Sometimes tough materials are the only things that will stand up to the forces of nature.

By Donna Gordon Blankinship

Remember the days when an eroding channel was most commonly fixed by pouring a load of concrete? With all the hard- and soft-armor solutions now available, those days are long gone. And the engineers and city planners benefiting from all the choices in today's marketplace say good riddance.

Terramesh installation on the Little Wekiva River

Concern for the environment is given credit for many of the changes in the way we approach these issues, according to engineers and planners interviewed for this article. Saving money is a happy side effect.

Stopping or repairing erosion in drainage ditches—or drains as they are often called in Michigan—is a common problem in the Great Lakes State, where high water tables have always been a natural force to deal with. The choices engineers have in dealing with these problems have grown in recent years. So when Kentwood, MI—a city just southeast of Grand Rapids—had some drain issues, officials decided to take a more creative approach than pulling up the concrete truck and letting it flow.

What they have done upstream and downstream in this area was just poured massive amounts of concrete to try to prevent the banks from falling in," explains Stephen Kepley, Kentwood's city engineer and director of engineering and inspections.

But that's not always the best choice, Kepley says, considering the amount of water flowing through these channels and the possibility of a bad installation letting water undermine the concrete structure and breaking it apart. Someday, he says, you may see the concrete floating downstream.

Beautiful Flexibility
Kepley was hoping to create a stable drain bank with a natural look and remembered a product he had used at a previous job that seemed to fit all his parameters—an environmentally friendly but structurally secure solution from a company called Armortec. Armortec's A-Jacks are interlocking concrete pieces that look a little like the jacks we used to play with as kids. They are connected together along the bank in a way that is flexible and permeable. They can be installed either randomly or in a uniform pattern. Space left between the units can provide habitat for fish below the water and can be filled with soil above the water for planting grasses, shrubs, and trees.

The concrete structures come in diameters of 24, 48, 72, 96, and 120 inches. When combined with bioengineering techniques and given a few seasons to grow, the A-Jacks can become completely invisible. The smaller units weigh around 40 pounds and can be put in place by human beings, but the larger ones weigh more than a ton and require a backhoe and sling for assembly.

The flexibility of A-Jacks and their ability to be completely covered with vegetation are the two things that interested Kepley in using them to shore up the Kentwood drain during the fall of 2004. The city wanted to have a natural slope and natural stream bottom but needed to make sure the toe of the slope was stabilized in a way that couldn't break apart and go downstream during storms or high water flows. Kepley says the city considered using riprap on the toe of the slope but felt it could break apart during a large storm. He says the way A-Jacks are locked together keeps them in place.

He advised other engineers to pay good attention to soil conditions before deciding what kind of hard-armor solution to adopt. If you're dealing with wet sand, the units will be more difficult to use, but they work well with regular sand or clay, Kepley says.

He also warned contractors that there is a learning curve to using the product, just as there is for any new approach. But once the learning curve is done, it's a relatively quick install," Kepley says.

The Learning Curve
Learning how to use new products is one of the drawbacks to trying some of these new hard-armor techniques, but Robert Gaylord, engineer with Singhofen & Associates in Orlando, FL, says the learning curve is worth the time when the results are attractive, environmentally friendly, and less expensive than traditional hard-armor solutions.

Several complicated issues faced by the Seminole County Public Works Department led Gaylord to take a look at Maccaferri's Terramesh product for an erosion problem on the Little Wekiva River. The steep and very tall embankment had experienced critical erosion, and city sewer pipes within the channel banks had been exposed and were leaking into the river. The department was hoping to retain a group of trees on the other side of the river, so moving the channel and building a more gradual slope was not an option in this project, Gaylord says.

Engineers had originally planned to build a gabion structure to solidify the shoreline, but some parts of the structure would have to be more than 20 feet tall, and traditional gabions would be difficult to install and uneconomical at that height.

The Terramesh mechanically stabilized earth system is designed for the tallest embankments. It is a combination of a gabion fascia with a soil reinforcing tail" element that is a double twist woven steel wire mesh with PVC coating; Gaylord describes it as a rock basket with a long tail. The tail is compacted within the structural backfill, reinforcing the structure but allowing it to remain flexible enough to move with the earth when necessary. Soil already on the site can be used for the backfill.

Gaylord says the system that was installed in the fall of 2003 proved it could stand up to any storm during the historic 2004 hurricane season, when the new wall withstood the wrath of Charley, Jeanne, Frances, and Ivan. You don't ever want your system to be tested like that," he says.

He is glad to see the way environmental concerns—like the desire not to remove the well-established trees in this project—are driving engineers and designers toward more creative solutions for erosion control, which also end up being more attractive and often less expensive at the same time.

It would have been cheaper to tear up all the trees and put in a rigid system," Gaylord says, but that would have looked just like a concrete wall, and that's not what the people in this Florida neighborhood wanted to see when they looked out their windows.

Erosion control along Lake Boone Trail

Gaylord says using a product like the Terramesh system definitely involves some training for those involved. He recommends getting as much expert advice from the manufacturer as possible and talking to other people who have installed it. He says it would also be best to hire a contractor with experience installing the product you choose, if that's possible.

One thing he wishes he had done differently on this project was to watch the installation more closely at the beginning, setting aside time each day for the first month to be there for questions and adjustments. Making yourself available to contractors is always important, especially when there's a new product to figure out.

