When it comes to choosing between biodegradable and synthetic blankets and mats, the project goals dictate the choice.
Turf reinforcement mats (TRMs), which often contain synthetic components, are often selected for applications that call for an approach that is heavier, longer-lasting, and more sustainable in harsh conditions or high-water velocities.
Erosion control blankets are often chosen for their lighter weight and lower cost. Other considerations include their faster breakdown, which is applicable when one is seeking a solution to last for just one growing season.
It used to be that most blankets consisted of natural materials, such as straw, excelsior, or coir sandwiched between plastic netting.
Now, many companies are marketing biodegradable products-those products that may have not nets, or nets that easily break down or are photodegradable. The choice here is often rooted in two concerns: wildlife entrapment in blankets and mats, and maintenance mowing equipment getting caught up in the erosion control product.
Long-Term Strength
A synthetic approach was the only option for Aqua Ohio when, two years ago, it sought a solution to the challenge of an overspill situation at Pine Lake Dam at a 700-acre lake owned by the company in New Springfield, OH.
“We had to provide emergency spillway capacity,” says Pete Kusky, a division engineer with Aqua Ohio. The concrete overspill area did not meet requirements for a 10-year rainfall when the lake overtops, as it does a few times a year, creating the potential for bank erosion.
The Ohio Department of Natural Resources sought a solution that would not only prevent erosion but would also be aesthetically pleasing. A grassy walkway around the lake was desirable, as the lake is located in a residential area.
“The lowest-cost option was to cut a new emergency spillway into the dam and, in doing so, allow water to flow over the earthen dam,” says Kusky. “It was a problem for erosion, and it came down to how it would be lined. We looked at other options, but the TRM was the lowest-cost option. Through the design process, we had to make sure it would withstand the velocities that would be expected.”
The solution came through Propex Landlok 300, a three-dimensional woven turf reinforcement mat, which was placed over rock riprap for bank reinforcement. The product served three purposes: it met the 10-year rainfall requirements, provided a reinforced vegetative solution, and met aesthetic requirements as well.
The second-generation woven TRM design locks the soil in place under high-flow conditions. After its installation, a May 2008 weather event caused overtopping through the spillway; Landlok 300 held up for its constant 24-hour duration.
The TRM’s tensile strength allows it to handle mower loads during maintenance over its long-term design life better than traditional first-generation stitch-bonded TRMs. First-generation Propex TRMs are constructed of a dense web of polypropylene fibers placed between two biaxially oriented nets. The first-generation TRMs are netted, fused, glued, or stitch-bonded.
Propex TRMs are made of 100% synthetic, ultraviolet-stabilized components and are completely nondegradable.
Biodegradable Options
Municipal entities are becoming increasingly concerned over the choice of materials for erosion control blankets and mats, not only for the potential of mowing and maintenance equipment getting caught up in the products, but also for the well-being of wildlife.
Holly Smit Kicklighter, an associate environmental planner for the city of San Diego, CA, expresses concern that wildlife could be entangled in certain types of erosion control blankets and nets much the same way as water birds are caught up in fishing nets, lines, and filament.
“Straw wattles can trap small reptiles almost like a gill net when the straw degrades and the wattles are no longer tightly surrounded by the netting,” Smit Kicklighter says.
Although the wattles are used as temporary best management practices, she is aware of some sites that were mass-graded more than five years ago, and the material is still evident there.
“Obviously, the temporary features should have some recommended replacement timeline specified by the industry or manufacturers, so they are not just expected to last through a development phasing program,” she says. “I also don’t think the orange biology delineation fencing is very attractive, but may be less of a wildlife nuisance.
“Maybe they can make that out of cornstarch,” she suggests, “because the required use time is usually only until grading is finished. I don’t know what they could use to make netting less detrimental to wildlife and still have it hold up.” Cornstarch has been used as a component of biodegradable products ranging from packing materials to stakes for anchoring erosion control blankets.
Chris Brennan, a staff member with San Diego’s Mitigation Monitoring Coordination, says some erosion control methods are a “huge problem in the field. I have seen lizards, snakes, birds, and rodents all killed by the netting left over from different types of erosion control materials-generally the black, nonbiodegradable half-inch grid plastic netting.”
Brennan says this type of erosion control “seems to be the worst for wildlife, as many animals can get halfway through and then not out-even snakes. Silt fences are generally a much more effective erosion control product than straw and fiber rolls, and they don’t trap wildlife. When building “˜rabbit fences’ around a developed property, quarter-inch grid fencing is much more wildlife friendly than half-inch-not that either one will keep out a determined rabbit.”
