In terms of durability, temporary, degradable erosion control blankets are a step above straw or hydraulically applied mulch and a few steps below nondegradable turf reinforcement mats (TRMs). Although some TRMs incorporate a biodegradable filler to help provide an ideal environment for seeds, most consist of layers of geosynthetic materials, such as polypropylene, polyvinyl chloride, or nylon and are resistant to ultraviolet degradation and soil chemicals. TRMs offer higher shear strength for channels or steep slopes where high-volume and high-velocity runoff is expected. Because they can last for years, they maintain a stabilizing grid even after vegetation takes hold and offer additional reinforcement in especially erosion-prone areas.
Degradable products are either completely biodegradable, such as coir (coconut fiber) or straw mats and logs, or biodegradable and photodegradable, as when coir, straw, wood excelsior, or other materials are used in conjunction with a lightweight photodegradable netting that typically lasts several months and sometimes two years or more-ideally until mature vegetation is established.
What determines when to use a degradable product? There are three primary reasons: minimal environmental impact, safety, and sometimes cost. Environmental concerns for some applications mandate complete degradation. Very often, however, a combination of solutions-degradable and permanent-is employed to achieve the most effective result at the most reasonable cost.
Bioengineering Tools: Coir Logs and Degradable Blankets
Degradable products are frequently selected for bioengineering applications, such as stabilizing streambanks, in which natural vegetation is expected to take over and hold the soil in place. Coir logs are especially suited for this purpose. Bob Swain, president of The Dawson Corporation in Clarksburg, NJ, used them after excavating 4,000 ft. of badly eroded streambank at the Radnor Valley Country Club and Golf Course in Radnor, PA. “The stream flows through what used to be a floodplain 100 years ago, and a lot of the golf course was built on the floodplain. So every time the stream floods, they lose part of the bank and the golf course,” he explains. “Around Philadelphia, as every other urban area, over the last 25-50 years there’s been a lot of additional development, so the upstream drainage now gets down to the golf course a lot faster than it did.”
Aggravating the erosion was the country club’s choice of vegetation along the streambank. “Historically all golf courses have turf going right down to the water interface, and anytime you have that in our neck of the woods, it’s a problem,” says Swain. Because it does poorly in a saturated root zone, turfgrass tends to die back from the edges of streams, allowing the bare soil to erode. “The stable grass that’s above it gets undercut, that whole thing falls in, and it’s a never-ending process. What we try to do is reengineer the whole edge.”
Swain used BioLog by BonTerra (now owned by Synthetic Industries of Chattanooga, TN) in combination with BonTerra’s C2 erosion control blanket. C2 is made of coir fiber sandwiched between long-lasting, UV-stabilized, photodegradable netting. It is designed for use on extreme slopes, as a channel liner, or for streambank rehabilitation-something halfway between a short-term biodegradable solution and a permanent TRM. “We installed BioLog at the toe of the slope and C2 up and toed into the top of the regraded slope, then did a wetland seed mix under the C2.” Aquatic emergent plugs placed in the BioLogs helped speed revegetation. Fortified with several species of Scirpus, Iris versicolor, Pontederia, Saggitaria, and rice cutgrass, the streambank is holding up well five years later. “It’s a lot more natural, a rougher type of edge than they were historically used to with the manicured grass right down to the water,” notes Swain, adding that the design varied the height of the vegetation depending on whether the golf course holes were parallel or perpendicular to the stream.
Steve Hurt, a construction manager with Environmental Technologies and Construction in Hunt Valley, MD, also uses coir logs for streambank and shoreline bioengineering. “In a lot of situations you use them in combination because the coir logs provide stability at the toe of the slope where the bank comes in contact with the most stress from either stream-flow events or, on a shoreline situation, wave events. The coir logs are intended to provide a substrate that other vegetation over a very long term can become established in and root into and provide physical support for the bank until you can get a very healthy root mat in behind them.” Because the coir logs eventually degrade completely, it’s critical that the vegetation that ultimately replaces them is well chosen. “We see a lot of problems with the design and installation of projects using coir-fiber logs. One of the most severe is the designs that call for no vegetation being planted in or immediately behind the coir log-in which case, once the log breaks down, there’s really nothing sturdy there to protect the shoreline-or call for ill-suited vegetation, such as herbaceous plugs of species that provide only aesthetic value,” Hurt observes.
