Putting Erosion Problems to Bed With Blankets

March 1, 2008

Water, water everywhere-not the best site on which to use water-based applications, such as hydroseeding. For slopes or areas where water is meant to run, such as drainage ditches or retaining ponds, a covering of erosion control blankets or turf reinforcement mats can solve many problems. Water can’t move them-indeed, they soak up water, adding weight, and become more immovable-offering longer-lasting protection for sown seed.

Great Cover for Runoff
John Ziliak, erosion control division manager of Daylight Land Management in Evansville, IN, uses EC blankets in many of his projects. “We do a lot of waterways for government programs on farm fields. The Natural Resources Conservation Service will pay for some of this work, as does the farmer or landowner. In an agricultural field, erosion is a problem; the natural drainage of the field might create a gully. We make a flat-bottom conveyance channel for the runoff and line the channel with erosion blankets and then seed with fescue. This channel slows the water and also keeps the agricultural runoff from going anywhere else.”

Daylight Land Management, which usually installs Western Excelsior Corp.’s products, also works on some industrial sites, and often uses other best management practices (BMPs), such as hydroseeding, on these more level sites. “On a 3:1 slope or better, we’ll cover the area with a blanket. Our company uses a little of both EC blankets and turf reinforcement mats [TRMs], but I haven’t used too many TRMs, because they’re more expensive,” Ziliak explains.

Photo: Illinois Tollway
Erosion control blankets along a slope near the Illinios Tollway

Although sloping is a consideration when choosing to use an EC blanket, Ziliak pays more attention to the speed of the water flowing down that terrain. “We might use a blanket on a slope of 2:1; it depends on the runoff’s velocity. I’m not concerned as much on slope, but if a lot of water goes over a slope, we have to look at different avenues. Whereas 90% of our EC blankets are called for on industrial sites, these areas are pretty flat, and we end up doing more hydroseeding and seeding than installing blankets.

“I also factor in the soil type as well as how much water will be moving across what I’m stabilizing,” he continues. “That is the greatest influence on what I try to use. A lot of the work I do is contract work, which has specifications on what type of blanket to use and where it goes, but generally, any ditch 3:1 or greater will receive double-net straw or coconut [coir] matting. We don’t use seeded blankets; we seed separately, not only because every job has its own specific seed mix, but also because I want seed-to-soil contact, which is very important for germination.”

Any maintenance problems with EC blankets? “Several people have complained about the blankets’ netting getting into their mowers, but now several companies are making biodegradable products netting, which should biodegrade in 60 days,” he says.

For flatter surfaces, Ziliak takes into consideration how much water is going to move over the area. “Splash erosion from the rain probably doesn’t need a blanket. Maybe just some straw mulch or hydroseeding is sufficient.”

He hasn’t had a really big blanket job recently, though. “We have been using quite a bit more BFM [bonded fiber matrix], mixed in with hydroseed, instead of blankets, a decision made primarily from a labor standpoint. BFM is easier to install, and the cost of installing an EC blanket is a “˜moving target’-the labor cost of installing blankets is higher, because you have to stake them down. In my area, select spots may have major erosion problems, but once you get them stabilized, that’s it. We don’t have the severity of hills to deal with as is found in some parts of the Ohio River Valley.”

Photo: Illinois Tollway
Erosion control blankets cover the slopes at the interchange of Interstate 55 and Interstate 355 in Bolingbrook, IL.

Blankets Keep Toll Roads Free of Sediment
Despite the fact that much of the Chicagoland area is tabletop flat, new road construction creates slopes that cannot be allowed to erode. Natural Creations of Joliet, IL, worked on the recently opened 12.5-mile south extension of the Veterans Memorial Tollway (formerly the North-South Tollway), Interstate 355. This major traffic corridor now runs from the I-290 extension in DuPage County’s Addison in the north to I-80 in Will County.

