Don’t Get Swept Away... Controlling Stream Erosion
We can all relate to some of the troubles the ancient Greek demi-god faced from that job. He had to fight the property owner to get the payment he’d been promised, and then the gods accused him of double billing because he made a profit.
Of course, times have changed a bit since then. Most of today’s work involving streams and rivers is about keeping them safely tucked in their beds. No doubt, the EPA would have sharp words for any contractor who tried to wash 30 years of sediment downstream.
Still, even if the goals are different, the core concepts are the same. Water causes erosion, and that erosion needs to be managed in order to avoid serious environmental problems. That takes a lot of work, but more importantly, it takes a lot of forethought, because it’s all too easy to shift a problem with running water, rather than fix it.
To figure out how this happens, and what we can all do to minimize it, I talked with some people who have experience protecting the banks of streams and rivers from erosion. The general consensus was that the first step in fighting erosion is to understand where the channel is currently, and where it’s going.
“Basically, the average stream or river is always in one of three different phases,” said Ryan Alltop, contruction division manager at ENCAP Inc., in Dekalb, Illinois. Either it’s ‘downcutting,’ where the sediment on the bottom is washing away; it’s eroding laterally, where it’s trying to expand its floodplain by eroding the riverbank, or it’s stable, and not trying to shift at all.
Achieving that stability will often eliminate erosion, but it isn’t the only way to prevent it. The first, most direct, option is to put down a material on the bank that’s strong enough to take whatever the river might throw at it, namely, hard armor.
Hard armor comes in many forms, but mostly, it’s a protective layer of stone or concrete, materials which will take the river eons to erode. There are enough options available that this subject is an article unto itself, but suffice it to say that whichever one you may choose will depend on the degree of protection required and the project’s budget.
Concrete walls, wire baskets filled with stone called gabions, riprap linings and even interlocking systems of articulated concrete blocks can be used to protect eroding slopes. However, hard armor isn’t right for every situation. Everyone I spoke with said that they use hard armor sometimes, but that it can cause complications if misused.
“When you put in riprap, concrete walls or even heavy wooden walls, especially in a stream, that redirects the natural flow of water and the energy of that flow,” said Patrick Skillings, vice president of Skillings Connolly in Lacey, Washington. “What that does is to deflect that water energy, which can cause erosion somewhere else,” he said.
That’s why Skillings tries to reserve hard armoring for those projects where failure is not an option.
“Where we fight the flow is typically up next to infrastructure,” he said. “You can’t let a bridge abutment or a road blow out. You do that and the road closes, people can’t access their homes, and EMS personnel can’t get to them if there’s an emergency.”
In Warrenton, Virginia, Reid Cook, senior stream specialist at Angler Environmental, recommends adding natural design elements to projects that call for hard armor. “Even in channels where there’s a clear infrastructure issue and we need to protect the channel immediately, the regulatory agencies are still reluctant to approve a plan that only employs hard armor,” he said. “A lot of them still want to see a positive benefit back to the stream channel.”
That’s why more and more companies are offering soft armor solutions alongside their hard armor options. Soft armoring has an even wider range of options, but they can be roughly categorized by how long they’re expected to stick around.
At the high end of this scale are transition mats and scour mats. These mats, generally constructed of plastic, are thick enough to provide protection similar to riprap, while still allowing vegetation to grow through.
In some areas, the flows aren’t quite as extreme, but the slope still needs lasting assistance. In these cases, Turf Reinforcement Mats, or TRMs, can provide a layer that will significantly cut down on erosion without impeding the regrowth of vegetation.
Revegetating slopes is the end goal when using soft armor, because there’s no solution for holding down dirt that’s more natural than letting plants and their root systems do the job for you. That’s why biodegradable Erosion Control Blankets, or ECBs, are often specified for streambank work. They prevent erosion and keep seeds in place long enough for them to get established, and then feed those roots as they decay.
One common solution is coir, a fiber made from the husks of coconuts, which can last for a few years, while providing a degree of strength that’s uncommon among natural fibers. “When we’re trying to use something that’s a softer technique, we’ll use coir wraps,” said Skillings. “Because they’re biodegradable, they’re natural, and they absorb the rain.”
The fabric is tough enough that coir logs filled with soil are sometimes used at the toes of embankments, in place of gabions. By building a slope up in lifts, each lift wrapped in coir, this type of soft armor provides a strong, natural solution that phases itself out while encouraging plant growth. Eventually, all that’s left is a natural, verdant slope, and that’s as green as anyone can ask for.
“The nice thing about that is you can take willow stakes and jam them right through the material,” Skillings said. This revegetation method, called ‘live staking,’ consists of taking a still-living tree branch, dipping it in a rooting hormone, and driving it into the ground, where it can then grow into a full tree.
