Choices in Channel Protection

March 1, 2003
Rushing waters eating away at road embankments or undermining bridge pillars is not a pretty sight. Preventing these disasters is often the job of an erosion control contractor, and the availability and variety of tools for the job are incredible. If you have a bank stabilization project, how do you know what products are available? How do you choose?Rock is the first choice many people think of, although it is not always the best choice. Rock has been used for channel protection for centuries, but it has drawbacks. If the rock is too small or too brittle, you will face heavy maintenance. Rock also can add a roughness coefficient that is not always desirable. Many times rock can be used, but the criteria for the size and type of rock must be carefully considered. Just throwing rock on top of a fabric lining is not a good solution. The hydraulics of the channel must be studied so an exact plan can be made.Other solutions exist for channel protection. They range from “very soft” products (such as turf mats) to “soft” products (such as Geoweb systems) to hard-armor revetments. This article considers hard-armor systems known as ACBs (articulated concrete blocks) and geogrid systems.Factors to ConsiderThere are several factors to consider when deciding on a channel protection product.Type of Stream and Speed of Water Flow. ACBs work exceptionally well in channels that carry a heavy amount of water at moderate to high speeds or streams that tend to “flash.” Streams with lower flows might be adequately protected with soft-armor products.Cost. ACBs and web systems usually cost more upfront than locally available rock. In the cost equation, however, you must also consider maintenance. The rock might require maintenance every few months, while ACBs and geogrid systems generally need little care. When maintenance is performed, the smooth surface provides easy access and clean-out of the streambed. Aesthetics. Open-cell ACBs or grids can be planted with grasses to provide a green bank that resembles a natural bank. They also allow infiltration of stormwater. Accessibility. If the site is not easily accessible to large trucks, hand-placed ACBs can be installed. Grids come in rolls that are much lighter than rock or ACBs.Strength. Because of their concrete ingredients, cabling systems, and smooth faces, ACBs have higher shear strength than rock or soft-armor solutions.Hand-Placed StrengthRussell Reeves, co-owner and operations manager of Mid South Erosion Control Services in Golden Meadow, LA, has used both hand-placed and mattress-type hard-armor products. In 2001, for a state flood-control project in Lake Charles, LA, Reeves’s team installed 33,000 ft.2 of Conlock II. Manufactured by Pavestone Inc. of Grapevine, TX, each hand-placed block has two ears and sockets that interlock for strength on contoured surfaces. The site was along the Antoine Gully on the discharge side of a pump station. During rainstorms, about half of the town’s sewers drained into the area and the stormwater was pumped into Antoine Gully. With heavy rainfall amounts, the area wouldn’t drain, and the water threatened bridges of I-10 where it crossed the gully. To prevent undermining the bridge columns, hard armor was specified for the project. Conlock II blocks were chosen instead of rock or mattress products because of the inaccessibility of the site. “There was no way an 18-wheeler could get down to the site,” says Reeves, “so we used hand-placed blocks and transported them down there with a four-wheel-drive forklift.” Project planners also wanted a smooth transition rather than the roughness of rock, and they wanted vegetation to grow. The open spaces in the blocks provide good footing for vegetation.The channel varied from 160 to 180 ft. wide with 3:1-sloped sides. Reeves’s six-man team spent about 10 days on the project, first laying down 7-oz. nonwoven fabric lining and then installing about 250 lin. ft. of Conlock II. The blocks were backfilled with onsite materials and then hydroseeded with native grasses. Another project Reeves contracted was for a gas pipeline company, Williams Energy Services. A tributary of Nonconnah Creek in the south part of Memphis, TN, drains a corporate area and flows into the creek. The section was experiencing heavy flooding. Erosion had exposed parts of two gas pipelines, and the company was looking for a solution to the problem area. Reeves soon saw the problem: One day after his team arrived to start work, a 2-in. rain caused the creek to rise 12 ft. and covered some of their equipment. The water receded very quickly, causing problems associated with both fast-rising and fast-receding streams. The work site was inaccessible to big trucks, so the project called for about 1,000 lin. ft. of hand-placed material. A nonwoven fabric was installed over the slope and bottom, and Conlock II blocks were placed on top. The blocks were backfilled and seeded with Bermuda grass and Kentucky bluegrass. “The company is very pleased with the outcome of the project,” says Reeves.
