Blankets and Rolls, Mats and Mulches
An increasing variety of options to cover the ground
When veteran landscaper Bart Hodges began working on the 1-mi.-long stream enhancement at Druid Hills Golf Club in Atlanta, GA, he knew he would be using a large assortment of erosion control products. The golf course was built in 1912 and today is considered the second oldest in Georgia. The problem at Druid Hills was that, as erosion control practices changed throughout the years, the stream underwent changes no longer considered ideal.
"In the past, they put riprap down the entire length of the stream," Hodges describes. "And at some point they built a wall all the way down to the stream edge. It was a very urbanized stream."
Hodges first assessed the site to determine what types of soil he'd be dealing with, the slope angles, and the shear stresses in the channel. It took the natural-resources and other permitting agencies two years to approve the stream-enhancement plans. In the meantime, Hodges researched the stream and how other experts had handled similar situations.
"This was 1 mile on both sides, and we haven't found a larger stream-restoration project in this part of the United States. We studied a lot of other projects, and we just couldn't find one of this magnitude."
Garden Management Consultants built some artificial waterfalls and created a large diversion pond to help control extra runoff. Turf-reinforcement mats were installed as an entrance to guide water toward the diversion pond.
To make matters worse, several places along the stream previously had concrete or shotcrete poured along the bottom of the canal. "In these areas, we used the RoLanka 3DTRM-CC mats for high-water flow areas. Then we added over that a natural river rock and planted spider lilies."
Along the majority of the stream, where riprap dominated, Hodges used coir logs and turf-reinforcement mats. "We used the coir rolls on the streambanks on the toe to the steep undercuts. And we did this for about three-quarters of a mile on both sides of the stream. In addition, we used other long-term mats, like the coir mat that has polypropylene wrapped around it—the [RoLanka] 3DTRM-PP and BioND-TRM100.
"For the velocity and shear stress that we had, we needed some very heavy mats. In some areas, these actually qualify as riprap. They're tough and very competitively priced."
The landscapers wrapped the mats halfway around the logs and allowed them to lie half-in and half-out of the stream. They pegged the logs down and planted native Bermuda grasses into the coir. In a good recycling scheme, another crew pulling out older materials from the site came across some old silt fence that still had intact posts, some of which were used for staking the coir logs. Hodges notes that, although the recommended fasteners for the mats are 8-in. staples, because of the rocky soil immediately below the surface, crews had to use cut rebar instead.
In addition to planting native grasses in the coir logs, the crew also inserted live and bare-root plants directly into the mats. "In some areas, we used larger plants," Hodges adds. "There we used two layers of mats. One was used as a base, and then there was a soil layer for a root zone and then the other mat. That's where the plants were inserted. The Pea Vine Creek Watershed Alliance and the Department of Water Resources inspected the project, and they were all very happy with it."
Erosion Control Blankets
Mel Mathews, CPESC, stresses the importance of proper installation for erosion control blankets in his research published by the Erosion Control Technology Council (ECTC). Good soil-surface preparation is the critical component for success, he says. To get full contact between the blanket and the soil surface, Mathews recommends first clearing away all rocks, dirt clods, trash debris, and vegetation. Blankets should be anchored with staples or stakes driven through the blanket and into the soil to remain flush with the soil surface.
Although Mathews cautions not to use erosion control blankets in areas where there is a possibility of the netting materials catching in mowers, Laurie Honnigford, executive director of ECTC, says guidelines can be amended depending on site conditions. "It all depends on the nets being used. If you know you'll have to mow it, then you have to request a shorter-term mat. But we do it all of the time at golf courses, and it's OK."
In deciding on the best erosion control methods for a given site, factors to consider include initial costs of the product, ease of installation, the amount of maintenance required, and the durability of the product. For example, lighter-weight erosion control blankets with biodegradable and photodegradable nettings are available for short-term applications. Honnigford notes, "Some estimates indicate that installation of turf-reinforcement mats can be one-third of the cost of 8 inches of rock riprap [yet provide] the same amount of erosion control."
Mulching is probably the oldest form of erosion control. Various materials have been used over the years to accelerate seed germination and plant establishment and to protect soil from loss due to splash and wind erosion.
