Bioengineering for Soil Stabilization

Jan. 1, 2005

From the vegetated reinforced soil slopes on the banks of Johnson Creek in Portland, OR, to an erosion control demonstration project on the banks of wind-whipped Lake Winneconne in Wisconsin, geosynthetic materials are going where they’ve never gone before in an effort to accomplish a much wider range of functions.

Photo: Winnebago County Land and Water Conservation Department A rock toe in front of the TRM protects the front edge of the shoreline.
Photo: Winnebago County Land and Water Conservation Department Crews install Deltalok bags at a demonstration site on a Wisconsin lake.

While the evaluation continues at the Lake Winneconne project in Winnebago County, the Johnson Creek project makes it clear that geosynthetic reinforcement materials are providing the extra muscle to make soil bioengineering a more viable option to stabilize and restore soils along streams, rivers, and shorelines.

The Vegetated Reinforced Soil Slopes (VRSS) system “uses living vegetation purposely arranged and imbedded in the ground to prevent shallow-mass movement and surficial erosion,” according to Robbin Sotir, a soil bioengineering consultant and owner of Robbin B. Sotir & Associates in Marietta, GA.

Geotextiles and geosynthetics weigh heavily in the equation, too. Sotir’s projects rely on geosynthetic reinforcement, geocomposite drains, and erosion control fabrics to safely and economically complete projects on grades of 45–70 degrees. “Areas where we couldn’t work before can work in comfort,” says Sotir, whose clients include municipalities, counties, the National Park Service, the US Army Corps of Engineers, the US Fish and Wildlife Service, and nonprofit organizations.

What should erosion control professionals and the general public make of soil bioengineering in a world that is, among other things, a monument to sophisticated engineering and manmade structure? Is the resurgence of soil bioengineering just a case of nostalgia, where everything old is new again? Is it an example where old approaches really are the best solutions? Or, is it a case where erosion control experts have gained so much new knowledge about engineered products and the use of plants in soil stabilization that we really are working with breakthrough approaches?

“It’s a combination of several of those things. First and foremost, what sometimes gets confused is the question of whether we’re going to do engineering or plants. It’s very important to understand that the engineering comes first. The engineering has to be right. The hydraulics, the alignment, the cross-sectional areas all have to be right. When that foundation is in right, then you bring in the ecological components,” says Sotir, whose recent work includes several erosion control demonstration projects on steep natural terrain in Hong Kong. “What we’re trying to do is bring them together.”

Photo: Robbin B. Sotir and Associates
Photo: Robbin B. Sotir and Associates
Photo: Robbin B. Sotir and Associates A tiered design with a sub-channel handles bank-high flows and provides waterfowl habitat at Portland’s Jonson Creek.

Sotir specializes in soil-bioengineering techniques for stabilizing soil, especially on steep slopes where retaining walls or some other “hard armor” methods have traditionally been employed for stabilization and to control erosion.

So-called soft-edged systems–those that use the manmade strength of geotextiles and the natural assets of vegetation to create a sum greater than the parts–produce more environmentally friendly impacts on water temperatures and landscapes, Sotir contends. “Walls tend to heat up and raise water and air temperatures to the detriment of aquatic life. A vegetative site will be cooler and offer air-cleansing benefits. In addition, an earthen VRSS system will absorb sound and function as a true noise-abatement device, as opposed to a wall, which transfers noise. It can also reduce stormwater runoff and reduce pollutants.”

As a result, one clear-cut advantage of soil bioengineered projects is that government permits are easier to secure than their hard-armor counterparts. Another benefit: Soft projects are frequently, though not always, less expensive to build and maintain than their hard-armor counterparts.

“Slope-facing requirements to prevent erosion and scour will depend on the anticipated velocities, shear stresses, type of backfill, slope angle and other design parameters related to reinforcement spacing and type,” wrote Sotir and B.R. Christopher, an Atlanta geotechnical engineer in a paper presented at the 2003 North American Geosynthetics Society Conference in Winnipeg, Manitoba, Canada. “Most often in highly confined urban channels reconstructed to handle additional stormwater runoff, the slope face (outer slope profile) or surface requires wrapping to prevent erosion. Additional treatments or selections with higher carbon content may be required on the face to protect and/or shade the geosynthetic and prevent ultraviolet light exposure that may degrade the geosynthetic over time.”

