Stabilizing the Unstable Site

May 20, 2014

There’s an erosion control blanket or turf reinforcement mat (TRM) for just about every disturbed slope, open area, and drainage ditch. And when they’re used with additional erosion control products, the combination can control virtually every erosion risk.

These products do more than stabilize the soil. They increase water infiltration, retain soil moisture, and protect newly planted seeds from heavy rain and wind.

Erosion control blankets are generally specified for flatter slopes, shorter slopes, less erodible soils, and areas with lower water flows. They’re made of such natural fibers as straw, coir, jute, coconut, or wood excelsior sewn in a net. The net may be biodegradable or photodegradable, designed to degrade within 6 to 24 months as the vegetation becomes established, or a non-degradable synthetic fiber. A few manufacturers offer netless blankets.

Turf reinforcement mats are designed for steeper slopes, longer slopes, highly erodible soils, and areas with higher water flows; they usually last for years instead of months. They are thicker and sturdier than blankets and offer greater protection, but they still provide the void space for soil and vegetation.

TRM fibers are UV stabilized and nondegradable synthetics such as polypropylene, polyethylene, or nylon. The netting that holds these fibers together is typically nondegradable as well.

Proper installation of blankets and mats is crucial. The site must be cleared to ensure that they have full contact with the soil surface. The blanket or mat must be anchored properly.

Additional erosion control products include coir logs and wattles, hard armor such as riprap and gabions, and hydraulically applied products such as bonded fiber matrix. Hydraulically applied products can provide a crust that cements soil particles and provide long-term protection from erosion. These products often consist of a mix of soil binders and mulch fibers, and also may contain seed and fertilizer.

If seed will be planted, the site should be disked to loosen the soil. Soil amendments and fertilizer should be added if necessary. According to the Idaho Best Management Practices Manual for Erosion Control Blankets and Mats, the area should be seeded before blankets are installed, but after TRMs are installed. Always follow the manufacturer’s guidelines.

The following three profiles show three very different projects and illustrate some of the ways blankets and TRMs can be used with additional erosion control products for very effective protection against erosion.

Flood Mitigation on the Assiniboine River
The Assiniboine River flood was part of a monstrous 1-in-300-year flood, according to the National Oceanic and Atmospheric Administration. On May 31, 2011, the river peaked at some 5 feet above normal, 60% higher than the rural area’s previous record in 1923. It overwhelmed the emergency ring dikes that were frantically built as the waters rose. The slow-draining Lake Manitoba, which flows into the Assiniboine, a tributary of the Red River, rose more than 4 feet in one afternoon. Entire communities were devastated.

The previous fall, several major rain events had saturated the soil in the Assiniboine River and Lake Manitoba watersheds. During the winter, a reservoir above the watershed was emptied to prepare for the floodwaters expected in the spring, the ground froze, and twice as much snow fell as in an average year. In the spring, there were major storm events, both snow and rain. The amount of rainfall was above average.

“Flood issues continue to be a problem with Lake Manitoba,” says Keith Porter, president of Mid-Canada Hydroseeding in Winnipeg, MB, which distributes and installs erosion control products in central Canada.

A project to mitigate future flooding of the Assiniboine was part of the Red River Floodway Expansion Project, one of the largest public infrastructure projects in Manitoba’s history. It included the construction of permanent dikes near the river and drainage ditches within the communities and towns that flooded to protect them from future floods.

Mid-Canada installed erosion control mats and revegetated the exposed soil of both the dikes and the drainage ditches in two of the towns: St. Laurent and Oak Point. The company used straw blankets from Erosion Control Blanket in Riverton, MB.

“We worked with the biologist responsible for the province’s infrastructure,” Porter says. “She was pro-straw. It’s grown here, and they’ve had good success with straw mats. They break down rather quickly and they provide enough erosion control.”

Work started the beginning of August 2013, and finished September 15.

The contractor, Camster Construction of St. Laurent, MB, removed the remains of the emergency dikes, then built permanent dikes between 7 and 10 feet long with slopes between 4:1 and 5:1. They were built closer to the river than the temporary dikes had been.

The slopes were designed to be long and gentle enough to not need blankets. Mid-Canada applied topsoil to the graded and harrowed soil and drill-seeded forage seed mixes.

Camster Construction built, graded, and harrowed the drainage ditches as well. The ditches run through the towns and have one-way culverts to prevent floodwater from flowing back upstream during a flood. They slope approximately 5 feet down from the edge of the roads at 3:1 or 2:1, depending on the width of the right of way.

Mid-Canada used a very creative, sustainable method to choose the seed mix. “These are farming communities,” Porter says. “We talked to the farmers in the area and there was a big push for forage mixes. We planted what they wanted, and they’ll maintain the ditches.”

The seed was supplied by DLF Pickseed, which has locations across Canada and Oregon. Seed included mostly grasses, with some alfalfa for forage, red clover for nitrogen, brome, and some others.

The soil is a very poor sandy clay, Porter notes. Mid-Canada placed some seed on the soil and harrowed it in, then combined the majority of the seed with the company’s proprietary product, Biotic Earth, and hydroseeded. Biotic Earth is a hydraulic growth medium that contains organic material, micronutrients, and growth stimulants to reduce soil crusting for improved water infiltration and plant establishment.

