Nov. 18, 2013 03:43

New Techniques for Streambank and Channel Stabilization

In an ideal world, when faced with a difficult soil erosion problem, a contractor would like to be able to use whatever tools, practices or materials the job calls for: hard armor, soft armor or something in-between.

However, in case you haven’t noticed, we don’t live in an ideal world. Not even close. Very often, what you will use and how you will use it will be dictated by ordinances, regulations, or by the owner of the property itself. This is the case even when you, in your professional evaluation of the situation, know that something else would really work better.

This is never more true than when it comes to streambank and channel stabilization work. Environmental as well as aesthetic considerations weigh heavily in these cases. Contractors who do this type of work must tread carefully, but still get good, lasting results.

“A lot of my work is somewhat dictated by what you can get permitted,” said Erik Jones, P.E., project manager for Fargo, North Dakota-based Houston Engineering, Inc. “I do most of my work in Minnesota. Their Department of Natural Resources (DNR) is one of the main permitting agencies. They have their pet ways of doing things. We know what they like to see from a permitting standpoint, so we try to make things go as smoothly as we possibly can.”

“I do very little work with government agencies,” says Craig Hernan, owner of Westwood Landscape Contractors in West Chicago, Illinois. “I’m probably cutting my own throat by doing that, because the majority of streambank stabilization work that’s done around here is through municipalities, counties or townships.”

Hernan cites this example as to why he shuns such work. “The village of Glen Ellen had a bioengineering project. They handed me a 121-page contract with lots of specifications and details that I didn’t agree with. I got a headache trying to read it all. If I’m going to do a job, I’ve got to know the property owner and we have to have a good relationship. I need to have the leeway to make changes on the fly, should we run across something we didn’t see during our initial observation of the site. And if I can’t have that, I can’t do the work, because I can’t guarantee it.”

Hernan feels that too often, the specs for such jobs “have been written by somebody who went to a seminar someplace. They came back with product information from one manufacturer or another who told them their product was the panacea for every single channel erosion control situation. That’s just not the way it works.”

“I think my biggest problem with these government contracts is that they waive everybody’s responsibility,” says Hernan. “If anything goes wrong, the engineers are absolved, the municipalities are absolved, everyone’s absolved— except the contractor.”

He describes a streambank job this past spring that he’d refused to bid on because he didn’t like the design that had been specified. Another contractor won that job, and probably wishes he hadn’t. “A 50-year rainfall channeled right through his section, and he didn’t have time to stabilize it. Now, he’s on the hook to repair it.”

Soft armor: Nature’s way?

Soft armor solutions abound. There are turf reinforcement mats (TRMs), erosion control blankets, (ECBs), geosynthetic grids, mats and logs made of coir fiber, and filter socks using everything from mulch to shredded rubber tires as filler. These can be used alone or in any combination. In fact, there are far more soft-armoring solutions than there are hardarmored ones. This is no doubt because permitting agencies overseeing streambanks favor more natural-looking approaches.

Even when hard armor, such as gabion baskets or articulated concrete block, is used, permitting entities want it to be vegetated. In her thirty years of experience, Kym Kelley, co-owner of Reno, Nevadabased Kelley Erosion Control, Inc., has used plenty of hard armor. “I used more gabions many, many years ago,” she says. “But they’re not specified too often up here (in the Lake Tahoe area) anymore. It’s more softscaping, wherever possible.”

“Being both a landscape contractor and a landscape architect, I appreciate aesthetics,” said Hernan. “So I try to stay away from hard armor as much as possible,” though he has also used gabion baskets in some of his streambank and flood control structures. “We’ve used them in some outside reaches of streams, due to the high velocities that were present. But primarily, we try to do as much as we can with the bioengineered products, using a combination of coir fiber logs and things like that, in combination with plant material.”

In Minnesota, the DNR favors the use of “toe-wood debris benches” on severely eroded streambanks and channels, according to Jones. Toe-wood debris consists of salvaged rootwads, stumps and branches from downed timbers. The benches are then covered with soil and topdressed with sod. “Then, above those benches, there’s slope work done to further establish vegetation. Sometimes that involves using biodegradable ECBs.”

Of course, Minnesota has lots of forests with plenty of timber to make toe-wood benches out of. But in places where timber is scarce, a contractor must seek other alternatives.

“Another solution is to use rock stream barbs (low rock structures that extend into stream flows to modify their flow patterns),” says Jones. “You point them upstream, usually about a third of the way across a channel, to deflect the flow away from the banks. Typically, you use them along the outside bends of streams or rivers, where you have a lot of the down-cutting that forms cliffs along the banks. Water spills over them at right angles. They allow you to refocus the energy of a stream towards a channel, as opposed to having it butt up against a failing slope.”

Over time, sediment will build up over these stream barbs, providing additional protection to the banks. In this way, sediment, something usually viewed as a negative, becomes a building block, strengthening streambanks.

Avoid taking a “sedimental journey”

“Our mantra up here is ‘Keep Lake Tahoe Blue’,” says Kelley. “A lot of funding around this area goes into stabilizing creeks and wetlands, trying to keep sediment from hitting the lake.”

Kelley’s company does a great deal of creek and stream restoration and wetlands mitigation work around the lake, quite a bit of it in environmentally-sensitive areas. On these jobs, she uses TRMs, coir logs and other biodegradable materials, along with nursery-grown plant material that’s already established, to provide immediate stabilization without waiting for seeds to germinate.

One of the biggest challenges in doing streambank stabilization, says Kelley, is keeping sediment out of a waterway while you’re working on it. “When you’re digging, putting in biologically-engineered products such as propagated sod, propagated coir logs, or native sod, you’ve got to be careful about getting the products installed in a timely manner without disturbing the creek.”

