Turf reinforcement mats offer an aesthetically pleasing and environmentally friendly solution to the problem of shear stress erosion. Shear stress erosion is the result of high-velocity water flow moving parallel to the plane of a surface. These stresses hinder vegetation growth and accelerate soil loss.
TRMs are stitched three-dimensional matrices and thermally-fused polymeric matrices made from non-degradable geosynthetics such as polypropylene, nylon or polyvinyl chloride netting or a composite of degradable and non-degradable materials. These products create a system resistant to high shear stresses and reinforce vegetation root structure as it grows and entangles with the matting’s matrix. As one of four rolled erosion control products, TRMs provide permanent erosion control in channels, ditches or other high-flow applications where erosive forces exceed the limits of natural vegetation. (See sidebar on page 44 for information about the other three RECPs).
“Once you put a TRM down you won’t need to remove it,” says Jill Pack, technical representative, North American Green. “Sometimes you may have to go back and perform maintenance work, but only if you are cutting vegetation or the product was damaged in some way. I have seen projects that look as good as when they were put down.”
According to the Environmental Protection Agency, TRMs are considered “soft engineering techniques” as opposed to the hard armoring use of concrete blocks, concrete paving, fiber-reinforced concrete paving or rock riprap. While these hard techniques present the highest level of erosion control, they take more labor and time to install and lack the filtering qualities of TRMs.
Typically TRMs withstand shear stresses of up to 12 pounds per square foot, but some manufacturers produce higher tolerance products. The Pyramat TRM by Propex, for example, is a high performance turf reinforcement mat designed to endure shear stresses higher than 15 pounds per square foot. North American Green’s VMax3 tolerates shear stresses of up to 14 pounds per square foot. (Results in the field may vary.)
There are also composite TRMs, which add a degradable material – such as a hydraulic mulch or wood excelsior – to the synthetic matrix. By the time it degrades, vegetation is well established.
Advantages of TRMs
Two advantages TRMs have over hard armoring techniques are cost and ease of installation. Hard armor costs $20 to $50 per square yard to install while TRMs cost $5 to $15 per square yard. Riprap installation requires the excavation of a bed, lining it with a geotextile and filling it with two or more layers of rock. This entire process – aside from installing geotextiles – requires heavy equipment.
According to the Erosion Control Technology Council, after riprap is installed, it can’t be safely mowed with riding equipment and must be maintained with weed-eaters or pesticide sprayers to eliminate weed growth. Conventional mowing equipment works best with TRM-reinforced channel linings and litter removal can be done by hand or with a power rake where heavy litter exists. Pack says that riprap poses a risk for injury in residential or commercial areas and along roadsides where riprap linings become hazards for pedestrians.
TRMs can be installed by hand and take little training to put them in place. (See sidebar on page 48 for installation instructions). Once the site is prepared, the TRM is rolled out over the ground, filled with soil, seeded and fertilized. This protects the seed bed until vegetation is established and allows it to grow into the permanent matrix.
Pack points out that TRMs don’t leave standing water and produce a natural, green environment. “Aesthetically, they’re just more pleasing,” she says. “In residential and commercial areas, it’s nicer to see green as opposed to rock or concrete.”
TRMs combined with the vegetation also provide a natural filter.
“When you have water running through vegetation it may contain sediment and pollutants such as fertilizers, herbicides, heavy metals and oils,” says Marc S. Theisen, director of business development for Profile Products. “The vegetation acts as a filter and trap for these contaminants before they are consumed by microbial activity.”
“In comparison, concrete captures 50 percent of the sediment from water flow and riprap performs at 75 percent,” says Melissa Chastney, regional manager, Propex. “A TRM provides twice the erosion protection of vegetation alone.”
But on the other hand…TRMs have some drawbacks.
While they are less expensive than hard armor techniques, they typically have less ability to handle shear stresses higher than 15 pounds per square foot or sites with constant water flow.
Sites with constant wave attack also reduce the effectiveness of TRMs, which aren’t designed to withstand wave heights of more than a foot. Theisen says high waves nip away vegetation with a “piranha effect.” “In coastal regions TRMs can be used as a transition zone,” he says, “but you don’t want to place them where there are direct wave attacks. In those instances, hard armor structures are best.”
TRMs also have problems in rocky or arid soil environments because anchoring becomes difficult. If no vegetation establishment exists, then ultraviolet degradation from direct sunlight will break TRMs down over time.
“You need vegetation for the TRMs to reach their optimum performance,” Theisen says. “Vegetation is vulnerable in the early days of its life cycle so you have to find ways to establish it so it can help absorb water back into the ground.”
Choices, choices, choices
If you don’t take the time to do your homework, finding the right TRM can be confusing. There are numerous TRMs on the market, and it’s difficult to make an appropriate product selection without a bit of help.
“The most important factor is performance,” says Kurt Kelsey, director of technical services, American Excelsior. “The TRM must withstand the shear conditions of the project. Check the design shear with the project engineer so you save money in product costs.”