Mid-Canada installed some 90,000 feet of S32 blanket from Erosion Control Blanket. “It should last a year and a half, long enough for the vegetation to become established,” Porter says. The blanket is a double-net, 100% agricultural straw fiber matrix with photodegradable nets and thread, designed for use on moderate slope and channel applications.

The project presented only one real challenge. “We’re pretty proficient in getting in and out and getting around construction sites, but in these residential communities there are some very confined spaces. The semis with the blankets couldn’t get in because of the low power lines, so we offloaded the blankets to flatbeds on the highway, one or two pallets at a time. That made it interesting.”

Erosion Control Blanket is a good, dependable supplier, he adds. “We follow the earthmovers, so I’m in the “right now” business, as in “I need you right now.” ECB has been fantastic. They’ll put an extra crew making the mats over the weekend if they have to. There’s no challenge too big for them. I’ve tried to surround myself with vendors like that.”

Wupatki Trails Channel
In late June 2010, the Schultz Fire scorched 15,000 acres of the Coconino National Forest near Flagstaff, AZ.

“The fire devastated the east side of the mountain,” says Terry Miller, co-owner of Eagle Environmental in Flagstaff, a wholesale distributor of a wide variety of erosion control materials. “All the vegetation was gone.”

Then the monsoons hit, dropping up to an inch of rain in 15 minutes. “The entire mountain came down into the subdivisions, including boulders as big as 4 feet in diameter,” Miller says. “There was major flood damage where the drainage channels are. They weren’t able to handle that kind of volume and amount of debris.”

Mitigation efforts began almost immediately, involving the Coconino County Flood District, the Arizona DOT and Department of Environmental Quality, the US Forest Service and the Federal Highway Administration, the Natural Resources Conservation Service, the city of Flagstaff, and private landowners.

Eagle Environmental provided the matting for one of the projects, the Wupatki Trails channel, which extends from the national forest to Lenox Road.

“Coconino County wanted something that would revegetate and able to handle a 100-year flood,” he says. “We were given the specs and decided on PP5, a heavy-duty TRM from Western Excelsior Corp. in Mancos, CO. It’s a great product. It has a job history in other parts of the country and around the world, a price point that’s going to put us at an advantage, and great tech support.”

Work took place in the summer of 2013-monsoon season again.

The weather was a concern throughout the project, Miller says. “They were working within the constraints of small drainages, in the midst of monsoons. If it rained, we were concerned that the hydroseeding would wash away, and they’d have to remanicure all slopes. That would have escalated the costs.”

By 2013, the drainage channel was covered in vegetation. Tiffany Construction of Phoenix cleared, smoothed, and graded the sandy, rocky soil.

Revegetation Services, a hydroseeding contractor and erosion control installer based in Mesa, AZ, subcontracted the rest of the job. The company used 13,000 to 14,000 square yards of the TRM, Miller says.

The project was divided into two sections that ran parallel to each other because of an “S” turn the channel takes, says Jake Anderson, the project manager for the lower section.

The upper section measured approximately 3,400 feet long, with 3:1 slopes 8 to 10 feet high, says Bill Kerr, the project manager for this section. The bottom of the channel was approximately 120 feet wide.

Revegetation Services harvested trees from the burn site and cut them into logs approximately 20 feet long to stabilize this part of the channel. The diameters ranged from 12 inches to as wide as 30 inches.

“The sizing and the fitting of the logs was a challenge,” Kerr says. “We sized and categorized them at the same time Tiffany was grading the soil. That made it a lot easier when we went to build the structures.”

Crews buried some of the logs in the slopes and then laid the PP5 TRM on the soil, parallel with the flow and pieced together, from the top of one slope to the top of the other. The mat is continuously woven for durability and strength. It has improved resistance to hydraulically induced shear, is UV-protected, and has a nominal longevity of more than 36 months.

Revegetation Services placed the majority of the logs on top of the TRM, sawing them by hand to get them to fit together tightly enough. Crews then stacked the logs, from the bottom of the slopes to the top, and ran cables through them to hold them in place.

The lower section of the project ended at Lenox Road. This section was 2,700 feet long and 10 feet wide at the bottom, with 3:1 slopes some 20 feet long, says Anderson.

Crews hydroseeded on the bottom and the slopes of the channel. The seed mix was chosen by the county and consultants.

“We used native seed, soil amendments, a soil conditioner, and a tackifier,” Anderson says.

Revegetation Services then installed 12,472 square yards of the TRM on top of the seed.

On some parts of this section of the channel, crews had to use natural materials only. Instead of installing the TRM, they spread wood chips from the trees that were used on the upper section as mulch.

“After we finished, we made one more pass with the hydroseeding equipment on the road that was cleared for us,” Anderson says.

Both sections of the project, which was completed in August 2013, were very straightforward. “We have a relationship with Revegetation Services, too,” Miller says. “It’s critical that the mats are installed correctly. They did a very good job.”