More government entities are specifying biodegradable erosion control blankets and mats. Such is the case in Delaware, where the state’s Department of Transportation mandates biodegradable products for highway work, says John Baker, a project manager for A-Del Construction in Newark, DE.
On two separate road projects, A-Del has applied ECS-2B double-net straw biodegradable blankets from East Coast Erosion Blankets. The blanket contains 70% agricultural straw and 30% coconut fiber sewn into two jute nets with biodegradable thread. It has longevity of 12 months and is applicable for 2:1 to 3:1 slopes.
In one project, from September 2006 to July 2008, the Delaware DOT was widening Route 1 by adding a third travel lane. By using ECS-2B on the project, “the installation was almost half the time it took compared to the jute net we were looking at,” Baker says. “We had immediate growth of the grass.”
The potential for wildlife to get caught in nets is one concern of the state’s DOT, Baker says.
“The state also likes the biodegradable net because it alleviates the problem of getting caught in the mower when they maintain the grass,” he adds.
The same technology is being applied in another road-widening project that was taking place over the summer on US 301, a two-lane truck route in Delaware that is being widened to four lanes with a grassy median.
“We constructed three large stormwater management ponds, and we’ve used the biodegradable net on all of them,” Baker says. “We had grass growing within a week. The grass right now looks like a golf course on the slopes of the pond. We’re also using it along the embankment of the highway as well the back slopes and in the median.”
Although Baker’s company utilizes biodegradable blankets and mats for its government work as mandated, Baker favors it for his company’s private work as well.
“We have used synthetic products in the past, but we’ve had so much luck with the biodegradable that we use it on almost all of the projects now,” he says.
American Excelsior Company’s Curlex NetFree excelsior blankets were designed to offer the benefits of the standard Curlex excelsior blanket-promoting vegetation growth while protecting topsoil from erosion-without the use of polypropylene netting that might be caught in maintenance equipment or entrap animals.
The product is a 100% biodegradable erosion control blanket composed of interlocking, curled, Great Lakes Aspen excelsior wood fibers stitched together with a 100% biodegradable thread.
The Iowa Department of Transportation is now specifying such products for its road projects. Curlex NetFree was used recently in a road project in Council Bluffs, IA, where the state’s DOT was expanding a road and building a new bridge.
“It was a unique project,” says Ole Skaar Jr., an agronomist in the roadside development section of the Iowa DOT. “It wasn’t quite rural; it was urban and semi-urban, and it would eventually be urban, but there were still some rural aspects to it.
“In the urban section of it, I used a bonded fiber matrix for a little better appearance. The semi-urban area is probably going to get mowed and maintained. I used a netless product, so when they go to maintain it and mow it, they’re not snagging the netting. It was a neater-looking project with the netless rather than with plastic netting that was left over for a year or two.”
Skaar used Curlex NetFree a few years ago when the DOT was building a roundabout on Highway 63. “It was the first time we used it, and we had big success there, so that’s why I have confidence using it in the Council Bluffs area,” he says.
Iowa has been going the way of other state transportation departments in specifying biodegradable blankets and mats, says Skaar.
“Most of the manufacturers and distributors are going to netting that’s either biodegradable or photodegradable,” he says.
The state will still use synthetic products when necessary, Skaar says.
“I’ll use them in a situation where I think might need protection for a couple of years,” he says. “I would rather have a product be firm for at least that long. If I think I may have trouble getting [vegetation] established with it in the first year and I’ve got either a large volume of water or a steep grade and I know I want protection for a longer amount of time, then I would steer away from biodegradable. That may be gone at the end of the first year, and if I still need protection the second year, it won’t be there.”
Environmental concerns were the focus when Landscape Development in Valencia, CA, chose to utilize RoLanka International’s BioD-Mat 90 blanket on a beach restoration project in Malibu, CA. The project began last summer and continued through this summer.
“The sand that is usually on the dunes there has continued to wash away over this past year,” says Tiffany Leo, a project associate. “It’s encroached onto homeowners’ properties and into the septic system, and in order to prevent that, we’ve had to build temporary berms to mitigate further erosion of their property.”
In the emergency permits issued by the city of Malibu, biodegradable products were mandated because of the site’s proximity to the Pacific Ocean.
Bio-D Mat 90 is woven from machine-spun bristle coir twines and is 100% biodegradable. The product has longevity of four to six years. The open weave allows for seeding before and after installation.