The Johnson Creek realignment occurred as a result of the Oregon Department of Transportation’s (DOT) highway and bridge construction plans. A relocated section of the stream was shortened by about 20% and its grade was increased by roughly the same amount. People who lived near the stream expressed worries that the realignment would reduce wildlife habitat and diminish water quality, Sotir says.

“The stream reach is in a highly visible location, and there was concern that the proposed riprap rock channel designed by the DOT would present a stark, sterile appearance and cause loss of habitat, water quality, and aesthetic benefits,” Sotir and Christopher observed.

In addition, evaluations of the proposed design concluded the “channel cross-section shape and gradient were too uniform and that the floodplain berms were too high.” The alternative was a tiered compound channel design, installed during the winter and spring of 1993-94, that included a sub-channel to handle bank-high flows during the wetter months and provide habitat for nesting waterfowl during drier months. A low-flow channel concentrates flows during drier months.

On the other side of the country, the Long Leaf Stream Flood Control Project in Wilmington, NC, was needed in 1999 when “continued development of the watershed caused the creek to deepen and widen, a result of increased stormwater runoff and high-concentration events (hurricanes). Properties along the stream were experiencing flooding, erosion, and bank failures. Additionally, loss of aesthetic values, riparian corridor vegetation, aquatic and terrestrial habitat, degradation of water quality, and increased maintenance needed to be addressed,” Sotir and Christopher wrote.

Photo: Robbin B. Sotir and Associates Long Leaf Stream’s VRSS system withstood a 500-year storm event.

The site featured added challenges of weakened banks of up to 18 feet high and a volatile sandy loam. When all the options were considered–grass slopes, riprap, soil bioengineering, and a concrete channel–Wilmington’s citizens chose to construct a VRSS system complemented by several other soil bioengineering techniques designed for a 100-year flood event. Their decision was validated in a few short months when two hurricanes, Dennis and Floyd, pounded the state and created a 500-year event. Aside from some toe failure that eventually required repairs, the system withstood the pounding and has continued to meet performance expectations.

The Oregon and North Carolina projects had different objectives, but in each case the result of installing the VRSS approach of structural and living elements was a low-cost, low-maintenance system of “safe steepened slopes that are aesthetically pleasing, provide habitat enhancement, air quality improvement, temperature modification, and flood resistance.”

Now in place for 10 years, it’s clear that the Johnson Creek realignment project has given Portland residents what they wanted. But beauty is in the eye of the beholder. Sotir recently ran into surprising resistance when she proposed a soil-bioengineered approach to a neighborhood flood-control project in a Midwestern state. “People said they didn’t want all these shrubs and bushes. They said, ‘We want a concrete channel, just like the rich folks upstream.'”

And, she acknowledges with bemusement, “I learned a long time ago that an engineer might prefer a wall over plants.”

Not as Easy as It Looks

In some instances, a potential hurdle to the acceptance of soil bioengineering is the ability to persuade the public that the method involves much more than simply digging a few holes and putting some plants in the ground.

Photo: Metamorphosis Hydroseeding and
Erosion Control
Asphalt is removed from a parking lot to make way for a landscaping project.

Thomas G. Franti, a surface water management engineer with the University of Nebraska Extension Service in Lincoln, has noted that the “use of bioengineering methods dates back to 12th century China, when brush bundles were used to stabilize slopes.”

“In the early 20th century, similar techniques were used in China to control flooding and erosion along the Yellow River. In Europe, especially Germany, bioengineering methods have been used for over 150 years. Documented use of bioengineering in the United States dates to the 1920s and ’30s,” Franti wrote in 1997. “Streambank stabilization, timber access road stabilization and slope restoration were common applications. After World War II, with increased access to earth-moving equipment and the development of new structural slope stabilization and erosion control methods, bioengineering practices all but disappeared. In the last 20 years bioengineering has been recognized as a re-emerging technique to provide erosion control, environmentally sound design and aesthetically pleasing structures.”