Working with the Illinois Department of Transportation (IDOT) and the Illinois Tollway, Craig Hyslop, Natural Creations’ registered landscape architect and estimator, rarely gets his choice of EC products: “They have definite blanket standards on their jobs. If there’s special provision, perhaps we can use something else. Ninety-five percent of Tollway jobs use EC blankets but, at times, also use other practices for permanent seeding.” Hyslop frequently uses blankets manufactured by Evansville, IN’s North American Green.

Other BMPs are often used in conjunction with blankets and mats. “The Tollway would do basic control with mats; then we’d either hydroseed or apply seed manually and fertilize. We don’t necessarily hydromulch it much.”

over is also applied: “Maybe some projects would use as much as 2,500 pounds of cover to the acre, where Tollway would apply maybe 400 pounds.”

According to Hyslop, IDOT and the Illinois Tollway usually use blankets that decompose in a shorter length of time. “Some sites have accelerated blankets, which contain an ingredient that makes blankets decompose faster; the netting will decompose in 90 days. Especially if you’re mowing in that area, you don’t want the netting to get caught in the mower. Standard netting lasts maybe 12 months. Areas around the airport might use a longer-life blanket, and for sites that are hard to hold or experience prolonged growth, you might use a 12-month blanket.”

Despite the cost, EC blankets are seen as an insurance policy: “Blankets don’t cause problems as [vegetation] establishes, as on steep Tollway slopes-you must prevent washout. With the heavy snows here, the blanket might have to stay there all winter long. Putting blankets everywhere they seed is the safest way to go. Some Tollway slopes are 2:1; the basic construction procedure now uses a 2.5:1 slope ratio. On a slope with a ratio like 3:1, maybe no blanket is needed, and at 4:1, we can stop using blankets; there are no real issues.”

Increasingly, the use of EC blankets is mandated: “The US Army Corps of Engineers and other government agencies are getting more strict about EC procedures,” Hyslop says. “Blankets are there in the specs. When we find line items in the bid, it states how many blankets they intend to use and what variety. They may not specify the manufacturer, but they specify the standards to use.

“Maybe the job will call for 20,000 square yards, for example-sometimes the specification is per acre but mostly per square yard. The bid will also detail what type of blanket to use, but it could be supplied by a variety of companies. We use North American Green, an established company. The standard on most highway projects in general is often the NAG-DS75, a specific weight of blanket that’s rated by how much rain and bad weather it can take. North American Green also makes heavier blankets that can handle more water flows.”

Photo: Register Nelson Environmental
Photo: Register Nelson Environmental
Coir mats line streambanks and channels in Georgia. Right, Natural river rock helps create riffles.

What factors are considered when deciding what blanket to use, and when? “You can’t just put down blankets with seed and do it during the wrong landscape season,” Hyslop explains. “A blanket put down in a drought? The seed will never come up through the blanket. During a freeze? The blanket could also kill the grass. You have to use it during the proper season. The worst time to seed in this area [USDA Zone 5] is June 1 or 15 through August 1 or 15. There’s a two-month period when it’s just too hot to seed. You also have to keep tabs on projected freezing dates. October 1 to 15 is the most dangerous time to have plants freeze, because they’re not dormant yet. Sometimes workers will return on November 15 to seed again. But here, the fall is unpredictable; in 1985, for example, on November 15 plants were already frozen down.”

Despite the mandated use of blankets, can the company sometimes use a less expensive BMP? “We don’t really ever see that anyplace unless it’s a 4:1 slope or flatter; that doesn’t happen on highways,” says Hyslop. “Once in a while, on rural highways, IDOT will use straw mulch, as there are drainage ditches instead of stormwater/sewer drains next to the highways, so there’s not much worry about sediment-which is a concern the cities have, with their sewer systems.”