Gaylord says the hardest part of this installation was the excessive rain at the beginning of the installation. With rain delays, it took crews nine months to complete the construction.

Falling Trees
Maccaferri helped with another complicated project in the Southeast, a streambank erosion problem on the west side of the city of Raleigh, NC. Fast-rushing water from a stormwater pipeline had been eroding the southern wall of the creek, creating a 20- to 25-foot-high cliff on one side.

This sheer wall of dirt used to be home to a lot of trees, but they were falling down the bank one by one, according to Samir Bahho, senior project engineer for the Raleigh Engineering Department. The city wanted to install a second large stormwater pipe and stop the trees from falling down the cliff.

At first engineers considered installing a cement wall, but because the wall would need to be 18 feet tall and about 80 feet long, the cost was prohibitive. They also considered the Terramesh system, but the slope of the cliff made this too difficult. This approach would have been less expensive, but the cost of temporary stabilization methods while the work was being completed would have added to the price.

Bahho says the City of Raleigh decided on a traditional gabion wall and what Maccaferri calls a Reno mat on the bottom of the creek. The gabion wall, which was installed in the spring of 2004, is 9 to 12 feet deep on the bottom and about 3 feet deep on top.

Bahho says they liked the idea of a gabion wall for several reasons. First, height is not a problem, as long as you have the room to make it as deep as it needs to be. Second, it is flexible, so if the dirt settles, the wall can move with it, although steel cables keep it attached to its other parts. Third, it can quickly and easily be seeded and made to look natural, unlike a concrete wall.

The Reno mattress is similar to a gabion wall except that it is only 1.5 feet high, about half the height of each mesh box of rocks making up the gabion wall. On the floor of the stream, the Reno mattress has a real advantage because fish habitat can be quickly reestablished. Bahho also liked the fact that neither of these products had to be dry installed, unlike a cement foundation.

Above and Below the Water
Dealing with water is always a challenge when trying to repair erosion damage, but sometimes it's even more of a problem than engineers and contractors are expecting. When Max Baldwin, erosion control specialist for Contech Construction Products, was helping the Oregon Department of Environmental Quality finish up a Willamette River project to protect a Superfund cleanup site from being disturbed by erosion, he was surprised at how much the water flow fluctuated during the course of installation. The fluctuation meant the company needed a lot more help from divers than it had expected. But the water also gave the project's planners one more reason to be glad they hadn't opted for the full hard-armor, more traditional approach, because concrete is difficult to pour in such a wet environment.

A combination of articulated blocks and turf-reinforcement mat protects the shoreline at this Superfund site.

The Oregon Department of Environmental Quality needed to armor the shoreline in northwest Portland to keep the cap in place that had been installed during the cleanup of an old creosoting plant. Contech suggested using a combination of hard- and soft-armor solutions to match the various characteristics of the shoreline. The goal was to find a permanent solution that could be vegetated.

For the area exposed to 2- to 3-foot waves, articulated blocks were suggested. Upstream, an area exposed to lighter waves was to be shored up with Pyramat, a turf-reinforcement mat that works on near-vertical slopes and in high-flow channels. Contech says the product is nondegradable and allows for rapid vegetative establishment. Turf-reinforcement mats are easier to install than hard-armor solutions, and the plants that grow on top of them serve an additional purpose of removing pollutants from runoff.

Baldwin says in this case the decision was based more on environmental concerns than aesthetics or economics, but he also notes that without the economic benefits there wouldn't have been money available to complete some of the erosion control projects his company has worked on. The Oregon project, however, was mandated because it was part of the cleanup at a Superfund site.

He notes that changes in state and federal laws to encourage more attention to the environmental impact of public construction projects have encouraged the engineering community to look more closely at some of the newer technologies. He hopes this attention to new approaches will help companies continue to improve their erosion control products. He would like to see installation simplified for some products and improvements continued to be made in products that often require specialization.

Saving Money
The flexibility these new products already have makes a big difference in municipality budgets because flexibility saves money. The Durham (Canada) Regional Conservation Authority saved about two days and between $4,000 and $5,000 on a small erosion control job on the Saugeen River by choosing International Erosion Control Systems' Cable Concrete mattress rather than loose rock, which would have required digging much deeper.

The mattress will not be visible after vegetation grows in.

The job, completed in July 2004, had two aims: to create a swale for runoff from a nearby park into the Saugeen River and to stabilize the area around a new natural-gas line. Appearance was important in this project, as were environmental issues, according to Cliff Hopkins, regional manager in Canada for International Erosion Control Systems.

The mattress, consisting of cable wire and concrete blocks, can be installed above or below the water level. Site preparation is minimal, and the mats can conform to bumps and bends in the surface. The open area within the mats allows for revegetation. They can also be driven over without damage.

One of the goals was to maintain fish habitat during construction, so half the river flow was diverted at a time while the work was completed on that side. Hopkins says the shape and flexibility of the mats saved money and time because of the way they are installed, which in this $20,000 project took only about four days. He notes that timing is crucial in a project that involves diverting water.

The project near London in northern Ontario, Canada, was completed in the spring of 2004; through the summer, the mat silted over and vegetation growth began. Hopkins says, by fall, a significant part of the bank had begun to grow.

Donna Gordon Blankinship is an author working in Seattle, WA.

EC - September/October 2005