Erosion prevention practices - turf reinforcement mats

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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

Water Quantity
Flow attenuation	Little or no design benefit
Runoff volume reduction	Little or no design benefit
Water Quality
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.