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Selecting a rolled erosion control product (RECP) for designing with vegetation.
By Roy J. Nelsen
Erosion due to water is a natural event resulting from both the detachment and transportation of soil particles. Accelerated erosion due to human activity and the removal of vegetation or cover from the land often results in drastically increased rates of soil loss.
Rolled erosion control products (RECPs) effectively function as a barrier against both the detachment and transportation phases of erosion until vegetation or reinforced vegetation assumes this function. The matrix fibers of RECPs must provide a cover material to effectively prevent detachment of soil particles by absorbing or dissipating the kinetic energy of raindrops. Furthermore, the blankets and mats must serve as mechanical barriers to dissipate the erosive forces of concentrated flows from acting on the soil surface, thus preventing the displacement and migration of soil particles.
While the matrix fibers of the RECPs perform the aforementioned functions, the nets and mechanical attachment of the fibers to the nets provide a system for stabilizing and retaining the matrix fibers to effectively reduce their mobility in flowing water. This mechanical stabilization allows the fibers to protect the underlying soil from erosive forces. Because erosion control blankets (ECBs) are temporary products, once an effective mature stand of vegetation is established, their erosion control function is no longer required.
When vegetation alone will not provide the required permanent erosion protection, turf reinforcement mats (TRMs) may be needed to provide additional mechanical stability for the root and stem structures of the vegetative cover. In the permanent phase of erosion protection, after a mature stand of vegetation is established, the erosion control function of TRMs shift. The mature stand of vegetation provides the primary form of cover, whereas the primary function of the TRM becomes reinforcement of the vegetation. Additionally, the TRM must also supply supplemental cover to the soil surface for any erosive forces penetrating the vegetation.
Define the application
There are a wide variety of RECPs available. The first step in selecting which RECP will provide the most cost-effective erosion protection for your project is defining the application and erosion processes. Specifically, is the application a slope where erosion is primarily caused by soil detachment from the kinetic energy contained in a raindrop and the subsequent transportation of soil in runoff? Or, is it a drainage channel where concentrated water flow is the culprit? Once the type of application is determined, consideration must also be given to selecting both the temporary and permanent forms of erosion protection.
Vegetation for permanent stabilization
Vegetation, either unreinforced or reinforced with a permanent RECP, is often the ideal permanent soil stabilization material due to its erosion control effectiveness, relatively low cost, easy maintenance, and aesthetic appeal. Vegetation reduces the impact of raindrops, reduces the velocity of runoff, holds soil in place, and is generally less expensive than structural controls. However, the area must be conducive to growing vegetation with the appropriate growth factors present including sufficiently fertile soils, proper pH, aerated soils, moisture and adequate sun exposure.
In most instances, unreinforced or RECP reinforced vegetation, once established, will provide the required erosion protection for slopes and channels common on most sites. However, when starting vegetative cover from seed, these areas often require temporary erosion protection and mulch to hold the soil and seed in place while regulating growth factors, such as temperature and moisture, to insure vegetation establishment. Providing temporary protection and mulch is an important function of temporary and permanent RECPs.
RECP options
By combining degradable and/or non-degradable components into a mechanically stabilized structure, RECPs provide either temporary erosion control or permanent vegetation reinforcement. Temporary RECPs, often referred to as erosion control blanket (ECBs), are defined by ASTM as “a degradable material, composed primarily of processed natural organic materials, manufactured or fabricated into rolls designed to reduce soil erosion and assist in the growth, establishment, and protection of vegetation.” In general, ECBs are manufactured by stitching matrix fibers, normally consisting of straw, straw/coconut, wood, or coconut fibers to or between nets.
Conversely, non-degradable RECPs, often refered to as turf reinforcement mats (TRMs), are defined by the Erosion Control Technology Council (ECTC) as a long term non-degradable RECP composed of UV stabilized, non-degradable, synthetic fibers, nettings and/or filaments processed into a 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. TRMs provide sufficient thickness, strength and void space to permit soil filling and/or retention and the development of vegetation within the matrix. Additionally, the permanent three-dimensional structure of TRMs can be incorporated with organic fibers. These type of TRMs are considered composite or C-TRMs. Through the addition of randomly oriented organic or synthetic fibers, C-TRMs further improve the immediate and permanent erosion control capabilities of the mat. The addition of organic fibers in C-TRMs can also substantially improve its vegetation establishment capabilities.
