Erosion prevention practices - turf reinforcement mats
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Contents
Turf Reinforcement Mats
Definition
Turf reinforcement mats (TRMs) are synthetic, non-degradable soil and seedbed covers of variable thickness designed to provide short-term protection against raindrop and wind erosion, permanent support for vegetation on slopes, and permanent armoring and vegetation support for ditches, swales, and channels. They are composed of UV stabilized, synthetic fibers, filaments, nettings and/or wire mesh that are processed into three dimensional reinforcement matrices – a design that serves permanent and critical applications where site conditions exceed the limits of mature natural vegetation. TRMs provide sufficient thickness, strength and void space to permit soil filling and/or retention and the development of vegetation within the matrix. TRM manufactures are also providing flexible growth media integrated into the mat.
Purpose and Function
TRMs, like erosion control blankets, are part of the larger group of rolled erosion control products. TRMs are used to provide temporary cover for bare soil, long-term support for vegetation, and permanent armoring against shear stress caused by flowing water. TRMs share many attributes with erosion control blankets, but are non-degradable, erosion protection aids. After installation, vegetation and soil provide shielding from the sun and the elements, preserving the synthetic components and maintaining the structural integrity of TRMs.
Applicability
TRMs can be used to prevent erosion and support vegetation on a wide variety of site slope and drainage features. They are typically used in conjunction with grass and other seed on steep slopes, in higher velocity ditches and channels, along shorelines, and for scour prevention and armoring at culvert outlets. Some of the thinner TRMs are designed to be placed directly over seeded bare soil areas, while some thicker open-celled/open weave products may be staked down first, then seeded, and then covered with a thin layer of topsoil. Because of the many different types of products, manufacturer’s specifications regarding use, installation, anchoring device selection, and maintenance must be followed precisely.
Site Applicability
TRMs are used to support permanent vegetation on longer, steeper slopes (e.g., more than 100 feet and 3H:1V) and in steeper, higher velocity ditches and channels (e.g., more than 10H:1V; velocities of up to 15 feet per second, and shear stress of up to 10 lbs per square foot). They are typically used when slope and channel conditions exceed the capabilities (e.g., manufacturer’s requirements) of erosion control blankets, but are not severe enough to justify terracing or retaining walls in slope applications, or harder armoring (e.g., articulated block, riprap, pavement, etc.) within channels.
Permit Applicability
As noted above, TRMs provide permanent support for vegetation on slopes, and permanent armoring for vegetated ditches, swales, and channels. They also provide protection against raindrop and wind erosion during the weeks between seeding and plant emergence. As such, TRMs are an integral part of the site’s permanent cover, which is defined in Appendix B of the MPCA Construction General Permit as “surface types that will prevent soil failure under erosive conditions.” The permit defines a uniform perennial vegetative cover as “evenly distributed, without large bare areas” and “with a density of 70 percent of the native background vegetative cover for the area,” which “must be established on all unpaved areas and areas not covered by permanent structures” in order to terminate permit coverage.
Besides supporting vegetation on bare areas and in ditches, TRMs are often used to construct permanent stormwater management systems, which are addressed in Part III.D of the MPCA permit. Components of permanent stormwater management systems that may be supported by TRMs include infiltration areas, vegetated ditches/channels, sedimentation basins, regional ponds, and vegetated buffers adjacent to surface waters. TRMs are extremely useful in ditch and channel stabilization that are required for certain areas by Part IV.B.4 of the permit, which states that the permittee “must stabilize the normal wetted perimeter of any temporary or permanent drainage ditch or swale that drains water from any portion of the construction site, or diverts water around the site, within 200 lineal feet from the property edge, or from the point of discharge into any surface water. Stabilization of the last 200 lineal feet must be completed within 24 hours after connecting to a surface water or property edge.”
For general site stabilization, Part IV.B.2 of the MPCA Construction General Permit requires that the permitte(s) “must stabilize all exposed soil areas (including stockpiles),” and notes that “(s)tabilization must be initiated immediately to limit soil erosion whenever any construction activity has permanently or temporarily ceased on any portion of the site and will not resume for a period exceeding 14 calendar days.” In addition, the permit requires that “(s)tabilization must be completed no later than 14 calendar days after the construction activity in that portion of the site has temporarily or permanently ceased.”
Effectiveness
When selected, sited, installed, and maintained properly, TRMs are effective in providing short-term protection against raindrop and wind erosion, permanent support for vegetation on slopes, and permanent armoring for vegetated ditches, swales, and channels. They generally reduce sheet, rill, and channel erosion by 90 percent or more. Effectiveness is dependent upon TRM type, surface preparation, installation practices (i.e., soil contact, staking pattern, seeding, etc.), and site conditions (e.g., slopes, soils, rainfall, etc.). Mowing vegetation over TRMs too low after installation (i.e., to the point where the TRM is exposed to sunlight) can reduce TRM effectiveness significantly over the long term and should be avoided. The expected performance for turf reinforcement mats table below summarizes expected performance for an array of typical water quantity and quality target constituents for TRMs.