“Another installation problem that frequently occurs with the coir logs is insufficiently linking the upstream log to the downstream log. By spending a lot of extra effort linking one log to another as you’re installing them, it avoids a weak point where, during a high-flow event, the tip of one log can become dislodged and work its way down.” When one log breaks loose, Hurt continues, it usually takes others with it, domino style. Lacing the logs together is a common way to attach them, and Hurt prefers nondegradable synthetic twine. “We haven’t found a natural twine that’s very satisfactory. The coir-fiber twine would be ideal because it lasts a very long time, but it does not take bends well, and we find that it becomes very brittle.” Other natural fibers tend to break down within the first season. “Even on projects where you’re trying very hard to keep the thing all natural, we’ll usually argue to go with synthetic on that little bit of work involved in linking them together.”
On recent shoreline projects where he has used KoirLogs by Nedia Enterprises of Jamaica, NY, Hurt has carefully considered which type of degradable mat to use in conjunction. “Mattings fall into to two broad categories: either the woven or the spun fiber,” he points out, adding that although spun-fiber products are commonly sewn together with polypropylene, more manufacturers are now offering mats with natural string instead. “Because of the shear stresses, most of the stream work that we’re involved with calls for the woven products, which tend to be stronger in that way and more resistant to stream flow. The spun-fiber products, in our experience, have been used more for projects involving slope stabilization, when you’re looking to get a nice solid ground cover and diffuse the power of overland flow and water hitting the surface.” Spun-fiber mats encourage faster revegetation as well. “Once you’ve seeded, the seed stays in place very nicely with them, whereas the woven products tend to have spaces between the weave that allow the seed to migrate or move around,” notes Hurt.
Because they are generally less expensive than TRMs-up to five times less-temporary erosion control products are often used for all parts of a project that don’t require the durability of a TRM. “In most cases, unless it’s used in a channel, most everyone, including the departments of transportation [DOTs], will specify a degradable mat. If it’s to be used on a slope or in a channel, you might use the higher-velocity blankets,” says Danny Marsh, president of Southwest Environment Services Inc. in Tyler, TX. Most DOTs, he adds, rely on an approved products list on which various manufacturers’ products are rated; the Texas DOT maintains one such list that is widely used in the industry.
Marsh describes a complex drainage channel project for the city of Longview, TX, for which he chose a combination of degradable and permanent matting. A newly constructed eight-barrel box culvert diverted water into the channel. At the bottom was a pilot channel constructed of gabion baskets. “The area coming out of the box culvert before it got to the pilot channel was all done with the nonbiodegradable mats, mainly because of the water velocity,” says Marsh. Southwest Environment Services installed 5,100 yd.3 of Earth Lock, a synthetic filament geomatrix containing biodegradable wood excelsior to aid in establishing vegetation. “All of the flat area and slopes coming out of the pilot channel basically had the standard mats on them.” About 22,000 yd.3 of excelsior blanket with photodegradable mesh on one side was used on the flat areas and 3:1 slopes. Because the specifications called for a product to be used in these areas that not only was completely degradable but also met a weight limit, Southwest Environment Services chose a blanket made of excelsior rather than straw. Both products used on the project were manufactured by Erosion Control Systems of Northport, AL.
Marsh has also used hydraulically applied mulches, which can cost up to a third less than degradable mats, in combination with rolled products. With mats, overlap adds to the cost-a fact that, Marsh points out, some people fail to consider when estimating a job. “You’ve got an 8-10% overlap factor, so if it calls for 1,000 square yards, you may be using 1,080-1,100 square yards of actual material. But if you’re using a spray-on material, you can actually use 1,000 square yards and get paid for 1,000 square yards.” He recalls a project for the city of Tyler, TX: The area was really odd in shape. “There were a lot of trees, and you would have wasted a lot of blanket. It just wasn’t feasible to put the blankets in there when you could go in with these spray-on materials and just shoot it and be done with it.”
Economics helped determine the choice of material at the construction site of the new Pelion High School in Pelion, SC. Original specifications called for a more permanent mat for a 77,000-ft.2, 30-ft.-high slope behind the school building. TerraJute, a lightweight woven blanket of photodegradable polypropylene manufactured by Webtec Inc. of Charlotte, NC, was later substituted. “I think it is a matter of cost more than anything,” states Tom Morey, a project manager with Martin Engineering in White Rock, SC. “The slope is hydroseeded first, and then we put on the material. It’s a pretty steep and sandy slope, and we haven’t had any problems.” He adds that the lovegrass planted on the slope has now taken hold.
Charlie Herndon, vice president of Herndon Inc. in Lugoff, SC, recently tackled two landfill projects in North Carolina with a combination of erosion control solutions. At the Cumberland County Landfill, Herndon explains, “There were approximately 25 acres of two-to-one and three-to-one slopes, some 200 feet from top to bottom, and on the top there was another 25-acre area draining water onto the slopes.” The Onslow County Landfill had less area to cover, but some of the slopes were 500 ft. high. Herndon covered the slopes on both projects with Synthetic Industries’ three-dimensional Landlok 450 TRM, which consists of polyolefin fibers bound between two high-strength, biaxially oriented nets. “At the bottom of some of these slopes, we would use another product in some channels-an excelsior erosion control blanket. Moving the water off these slopes is your most important objective. Get it from the top to the bottom, and when you get it to the bottom you can control it in these channels with the temporary mat.”