It’s good to see that the county is stepping up, he adds. “We all have constraints with revenue. They’re doing their best to address the problems-and using products that will give the best quality to protect the residents.”

Enterprise South Mass Leveling for Volkswagen Assembly Plant
In 1942, the US Army Corps of Engineers carved out a site in the green, heavily wooded hills outside Chattanooga, TN, for a facility that produced TNT for World War II. The facility continued to produce explosives for the two successive conflicts in Korea and Vietnam.

Now, the 1,200-acre Enterprise South Industrial Park occupies part of the site. It’s home to the Tennessee DOT, distribution warehouses for companies such as Amazon, and a Volkswagen assembly plant.

In March 2013, land clearing began for the construction of new buildings and parking lots for the plant.

“We did all the erosion control, matting, seeding, silt fence, and wattles,” says Peyton Womble, owner of Womble LLC in Henderson, TN, which works on large industrial and commercial projects throughout Tennessee and Northern Alabama.

The company used three different products from East Coast Erosion Blankets, which is headquartered in Bernville, PA: double net straw matting, coconut TRM, and straw-coconut TRM.

“The key to being successful in this matting industry is to provide quality work, service, and products to the client, with no mistakes,” Womble says. “In order to do this you have to have great suppliers with great products. East Coast Erosion Blankets is one of them.”

First, Womble installed 21,000 linear feet of wire-backed silt fence around the entire 240-acre site. Then Plateau Excavation of Austell, GA, cleared the brush and grass, drained and filled some low, wet areas, and leveled the site. Plateau also dug drainage ditches with 3:1 slopes on the perimeter of the flat building areas.

“Plateau started with stripping off 50 acres at once,” Womble says. “They’d get 10 acres of the clay soils graded and top them with topsoil from elsewhere on the site. As soon as they got the topsoil done, we’d come in and stabilize. After the stabilization was complete, Plateau would be allowed to strip off another 10 acres. This process carried on throughout the job, so there was no time to stall. There were millions of dollars of equipment waiting on us to complete the area that was ready so they could proceed with production on the next area.”

When the 50 acres were complete, Plateau would strip off another 50 acres.

On the flat areas, Womble spread seed, lime, and fertilizer by broadcast method, and then lightly disked them in. Crews covered this with blown wheat straw and crimped in the straw.

They covered all 3:1 slopes and the sides of all permanent drainage ditches with some 125,000 square yards of ECS-2 Double Net Straw matting. This matting is made with 100% agricultural straw and two polypropylene nets sewn together with degradable thread.

In the center of the permanent drainage ditches, where the majority of the flow will be, crews installed 9,200 square yards of ECC-3 TRM, which consists of 100% coconut fiber. In the temporary drainage ditches, they installed 19,100 square yards of ECSC-3 TRM, which is made of 70% agricultural straw and 30% coconut fiber. Both these mats are made with three polypropylene nets sewn together with UV-stabilized thread and are suitable for slopes as steep as 1:1 and for high-flow channels.

Crews also used 42,000 square feet of 20-inch straw wattles from L&M Supply in Willacoochee, GA. They placed them around the top edges of the permanent drainage ditches to filter the water before it entered the ditches, as well as on the long 3:1 slopes about 20 feet apart to slow the water flow down the slope.

England Erosion and Seed Supply in Nashville supplied the seed mixes, which were TDOT-approved. The project required permanent seed mix for a total of 1,161,600 square yards, as well as temporary seeding over 798,600 square yards.

“We used temporary seed only when we had to,” Womble says. “When it got to be winter we used temporary and permanent seed.”

Short-term vegetation from the temporary seed covers the ground more quickly than permanent vegetation, and covering the ground quickly was a priority in this project. As much as 2 inches of rain can fall within an hour in Tennessee in the spring and summer, and occasionally 5 inches can fall in 24 hours. That much rain would wash topsoil away, requiring Plateau to regrade and truck more topsoil in.

Womble’s office was about two and a half hours from the job site, so he would discuss with Plateau during the day whether his crew would go out the next day or not. Sometimes Womble would send them out in the morning; other times he’d send them out the evening before and put them up in a motel.

If scattered showers were predicted, the crew would go to work and watch the radar as they were working, he says. “If it started to downpour, we’d sit in the truck for a while to see how long it would last, because in Tennessee, scattered showers can last either five minutes or five hours. You never know.”

If rain was predicted but didn’t come, Womble often would have to mobilize with little warning. The company had stock in its warehouse and onsite but occasionally had to get supplies shipped on very short notice.

“When you’re working with quality material and great suppliers such as East Coast Erosion Blankets, England Erosion and Seed Supply, and L&M Supply, you don’t have to worry if they have it in stock or when they can get it to you. That means a lot when you have to meet the extremely tight schedules on these industrial projects,” he says.

The erosion control portion of the project was completed in January 2014.

“The job went pretty smoothly overall,” Womble says. “My guys have been with us for a while. They do good work. And we’ve worked with the contractor for seven or eight years, so we know how each other works.”

About the Author

Janet Aird

Janet Aird is a writer specializing in agricultural and landscaping topics.