The clever but decidedly low-tech methods of 12th century China and successive generations of the technique may have served to shape inaccurate perceptions about soil bioengineering. “The fact that we’ve been using vegetation for thousands of years to stabilize soil leads people to think it’s very easy to do, but there’s a difference between concept and execution,” Sotir says. “You have to bring all the aspects together, from making sure you have a good, strong site assessment to putting together the right team to having the proper onsite supervision. We could have a have a great design, but if we’re not onsite we could end up with a bad construction job and the project would fail. It’s not always easy to get people to accept the idea that you have to have good, strong on-site supervision, but if you don’t do that you really open yourself and the client up to a lot of problems. It’s an insurance policy for everyone.”

The project doesn’t end when the installation is complete. Sotir emphasizes that site evaluation and monitoring must be conducted for at least two years–and appropriate maintenance provided–to ensure the project’s results.

That’s the situation that Keith Marquardt, project technician with the Winnebago County Land and Water Conservation Department, conservation technician Chad Casper, and their colleagues find themselves in with the demonstration project near a public boat landing in the Winnebago County Park in Winneconne. The department built public interest in the project at a 2003 Shoreline Forum sponsored by the University of Wisconsin Extension Service, the Wisconsin Department of Natural Resources (DNR), and the Fox Wolf Watershed Alliance and through other avenues of communications. (Information about the project can be found online at

“We’re testing the long-term effectiveness of several different types of shoreline-protection methods and geosynthetic products that have traditionally not been used in the watershed, and we wanted the project to be in a spot where people could see it,” Marquardt says. “We chose that spot, too, because Lake Winneconne has been designated as a high-energy environment.”

The Wisconsin DNR designates waterways in three energy categories–high, medium, and low–based on the size of waves, boat wakes, and ice shoves. All three forces pose serious threats to shoreline integrity. (Medium-energy waterways are identified using a model of indicators, which help state officials decide if they will issue permits for hard-edge systems. Alternative techniques are required on low-energy bodies of water.) Rock riprap has been the traditional choice to protect eroding shorelines and streambanks in the area, Marquardt notes.

Turtle-Friendly Waterfront

“Rock is cheap. It’s relatively easy to install; if you’re trained to operate a dump truck properly, you can do it. And it’s certainly effective,” Marquardt explains. “But it destroys the natural vegetation along the shoreline and it creates barriers for shoreline birds, mammals, and reptiles, especially nesting turtles. We saw new state regulations coming and still, being conservationists, we felt we needed to be in the forefront. We felt we needed to try alternatives to our routine methods so we contacted several companies. Their products are going to get a good workout at the Lake Winneconne location.”

Winnebago County conservation officials installed the four systems in sections of approximately 60 feet, separated by 25-foot sections of the existing riprap. Those systems are:

  • Deltalok bags, which are made of fabric and filled with soil. The bags are stacked atop interlocking blocks of concrete along the shore to prevent erosion, and plants were added to provide wildlife habitat.
  • Gabion baskets, wire-mesh structures filled with rock and soil. The baskets were lined with coconut fiber to hold soil in place and allow plants to grow. Dogwoods were planted in one area between two rows of overlapping baskets. In the other section, a gabion mattress was installed and planted with several different species of plants. Once the plants become more mature, this type of shoreline protection should provide more habitat than traditional riprap.
  • Recyclex, a turf reinforcement mat made from 100% recycled post-consumer goods—green soda bottles. There are 20 green soda bottles in every pound of the TRM. A portion of the Recyclex mat was left unrolled to create a “log.”
  • Turf reinforcement mats to protect the soil. Vegetation was planted into the fabric to further stabilize the soil and to provide habitat. A rock toe was placed at the water line to protect the front edge while the vegetation becomes established.

“The demonstration as it relates to the turf reinforcement mats is whether the mats can hold up against the conditions,” Marquardt says.