Hay, straw, composted vegetation, recycled newspapers, and other fibrous materials help cool the soil, decrease the heat stress on plants, and reduce evaporation. During the winter, mulches provide insulation for dormant vegetation and soils. As spring temperatures warm the frozen ground and mulch, decomposition releases organic matter that again acts to enhance soil stabilization once it's incorporated into the soil.
On sites with steeper slopes, mulches often are applied with the help of a tackifier to hold them in place. Tackifiers not only anchor the mulch to the ground surface but also bond the fibers together. Many types of tackifiers are available, composed of petroleum distillates, emulsions of copolymer acrylates, latexes and polyvinyl acetates, or asphaltic emulsions. Some tackifiers are composed of dry, powdered, vegetative gums derived from psyllium, guar, and sodium alginase.
Another category of mulches includes hydraulically applied wood fibers, wood cellulose, paper pulp, and recycled cardboard. As hydromulch is applied, seed mixes, fertilizer, and other soil amendments can be added directly to the slurry before it is sprayed on. Hydraulic mulches with added tackifiers have the advantage of adhering directly to the soil surface and often are used to spray in hard-to-reach areas.
The Bonneville Dam Corner-Collector Facility
Working around water and sensitive aquatic habitats calls for effective erosion control measures. Alex Zimmerman of CSI Geosynthetics recently worked with the US Army Corps of Engineers on a project designed to help the passage of salmon in the Columbia Gorge in Oregon and Washington. The $32 million project called for construction of a corner-collector surface-flow bypass system at the Bonneville Dam on the Columbia River.
Federal biologists expect the corner-collector facility to provide a 1 to 3% increase in juvenile salmon survival past the Bonneville second powerhouse. In earlier tests performed by the corps, about 40% of the juvenile fish in the area directly upstream (in the forebay) of the second powerhouse passed through the existing ice and trash chute during their downstream migration. Corps representatives estimated that "by modifying the chute into a high-flow surface-bypass system (a corner collector), about 50 to 60% of the juvenile fish entering the forebay will be guided into the corner collector, passed through without injury, and returned safely to the river at a location farther downstream."
Plans for the corner-collector facility included construction of a 2,800-ft.-long transportation channel, a 500-ft.-long outfall channel, and a plunge pool. Modifications to the ice and trash chute also will aid in safe fish passage.
"We had 10 acres of soil to cover," explains Zimmerman. "They had to excavate a huge area to get down to the river. The stockpile from that had to overwinter on Cascades Island, located in the center of the Columbia River. The two sections of the Bonneville Dam join on the island."
Zimmerman was tasked with recommending the measures for the stockpiled-soil erosion prevention. His company also was involved in water treatment with chitosan-enhanced sand filtration on the project. "Our company worked on the subgrade stabilization, drainage, and plunge-pool armoring measures as well," he adds.
After assessing the environmental aspects of the site, Zimmerman chose Soil Guard from Mat Inc. to protect the enormous mound of soil from blowing away in the abyss. "With the power lines and high gorge winds, [applying] plastic cover or straw was not possible. This was the only option that could do anything in that situation."
Soil Guard is a hydraulically applied erosion control blanket that binds in conformation to the ground. When dry, it forms a bonded fiber matrix that can be moistened repeatedly to keep seed and soil from washing out. Soil Guard is a temporary erosion control measure that decomposes slowly, usually after vegetation has grown in.
Applying at a rate of 60 bags per acre, Northwest Hydromulchers used 3,000 lb. of Soil Guard at the site, applying it to the vertical cuts that extended from the excavations to the slide. They also applied it on the road that was cut out to move equipment down to the fish slide.
"The biggest challenge was the wind," recalls Barry Cook, owner of Northwest Hydromulchers for more than 23 years. High winds forced the crew to use hoses to apply the mix, "and when you use lots of hoses, the discharge pressure is affected. The bonded fiber matrix is unique in that it's necessary to maintain the water-to-product ratio. You can't manipulate the mix; you can only manipulate the water. But if you do that, you're stuck with going back over it several times."
To solve the problem at the Bonneville Dam, Cook's crew had to layer the slurry in several applications. Working in inclement weather, they took care with the applications to ensure extremely good soil-to-product contact so the manufacturer's specifications would be met. "That's the great thing about this product," Cook concludes. "Wet or dry, it retains its structural integrity. You just don't get any separation from the soil surfaces."