Herndon rarely sees projects where degradable rather than permanent products are specified for environmental reasons. “I haven’t heard that anybody would object to the permanent. If there’s any objection, it would be because they didn’t want to spend the money for the permanent. In a lot of cases they should have-and probably wish they had-spent the money when they have to come back in and go to all that expense to repair and redo the job.”
Two thousand miles away, the Wyoming DOT faced an altogether different scenario when it reconstructed more than 10 mi. of Highway 14 near Yellowstone National Park. Revegetating the dry, rocky volcanic soils on bare roadside slopes was a priority. The park has a comprehensive erosion control plan, and excessive sediment from the easily eroded soil would have threatened the abundant fish population in the park’s waterways. The well being of the park’s larger animals, however-mountain sheep, bison, elk, mule deer, and grizzly bear-led to an agreement four years ago between the US Forest Service and the Cheyenne office of the Federal Highway Administration: Only 100% biodegradable erosion control products can be used in the park or near its boundary.
“One problem we had with our plastic nets is that either elk or deer will tangle up with it,” notes John Samson, an agronomist with the Wyoming DOT Environmental Services. “It can actually hang onto their hock, and they’ll drag it and rip it out, which is of course an unsightly thing. Also, it doesn’t do our blankets any good. We’ve found that with the biodegradable jute nets, it acts more like a paper punch, so that they punch right through but don’t really drag as they walk down the slope. The blanket stays in place, and the animals are not walking around with plastic hair nets.”Samson quickly adds that when animals did drag the nonbiodegradable netting, it would tear easily-especially if it had partially photodegraded-quickly freeing the animal as the netting snagged on rocks or vegetation. The litter problem, however, was significant, and crews were sent out to pick up pieces of netting that the animals had dragged out of position. Some photodegradable products also took longer than expected to break down, Samson says. “They’re under snow a good share of the season.”
BioNet erosion control blankets, manufactured by North American Green of Evansville, IN, were chosen for the first phase of the Highway 14 project. A lighter-weight SC150BN blanket-consisting of a 70% straw and 30% coconut fiber matrix bonded with biodegradable thread and layered between biodegradable jute netting-was installed on roadside slopes. A denser, 100% coconut-fiber C125BN blanket was used in the drainage swales beside the roadway, which receive heavy runoff from snowmelt. The coconut fiber gives BioNet blankets a longer life-18 and 24 months, respectively-than 100% straw blankets, which was important in the dry climate where vegetation often takes more than one season to become fully established.
Compared to other temporary erosion control blankets (not TRMs), the Wyoming DOT has found that 100% biodegradable erosion control mats are more expensive. Samson estimates the installed cost can be up to 30% more. “The reason we haven’t gone 100% to it is cost,” says Samson. Although the Wyoming DOT uses a variety of products, not all of them biodegradable, outside the Yellowstone area, the park is something special: “It has the large, four-legged animals still easily visible from a roadway. And it demands that we change our designs to allow for that. Certainly we use rolled erosion products to control possible stormwater problems during and after construction, and we’re just trying to modify that proven technique in a more wild-game-sensitive type of material.”
Temporary biodegradable products have another advantage at Yellowstone over heavier TRMs. “We try to do visual mitigation, and in doing that we tend to leave irregular, contoured slopes and boulder groupings,” says Samson. “Those can present a bit of a problem in trying to get the blankets on and over and fastened. The biodegradable mats are a lot more flexible. The plastic netting-especially in late fall when you have freezing temperatures-tends to tent more and cause an installation problem in getting it to conform to the topography. That’s particularly pronounced when we’re doing late fall applications in freezing temperatures.”
Not Just the Mat, But How It’s Fastened: Safety Issues
On some projects, the fasteners used to hold an erosion control blanket in place cause more concern than the blanket itself. When Erosion Control Specialists Inc. of Fort Worth, TX, installed 57,000 yd.2 of erosion control blanket on the north end of a runway extension at Tulsa International Airport, conventional steel staples were out of the question because of the danger of foreign-object damage-the risk of one of the staples coming loose and being ingested into a jet engine on take-off or landing.