The 12.5-mile project was literally blanketed: “The new I-355 Tollway is good example-easily 3.4 million square yards of blankets were used, and our company did a big percentage of that. Of the 18 jobs that were let out to bid in that area, we did seven of them. Those jobs almost exclusively used blankets on everything for permanent seeding. The blankets’ cost relative to the overall varies a lot. Once you get past the topsoil aspect of the job, often the costs of EC blankets are 25% to 50%, including installation. Then, of course, you have to seed; we never use seeded blankets, as different seeds are called for, depending upon the site-the seeds can be grass, wetland plants, prairie grasses, or wildflowers.”

Problems with maintenance can occur when using EC blankets on occasion. “Sometimes the netting gets caught when crews are mowing. In some cases they want that ripped off once it starts to deteriorate, after three to six months. But that’s if they mow it short; maybe crews won’t ever mow some of the areas, especially if they’re left natural. Of course, on small, busy streets, mowing’s not a problem; crews still use sod 50% of the time.”

In New Construction, Stopping Problems Before They Start
On new home construction, with lots of heavy-machine traffic, blankets help secure the soil, especially in areas designed for drainage.

In the Indianapolis area, Scott Rau’s Primary Grounds LLC works on many residential projects. “We’ve done a lot for Drees Homes and also Projects Plus, both the engineering outfit and developer. Right now, we’re working on the Villages of Honey Creek in Greenwood, Indiana, which is a new development created in former farmland. We put down more than 20,000 square yards of blanket on this project.”

Rau’s firm is called in during the early stages. “We come in and it’s nothing but dirt. We do erosion control from there. Developers dig out ponds, and we do erosion control and silt fence to county specifications-in this instance, Johnson County, south of Indianapolis. When we go in, and how much we do, depends upon the county or locality-whether we go right in or wait until roads and curbs are in. For example, in the town of Avon, developers must have silt fence in before any digging is done.

“In Honey Creek, which probably covers 40 acres, we were first working on drainage ponds and lakes, which of course also have to be attractive. Crews had to stake out the entire property; some of the older farm ponds were reworked, to improve drainage. We worked all over the site, working specific areas and times, as the construction progressed. Putting in silt fences took a week. Right now, we’re waiting for builders to buy up lots and start building single-family homes-a process stalled due to market economics. We’re still having to put a lot of temporary seeding down, and we’re performing construction containment-maintenance is ongoing. We do a lot of damage repair. Contractors are sometimes not kind to silt fences,” he chuckles.

Primary Grounds uses double-netted straw 0.5-inch blankets from Oakwood, OH’s Enviroscape ECM Ltd. “We also do temporary and permanent seeding with turf reinforcement mats, but overall, we use more blankets,” Rau says.

Of course, if the slope is more than 45 degrees, we have to add other EC controls to the blankets. In this area, the soil is clay-and when choosing which EC product to use, we take into consideration the soil type, site conditions, if it’s going to be a temporary or permanent seeded area, the slope or terrain, the type of traffic it will have, and, of course, the cost.”

A less-expensive BMP can sometimes be used. “When erosion is not a big factor, and it’s the right time of the year, we can overseed, put seed in the ground, and not just on the top. Not a big slope? No blanket. And we seed separately-no seeded blankets, because all projects are different; some are temporary seed, and some are permanent.”

Mowing is definitely a factor in a residential area: “Sure, blankets can get tangled in mowers. We’re now using a three-month blanket, and they break down quicker. The one-year blankets last longer, but people don’t like them even though we get the results we’re looking for. If you use a Bush Hog, it’s not as much of a problem. Mowing and cutting it shorter is hard to do when an EC blanket is in place. Even we can have problems during seeding; our equipment gets into the straw, and net strings get caught in our bearings. To eliminate that problem, coconut [blankets] might be better-but they cost much more money.”

Broadcasting Seed-A Sound Option
Enviroscape products are also used down the road in Trafalgar, IN, by Erosion Control & Land Maintenance LLC.

“We use blankets and mats in conjunction with other BMPs, such as riprap,” reports Owner Matt Davidson. “I rarely hydroseed; Purdue University did a study that indicated broadcasting seed is just as effective.”

