Turf Reinforcement Mattings
An EPA-recognized stormwater BMP
By
Roy J Nelsen,
Kurt Chirbas,
Mark Myrowich
Geological erosion is a natural event resulting from the detachment and transportation of soil particles by water. Accelerated erosion, caused by removing vegetation and cover during construction, results in drastically increased rates of soil loss. Pollutants, such as fertilizers, pesticides, and petroleum products, that are attached to eroded sediments can be transported. The EPA has listed erosion as the single largest impairment to water quality in rivers.
Historically, a great deal of time and money has been spent designing and implementing sediment control devices. It has been shown repeatedly that while sediment control is reactive, effective erosion control is proactive keeping soil and associated hazards out of the water in the first place. A good example is the use of rolled erosion control products (RECPs), which reduce detachment of soil particles and therefore reduce sediment load in runoff, diminishing demands and costs associated with sediment control devices.
 |
 |
 |
| Turf reinforcement mats improve the performance of vegetation, allowing the use of "green solutions" to meet a diversity of erosion control needs with aesthetically pleasing results. |
There are a wide variety of RECPs that afford cost-effective solutions to nearly any erosion problem. Varying from temporary erosion control blankets (ECBs) to permanent turf reinforcement mats (TRMs), RECPs in combination with vegetation are sometimes referred to as "green engineering" or "soft armor." The maximum use of these living plant/manmade systems can provide effective temporary and permanent erosion control.
Vegetation offers an excellent form of erosion control and cover (Temple 1980); however, increased concentration of runoff from additional impervious surfaces may result in flows exceeding the vegetation's natural erosion control capabilities. Riprap and concrete have been the traditional "hard armor" materials and were once thought to be the only suitable solutions for lining high-velocity channels and high-shear-stress areas. Before the advent of TRMs, vegetative linings were simply out of the question for these highly erosive conditions where expected velocities exceeded 6.8 ft/sec (2.1 m/sec) (Chow 1959), or where shear stresses exceeded 3.7 lb/ft2 (177 Pascal [Pa]) (Chen and Cotton 1988). However, modern TRMs have the proven ability to substantially increase the erosion resistance of vegetation, enabling its use in areas where high velocities/shear stresses are prevalent (Hewlett et al. 1987, Northcutt 1996).
Design Considerations
Water flowing over a surface causes tractive force or shear stress (units of pounds per square foot, or Pascal) between the water and the soil surface. When shear stress exerted by the flow exceeds the permissible shear stress of the surface material (such as riprap, soil, matting, or vegetation), the movement or loss of these materials is likely, and significant damage can occur to the channel within a few hours. The level of shear stress generated by a flow is dependent on the discharge, depth of flow, slope or energy gradient, surface geometry, hydraulic roughness of the liner, and underlying soil type. Although there are numerous methods used to assess or determine the erosive force of water, these are some of the primary equations and methods:
- Manning's Equation
- Continuity Equation
- Shear Stress Equation
- Revised Universal Soil Loss Equation
It is well documented that during flow, shear stress at a given velocity does in fact predict when erosion will occur, and that shear stress is superior to velocity in predicting this failure. Therefore, the standard hydraulic test to determine performance properties of an erosion control technique is related to permissible shear.
Any erosion control material's permissible shear stress/performance can be established by large-scale research. Typically, in establishing performance values for TRMs, flumes are used that are normally 2–4 feet (0.61–1.2 meters) wide and 15–85 feet (4.5–25.9 meters) long and that can create shear forces of 0.5–20 lb/ft2 (24–960 Pa) for durations of 30 minutes to 60 hours. Several of the laboratories that have been used to establish TRMs' permissible shear stress and design values for RECP manufacturers include those at Utah State University in Logan, and Colorado State University in Fort Collins. Bench-scale testing programs are also available to determine index/physical parameters through laboratories such as TRI in Austin, TX.
 |
| Vegetation provides wildlife habitat. |
Use of Riprap
The purpose of rock or any erosion control system in a channel is to reduce the energy (velocity, shear stress, and/or impact) of water during concentrated flow and to minimize soil displacement. Depending on the diameter of the rock used and its hydraulic roughness, the rock might significantly reduce flow velocities but increase shear stress because of the subsequent increase in flow depth. The use of riprap that is not well graded results in discontinuous contact with the soil, which has shown to be ineffective in minimizing erosion or soil displacement.