Jeff Herlocker of Chicago is a regional manager for SI Geosolutions, the firm that provided the TRMs for the project. He notes, “This is a demonstration that we feel comfortable enough as a company that the products will perform well. The geotextile backing is very important, because if the TRMs are very open the soil particles can still get pulled through the mats and there will still be migration of the soil particles underneath. The geotextile material really provides that layer of protection.” He notes that excessive wave action can undermine TRMs, causing crevices. “That’s why it’s important to do all you can to encourage the germination of the seeds you plant,” he explains.

Marquardt says county officials expect to monitor the four projects for five years, but the project has already demonstrated one of the challenges that soft-edge techniques can pose: Initial progress is at the mercy of the growing conditions for chosen plants.

“The trick has been to get the vegetation established. We’re a couple steps away from that because we’ve had both extremes. The project went in late August of last year, which is late in our season anyway, but we were also experiencing drought conditions. This year, the site was underwater,” Marquardt recalls. “The trick has been to deal with the wave action this year, so companies have set up wave arresters on some spots of the project. I also want to work a little with polymers and I want to work more closely with the botanists to bring in more ideas because there is a real science to plant selection. But there is definitely a future here for the geotextiles and the soft systems.”

Marquardt has also learned a lesson about backfilling the geotextile devices, noting, “I got the clay free from our county highway department. Because this area has a lot of clay, that’s what we ended up with. Clay compacts, and that’s just what it did there. When the water came up this spring, chunks of the clay were just falling off and taking grass with it. We won’t do that again.”

Photo: Metamorphosis Hydroseeding and
Erosion Control
Gravel and plants requiring little water complete the landscaping.

On the plus side, Marquardt has already seen more wildlife in the area–particularly shoreline birds.

Noting that 30% of Winnebago County is covered with water, Marquardt says the Land and Water Conversation Department is hopeful that the alternative erosion control methods capture the public’s imagination and work their way into use. “We have a list of interested people with the goal of cost-sharing with them to put in vegetative buffers, because nonpoint-source pollution is a big issue for us. Vegetative systems keep out some of the sediments and they obviously take care of some of the runoff.”

He adds, “I can’t thank the manufacturers and suppliers enough for their willingness to test their products in situations that may not be the right application. That’s going to help us gain a lot of helpful information during this process.”

Tried and True

Of course, geotextile materials have a proven track record of results on smaller projects, too, notes David Franklin, vice president of Metamorphosis Hydroseeding and Erosion Control in Napa, CA. The firm recently alleviated a client’s stormwater runoff problems by converting a hard-surfaced area into an environmentally friendlier soft system.

“One of the main issues we were dealing with instead of erosion on the site was stormwater runoff, both in quantity and quality, that had an impact downstream,” he says. “What we did was reverse the trend. We removed the impermeable surface and replaced it with a functional and aesthetically pleasing dry creek swale runoff structure.” Anchoring the system was 3,600 square feet of woven geotextile material. An immediate positive benefit was the control of weeds on the site.

“I’ve used the product before where we install it and cover it with 3 to 6 inches of good soil, knowing the vegetation we planted on top will grow down through it while the weeds we’ve covered with it almost never come up from below. Another option is to cut holes in the material and plant the plants through it,” he explains. “The gravel, mulch and stones in the dry creek bed remain separated from the soil of the construction area so, ultimately, the gravel isn’t lost into the soil. The geotextile acts as a divider and it acts as a transfer of oxygen and moisture that, combined with the drag of gravel, increases infiltration.”

Metamorphosis performs a large number of creek bank restoration projects for its clients, relying on various geotextiles for support. “For years, that work has been stone-filled cages and cement walls, but most people I interact with now have a good appreciation for more benign, bioengineered products where a lot of plants are doing the work,” Franklin observes.

Sotir would like to see that sentiment spread to a broader audience. “There are sites where a conventional approach is more appropriate and others where a mix is right. Appropriately combined, the final solution offers considerable improvement over either system alone. But there are areas where I’d like to see soil bioengineering used more because it’s a better fit. Especially in some of the national parks, you’ll see a hard system where a softer, more natural approach would have been best.”

In the end, she emphasizes, the question for erosion control professionals is whether a particular method and material gets the job done. And, with the exception of some heavily built-up urban situations, Sotir concludes, soil bioengineering is a viable answer. She adds, “I don’t believe there are as many limitations anymore.”