Manufacturers provide free online design software programs to help with product selection on a project-by-project basis. (See sidebar on page 46 for more details about software). In addition, some manufacturers offer field support and visits to your site to help recommend the right product.
Other forms of rolled erosion control
Turf reinforcement mats are one of four rolled erosion control products:
Anchor loose fiber mulches such as straw or hay with two-dimensional mulch control netting. They are woven from natural fibers or nettings rolled over seeded and mulched areas and stapled or stacked into place. MCNs provide more erosion control than hydraulically applied mulches. According to the Erosion Control Technology Council, MCNs are best suited for site conditions where other forms of erosion control aren’t necessary.
Open weave textiles form a closely knit two-dimensional matrix with woven or processed polyolefin yarns. The construction of these materials enables them to provide erosion control with or without underlying loose mulch layers. OWTs provide a high-strength solution that reinforces underlay for sod.
Erosion control blankets (ECB) are organic or synthetic fibers woven or glued with nettings or meshes. Made from straw, wood excelsior, coconut, polypropylene or combinations of those fibers stitched or glued together, these blankets roll out over soil and then are anchored with spikes or staples. They work best on sites that need durable erosion protection and on applications such as gradual to steep slopes, low to moderate flow channels and low-impact shore linings. ECBs are degradable and temporary and generally limited to areas where natural, unreinforced vegetation alone can’t provide long-term soil stabilization.
Many companies manufacturing turf reinforcement mats also offer software to help determine the right product through channel or slope shear stresses calculation.
Once you key in your site condition data, the program runs the hydraulic or soil loss calculations and generates a list of products acceptable for site conditions.
Manufacturers offer this software in several forms, including CDs and Internet downloads.
Design software resources include:
North American Green:
Erosion Control Technology Council:
Installing turf reinforcement mats
The Erosion Control Technology Council website has suggested guidelines for turf reinforcement mat installation. To download the video or order the DVD of detailed RECP installation guidelines, go to www.ectc.org/DVD.asp or contact them at (651) 554-1895.
Installation on a channel:
Step 1: Prepare the site by grading the soil to a smooth profile, removing any gullies, rivulets, stones or sticks. Fill any voids to make sure the soil is compact.
Step 2: Install the TRM by rolling the product out in the direction of the water flow. Start the mat at the center of the channel and work your way up the sides. Try to minimize seams at the bottom and avoid seams in concentrated water flow.
Step 3: Place seed on top of the TRM. Match seed with the right conditions. Spread seeds by hand or hydroseeding machine for optimum germination, root system development, vegetation density and long-term functionality.
Step 4: Place soil completely over the TRM and make sure it’s properly compacted or hydraulically infill the mat with a flexible growth medium. You may want to consider another type of rolled erosion control product to aid in germination.
Step 5: Dig an anchor trench at the beginning of the channel. The trench should be 6 inches wide by 6 inches deep and run perpendicular to the direction of the water flow across the width of the channel. Then place the TRM and start stapling 30 inches upstream of the anchor trench and secure staples every 12 inches. Backfill the trench with soil and make sure it is compacted. As you are rolling out the TRM, walk it down the channel and guide it as you go. Kicking it or letting it roll down unaided may damage the TRM or pull it out of the anchor trench.
Step 6: Secure the TRM with staples every 36 inches down the channel’s length and 24 inches across the width. The higher the water flow, the more staples required to anchor the TRM. Apply staples by hand or with a staple gun.
Step 7: If the matting ends and you have to start rolling another, make sure to handle shingle overlap properly by placing the end of the upstream mat over the beginning of the downstream mat. This is best accomplished with two rows of staples every 4 inches in a staggered pattern across the width of the mat. This technique is known as a check slot. There should be no more than 4 inches of overlap.
Step 8: Lay the TRM horizontally across the face of channel outlet, overlapping the matting over the trench at its base. Then cut an opening and secure the mat around the opening of the outlet and secure.
Step 9: Install check slots every 25 inches down the length of the channel. This is important because they keep water flow from getting underneath the TRMs, causing further erosion. Once you have secured the length, secure the terminal end of the channel with a check slot.
Installation on a slope or shoreline:
The steps for installing TRMs on a slope or shoreline are the same as channel installation, with the exception of a few key steps. During slope installation, it is important to dig an anchor trench 6 inches wide and 6 inches deep at the top of the slope to hold the TRM in place. This is also important if there is the possibility of water flow from an upland source. Secure the TRM with one staple every 3 to 5 feet vertically, staggered 18 to 24 inches horizontally. Anchor the TRM 2 feet past the foot of the slope or up to vegetation at the end of the slope. Use a check slot to secure the toe anchoring.
For shorelines, lower the waterline if necessary, install and secure the TRM and dig an anchor trench to handle any upland water flow. At the toe anchoring, dig a trench 6 inches wide and 12 inches deep and secure the matting. The toe anchoring may be located beneath the water line so it is important to plan accordingly. Once the toe anchor is in place, secure with one staple every foot and backfill.
Once installed, TRMs require little maintenance, but it is a good idea to water vegetation as needed and inspect the matting for rill or gully erosion until vegetation is fully established.