Oregon and Washington fisheries agencies, the National Marine Fisheries Service, the Bonneville Power Administration, local tribes, and the corps have all endorsed the project. "Construction is essentially complete," says Matt Rabe, a spokesman for the corps. "We hope to have it operational by the spring 2004 juvenile salmon migration."
Rolled Erosion Control Products
Although the term seems to describe such products as coir or straw logs, rolled erosion control products (RECP) are actually a diverse group of products, including erosion control blankets and meshes, geosynthetic mats, and erosion control nets.
Erosion control nets are made of woven fibers and usually are used in areas where heavier erosion control blankets are not required. Installation is quite basic: The netting is rolled out over loose mulch material, such as layers of straw, and then is stapled in place. Erosion control nets are considered a short-term product and do not offer the same structure as machine-constructed blankets.
Erosion control meshes, on the other hand, are woven or prestitched polyolefin yarns that form a two-dimensional matrix. They can be used with or without an underlayer of mulch material. Used on slopes or to stabilize the underlayer of turf, erosion control meshes offer more strength than netting does.
High-strength, 3-D matrices compose the RECP group called geosynthetic mats or turf-reinforcement mats. These mats are manufactured from an assortment of UV-stabilized filaments and fibrous strands into a permanent material. They are designed for more permanent applications and for use in such places as drainage channels where they are required to withstand greater velocities and shear stresses than the natural, mature vegetation can endure. In essence, these mats, along with installed vegetation, take the place of hard armors, such as riprap, and often are more cost-effective and environmentally accepted than nonvegetated linings.
ECTC publishes definitions and uses of various types of RECPs at www.ectc.org.
From Green Soda Bottles to Land Reclamation
As part of the federal Surface Mining Control and Reclamation Act, land used for surface mining must be returned to a state that meets or exceeds premining conditions. At a mining operation in eastern Texas that encompasses 44,106 ac., approximately 12,000 ac. are in various stages of reclamation. The process creates hillsides, slopes, ditches, and drainage channels and establishes permanent vegetation on the site. Reclamation of the land for postmining uses generally begins with soil leveling; after the grading is done surface water drainage networks are created.
The channels at the site experience concentrated peak water flows during rainfall events. Turf-reinforcement mats are used to help newly established vegetation withstand the anticipated shear stress. A permanent turf-reinforcement mat employed here, Recyclex from American Excelsior Company, is produced from 100% recycled postconsumer goods—specifically green soda bottles.
A seed mixture of brown top millet (Panicum ramosum) and common Bermuda grass (Cynodon dactylon) was used on reclamation projects at the mine. Brown top millet is an annual grass that provides temporary erosion control because of its rapid germination. The millet is used in conjunction with the slower-germinating and perennial Bermuda grass. The vegetation easily grew into and through the Recyclex matrix—an important feature because, if vegetation struggles to penetrate the turf-reinforcement mat, it can cause the mat to be lifted upward. This process sometimes is referred to as "product tenting" and results in the loss of contact between the product and the soil surface.
Tenting also can occur because of repeated wetting and drying of the mat. Ideally reclamation-project engineers want products with fibers that do not absorb water and thus prevent tenting. Another common problem with these installations is the "tiger-trap effect"—or the subgrade washing out from underneath the product. While no project is immune to the problem, these turf-reinforcement-mat installations fall victim to it infrequently because of the strength of the matrix and the continuous contact between the product and the subgrade.
In previous applications, straw bales were used in an effort to reduce flow velocity in the drainageways along the terraces. The bales were unsuccessful in the loose, sandy soil because the water scoured around and under the structures before the bales could reduce flow velocity. When the Recyclex was installed and stapled in place, it was allowed to naturally fill soil from the land area contributing to the drainageway. The fibers on the bottom of the blanket buried themselves in the subgrade to form a synthetic root system, and the fibers on top of the product grabbed the soil particles that moved across it. Once a seedbed was established, natural vegetation became stabilized in the matrix. To date, more than 2,000 rolls of the turf-reinforcement mat have been installed at the mine site.
Author's Bio: Guest author Linda Robinson is a journalist specializing is a journalist specializing in agriculture and land-use planning.