Rush Creek before ACB placement
With blocks in placeDave Linder of Linder Construction in Fort Worth, TX, also uses the Conlock II block system. The Army Corps of Engineers (USACE) needed to prevent flooding of a business park along Rush Creek in Arlington, TX. The creek also passed under a state highway. Project planners specified ACBs instead of rock because of the tendency of rock to retain tiny pools of water, creating breeding grounds for mosquitoes. Linder placed a 9-in. rock base, fabric lining, and Conlock II blocks down the channel bed and along the 2:1-sloped sides for a distance of about 75 ft. An 18-ton concrete toe was put on top. At the end of the armor, energy dissipaters were placed before the point where the flow passed into the natural channel. The blocks allow maintenance crews to run equipment down the channel for clean-out, and the flooding problem has been solved. The ACBs also stay in place over the expansive soil that is in the area.One Fort Worth – area job originally specified rock-filled gabions for drainage through a residential area. However, Linder states that even though his bid with modular block walls and Conlock was $40,000 over the rock bid, his company was selected. “We got the job because it looks good,” Linder says. Loves Park Creek in Loves Park, IL, drains stormwater in a residential area. At one time, parts of the creek had been surfaced with concrete. Homeowners were not pleased with the look, and over time and due to freezing and thawing cycles, the concrete had broken apart.USACE proposed using Conlock II to replace the channel’s old concrete, and the plan was approved. More than 274,000 ft.2 of the ACBs were placed; homeowners are happy about the green look, and stormwater runoff is under better control. Rock riprap would have needed to cover the channel at a depth of 4.5 ft. to provide the same erosion control. The final cost of $1.1 million was actually about half of the estimated cost of riprap. Mattress ProductsMattress products include ACBs that lock together but are also attached by cables running both longitudinally and laterally. A section is put in place by a crane, and then the cables are attached to the next section. The cables are usually stainless steel, galvanized steel, or polyester. The mattresses can be installed along a sloped side or under the water line. For a Fort Worth project to protect the bridge embankments on the Trinity River, Mid South Erosion Control Services installed Armorflex, made by Armortec of Norcross, GA. Armorflex consists of concrete blocks made from 4,000-psi concrete and varying in thickness from 4.75 to 9 in. The blocks interlock and are connected by cables into mattresses that can be placed by a crane with a spreading bar, even under the water line. The blocks are placed over a fabric lining and have open spaces that can be backfilled and planted. The flexibility of the articulated system allows for placement on contours and also gives excellent freeze and thaw heaving protection. Blocks can be closed-cell with little infiltration or open-cell with about 20% open area. Mid South installed 70,000 ft.2 of Armorflex over a fabric base.
An ongoing project by Mid South is taking place along the Medina River near San Antonio, TX. During heavy rains last year, a bridge washed out on County Road 471. To prevent this from happening again, Reeves says the company is doing fabric-forming techniques at the bottom of the riverbed, topped by ACBs.For an embankment overtopping at a dam spillway, Ayres & Associates of Eau Claire, WI, also chose Armorflex supplied by Construction Fabrics and Materials Corporation of Cottage Grove, WI. The hydroelectric project no longer had the spillway capacity required by new regulations. Dean Steines, water resources engineer for Ayres & Associates, says several ways to increase the spillway were considered, including fused-plug, cast-in-place concrete and overtopping protection with ACBs. The ACBs proved to be the most economical.The project is just passing from design stage into the construction stage. The plan calls for a toe with a stilling basin just below the ACBs. The spillway area will be used as a road, and the ACBs will provide a smooth surface. Although the spillway is a minor one that is seldom used, protection is still needed. The open cells were chosen so the area can be reseeded after construction. Future maintenance should be minimal, improving the cost of the project over the long term.Ayres & Associates is also designing a project for the Fish and Wildlife Service in Iowa. A marsh area and system of ponds are separated from the Mississippi River by a low levee. During high-water events, the levee is overtopped and the marsh area receives new water. But occasionally, according to Steines, the levee is “blown out.” To keep the advantage of the water overtopping the levee and also to provide strength, ACBs will be installed on the top and sides of the levee and tie into riprap across the bottom. The expected conditions are not extreme, but the water velocity will be fairly high at times.