A filtering system, such as a woven or nonwoven geotextile, is required beneath the rock in an effort to minimize soil displacement. In this instance the planar fabric is used for erosion control and the rock is generally considered "ballast" for holding the fabric in place. However, if the riprap is not well graded or if there are minimal contact points with the geotextile, the fabric tends to lift from the ground surface due to hydraulic loading. Once the fabric is separated from the soil surface, soil begins to move or be removed.
Typically, riprap gradation design will fall in the range of D100/D50 and D50/D20 with a riprap thickness of 1.5 to 3.0 times the mean riprap diameter. The subscripted number in this riprap nomenclature denotes the percentage of rock that must be at the noted diameter. While rock size is an important consideration, the shape of rock must also be considered in its selection. Because of its interlocking capabilities, angular rock allows greater resistance to flow than rounded stones. More importantly, for geotechnical stability reasons, rounded rock should not be placed on slopes steeper than 3:1 (Chen and Cotton 1988).
What Is a TRM?
TRMs have been used for more than 35 years to improve vegetation's resistance to erosion by increasing the permissible shear of the vegetative cover. The Erosion Control Technology Council (ECTC), an organization with a mission to develop performance standards, uniform testing procedures, and guidance on the application and installation of RECPs, defines TRMs as
a rolled erosion control product composed of non-degradable synthetic fibers, filaments, nets, wire mesh and/or other elements, processed into a permanent, three-dimensional matrix of sufficient thickness. TRMs, which may be supplemented with degradable components, are designed to impart immediate erosion protection, enhance vegetation establishment and provide long-term functionality by permanently reinforcing vegetation during and after maturation. . . . TRMs are typically used in hydraulic applications, such as high flow ditches and channels, steep slopes, stream banks, and shorelines, where erosive forces may exceed the limits of natural, unreinforced vegetation or in areas where limited vegetation establishment is anticipated.
It is important to note that TRMs must contain a permanent three-dimensional structure for stem and root reinforcement and further enhance the vegetation's erosion control performance. Those materials containing only a permanent two-dimensional structure, such as geogrid, do not provide reinforcement but only maintenance of the vegetation's natural performance levels as noted in the CIRIA 116 Report, Design of Reinforced Grass Waterways (Hewlett et al. 1987).
| Performance of Vegetation With and Without TRMs |
 |
| Adapted from "Green Engineering Optimizing Erosion Control with Vegetation and RECPs" (Sprague 1999). |
How TRMs Work
The effectiveness of TRMs has been established by the EPA in its Stormwater Technology Fact Sheet entitled "Turf Reinforcement Mats" to provide an effective, economical, and aesthetically pleasing alternative to hard armor (such as rock riprap) for erosion control in areas where erosive forces exceed the capabilities of naturally occurring vegetation. TRM-reinforced vegetation also affords a corollary benefit—sediment/pollutant removal from stormwater runoff.
Despite variations in materials, TRMs have proven their performance capabilities over the past 35 years in both qualitative field applications and quantitative laboratory research. The vegetation and the permanent three-dimensional structure of TRMs form a symbiotic interaction that provides reinforcement to the stem and root structures. It is important to emphasize, however, that TRMs should not be used under constant high-velocity flow conditions or in any location that will not allow vegetation establishment. TRMs only work in conjunction with vegetation (Sprague 1999). With permissible shear stresses up to 10 lb/ft2 (478 Pa) as noted by the ECTC, geosynthetically reinforced turf can provide similar erosion resistance as 30-inch (0.76-meter ) rock riprap (FHWA 1988).
A secondary and temporary function of TRMs is to collect sediment and act as a barrier to minimize soil displacement during hydraulic flows before and during the time vegetation is being established. The performance provided by TRMs increases as the vegetation becomes established and the vegetation's stem and root structures become entangled within the three-dimensional structure of mattings.
Reinforced Vegetation Benefits
Having discussed the general properties, mechanism, and performance of riprap and TRMs on a permissible shear basis, we now need to discuss the true comparison of using riprap and TRMs. There are many additional advantages and objectives that can be satisfied using a TRM versus standard riprap than simply meeting the permissible shear requirement.
