Photo: Nathan D. Maier Consulting Engineers

Chamberlain Crossing before construction.

Photo: Nathan D. Maier Consulting Engineers

Grass was planted at the site to maintain a natural look and prevent further erosion

Creating a stable environment for vegetation to establish itself can slow the water’s flow, while concrete block protection can increase velocity. Channel protection can assume a variety of forms, but ultimately its success depends on making the right choices.

Soft armor, including the use of geosynthetic fabrics combined with a revegetation strategy, can be a good option for channels with slow flows and a low volume. Hard armor, such as articulated concrete blocks, can be a good option for channels directing large volumes of water when increasing the water’s velocity is also desired.

Determining whether or not to select hard or soft armor for a project, however, is typically a site-specific decision.

Whether they are sandy, loamy, clay-like, or rocky, soils have different allowable shear stresses, explains Rae Van Hoven, the drainage design section manager at the Santa Fe–based New Mexico Department of Transportation (NMDOT).

Often, combining soft and hard armor in your site’s channel protection plan is a viable solution for a challenging channel. “Using a combination of soft- and hard-armor erosion control systems is cost-effective and probably more efficient,” notes Van Hoven.

 “Based on the specific site’s theoretical tractive shear forces and related velocity, the designer selects a material that can meet that specific hydraulic environment,” explains Richard Bodie of Dallas-based Pavestone. “Generally speaking, you would use soft armor like vegetated slopes for minimum flow conditions, geotextile materials for medium flows, and concrete materials for maximum flow conditions.”       

Soil conditions can also affect the choice between hard and soft armor. “Soil types are evaluated to select the geotextile underlayment,” says Bodie. “An articulating concrete block system is composed of a concrete block and a geotextile. The block provides the stability against hydraulic forces and the geotextile provides the retention of the soil.”

When stormwater runoff carries unwanted urban pollutants, hard armor may be a consideration, especially if those contaminants threaten to infiltrate the soil of the channel bed.

Planning for Flooding in an Overflow Swale
Chamberlain Crossing in Mesquite, TX, is an example of a project that required a combination of soft and hard armor for channel protection. Adjacent to a major creek, South Mesquite Creek, the channel is similar to “an overflow swale that only gets water in a major flood,” describesMark W. Roberts, a project manager at Dallas-based Nathan D. Maier. The company sought a solution that could handle the volume and velocity of water at the site that had eroded since the company initially completed work at the site in the late 1980s, while allowing the area to keep its park-like atmosphere. The channel was mainly used to convey water, not treat it.

“From a water-quality standpoint, it’s adjacent to a capped-off sanitary landfill,” says Roberts. “Part of the desire was to stop the erosion from going down any farther because it might endanger the integrity of that landfill.”

The company studied zoning maps and determined what the ultimate discharge would be if the watershed area was completely built, says Roberts. Bodie explains that storm event calculations are based on historical storm data for a given region.

 “We designed it for the 100-year ultimate,” says Roberts, noting that a combination of geofabrics and articulated concrete segments was used. “There was a concrete pilot going through the area to carry the low flows. It was eroding out on both sides of that channel.”

 The concrete pieces were placed against the channel for about 10 feet, “and then we transitioned to the fabrics in the areas where there was less shear,” explains Roberts. “We used Conlock II 6VM-30, which is a Pavestone product.”

The channel protection at Chamberlain Crossing is now difficult to find since grass has been planted there. This was part of the design that hoped to successfully maintain a natural look. Vegetation will also help prevent the soil from washing away, aiding with further stabilization.

Preserving Turkey Creek
Flood control while maintaining a natural aesthetic can be a concern with channel protection. The project at Turkey Creek, a waterway along Interstate 35 in Kansas City, KS, in an area that has flooded sporadically for the past 100 years, was one such example.

Geotextile fabric protects the channel in areas of less shear stress.

“They have taken a smaller channel and created a much larger channel. The political process was more of the challenge than anything,” says John Kahl, the project manager and also the president of Terra Technologies based in Overland Park, KS, referring to selling the concept.

Several organizations were involved with the project, begun at the time I-35 was being widened.

“We were working as a subconsultant to HNTB,” says Kahl. “While the erosion control applications were designed by Terra Technologies Inc., HNTB had the overall design responsibility for the project. The US Army Corps of Engineers funded the channel design, but the Kansas Department of Transportation is constructing and funding this phase of the flood control improvements. We’re all working collectively.”

Schrimpf Landscaping of Jefferson City, MO, was the contractor that constructed the erosion control applications, and is working as a subcontractor of Kansas City, MO–based Clarkson Construction Co.

The Environmental Protection Agency voiced concern over the Army Corps’ initial plan, which called for hard armor, mostly riprap, says Kahl. “They wanted to do something to retain or restore or enhance the natural qualities of the area,” he says. In places, Turkey Creek had to be moved 40 to 50 feet from its prior location because of the I-35 expansion, and the channel had to be bigger and deeper than it was initially. Kahl explained that in constructing the necessary flood control, riparian systems, trees, and grass had to be initially stripped from the site. It was important, he notes, for the channel protection design to also put something back into the environment.

