When David T. Williams gets a call to begin consulting on a streambank stabilization project, it could be for a variety of reasons. “If the river is misbehaving, or it’s going to look ugly, or it’s going to expose some infrastructure-a pipeline-or maybe for home protection,” or a combination of those things, he says. For Williams, who is president of DTW and Associates, a civil engineering firm based in Fort Collins, CO, the solution “depends on the situation.” And the methods available to achieve the goal of a secure streambank are perhaps equally diverse.
With extensive experience designing streambank stabilization projects, Williams says the work usually starts down one or two main pathways. Some projects, he says, fall under the category of streambank protection, which will tap mostly the expertise of engineers. On the other hand, streambank restoration can be considered multidisciplinary and calls on the skills and expertise of not just engineers but also ecologists, biologists, and others.
For Williams, who recently hosted a Forester University webinar on stream restoration, it’s more than a matter of painting with the actual landscape. Stabilization projects, he says, must arrive at a definite purpose that is often much bigger and more critical than striving for beauty.
“You also have to identify the objectives,” he notes.
“Are they to move the creek back to where it was-then what can you do to get it to stay there? And, while you’re at it, what else can you do to make the stream better?” However, it all starts with identifying “a stream that is deteriorating.” Sometimes that’s more obvious than others, and yet on some occasions there is little warning that a stream is facing troubles-that is, until it’s hit with the kind of surprise Mother Nature alone can dish up.
Taming a Wild Cougar
Canmore, AB, is normally a quiet, scenic place to live or vacation, according to Matt McArthur. Amidst the craggy eastern slopes of the Canadian Rocky Mountains, the town serves as a gateway to the world-famous Banff National Park, one of the most favored destinations in Canada’s Rockies. McArthur says that like so many of his neighbors in Ontario, he’s visited the region frequently over the years to enjoy top-notch winter sports, unparalleled mountain vistas, and a restorative atmosphere. Recently, though, as an employee of International Erosion Control Systems (IECS), he has been back and forth between Alberta and Ontario on numerous trips for a different purpose-to help tame Cougar Creek.
He says the creek went completely wild June 19 through 21 last year as summer made an abrupt appearance, bringing a massive three-day-long rainstorm coupled with an incredibly rapid snowmelt.
Raging from the deluge that dumped nearly 10 inches (250 millimeters) of precipitation on the region at the same time a sudden snowmelt shaved off 3 feet of snow pack from the mountainsides in a matter of hours, Cougar Creek jumped its banks. And it did much more. It turned into a raging monster, rushing down the hillsides with so much force it carried boulders and chunks of debris the size of Chevys through the heart of Canmore. Newspaper reports said that trees that were not carried away by the tumultuous flow now bear scars 30 feet high on their trunks where debris tore through the riparian woodlands along the creek’s bank. Scores of homes were damaged, landmarks were destroyed, and the town suffered a major blow to its economy and morale.
BGC Engineering, the firm contracted to perform a forensic analysis in the aftermath of the catastrophe, termed the disaster a “debris flood,” defined as “a very rapid surging flow of water heavily charged with debris in a steep channel.” According to BGC, the transition from water flow to a debris flood is usually defined as the point at when at least “3 to 10% of the volume of a flow is made up of sediment.” These kinds of floods, BGC reports, “typically occur on creeks with a channel gradient of between 3 and 30%,” which would easily characterize a mountain stream such as Cougar Creek.
The report notes, “The creek has developed an extensive alluvial fan since deglaciation of the area on the order of 10,000 years ago. An alluvial fan is a fan-shaped deposit of sediment crossed and built up by streams where they flow from a steep mountain channel onto the dramatically reduced gradient of a valley floor.”
The report continues, “Streamflows and associated transported sediment come from a single point source at the apex of the fan. As the stream loses confinement and the channel gradient decreases, it drops coarse-grained material.” The sediment deposited raises the elevation of the streambed as it traverses the valley floor, and “this aggradation reduces the capacity of the channel and forces it to change direction, thereby gradually building up a shallow conical fan shape. Over time, the creek moves to occupy many positions on the fan surface.”
Because of the unpredictability of the alluvial process during a catastrophic flood, the town decided it could not risk a recurrence. Any location on Cougar Creek’s 3.1-kilometer-square alluvial fan could be at peril, and that included a lot of valuable assets. In fact, the fan is almost entirely developed, according to BGC’s report. The land uses include 1,417 residential homes, a number of lodges and resorts between Highway 1 and Highway 1A, a light industrial area, numerous roads including a crossing of the creek at Elk Run Boulevard, one elementary school, a heliport, a recreational area that includes an off-leash dog park and three baseball fields, the Canadian Pacific rail line, and various municipal infrastructure such as telecommunication lines, sewers, and powerlines.
In the short term, the objective of the project on Cougar Creek was specific: to keep the channel from migrating to any new location until a comprehensive plan could be implemented that would permanently reduce the risk of damage from flooding to a level the people of Canmore would find acceptable. In the long term, the creek bank stabilization project is to become an integral component of an overall flood prevention strategy for the town of Canmore.
Hugging the Ground
Adding urgency to finding a solution, Cougar Creek rises with snowmelt each year during early spring, imposing a strict deadline for successful completion of stabilization measures to have protection in place in advance of the next predicted snowmelt.