Aesthetics
People have very different viewpoints when it comes to aesthetics, but one thing people living in areas where there are large expanses of hardscapes (buildings, concrete, asphalt) agree on is the need for "green" areas. As our infrastructure continues to expand, hardscapes also continue to increase. Reinforced vegetation offers a softer, natural-looking relief in built-out areas.
Water Quality, Aquatic/Wildlife Habitat
Vegetated areas provide habitat, cover, food sources, and nesting areas for wildlife. They also improve aquatic habitat by reducing thermal pollution and removing sediment and other pollutants from runoff.
Vegetation is an effective form of erosion control because of its hydraulic roughness characteristics and the cover it provides the soil surface. Depending on the vegetation's characteristics that influence hydraulic roughness (height, density, and growth habit), vegetation dissipates energy of flow and therefore results in the slowing of flow. As water slows, its energy is reduced and the water is less able to keep material entrained. Vegetation can also play a key roll in the phytoremediation of various pollutants attached to sediment as it settles out of the flow.
Slowing the flow allows for increased infiltration, groundwater recharge, and natural percolation through the soil profile. As compared to rock or concrete, reinforced vegetation does not heat and maintain thermal radiation and does not transmit heat into runoff that is then conveyed to receiving waters (USEPA 1999).
Although TRMs do not directly improve water quality, they do increase the erosion resistance of vegetation and therefore allow its use in many areas not previously applicable.
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Construction Impact
Riprap is typically placed on a prepared soil surface by large equipment like a front-end loader or an excavator when larger boulders are required. For riprap to be effective erosion control, a well-graded mass with minimum voids is essential. In contrast, TRMs are normally sold in lightweight rolls easily installed by hand without the need for large equipment.
Economic and Performance Comparison
Providing a service that is both functional and economically sound is the basis for the fundamental theory of value-engineering. Through the use of TRM-reinforced vegetation instead of riprap, designers and engineers can achieve effective erosion control while reducing total project costs. Tables 1 and 2 provide approximate comparisons of performance and cost for both types of material. The economic comparison for individual projects may differ according to variations in geographic location, labor rates, and material costs.
Author's Bio: Roy J. Nelsen, CPESC; is a member of the Erosion Control Technology Council.
Author's Bio: Kurt Chirbas, P.E.; is a member of the Erosion Control Technology Council.
Author's Bio: Mark Myrowich, CPESC, is a member of the Erosion Control Technology Council.
March-April 2005
Turf Reinforcement Mattings
An EPA-recognized stormwater BMP
By
Roy J Nelsen,
Kurt Chirbas,
Mark Myrowich
Geological erosion is a natural event resulting from the detachment and transportation of soil particles by water. Accelerated erosion, caused by removing vegetation and cover during construction, results in drastically increased rates of soil loss. Pollutants, such as fertilizers, pesticides, and petroleum products, that are attached to eroded sediments can be transported. The EPA has listed erosion as the single largest impairment to water quality in rivers.
Historically, a great deal of time and money has been spent designing and implementing sediment control devices. It has been shown repeatedly that while sediment control is reactive, effective erosion control is proactive keeping soil and associated hazards out of the water in the first place. A good example is the use of rolled erosion control products (RECPs), which reduce detachment of soil particles and therefore reduce sediment load in runoff, diminishing demands and costs associated with sediment control devices.
|
|
|
| Turf reinforcement mats improve the performance of vegetation, allowing the use of "green solutions" to meet a diversity of erosion control needs with aesthetically pleasing results. |
There are a wide variety of RECPs that afford cost-effective solutions to nearly any erosion problem. Varying from temporary erosion control blankets (ECBs) to permanent turf reinforcement mats (TRMs), RECPs in combination with vegetation are sometimes referred to as "green engineering" or "soft armor." The maximum use of these living plant/manmade systems can provide effective temporary and permanent erosion control.
Vegetation offers an excellent form of erosion control and cover (Temple 1980); however, increased concentration of runoff from additional impervious surfaces may result in flows exceeding the vegetation's natural erosion control capabilities. Riprap and concrete have been the traditional "hard armor" materials and were once thought to be the only suitable solutions for lining high-velocity channels and high-shear-stress areas. Before the advent of TRMs, vegetative linings were simply out of the question for these highly erosive conditions where expected velocities exceeded 6.8 ft/sec (2.1 m/sec) (Chow 1959), or where shear stresses exceeded 3.7 lb/ft2 (177 Pascal [Pa]) (Chen and Cotton 1988). However, modern TRMs have the proven ability to substantially increase the erosion resistance of vegetation, enabling its use in areas where high velocities/shear stresses are prevalent (Hewlett et al. 1987, Northcutt 1996).
