Flexible and durable options
Not too long ago, poured concrete and stone masonry were about the only choices engineers had when they needed to stabilize banks and prevent erosion in channels with high shear stresses. They succeed, but at a cost: Because they don’t allow stormwater to infiltrate into the soil, contaminants are carried along with the water to the outfalls, which often face greater erosion problems than the channels themselves because of the velocity the stormwater has built up along the way. In addition, these channels can look unnatural and interfere with wildlife habitat.
Engineers still determine the most appropriate channel lining based on the volume and velocity of stormwater in a given time frame, as well as the soil type, slope, and size of the site. However, a considerable amount of research and development has taken place through the years, and it’s given them a wide variety of choices to work with.
“There are appropriate solutions to any erosion control project,” says Patrick Dayan, an engineer with Miller Legg, a statewide consulting firm headquartered in Pembroke Pines, FL. “But there are also unknowns, such as site constraints, that drive the project at hand. Sometimes your vision gets shifted.”
Among the effective solutions are articulated concrete block (ACB). They stabilize the channel, slow the velocity of stormwater, and prevent erosion, even in high-velocity channels, as well as allowing water to move between the surface and the subgrade and letting vegetation to grow through them. They’re also flexible, resistant to freeze-thaw cycles, and aesthetically pleasing, and they allow for animal habitat. At the same time, they need little or no maintenance—they may need to be mowed occasionally—and can withstand being driven on. They all work best installed over a permeable geotextile.
Veronica Foster, senior engineer at Golder Associates Inc. in Mt. Laurel, NJ, has been specializing in this area for more than 10 years. “We know products of this nature have a proven record of working effectively,” she says.
Concrete block systems are blocks of either open or closed cells that form a mat. The mats are connected by cables or ropes, which give them their flexibility. Open cells and the spaces between the mats allow water to permeate, soil to infill, and vegetation to grow.
Indian River Generating Station
The Indian River Generating Station in Sussex County, DE, which is owned by NRG Energy Inc., generates electricity from coal. Byproducts, including fly ash and bottom ash, are disposed of in a landfill on the site that will be covered with an impermeable geosynthetic cap once the filling attains the maximum permitted grades to prevent erosion of the ash.
Stormwater from the landfill flows into a manmade perimeter channel that surrounds it. The channel drains into a forebay, a half-acre stormwater pond, where sediment settles out before the water goes into a larger pond, where it infiltrates, says Veronica Foster, who designed the project and specified Cable Concrete to armor the channel.
Preston Ayars, senior project manager for George and Lynch Inc., in Dover, DE, the contracting firm that built the channel in 2008, chose the closed-cell Cable Concrete system from International Erosion Control Systems Inc. in Ontario, Canada.
“The channel is essentially a hard-armored swale,” Ayars says. It was built to convey stormwater runoff from the landfill, its only source of stormwater. The underlying channel surface has sandy soils, a flat bottom 14 feet across, and 3:1 side slopes 1 to 2 feet high—higher on the curved portions than on the straighter ones.
Foster specified Cable Concrete for a number of reasons. The choice depends on the natural grades at the site and the regulations, she says. “But even when local regulations are for a 10-year storm, we design for a 25-year, 24-hour storm.”
A major consideration was the natural flatness of the land, resulting in channel slopes as little as one-half-percent. Shallower slopes require a larger channel than steeper ones and are a lot less forgiving: If a rock is in the wrong spot, you’ve created a pool. Cable Concrete is smoother than regular grass-lined channels, and smoother surfaces help convey stormwater faster than rougher ones.
In addition, she says, “If I used a more traditional lining protection to provide similar armoring, I’d have to use riprap, from 9 inches to 20 inches, and then I might not have the slope, or I’d have to add fill to make the slope higher. Cable Concrete is only 4.5 inches tall, and it can handle higher velocities.” The terrain was a challenge for equipment operators working on the project as well, because it was hard for them to construct such shallow slopes. They had to be very skilled, she says.
Before the vegetation is established, this channel can handle high amounts of runoff because it can convey stormwater quickly to the forebay. Over time, as sediment fills the crevices and vegetation takes root, it will begin to look less like an engineered channel and more like a natural one. If a very intense storm causes some fill in the crevices to erode, the sediment will settle in forebay.
Instead of installing the Cable Concrete over the geotextile, International Erosion Control Systems poured concrete into approximately 16 prefabricated molds to produce more than 400 mats. Each mold was a grid of 15.5-inch by 15.5-inch squares that created an 8- by 16-foot mattress, which was placed in the channel during installation. The top of each square was only 11.5 inches square, though.
“It looks like a pyramid with the top cut off,” Ayars says. “This creates space between each block for sediment to deposit and vegetation to grow, as well as the flexibility to allow the mattress to form-fit the swale bottom and sloped sides.” Crews set long stainless steel cables into the molds to join the blocks together, then filled them with concrete.
Finally, they placed an 8-ounce nonwoven geotextile made of 100% polypropylene staple filaments, US205NW from US Fabrics in Cincinnati, OH, on the base while still curing and attached it to the concrete with wires. They cut it two feet longer than the mats on three sides to provide an overlap on which to place the adjoining mats and prevent the system from being undermined.
“Having the geotextile preattached to the mattress is a little more labor intensive during fabrication, but you don’t have to worry about someone else rolling out the material and getting in the way, or about having the material blowing around during installation,” Ayars says.
The next day, crews flipped the mold over and had a concrete mattress with a geotextile base that conformed perfectly to the terrain, ready to install into the swale. They began laying the mats at the downstream end, allowing six inches as freeboard—the factor of safety above the design depth for intense storms.
“Because the mats are 16 feet long and poured in 15-inch sections, they’re very flexible,” he says. “We had to lift them from four points with a spreader bar or else they’d fold in on themselves.”
The channels will need very little maintenance, Ayars says. They can be cleaned up with a maintenance vehicle if too much sediment starts to form or if the vegetation needs mowing. If a mat needed repair, crews could dig the sediment out and pour in a little concrete.
The price is comparable to riprap; however, Cable Concrete requires much less fill to create the swale’s sloped sides. The entire project took about four months to complete; the Cable Concrete component about one month.
“It was pretty easy,” he says. “We’d never done it before and it went quicker than we’d thought. The learning curve was very short, which helped create an on-time project for the owner.”
View original article here: Channel Linings