The company found an alternative with Turf Tacs, corn-based plastic staples. “They degrade rather quickly, depending on the weather and soil conditions,” says Thurman C. “Corky” Pierce Jr., CEO and president of Erosion Control Specialists. “In fact, this was the largest job that they’d ever been used on.” Manufactured by EcoTurf Inc. of Chicago, the L- or U-shaped Turf Tacs usually biodegrade in three to eight weeks after installation and are designed to prevent damage to mowing equipment. They were used on this project to tack down the medium-weight Curlex II QuickGrass, a blanket of curled wood excelsior covered top and bottom with photodegradable extruded plastic mesh, manufactured by American Excelsior Company of Arlington, TX.
A specially designed stapler called the Turf Tacker, also by EcoTurf, allows installers to place the staples from a standing position. “We had a few problems using the Turf Tacs when the heavy clay would become crusted over and very hard to penetrate,” reports Thurman. Once in place, however, they did the job, holding the blanket in place under less-than-ideal conditions. Even with 65-mph winds during parts of the installation, the Turf Tacs secured the partially installed blanket.
It doesn’t take a risk as dramatic as aircraft damage to make contractors opt for an alternative to steel staples. Schools and heavily trafficked public areas pose safety concerns as well, says Marsh of Southwest Environment Services. “When you put down a soil-retention blanket, you pin it down with sod staples. Now let’s say you’re putting this on a slope in a schoolyard. And let’s say the guy goes out there, tries to mow it, and gets his tires caught in that netting and it pulls up a sod staple. It’s like a missile; somebody could get hurt.” In such cases, one option may be to forego the mat altogether and opt for spray-on erosion control-bonded fiber matrix or hydromulch. “You don’t have the residual netting to deal with.” While many such products are completely biodegradable, they are short-lived compared to most blankets and mats.
Such easily degradable fasteners as Turf Tacs might be too fleeting for some applications, points out Hurt of Environmental Technologies and Construction. “With the coconut-fiber products in particular, you’re looking for the product to be there for a number of years providing support to the vegetation, so you want something that’s going to be there for a while. The steel pins, once they’ve been in place for a month or two, start to rust, and that rust ends up forming a very, very secure connection to the ground.” It makes them very difficult to pull out.“I’ve seen some of the degradable fasteners but never really used them because I’ve never perceived a problem with using the steel pins,” Hurt continues. “We have done projects in public parks where there are safety issues with the steel pins, and in those cases we go with wood stakes and have been pretty happy with those. In my experience it tends to be fairly difficult to mow over these geotextiles anyway unless the installation is absolutely perfect, because the actual fabric itself tends to want to get up and into the mowers. One of the nice things about some of these [degradable] products, especially the woven products, is that they lie tighter on the ground. As they degrade, they just sort of turn into dirt, as opposed to the ones with the plastic netting, which are a mower’s nightmare.”
Softer Solutions
Beyond safety, cost, and specific wildlife concerns, degradable solutions seem enjoy a better image with some, while others are wary of their effectiveness. “Most regulatory agencies we deal with are much more amenable to a softer, biological, environmentally sensitive approach rather than a hardscape with gabions, riprap, or concrete or seawall or something like that. They look much more favorably on projects that incorporate a softer solution,” says Swain of The Dawson Corporation.
Samson sees the degradable products catching on because of how they drape when installed, making them ideal for uneven surfaces. “I think it’s a trend. You’ll see even urban landscape architects are probably going to use more irregular berms and boulder groupings.”
“I think there are different camps among the designers,” observes Hurt. “With a degradable solution there’s always a degree of risk involved; somewhere down the line something will come along that disturbs the vegetation, and then you’re left with a relatively unprotected area. If you go in with a permanent physical barrier, you at least have the synthetic to hold the soil in place until new vegetation establishes. So it’s a trade-off between the degradable products, which offer a much more pleasing aesthetic and environmentally conscious project, and the degree of assurance that a synthetic product’s going to provide.” To some, he says, the risk of unexpected subsurface conditions and unforeseeable weather extremes “kind of makes it worthwhile overdesigning and overconstructing the thing, using a one-grade-higher product than what you’d expect the project to require, just to be safe.”
Of nondegradable geosynthetics, says Swain, “There’s a ton of stuff out there that will do a very good job of holding the soil. But it’s going to be there forever, and you could have potential problems down the road depending on the application.” For some jobs, he allows, geosynthetics are ideal. “If you’re trying to stabilize something and build a road or a parking lot on it, engineering fabric is the way to go. But in a bioengineered solution, we always try to opt for a biodegradable product. Biodegradable products and a bioengineered solution are not a panacea for every single application, either. Where the erosion or the force of the stream or the ocean is just too great, you have to go with a more engineered solution. You have to know the elements that you’re dealing with and design accordingly.”