Photo: Register Nelson Environmental
Photo: Register Nelson Environmental
Restored stream sections on a former Georgia cattle farm in the foothills of the Appalachians.

As for the blankets’ limitations, Davidson heeds the advice of his Enviroscape rep. “Blankets for even a 3:1 slope usually have coconut [coir fiber] in them. I could use blankets on [slopes of] 45 degrees up to 60, if I had the right product. I would feel confident the blankets could do the job. Of course, it depends upon the soil type. Indiana’s soil is weird-it ranges from brown clay and sand in the north to loamy soil in other places. I also have to know what traffic is going across the area. If I’m working on a retention pond and there’s no swale to the field, I have to take that into consideration. What I can do also depends upon the seed rate-I might use 9 pounds per square foot, because if we can get it in quicker, we won’t need a strong blanket. And, with as dry as summer 2007 was, I was watering a lot.”

There are, however, times when a less expensive BMP can be used. “That has everything to do with the traffic. Is the lot flat? I won’t use the blanket unless the homeowner wants to use them. Is there standing water near the drainage pond? Blankets! I can use a little bit of everything in a job. We’re doing a mound now for a commercial site, and we covered 38 acres with blown seed.”

How much area is covered by blankets depends on the building site. “If it’s hilly, like where we’ve been working lately, the average home site requires two 16 wides. The soil was eroding, and the homeowners hadn’t yet seeded the backyard; we put in two blankets down the yard to the utility easement. In most projects, the cost of the blankets relative to the overall project is about one-tenth; around these homes, it was maybe a fifth.”

Maintenance problems do occur. “We sometimes get them. The blankets blend into an old part of a lawn; then the stronger, older grass pushes it up; and it’s grabbed by the mower. Our solution is to cut it out with a box knife and take if off the site,” Davidson concludes.

Reinforcing Repairs
In Streamwood, IL, Best Lawns Inc.’s projects are mostly multifamily residential. “We make initial plantings,” says Vice President Jim Scheffres. “Homeowners’ associations take care of maintenance. We also bid and give them services for replanting restoration and hardscaping. As for using blankets, we put them mostly on driveway edges, where it has been matted and worn down, along curbs and sidewalks where soil level is low and needs fill, and during re-seeding. We also use blankets a lot when our tree service removes trees and those areas need restoration.

“Of course, they’re used for any repairs on a slope, whether the problem is caused by conditions in the Fox River Valley or manmade. We might also use blankets on a hard spot, to keep sod moist.”

Scheffres’s firm uses Mat Inc.’s Grass-Mat wood-and-corn fiber blankets, which he purchases from his local distributor. “We just used blankets recently on drain tile repairs. We trenched out 18 inches to 2 feet, put in a mat, and stapled it down before backfilling. We also use them in back of retaining walls. For example, in Lexington Village in Schaumburg, which contains 364 units, we did some restoration to a retaining wall. We had to replace old railroad tie timbers. You can’t rebuild with them, so we put in a concrete block wall, which is more decorative and also serves the purpose. The blankets aided the vegetation regrowth. I’d say the blankets were about one-third of the cost of that project, including labor.”

In some applications, Scheffres can use other BMPs with the blankets.

“Penmulch, pellets that expand with water, is another product we use. However, blankets can work anytime of the year. We let nature take care of it, mostly, but in the summer you will have to water it. We usually get germination in all cases, usually in seven to nine days. We don’t do hydroseeding as much as we used to, as it’s harder to get a hose where we need to in a wild area or around buildings. Hydroseeding is more convenient in the front yards, where we’re parked at the curb, or in extremely large areas. Rolls of mat and blankets are more mobile; we can take wheelbarrows of them to where we want and then use 4-inch sod staples to secure the mats. We keep the mowers away from blanketed areas until the grass comes up and pops up the blanket. Normally, though, blankets biodegrade into the soil, unless it’s in a place people are walking through-then we take up the blanket,” Scheffres says.

Stabilizing Banks, Lining Channels
Blankets work well in streambank restoration, as Georgia’s Register Nelson Environmental Inc. can attest.

