Enviroscape ECM Ltd., says that if a blanket is stapled properly, the potential for vegetation lifting the blanket is minimal. RECP-reinforced grass can be mowed as if the area was seeded and covered with straw. Maintenance issues usually arise with the thread and not the netting, he concludes. Kelsey says that the proper use-in terms of patterns and quantities-of anchoring devices such as turf staples allows vegetation to grow up through the ECB while contact with the subgrade is maintained. Areas that require early mowing typically utilize netless ECBs or ECBs with rapid-degrade netting, Kelsey adds. Biodegradable staples are typically safer than conventional steel wire staples, because if they are struck by a mower, they will simply break apart rather than potentially being projected from underneath the mower.
Pack points out that since RECPs are installed at the ground surface, mowers generally do not come into contact with them. Fasteners such as staples and stakes used to install RECPs should be selected based on soil conditions, matting, and required performance, as well as on planned site use. Fasteners that install flush to the ground level should have no adverse safety issues, Pack adds.
“The main thing is to ensure proper installation in accordance with the manufacturer’s recommendations and ensure that the installers utilize the appropriate material that’s been specified in the plans,” says Seawell. “There are so many different variables with blankets, but as far as allowing erosion under the blankets, that will occur without your seedbed prep, implementing tracking on slopes by running the bulldozer up the slopes, and just making sure that they’re installed properly.”
Knowledge-Enhancing Resources
Contractors and engineers can enhance their knowledge of RECPs by using several sources, including the Erosion Control Technology Council (ECTC) Quality Data Oversight and Review (QDOR) program, www.qdor.org, and ErosionLab, www.erosionlab.com, a large-scale erosion and sediment control research and testing laboratory in Rice Lake, WI.
The QDOR Guidance Manual provides requirements for RECPs based upon standards such as AASHTO-NTPEP, ASTM D 6459, and ASTM D6460. The program is designed to assist government agencies, engineers, specifiers and inspectors who use rolled erosion control products effectively in their erosion control and stormwater projects. Products bearing the QDOR seal have been manufactured and tested according to industry standards, similarly to the Underwriters Laboratory (UL) label. The guidance manual describes the program.
All products submitted into the QDOR program must have data verified by the laboratory conducting the testing and the data are compiled with an online data-collection system. Approved products are allowed to use the official QDOR logo on the products and product- specific literature and packaging materials. Requirements will be upgraded and changed as RECP technology and the erosion control industry evolve.
The stated goal of ErosionLab is advancement of the industry and, to that end, it hosts several educational tours each year, notes Kelsey. Other activities include testing of erosion and sediment control products; research and development processes; and the development of technical papers, presentations, and test reports.
The lab has three primary facilities: the Rainfall Erosion Facility, the Channel Erosion Research Facility, and the Sediment Control Facility. Numerous research projects have been undertaken as well, including a study of the movement of available nitrogen in soils covered with erosion control products and soils that are unprotected, and a vegetation study that yielded vegetation density, total biomass production, and soil moisture enhancement ability for ECBs.