Design Considerations
Water flowing over a surface causes tractive force or shear stress (units of pounds per square foot, or Pascal) between the water and the soil surface. When shear stress exerted by the flow exceeds the permissible shear stress of the surface material (such as riprap, soil, matting, or vegetation), the movement or loss of these materials is likely, and significant damage can occur to the channel within a few hours. The level of shear stress generated by a flow is dependent on the discharge, depth of flow, slope or energy gradient, surface geometry, hydraulic roughness of the liner, and underlying soil type. Although there are numerous methods used to assess or determine the erosive force of water, these are some of the primary equations and methods:
- Manning's Equation
- Continuity Equation
- Shear Stress Equation
- Revised Universal Soil Loss Equation
It is well documented that during flow, shear stress at a given velocity does in fact predict when erosion will occur, and that shear stress is superior to velocity in predicting this failure. Therefore, the standard hydraulic test to determine performance properties of an erosion control technique is related to permissible shear.
Any erosion control material's permissible shear stress/performance can be established by large-scale research. Typically, in establishing performance values for TRMs, flumes are used that are normally 2–4 feet (0.61–1.2 meters) wide and 15–85 feet (4.5–25.9 meters) long and that can create shear forces of 0.5–20 lb/ft2 (24–960 Pa) for durations of 30 minutes to 60 hours. Several of the laboratories that have been used to establish TRMs' permissible shear stress and design values for RECP manufacturers include those at Utah State University in Logan, and Colorado State University in Fort Collins. Bench-scale testing programs are also available to determine index/physical parameters through laboratories such as TRI in Austin, TX.
|
| Vegetation provides wildlife habitat. |
Use of Riprap
The purpose of rock or any erosion control system in a channel is to reduce the energy (velocity, shear stress, and/or impact) of water during concentrated flow and to minimize soil displacement. Depending on the diameter of the rock used and its hydraulic roughness, the rock might significantly reduce flow velocities but increase shear stress because of the subsequent increase in flow depth. The use of riprap that is not well graded results in discontinuous contact with the soil, which has shown to be ineffective in minimizing erosion or soil displacement.
A filtering system, such as a woven or nonwoven geotextile, is required beneath the rock in an effort to minimize soil displacement. In this instance the planar fabric is used for erosion control and the rock is generally considered "ballast" for holding the fabric in place. However, if the riprap is not well graded or if there are minimal contact points with the geotextile, the fabric tends to lift from the ground surface due to hydraulic loading. Once the fabric is separated from the soil surface, soil begins to move or be removed.
Typically, riprap gradation design will fall in the range of D100/D50 and D50/D20 with a riprap thickness of 1.5 to 3.0 times the mean riprap diameter. The subscripted number in this riprap nomenclature denotes the percentage of rock that must be at the noted diameter. While rock size is an important consideration, the shape of rock must also be considered in its selection. Because of its interlocking capabilities, angular rock allows greater resistance to flow than rounded stones. More importantly, for geotechnical stability reasons, rounded rock should not be placed on slopes steeper than 3:1 (Chen and Cotton 1988).
What Is a TRM?
TRMs have been used for more than 35 years to improve vegetation's resistance to erosion by increasing the permissible shear of the vegetative cover. The Erosion Control Technology Council (ECTC), an organization with a mission to develop performance standards, uniform testing procedures, and guidance on the application and installation of RECPs, defines TRMs as
a rolled erosion control product composed of non-degradable synthetic fibers, filaments, nets, wire mesh and/or other elements, processed into a permanent, three-dimensional matrix of sufficient thickness. TRMs, which may be supplemented with degradable components, are designed to impart immediate erosion protection, enhance vegetation establishment and provide long-term functionality by permanently reinforcing vegetation during and after maturation. . . . TRMs are typically used in hydraulic applications, such as high flow ditches and channels, steep slopes, stream banks, and shorelines, where erosive forces may exceed the limits of natural, unreinforced vegetation or in areas where limited vegetation establishment is anticipated.