“We performed stream restoration on sections of seven streams at the Jenny Creek Mitigation Bank, located in White County, Georgia, north of Cleveland, in the foothills of the Appalachians,” explains the environmental consulting firm’s Restoration Projects Manager Josh Goldsmith. “Jenny Creek originates on Horse Trough Mountain, making a 1.4-square-mile watershed. It eventually flows into the Chattahoochee River. The site where we undertook the restoration was a former cattle farm; all of the streams onsite had been channelized, and cattle had been in the creeks for 50 years. When the farm ceased operation, the creek areas were protected in perpetuity via restrictive covenant, with some work by the US Army Corps of Engineers, and restored back to a natural state. There are a total of seven creeks onsite, four of which had channels relocated, and three streams received streambank stabilization.”

In addition to his crews, Goldsmith worked closely with Register Nelson’s Special Projects Manager Mark S. Nelson and with an environmental scientist, Thomas Krebs.

“On certain parts of four creeks, we excavated a brand-new channel,” Goldsmith explains. “With some, we raised the creek bed up with natural river stone, so when heavy rains come, the water can access its natural floodplain.”

The banks and some channels were lined with RoLanka International Inc.’s BioD-Mat 40 and BioD-Mat 70 mats and with some of its jute matting. Of RoLanka’s coir mats, Goldsmith notes,”They retain water well on the banks so seed germination can take place. We stapled the blankets down not only for erosion control, but also for the creatures that will inhabit the creek-it gives them something to hang onto, instead of slippery red Georgia clay, when they climb the banks.” In total, about 4,000 yards of EC fabric were laid out over an area approximately 4,000 feet long (blankets were laid through the channel and up bank sides).

Actual construction time was two months, although from the initial planning stages to completion the time elapsed was a year and a half. Goldsmith estimates the cost of the blankets relative to the overall project was about 25%.

Other BMPs were also used in the project. “We took the streams where we completed restoration “˜offline’ until we had sufficient vegetation to hold the soil. We put up silt fence in a few places where we had loose dirt, but mostly we tried to dig channels offline. We broadcast early-germinating vegetation over bare dirt and blankets the same day it was exposed. Our crews tried very hard not to get dirt in the creek-we have a very good backhoe operator.” The slopes were covered with EC blankets; on this site, he notes, “2.4% was the steepest stream slope. We tried to lay most of our banks back between 1:1 and 4:1.”

Goldsmith’s company considers several factors when deciding which EC product to use. “We use what we feel will work best on a particular site. We like coconut because it lasts seven years, and it’s strong. We haven’t had any problems yet with RoLanka EC blankets, which keep the soil moist and help grass seed germination. To keep streams running clear, we lined the entire channel with BioD-Mat 70 and then placed rocks on top to create riffles on the stream, which also keeps it clean. The additional roughness of the blanket causes heavy sediment to fall out and stay at the bottom of the stream.”

On occasion, the company can use a less expensive BMP instead of an EC blanket. “On top of the banks, if we don’t see active water, we might use jute fiber mats, which last six months to a year-they are there just long enough until the grass takes hold. We usually seed by hand, but we might do hydroseeding where a slope is too steep for us to safely hand seed.”

The company doesn’t use pre-seeded blankets. “We seed separately. Blankets are first and foremost structural. What seed we use depends on the season; in winter we use one form of grass, another in summer. We also plant native trees and shrubs; we cut a hole for putting them in and then staple the blanket back around the plant, securing the blanket with live stakes-willow, dogwood, et cetera.”

Extensive maintenance isn’t necessary on these types of projects. “We never mow-this is supposed to be a natural site. There might be some fabric pull-up after a big storm, but with proper installation, that shouldn’t happen. We don’t maintain it anymore except to get rid of invasive plants,” Goldsmith concludes. 
About the Author

Janis Keating

Janis Keating is a frequent contributor to Forester Media, Inc. publications.