After studying the combination of soils in the area and channel hydraulic modeling, the company used monofilament turf reinforcement mats (TRMs) by Colbond with North American headquarters in Enka, NC. The company’s Enkamat products feature polyamide filaments that are connected, with 95% of the matrix that is created being open space for plant roots to thrive and become entwined with the TRM, further stabilizing the cover.

Photo: Terra Technologies

The Turkey Creek flood control project took place during the widening of an adjacent interstate highway.

“It produced effective erosion control for both the short and the long term and provides long-term support to the soil system,” says Kahl. “What we like most about it is that it doesn’t impede vegetation. It’s black and it absorbs heat from the sunshine so it extends the germination cycle.” This was helpful also because approximately 20 acres were seeded with native grasses, which tend to be sensitive to growing conditions, he says.

Installation of vegetation also included 30,000 containerized plants and plugs.

“In the zones of high shear, we specify high-tensile TRMs; in low shear, monofilament TRMs,” says Kahl, adding that in the latter case he used 70/20. “In zones of lower shear, we just seeded the areas and covered them with an erosion control blanket to provide coverage while the vegetation becomes established. The project entails the use of 49,346 square yards of erosion control blanket, 8,608 square yards of TRM, and 11,865 square yards of high-tensile TRM.”

Additionally, thetoe of the slope stabilization covered only by the erosion control blanket was managed by the use of approximately 2,000 linear feet of coir logs.

The watershed itself remains fully developed, but the existing channel “behaved differently than this constructed improvement,” says Kahl.

A large amount of bedrock found during the excavation posed another challenge.

“At some point the sideslopes leave bedrock and go back into soils,” says Kahl, who adds that the soil has become very thin, making it difficult to reinforce with vegetation-dependent systems. “So we constructed a gabion basket right at the edge of that. Above the gabion basket, we worked with the TRM. The gabion resists erosion for the first 3 feet of the application; then once the gabion stops, you have at least 3 feet of soil behind the gabion that we can count on to hold or resist erosion.”

The contractor is required to maintain the site for one year, “culling and controlling undesirable volunteer vegetation to expedite the establishment of the designed vegetation,” explains Kahl.

“This phase of the channel project is approximately 5,200 feet in length,” he says, referring to the part of the project with which his company was involved. “This is the largest biotechnical stabilization project of which I am aware from the perspective of total quantity of engineered materials utilized with a constructed cost of approximately $1.3 million for the biotechnical applications. This cost does not include consideration for the excavation and embankment required to configure the channel that is to be biotechnically stabilized.” The overall project is expected to continue for several years.

Political Considerations: A Vote for a Vegetated Channel
Aesthetics were very important for another ongoing project. Located in Santee, CA, the Forester Creek Improvement Project involves a bike trail that meanders alongside the entire length of a channel that drains a large watershed and stretches through downtown.

Photo:City of Austin

Combining hard and soft armor helped prevent washout at Halls Bayou.

“For the most part, the water channel is being widened, spreading the flow out. It’s going to be vegetated. We’re using freshwater marsh plants and some willow on the edges of the channel. It will look pretty natural when it’s finished,” says Keith Gallistel, project manager for design at San Diego–based Earth Tech Inc., adding that there is a trickle flow for most of the year, though the flow increases during the rainy season.

A large section of the site that is primarily used to convey water will be vegetated. “The plan was designed with articulated concrete block. It will allow vegetation to grow through the block,” says Gallistel, explaining that he selected Carlsbad, CA–based Soil Retention Products’ interlocking concrete block for use in the project. “The velocities are kept low because of the channel being a vegetated channel. The project was designed using metric measurements. I think we’re keeping the velocity down to 2 to 2.5 meters per second.”
Many underground utilities posed a challenge for the Forester Creek project. “The sanitary sewer ran in an area that would be widened for the creek and had to be relocated,” says Gallistel. “In several important places, it’s several hundred feet wide.”

Cost wasn’t a consideration in choosing the creek lining, but environmental policy had an effect on the choices made for channel protection. “It’s hard politically to build hard channels anymore,” says Gallistel, adding that funds were contributed for the project in return for making it a naturalized, vegetated channel. “Environmental concerns are such that it’s just out of the question.”

Channel protection is also often a concern for state transportation departments, particularly because of their occasional proximity to roadways. If the water can’t be moved from roadway expansions, flooding because of streambank erosion can pose a risk for motorists.

“At this time we usually recommend the use of wire-enclosed riprap for channels that receive heavy flows and high velocities. We are gradually recommending the use of available new products,” says Van Hoven. “Several years ago, we did a comparison between traditional riprap and synthetic liner for a high-velocity flow; the cost was about the same. We compared geogrid filled with gravel versus riprap and the cost did not vary.”

Photo: Soil Stabilization Products

Constructing an effective channel system at West Bouldin Creek

NMDOT’s policy requires a site’s developer to regulate its flow. “The policy at the department is for the developer to maintain the existing drainage condition and not allow any increase in runoff,” explains Van Hoven.

State and Military Requirements
Similarly to NMDOT, the Tennessee Department of Transportation studies a site’s shear stress and water velocity when determining the best channel protection method.