It is important to note that TRMs must contain a permanent three-dimensional structure for stem and root reinforcement and further enhance the vegetation's erosion control performance. Those materials containing only a permanent two-dimensional structure, such as geogrid, do not provide reinforcement but only maintenance of the vegetation's natural performance levels as noted in the CIRIA 116 Report, Design of Reinforced Grass Waterways (Hewlett et al. 1987).
| Performance of Vegetation With and Without TRMs |
|
| Adapted from "Green Engineering Optimizing Erosion Control with Vegetation and RECPs" (Sprague 1999). |
How TRMs Work
The effectiveness of TRMs has been established by the EPA in its Stormwater Technology Fact Sheet entitled "Turf Reinforcement Mats" to provide an effective, economical, and aesthetically pleasing alternative to hard armor (such as rock riprap) for erosion control in areas where erosive forces exceed the capabilities of naturally occurring vegetation. TRM-reinforced vegetation also affords a corollary benefit—sediment/pollutant removal from stormwater runoff.
Despite variations in materials, TRMs have proven their performance capabilities over the past 35 years in both qualitative field applications and quantitative laboratory research. The vegetation and the permanent three-dimensional structure of TRMs form a symbiotic interaction that provides reinforcement to the stem and root structures. It is important to emphasize, however, that TRMs should not be used under constant high-velocity flow conditions or in any location that will not allow vegetation establishment. TRMs only work in conjunction with vegetation (Sprague 1999). With permissible shear stresses up to 10 lb/ft2 (478 Pa) as noted by the ECTC, geosynthetically reinforced turf can provide similar erosion resistance as 30-inch (0.76-meter ) rock riprap (FHWA 1988).
A secondary and temporary function of TRMs is to collect sediment and act as a barrier to minimize soil displacement during hydraulic flows before and during the time vegetation is being established. The performance provided by TRMs increases as the vegetation becomes established and the vegetation's stem and root structures become entangled within the three-dimensional structure of mattings.
Reinforced Vegetation Benefits
Having discussed the general properties, mechanism, and performance of riprap and TRMs on a permissible shear basis, we now need to discuss the true comparison of using riprap and TRMs. There are many additional advantages and objectives that can be satisfied using a TRM versus standard riprap than simply meeting the permissible shear requirement.
Aesthetics
People have very different viewpoints when it comes to aesthetics, but one thing people living in areas where there are large expanses of hardscapes (buildings, concrete, asphalt) agree on is the need for "green" areas. As our infrastructure continues to expand, hardscapes also continue to increase. Reinforced vegetation offers a softer, natural-looking relief in built-out areas.
Water Quality, Aquatic/Wildlife Habitat
Vegetated areas provide habitat, cover, food sources, and nesting areas for wildlife. They also improve aquatic habitat by reducing thermal pollution and removing sediment and other pollutants from runoff.
Vegetation is an effective form of erosion control because of its hydraulic roughness characteristics and the cover it provides the soil surface. Depending on the vegetation's characteristics that influence hydraulic roughness (height, density, and growth habit), vegetation dissipates energy of flow and therefore results in the slowing of flow. As water slows, its energy is reduced and the water is less able to keep material entrained. Vegetation can also play a key roll in the phytoremediation of various pollutants attached to sediment as it settles out of the flow.
Slowing the flow allows for increased infiltration, groundwater recharge, and natural percolation through the soil profile. As compared to rock or concrete, reinforced vegetation does not heat and maintain thermal radiation and does not transmit heat into runoff that is then conveyed to receiving waters (USEPA 1999).
Although TRMs do not directly improve water quality, they do increase the erosion resistance of vegetation and therefore allow its use in many areas not previously applicable.
Construction Impact
Riprap is typically placed on a prepared soil surface by large equipment like a front-end loader or an excavator when larger boulders are required. For riprap to be effective erosion control, a well-graded mass with minimum voids is essential. In contrast, TRMs are normally sold in lightweight rolls easily installed by hand without the need for large equipment.
Economic and Performance Comparison
Providing a service that is both functional and economically sound is the basis for the fundamental theory of value-engineering. Through the use of TRM-reinforced vegetation instead of riprap, designers and engineers can achieve effective erosion control while reducing total project costs. Tables 1 and 2 provide approximate comparisons of performance and cost for both types of material. The economic comparison for individual projects may differ according to variations in geographic location, labor rates, and material costs.