Needless to say, some trees take to this process better than others. Cook uses willows and alders close to the water, because they grow quickly, and having wet feet from the nearby stream won’t kill them off. Different climatic zones will call for different trees, but a native tree that can be live-staked, tolerate the water, and grow quickly, is ideal.
Tree roots are excellent at curbing erosion, which may be why another soft armor technique eliminates the middleman entirely. “One of the most common things to do is to place large, woody debris, or logs with the root wads still attached, along a bank,” Skillings said. That debris, once anchored into the soil with a deadman anchor or spiral nail, may provide enough erosion protection to let the slope grow back.
That brings us to another important method of controlling stream erosion: controlling the slope of the bank. “When a stream has already downcut, we usually have a very tall streambank that’s almost a 1:1 vertical bank,” said Alltop. “In those cases, one technique that we often employ is to pull the bank back to a more stable slope.”
This regrading commonly consists of pulling the slope back to a 3:1 or gentler angle. An easier slope doesn’t push the geotechnical limits of the soil as much, meaning you can use softer armor and revegetation where you might have otherwise needed concrete.
That isn’t the only reason to regrade, though, and sometimes Alltop will plane a slope down to near flatness. “We sometimes go as low as 12:1, because it allows for a floodplain to be created,” he said. This gives the stream more room for excess water when it floods, lessening the erosive effects of each storm.
Another solution for a channel in the wrong phase is to take the wind out of erosion’s sails, cutting it off mid-stream. By installing structures in the streambed itself, you can change how the water flows, and reduce the stress that the channel’s pattern puts on the banks.
One such structure is called a stream barb. “Stream barbs are instream structures which are typically pointed into the current,” said Alltop. “They’re often installed on the outside bend of a stream, and they shift the thalweg, which is the heaviest flow channel of the stream, out to the middle.” That serves to protect the outside bend, and can help counterbalance in areas where development around the stream has caused a phase shift.
Of course, a stream barb will do little to actually slow a stream down. If changes upstream have in creased the stream’s flow, there are some other in-channel solutions you can use. Rock riffles and cross vanes both serve to combat the downcutting that can occur when a stream starts feeling the need for speed.
“Cross vanes are the ones people would be most familiar with,” said Cook. “They redirect water off of the banks, but they also help contain the grade in the channel.” A cross vane looks like a blunted V- shape, with the flat tip pointed upstream in the center. The cross vane concentrates the energy of the stream in the middle of the channel, and a depression carved out behind it allows the energy to create a small pool, rather than erode the bank.
Rock riffles have more of an inverted U-shape, and rather than creating a hollow to concentrate the flow, they create a sudden rise that forces the water to roll over. Alltop usually counts on an upstream slope of 2:1 or 3:1 and then a tapering downstream slope of between 4:1 and 6:1 to get the job done. “When you get it right, you can see the actual movement of the water on the surface change,” he said.
This is why some in-stream structures are easier to install in the wet, because you can actually see the effect of your actions in real time.
There are a couple of other guidelines to keep in mind when installing these two stream structures. The heaviest materials go where the pressures are greatest—usually in the center—and the entire structure is typically keyed into the bank for a few feet, to ensure that the water doesn’t just sweep around the outside.
What all these specifications make plain is that correcting stream and river erosion is not as simple as it may seem. With this work, you are not only protecting a particular property, but an entire waterway. Local riparian ecosystems can be affected by which BMPs you employ and the degree of care you take.
If a stream you’re going to work on currently supports wildlife, that will also limit when you can work on it, and the kind of measures you can take. You likely won’t get a permit to work dry, if that will leave no water for local salmon to spawn in. Depending on where you’re working, the work windows can be very limited, effectively rendering this a seasonal service.
Municipal, state and federal agencies are increasingly aware of what thoughtful stream management can do for the environment, and the regulatory climate shows it. It’s not just the EPA and its state department equivalents, either. State fish and wildlife bureaus, municipal public works departments, tribal agencies, councils, land trusts, land stewards and, of course, property owners, all have a stake in the game.
Controlling streambank erosion is sometimes as much an art as a science, and on delicate jobs a little finesse can make a big difference. It means that you can always benefit by doing some homework before taking on a job, but it also means that the work itself is that much more rewarding.
Better yet, in this regulatory climate, the state agencies are tripping over themselves to publish information for contractors interested in providing a higher quality service. It’s easier to access the knowledge you need than ever before.
With eco-green practices, and water quality in particular drawing more public attention, now is an excellent time to consider making streambank erosion control part of your game.