Expected performance for turf reinforcement mats
Link to this table
| Flow attenuation | Little or no design benefit |
| Runoff volume reduction | Little or no design benefit |
| Pollution prevention | |
| Soil erosion | Primary design benefit |
| Sediment control | Little or no design benefit |
| Nutrient loading | Primary design benefit |
| Pollutant removal | |
| Total suspended solids | Primary design benefit |
| Total phosphorus | Primary design benefit |
| Heavy metals | Secondary design benefit |
| Floatables | Little or no design benefit |
| Oil and grease | Little or no design benefit |
Planning Considerations
TRMs are designed and fabricated to address specific site conditions, such as slope range and length, water velocity, and shear stress, UV exposure, seed type, and post-installation soil fill (see Table 5 2 for example specifications). As such, it is critical to observe the manufacturer’s requirements when selecting, siting, installing, and maintaining a TRM to achieve optimal performance. For example, TRM installation procedures may include applying the mat over bare seeded ground, or installing the TRM over the bare ground first followed by seed application and a topsoil cover over the TRM. Some applications specify for the seed to be mixed with the fill soil prior to spreading. In cases where a TRM is installed prior to seeding and backfilling with soil, some applications may require the installation of an erosion control blanket to protect the seed and soil within the three-dimensional matrix of the mat during the time between seeding and establishment of dense growth.
Regardless of the order required for mat installation, seeding, and topsoil cover, the need for proper preparation of the bare soil area or channel remains constant:
- Plan final surface preparation, seeding, and mat installation during dry periods.
- Assemble mats, anchors, dry topsoil cover, seed, and other needed materials.
- Establish final grade for the ditch, channel, swale, or bare ground to be seeded and protected.
- Test soil and adjust pH and fertility according to seed needs; avoid fertilizing ditches/channels.
- Remove all rocks larger than 2 inches and all sticks, limbs, protruding roots, and other debris.
- Ensure all surfaces are smooth and consistent, with a crumbly texture; rake rough spots as needed.
- Install seed, TRM, anchors, and top dressing of fill soil as directed by manufacturer’s instructions.
- Note requirements for using check slots to anchor top-of-slope and in-channel applications.
The current list of MnDOT approved/qualified products for TRMs can be found on the MnDOT website: http://www.dot.state.mn.us/products/erosioncontrolandlandscaping/turf-mats.html
Design
TRMs are synthetic, non-degradable mats that are typically buried to add stability to soils. They come in a wide range of designs and are valuable soil stabilizers on slope and channel-lining applications. TRMs are designed to be permanent and often are filled with soil and vegetated when installed. A TRM may have a biodegradable component intermixed with the synthetic portion to aid plant establishment.
Mats should be selected for the expected velocity and shear stress. TRMs with non-degrading, three-dimensional matrices can withstand velocities and shear stress values up to 15 feet per second and 10 lb per square foot, respectively. However, beyond these thresholds, vegetated structures such as articulated block, cable concrete, and cribwalls should be considered. Required minimum thickness and area holding capacity of the TRM should be defined by the manufacturer.
Once finish grade is established, the area should be seeded, the TRM installed and, if appropriate, immediately filled with topsoil. The finish surface is normally seeded and covered with an erosion control blanket or hydraulically applied mulch to keep the soil from eroding and aid in germination of a permanent stand of vegetation. Manufacturer’s recommendations should be followed for specific applications.
Some TRMs contain supplemental degradable components; all have a permanent three-dimensional structure with high tensile strength that functions as a matrix for securing plant roots, stems, and soils. TRMs and their constituent vegetation form a continuous composite, which becomes a unified, living mat. The resulting synergism increases root system lateral strength, reducing plant dislodgement under high velocity, high shear stress flows. The TRM’s permanent structure also consolidates and protects the soil in which the plants are anchored, preventing it from being stripped out of the vegetative cover and thus weakening root support.
TRMs are often employed as a “green” alternative to stone riprap, pavement, and other forms of hard armoring. They are typically used in a manner that optimizes plant root interaction with the mat structure. Typical installations involve rolling out and anchoring the TRM in intimate contact with the soil surface.
There are two general methods of TRM installation, depending upon the type of mat used. One method involves directly applying the TRM over a freshly seeded soil surface to allow vegetation to develop upwards through the mat structure. In this scenario, the TRM initially acts to prevent both erosion of the soil’s plant root structure and dislodgement of individual plants from the soil surface. As natural sedimentation processes eventually fill the spaces in the mat, successive stands of vegetation can grow down into and/or through the underlying mat structure for long-term root reinforcement.
The second method for installing TRMs is to first unroll the product, then cover with a mix of fine soil and the prescribed seed mix. In this type of installation, the vegetation immediately roots down into and/or through the matting structure for both immediate and permanent reinforcement.
Specific installation instructions after surface preparation include:
- Seed bare area, if called for by manufacturer’s instructions.