“We’ve been building these temporary sand berms, encapsulating them in various fabrics such as the RoLanka fabric to make it more semi-permanent in a way that it’s not going to be washing away every day and has to be maintained-either daily or weekly. It gives homeowners a longer-term of stabilization of their property,” says Leo.
The erosion control project is a temporary approach until a long-term solution can be provided. Prior to this effort, homeowners were building temporary berms using burlap sandbags.
“As the high tides come in, the sandbags just get washed away,” Leo says. “We had to come up with a different type of engineering to stabilize it better. That’s when we decided to encapsulate them with these various fabrics, so that it gave it a temporary sea wall effect.”
The approach has worked well, Leo notes. “We had to go through some alterations to find the right type of fabrics that would hold up,” she says. “In the beginning, we had fabrics that would deteriorate with the saltwater, the sun, and the wave action within months. We had to come up with a stronger material made of natural fibers, which is something that was more challenging, and that’s when we did some more research and came upon the BioD-Mat 90.”
Because of the proximity of the project area to the ocean, any product breaking down-be it biodegradable or synthetic-would end up there, Leo notes.
“The city would rather have biodegradable that’s not harmful to the marine life as opposed to something that’s plastic or whatever else might be out there harming the marine life,” Leo says. “The goal was to mitigate the problem of receding sand dunes as well as maintaining the ocean the best we can.”
Presently, research is being conducted to determine what is causing the erosion so a permanent solution can be found.
“People have lived there for more than 50 years, and no one has ever seen it rise so much and pull so much sand away from their properties as it has for the past year and a half,” says Leo. “Half the concern is figuring out what the cause of it is first, and then how they’re going to find a permanent solution.”
Landscape Development will use either biodegradable or synthetic products, depending on the project’s needs, Leo notes.
Of the advantages of biodegradable products, Calista Santha, president of RoLanka, makes the point that the ultimate objective of erosion and sediment control is to establish sustainable mature vegetation. “Natural products hold moisture, support seed germination and plant growth,” she says, adding other advantages of biodegradable products include performance, aesthetical appeal, and environmental friendliness.
Additionally, completely natural products are 100% biodegradable, and as they degrade they provide mulch that further improves soil conditions to develop sustainable mature vegetation, Santha says.
“They do not provide any harm to our natural resources-water, land and wildlife,” she adds.
She also notes that sediment control is a temporary measure and the products used often need to be removed after use and hauled to landfills.
“This not only increases the cost of sediment control and landfill maintenance, but also creates the unnecessary risk of sediment pollution during removal,” says Santha. “When 100% natural products are used for sediment control, they can be left out to decay.”
When erosion control material tangles up with construction and maintenance equipment, Santha says, “This not only damages the product, but also damages the growing vegetation.”
In stream restoration, biodegradable mats do not have fixed joints and have twines that move independently; thus, there is no interference with wildlife movement, Santha says.
Mine Restoration
Mining waste mitigation has presented a challenge for years as the United States Department of Agriculture and United States Forest Service (USFS) work in conjunction to cull the data on the extent of the contamination problem in an effort to address it.
Case in point: the agencies have for years been working on the Gallatin National Forest’s New World Mining District Response and Restoration project in Montana.
Part of addressing the challenge involves revegetation research on what are essentially high-elevation mine disturbances, which call for specific modes of species selection, fertilization, planting seasons, organic amendments, acid soil amendments, and surface soil treatments.
North American Green first got involved with the design for the project revegetation work in 1997, says the company’s Phil Davis. “Part of the research involved selecting vegetation that would grow at a high elevation of approximately 10,000 feet with very poor soils and in a short growing season-it was usually snowed in from mid-September to July,” he says.
The USFS successfully used North American Green’s photodegradable blankets on the test plots, but there wasn’t enough UV exposure to degrade the plastic netting, says Davis.
The Forest Service then turned to North American Green’s BioNet erosion control blankets. The product is 100% biodegradable. The jute netting and dense mulch layer increases water absorption capacity for effective erosion control, soil temperature regulation, and moisture retention to promote seed germination and early plant growth.
The jute top netting uses a leno weave to offer a higher level of tensile strength, durability, fiber retention, and erosion protection under severe conditions.
Because the interwoven strands of the BioNet netting can move independently of each other, wildlife entrapment is minimized. The construction also allows the use of live stakes and the installation of trees, shrubs, and other plantings through the blanket without compromising erosion control performance.
“The netting breaks down from exposure to moisture and microorganisms,” says Davis. “The USFS report said it was critical to use BioNet erosion control blankets to ensure maximum revegetation with the native seed mix.”