The selection of the proper materials for your application is dependent on the erosive forces needing to be negated and how long this protection is required. Because of the diversity of materials used in RECP construction, they can have a wide range of functional longevities. While ECBs may provide from 60 days to 36 months of protection against erosion, TRMs typically provide similar temporary protection as ECBs with their surface application, but also provide permanent turf reinforcement and supplemental erosion control.
Temporary erosion control materials
To provide temporary erosion control and mulch directly after seeding and during vegetation establishment where the vegetation alone will provide the permanent form of erosion control, ECBs may be used. ECBs are temporary best management practices that incorporate degradable synthetic and/or organic components into a mechanically stabilized structure. There are many different types of temporary ECBs available, each with its own specific range of cost-effective applications.
The nets used in ECB construction are normally manufactured from either photodegradable or 100 percent biodegradable materials. Whereas, the matrix fibers are predominately composed of organic fibers including straw, straw/coconut, or coconut fibers. The types of nets and organic matrices used will govern the functional longevity and erosion control performance of ECBs. Erosion control blankets are effective for applications ranging from mild slopes and swales to steep slopes, high flow channels and other highly erodible areas.
Geosynthetic turf reinforcement mats
Though well-established vegetation offers excellent erosion control capabilities, areas exposed to significant hydraulic forces such as high flow channels and steep slopes exposed to large volumes of runoff, may require supplemental reinforcement for permanent stability. Geosyn-thetic turf reinforcement mattings are designed to work in conjunction with vegetation to increase its erosion resistance and provide the required additional reinforcement.
These mats utilize a heavy duty, non-degradable, three-dimensional structure to provide permanent reinforcement for the vegetation roots and stems. Furthermore, composite-turf reinforcement mats (C-TRMs) often contain a fiber matrix sewn into the netting structure to provide immediate erosion control and long-term to permanent erosion protection.
Permanent non-degradable TRMs, and more specifically C-TRMs, form a symbiotic relationship with plants by providing immediate erosion control until the vegetation becomes established and then supplemental cover and reinforcement for the stem and root structures. The synergistic effects of permanent synthetic and temporary organic materials allow C-TRMs to offer improved immediate to long-term erosion control, vegetation establishment and permanent vegetation reinforcement.
Reinforcement of the vegetation ensures its long-term stability by allowing it to resist high shear stress flows. In fact, reinforced vegetation has proven to provide shear resistance once thought only achievable through the use of large diameter rock riprap or other hard armor at considerable cost savings. The use of TRM reinforced vegetation furnishes a value engineering option to hard armor alternatives by effectively reducing construction times, material costs, and equipment requirements for protecting critical channels.
Designing with erosion control materials
Historically, erosion control materials selection and design was conducted using a variety of procedures and charts available from a multitude of public agencies. Today there are computer programs available that have been developed by drawing upon the knowledge, experience, and databases developed from the many years of research and data collection by soil scientists, engineers, and other erosion and sediment control professionals. Through the scientific adaptation and computerization of this knowledge, along with specific test data on commercially available products, a select few RECP manufacturers have developed software to assist in the design and selection of RECPs for cost-effective temporary and permanent erosion control.
Any software utilized must contain accurate and substantiated design information on the performance research for the RECPs or other materials. Additionally, the program must be based on accepted design procedures. Design procedures for determining the erosive energy in rainfall and concentrated runoff have been developed and published by various agencies in the United States including the Federal Highway Administration and Department of Agriculture.
To improve the accuracy and effectiveness of RECPs, vegetation or RECP-reinforced vegetation for any slope or channel design, the software program must integrate well-established calculations and procedures as well as product specific performance data. Through the incorporation of various methodologies into a specific design software, engineers are provided with improved slope and channel erosion control designs. Technology and procedures often included in the design of RECP and vegetative erosion control methods include procedures from the:
- Federal Highway Administration’s Hydraulic Engineering Circular #15 (Chen and Cotton, 1988) “Design of Roadside Channels with Flexible Linings.”
- United States Department of Agriculture’s Agriculture Handbook #667 (Temple, et al., 1987) “Stability Design of Grass-Lined Open Channels.”
- United States Department of Agriculture’s Agriculture Handbook 703 (Renard, et al., 1997) “Predicting Soil Erosion by Water: A Guide to Conservation Planning with the Revised Universal Soil Loss Equation (RUSLE).”
Conclusion
Effective erosion control methods should be practiced before, during and after land disturbing activities and construction or renovation projects. Otherwise, maintenance and reconstruction costs may be incurred. Furthermore, public environmental awareness demands that site developers and engineers work with nature and not against it. Utilizing proper erosion control materials including vegetation and rolled erosion control products will allow the accomplishment of erosion control tasks in an effective and economical manner.