- Unroll from upslope to downslope, overlapping ends of the upslope roll at least 7 inches over the downslope roll. Overlap adjacent sides by at least 4 inches.
- Ensure mats lie flat with good contact on the soil and no bulges; do not stretch TRMs.
- For channels, anchor mats in a 6-inch backfilled slot at the top of the banks.
- Shingle and overlap edges parallel to the water flow by at least 4 inches.
- Overlap edges perpendicular to the water flow by at least 7 inches.
- Use only anchors specified by manufacturer’s instructions.
- For high velocity ditches and channels, space anchors at 1.5 feet below the waterline.
- Anchor every 3 feet on slopes, and every 2 feet below flow lines in ditches and channels.
- Use longer anchors in loose soil; use degradable stakes instead of metal staples near runways.
- For slopes steeper than 2H:1V and longer than 100 feet, and for ditches with slopes greater than 10H:1V, use 6 inch check slots every 100 feet on the contour / across the channel.
- Spread seed and/or seed and soil mix over installed mat, if applicable.
If soil cover is applied, use a sandy clay loam, topsoil, or other appropriate media as directed by the manufacturer’s instructions. Use only dry soil, which can be spread easily. Using a flat tool or the backside of a rake, lightly spread no more than ½ to ¾ inch of soil onto the mat, completely filling the voids. If equipment must operate on the mat, use rubber-tired types only (i.e., no treaded or tracked equipment). Do not make sharp turns and avoid rapid movements that cause the mat to bunch up. Keep equipment off mats if the soil is wet. Apply the soil cover until only the top of the mat is exposed – do not cover mat completely. Broadcast additional seed and apply a degradable erosion control blanket over the soil-filled blanket if required.
U-shaped wire staples or metal geotextile pins can be used to anchor mats to the ground surface. Wire staples should be 8 gauge or thicker; metal pins should be at least 0.2 inches diameter steel with a 1.2 inch stall washer. Degradable stakes can be used on flatter areas, and where metal or other anchor types might pose problems if accidently dislodged (e.g., near airport runways). Otherwise wire staples, metal pins with washers, or percussion driven anchors are recommended for slopes, bank areas, and channel applications. Use 6-inch anchors for rocky or clayey soils, 12-inch anchors for looser clays and silty soils, and 18- or 24-inch anchors for sandy silts and loose sands. Follow the manufacturer’s instructions for anchor spacing and configuration, with at least one anchor per square yard on flatter ground, 2 anchors per square yard on steep slopes, and 3 anchors per square yard in channels and on shorelines. Drive anchors in until they are flush with the top of the mat – do not countersink or bury.
Erosion Control Technology Council standard specifications for permanent turf reinforcement mats
Link to this table
| Type1 | Product Description | Material Composition | Minimum Tensile Strength2,3 | Minimum Thickness
(ASTM D 6525) |
UV Stability
(ASTM D 4355 @ 500 Hours) |
Channel Applications
Permissible Shear Stress4, 5 |
|---|---|---|---|---|---|---|
| 5.A | Turf Reinforcement Mat | Long term, non-degradable rolled erosion control product composed of UV stabilized, non-degradable, synthetic fibers, filaments, nettings and/or wire mesh processed into three dimensional reinforcement matrices designed for permanent and critical hydraulic applications where design discharges exert velocities and shear stresses that exceed the limits of mature, natural vegetation. Turf reinforcement mats provide sufficient thickness, strength and void space to permit soil filling and/or retention and the development of vegetation within the matrix. | 125 lbs/ft
(1.82 kN/m) |
0.25 inches
(6.35 mm) |
80% | = 6.0 lbs/ft2 (288 Pa) |
| 5.B | Turf Reinforcement Mat | 150 lbs/ft
(2.19 kN/m) |
0.25 inches
(6.35 mm) |
80% | = 8.0 lbs/ft2 (384 Pa) | |
| 5.C | Turf Reinforcement Mat | 175 lbs/ft
(2.55 kN/m) |
0.25 inches
(6.35 mm) |
80% | = 10.0 lbs/ft2 (480 Pa) |
1For applications in channels and on slopes not exceeding 0.5:1 (H:V) where vegetation alone will not sustain expected flow conditions and/or provide sufficient long-term erosion protection. For TRMs containing degradable components, all property values must be obtained on the non-degradable portion of the matting alone.
2Minimum Average Roll Values, machine direction only for tensile strength determination using ASTM D6818 (Supersedes Mod. ASTM D5035 for RECPs)
3Field conditions with high loading and/or high survivability requirements may warrant the use of a TRM with a tensile strength of 44 kN/m (3,000 lb/ft) or greater.
4Shear stress that fully vegetated TRM can sustain without physical damage or excess erosion > 12.7 mm (0.5 in.) soil loss] during a 30-minute flow event in large scale testing.
5Acceptable large-scale testing protocol may include ASTM D6460 or other independent testing deemed acceptable by the engineer.