“Where turf reinforcement mats are used, an additional check is made of the adequacy of the unvegetated lining. In general, an unvegetated TRM cannot withstand a shear stress greater than 3.0 pounds per square foot,” says Kim Keelor, public information officer for the Tennessee Department of Transportation (TDOT) based in Nashville. “Since the vegetation should grow in within one to two seasons, the unvegetated liner may be checked using the two-year flow rate. In addition, since the roughness characteristics are different for an unvegetated liner, this check is performed using an unvegetated n-value.”  

When the check finds the shear stress will be greater than 3.0 pounds per square foot, she explains, the spreadsheet used by TDOT automatically selects a composite lining using both riprap and the TRM.

“The height of the riprap above the channel bottom will be equal to the two-year depth of flow on the riprap [using riprap n-values], and the overall flow characteristics in the channel are computed using the composite n-value,” explains Keelor. “Where the bed of the stream has exposed bedrock, it is assumed that the bottom of the channel will not be subject to significant erosion, and the designed lining will begin at the top of the rock layer.”

This flow depth is also calculated based on a composite n-value.

“Where the composite channel will consist of rock and TRMs, riprap may also be added up to the two-year flow depth,” notes Keelor, “if it is found that the shear for the two-year flow rate on the unvegetated TRM will exceed 3.0 pounds per square foot.”

Maine’s Department of Environmental Protection encourages the use of an erosion control blanket at the base of a ditch or swale, at least for as long as it takes for vegetation to become established.

Normal-condition nonwoven geotextiles, which are often characterized by their good permeability, are recommended for erosion prevention when the material will not be lowered more than 5 feet onto the geotextile, when there are no sharp or angular aggregates used, and as long as the trench depth isn’t more than 10 feet, as outlined in the organization’s publication “Maine Erosion and Sediment Control BMPs.” When trenches are deeper than 10 feet or angular sharp aggregates are used, the organization recommends using heavier geotextiles.

As state organizations move toward the future of channel control protection, additional considerations are being planned. One of these issues is water quality.

“We do design channels for the water quantity,” says Keelor. “We are not designing them for water quality at this moment. But soon we will.”

Military sites have entirely different issues to address regarding channel protection. A channel project has to be cost-effective for both the military and the land managers and endure heavy wheeled and tracked vehicles such as tanks and humvees, explains Heidi Howard, a natural resource specialist for the Army Corps of Engineers in Champaign, IL.

“We look at the soil properties, the hydraulic flows, what type of training takes place there,” she says, stressing that the corps also looks at the site’s relation to the local community. “We’re very conscientious of our neighbors, and we try to be really aware of soil erosion, wind erosion, noise control—that type of situation.”

Many times, hard armor is not a consideration because, with training activities taking place and possible ricochet potential, it becomes a safety matter.

Howard tends to favor soft armor for most military projects. “It tends to be reasonably priced,” she says. “[The military] has its own in-house labor that it can use to rehabilitate a site.

“We recently worked on a multipurpose training range at Camp Atterbury, Indiana. They had just recently constructed a training range. They experienced significant erosion on one of their stationary targetry berms. We helped them pick out a combination of erosion control blankets, some check dams.”

The site had a south-facing slope and some extreme conditions, posing considerable erosion control concerns. “They were never able to get proper vegetation established,” says Howard. “It didn’t have a good layer of topsoil.”

The site was revegetated with grasses, including fescues, native big blue, and buffalo grass, by hydroseeding because of its slope. The blankets used contained coconut fiber, says Howard, “and also we did a straw blanket.”

Another reason hard armor wasn’t a consideration for this site was because of a need not to increase the water’s flow rate.

The West Bouldin Creek project in Austin, TX, is an example of an effective channel system for a site with short-duration high flows. When the outer bank beside the creek, which is adjacent to a roadway, began to erode, threatening to compromise that roadway, Presto Products Co. provided preliminary design assistance to the Watershed Protection Department of the City of Austin, along with the Merced, CA–based Soil Stabilization Products Co., which offered the department product support. The final design included a Geoweb cellular confinement system, with a sand-colored face and openings that allowed for native shrub plantings for a natural look. The perforations in the Geoweb system also encourage root lock-up, stabilizing the vegetation while allowing drainage through the system, explains Patricia Stelter of Presto Products. Presto is a business of Alcoa that’s based in Appleton, WI.

Combining hard and soft armor was one way washout was prevented at the Halls Bayou project in Houston, according to Stelter. A combination of Geoweb systems was placed on the uppermost slopes and in a stacked configuration at the very bottom of the channel with the addition of concrete.

“The reason concrete was used in the bottom few cells was to prevent washout of infill,” says Stelter, noting that high flows occurred somewhat frequently. “Also, by using the stacked configuration with concrete as opposed to single layering in a trapezoidal-type channel, they were able to maintain the steeper vertical profile of the channel, limiting more land-use requirements.”

As site issues become more challenging, it’s important for an erosion control plan to grow with development.

While water velocity and shear stress are key to determining appropriate channel protection, taking all a site’s needs into consideration when setting your channel protection strategy at a project’s outset can save time and money by completion.