Armorflex along the Stevens Creek ReservoirArmorflex was the product of choice for an award-winning project in northern California near Cupertino. A portion of Stevens Canyon Road between the upper portions of Stevens Creek Reservoir and Mt. Eden Road experienced flooding from the adjacent Stevens Creek, which carries runoff to a reservoir. In the summer of 2001, an interim project was constructed, designed to accommodate a 10-year rainfall event by raising the road on the existing alignment. Nolte Associates of San Jose, CA, was chosen for the job. Chris Metzger, P.E., vice president with Nolte, describes the hydraulic analyses performed to determine the best course of action. “We found that the rock riprap size required was quite large – up to 4-ton rock – that it would be under the roadway section, and that it could reduce the channel cross-section. Therefore, a slope protection that could withstand velocities over 15 feet per second and that had a low profile was desired.”About 20,000 ft. 2 of Armorflex was anchored along the creek. The project received the 2001 Project of the Year from the South Bay Area Chapter of the American Public Works Association. In a huge project in Florida, more than 1 million ft.2 of ACBs were installed in a canal in the Lake Worth Drainage District of Palm Beach County. It was the largest ACB project yet in the state. The canal, which links two recreational lakes in the area, carries boat traffic and stormwater runoff. The channel banks had to withstand not only water velocities but also waves from boat wakes. Hard armor was needed to control erosion; Petraflex was the type chosen. Manufactured by Petratech Inc. of Minneapolis, MN, the articulated block system is cabled in two directions, giving strength and flexibility.The 2.5 lin. mi. of canal required mattresses of two sizes: 8 ft. wide x 36 ft. long and 8 ft. wide x 24 ft. long. Trucks delivered the Petraflex sections to the site each day. The sections were placed in the canal and under the water level by a crane using a spreader tool. The mattresses were installed over a liner, and the cables of each were attached to the adjacent mattress. Crews fit the mattresses around curves, culverts, bridges, and homes. In a few months, vegetation filled the voids in the blocks. Web-Type Soft Armor
Some soft-armor systems consist of three-dimensional web or gridlike structures of high-density polyethylene or similar material. The grids, once unrolled on-site, can be filled with rock, concrete, or soil that can then be seeded. Such products include the Geoweb Cellular Confinement System by Presto Products of Appleton, WI, and Slopetame2 by Invisible Structures of Golden, CO. Peak storm flows had increased significantly in Ontario, Canada’s Ruisseau Leamy channel after nearby residential construction. The City of Hull built a standard retaining wall for the steepest part of the slope and installed Geoweb CCS and a geotextile lining over the rest of the channel. Under the water line, the Geoweb sections were filled with rock; above the water, they were filled with sand and then seeded. Individual sections were anchored to the bank with J-pins. A stretch of the San Antonio River in Texas also was stabilized with Geoweb CCS. Southwestern Bell Telephone Company needed to stabilize a section of riverbank across from the historic Brackenridge Park, so aesthetics was a prime consideration. Geotextile fabric was placed on the prepared bank, and 8-ft. x 20-ft. x 8-in. J-pins were used to anchor the Geoweb sections. The flexibility of the product allows for curvature around the river edge. The cells were infilled with soil and seeded; the riverbank is stabilized but presents a natural-looking green cover. Stabilizing the bank of the Chattahoochee River in Atlanta, GA, was a challenge for Beers Construction. The project originally called for an ACB system, but the client, Total System Services, requested the beauty of a grassy look without the concrete frames. Beers estimated that installing Slopetame2 would cost less than a third compared to ACBs, so the project advanced. Fabric backing was laid on the bank, and the Slopetame2 system was anchored with Duckbill anchors. The cells were infilled with sand and overlaid with Bermuda sod. The grass roots reached a 6-ft. depth in about a month. Slopetame2 can also be shipped in rolls with sod already in place.Buy the ComboFor many projects, a combination of hard- and soft-armor products works best. You might need to use a heavy-armor system in the channel bed and lower sides, tying it into a softer product, such as a vegetation-filled geogrid.