“There was a lot of down-cutting in this creek,” says Kelley, “eroding the banks and transporting sediment into the lake.” To stop this, they used bricks of wetlands sod dug out of a meadow in Northern California. “They were about eight inches

thick, with grass growing in them. They’re extremely heavy, so they take a lot of manpower to put in. We picked up the chunks of sod with a loader and placed them along the edges of the creek. It provided instant armoring.”

Propagated coir logs, twelve inches thick, were also used. “Before we installed the logs, we drilled holes into them and implanted plugs,” said Kelley. “Growth was already getting established in the logs when we installed them along the edges of the creek.”

Seeds were hand-cast over the site afterwards.

Hard armor

Natural solutions work very well in many instances. Sometimes, however, you have to go to the hard stuff. “You definitely need hard armor solutions in high energy, high velocity water flows,” said Ken Wilson, president of Midwest Construction Products in Fort Myers, Florida.

Hard armor was clearly called for in the case of a deteriorating bridge in the Rock Creek area of Fairway, Kansas. The area was closed for over a year because of the collapsing bridge, and the creek often flooded during severe storms.

The bridge was replaced, but afterwards, the banks needed stabilizing. Rock Creek is an urban drainage channel, very susceptible to rapid changes in flow rates and depths. In order to maintain the stabilization of the channel’s banks during high flows, an articulated concrete block system was installed. Openings in the cells were planted with plugs, to eventually give the channel a vegetated look.

“I had some reservations at first, and was afraid that the channel would look manufactured,” said Bill Stogsdill, director of Public Works for the city of Fairway. “After installation and planting, however, the result exceeded my expectations. Bank stability has been maintained through high water velocities and still looks natural, with native grass plantings in the open cells.”

Transition mats

There is another category of products between soft and hard armor, and that’s the transition mat. Bill Murphy, P.E., engineered products manager for Hanes Geo Components, Inc., in Winston-Salem, North Carolina, explains that transition mats are used on culvert outlets, overflow structures and dams.

They are used to avoid scour when going from hard-armored, concretized areas such as culverts or bridges to natural downstream channels. “This is typically the point at which severe erosion occurs, where high shear forces create scour holes. Transition mats are used to transfer from hard surfaces to softer-armored solutions, such as seeded TRMs.”

Murphy explains that there has always been a concern in the erosion-control industry about how to address these transitional areas. Transition mats are the answer to that problem. Like TRMs, they can be fully vegetated, so channels can have a natural appearance but still have the benefit of hard-armored protection where needed.

Murphy cites an example of where their mats were used successfully, on a stream that flows through a golf course in Iowa. “Several times, riprap had been installed and re-installed along the banks of this stream; but the riprap continued to fail.”

Plans were afoot to line the streambank with concrete. The owners were preparing to do just that, when the golf courses’ board voted the proposal down unanimously. The transition mats that were used instead provided effective and much more aesthetically pleasing results.

A channel project Jones worked on recently is another good example. “We worked on a small stream, a tributary to Wolverton Creek near Comstock, Minnesota, that had become a sort of mini Grand Canyon, having down-cut about ten feet so far, continuing to go lower and lower. This had to be stopped.”

Why? This particular channel was next to a road. Eventually, the down-cutting would have undermined that road. He explained that if you don’t stop down-cutting, it just keeps going, deepening and widening the channel, threatening whatever man-made structures happen to be adjacent.

Jones explains the process. “As the down-cutting continues, it creates more and more sediment. All that sediment has to go someplace. Gullies are formed, and the flow rate can be so high that a natural stream can’t convey the sediment very well. And so, over time, the bottom of the stream starts filling up, but still has to handle the same amount of flow. The stream will want to get wider to compensate, and when it does that, you start losing your slopes. That creates other issues.”

“The soils won’t hold up if the walls of your channel become vertical and deep; eventually they’ll fall in,” says Jones. “Every time it rains, it pushes more sediment out into the main channel. It’s kind of a vicious cycle. Given enough time, decades and decades, maybe it would correct itself, but there’d be a lot of other repercussions in the interim.” Repercussions, such as roads caving in, that we don’t want to live with.

To solve the problem, Jones and his crew put in a series of eight rock riffles (U-shaped structures made of rocks or boulders). “Each one of these stepped the water up a foot, ultimately bringing the channel back up to the level it had been in the past.”

“If you have a fast-moving, swift channel,” Jones advises, “you need to start looking at grade control within that channel, and that involves putting in a rock riffle. The bottom of the U should be upstream from the arms of the U.”

Rock drop structures can also be used in such cases. “You can use these within the channel to kind of stairstep the water down,” he said.

An abundance of products

Things have changed dramatically over the last couple of decades, in terms of what erosion control professionals doing streambank and channel stabilization work now have access to, Hernan says.

“Twenty-five years ago, we had the simple Curlex ECBs, straw blankets, and a synthetic TRM that I understood the purpose of but could never get germination in. Now, we have all these different TRMs, coir mats and other things that last for varying durations of time, in dozens of different varieties, depending on your flow velocities and what types of seed germination you’re looking for.”

Competition drives innovation. Someone is always out there trying to develop something better. As Hernan observes, “When you do this kind of work, you’re always saying, ‘There’s got to be a better way. What if we had this?’ Sooner or later, if you talk to the right people, that product will show up in the market.”

We’ll let Hernan have the last word. “One of the things that I’ve found early on in this work is that you have to be aware of every single product that’s out there. It’s your knowledge of those products and the combinations of them that will make for successful, stable streambanks that will stand up to whatever Mother Nature can throw at them.”

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