Canmor’s design for short-term emergency measures went beyond just shoring up the creek banks. It also includes a strategy to mitigate debris flooding in the future, starting with the installation of a debris net higher up on the mountain to try to stem the rubble at its source. For streambank protection, the town chose to install articulated concrete block, a technique with the capacity to withstand the impact of a debris flood of the type Canmore had just experienced. During the bidding process, Canmore settled on International Erosion Control Systems (IECS) Cable Concrete CC70 closed-cell block.
Richard Bremner, president of Bremner Engineering and Construction, the firm contracted to install the bank protection, says IECS immediately “erected a building to stay out of the elements, and brought in forms for casting the articulated mats from Ontario and all over the USA. They hauled them all to Canmore and set up a production facility. Producing them locally reduces the haul cost, whereas if they had been manufactured in Calgary that would have been much more substantial.”
McArthur says IECS Cable Concrete systems use a wet-casting technique that embeds the cabling within each panel of blocks as they are poured. Unique among articulated block systems, the cables in ICES mattresses traverse the block in both longitudinal and lateral directions to provide a flexible interlocking system.
The plans call for 50,000 square meters of armoring on both sides of the channel stretching for a kilometer on each side as it flows through the highly populated ski town.
The crews prepared the side banks by grading and compacting to achieve a 2:1 slope and “since January have been doing full-time installation of Cable Concrete mats,” McArthur says. “Richard’s crews are laying approximately 800 square meters per day. The speed of installation is quite a bit quicker than with gabions. Each panel is 12 meters, so you’re not hand-placing or dropping rock like riprap.”
He explains, “The mat needs to have intimate contact with the ground, so the surface needs to be as smooth as possible.” Crews are carefully removing stones and roots, and they must take care to avoid dragging or ripping the pre-installed sheet of geotextile fabric on the underside of the Cable Concrete mats. The 18- by 16-foot sections of mat are secured together with stainless steel clamps.
Installation requires a crane with the capacity to lift an 18- by 16-foot section of mat weighing 9,600 pounds, as well as typical excavation equipment to prepare the surface.
At the top of the bank, contractors established a four-block key-in at a 1:1 slope. The bottom is keyed in 10 blocks below the graded streambed, continuing the 2:1 slope of the restored bank into the subsurface. When the installation is complete, a layer of topsoil will be placed on top of the mat to give the fortified creek bank a natural appearance.
Bremner says ICES was very involved from the design phase through construction. “Initially, they were onsite for three or four days just to make sure we were going in the right direction. They visit the site once or twice a month to satisfy themselves that the installation is going satisfactorily.”
By the time of his visit in February 2014, McArthur says, Bremner Engineering and Construction workers “had 500 pieces, or 5,000 square meters, of protection installed.” And Bremner says they are on schedule for completion by the spring deadline.
Beyond Water
Erosive flows can range from nearly volcanic in their fury down to a mere trickle, which the average homeowner knows can do considerable damage if flowing in the wrong place and given enough time. Needless to say, there are a lot of instances in between, giving professionals the leeway to choose from a wide and ever-expanding array of streambank stabilization techniques. One big choice is between hard armoring and softer alternatives.
Because engineers must consider not only the force of the water and what might be tumbling through it, but also what is on the shore behind the banks demanding protection, the choice is not always simple. In short, Williams says, “If it’s going into farmland I may be willing to try something softer, but if it’s next to my nuclear plant I’m not going to take a risk, and I would choose hard armoring.”
Crossroads at the Creek
Frisco, TX, is growing and changing. What was once rural farmland is now a suburb of Dallas and is rapidly being paved and built up with homes, roads, institutions, and businesses. That increased impervious area will result in higher intensity stormwater runoff from the new land uses during any given storm. David Chacon, Frisco’s city engineer, says that as the city plans for growth, a number of adjustments will have to be made to accommodate changes in local hydrology. Quite a few of these changes center around infrastructure on the banks of Stewart Creek.
Chacon says Stewart Creek almost always carries a base flow, although sometimes not much more than a trickle. It generally courses a flatland area where one might find a few beaver dams along creeks and tributaries. However, a good quarter inch of rain in a few hours can turn Stewart Creek into a torrent. Within minutes, he says, it can rise to 4 feet deep, and that’s even without the additional impervious build-out expected in coming years.
Jason Fernberg, area manager for Maccaferri, a leading supplier of gabion erosion control systems, who is familiar with the geography in the region, says the stream’s volatile temperament is consequence of highly water-resistant clay soils in this part of Texas. “I always say to people, around here, “˜When it rains it’s like a toilet bowl: The water rises real fast, and then it goes down real fast,'” he notes.
On its way down, he says, stormwater runoff in Stewart Creek rips saturated clay soils in clumps from the bank. Chacon says a new outfall that has been added would have the capacity and the realistic potential to deliver up to 43,000 cubic feet per minute of stormwater, “the 100 year build-out flow,” into this particular bend on Stewart Creek, potentially exacerbating the erosion threat.
There was already quite a bit of infrastructure centered on the spot that could be placed in jeopardy if the creek were permitted to meander or downcut as a result of the added volume of water. Plans also called for the creek to be rerouted under a railroad right of way, which would require additional grading and armoring. Stonebrook Parkway, a major thoroughfare in the area, a private school, and adjacent parking lot and facilities on the opposite bank would also require whatever protection could be afforded.
The initial concept to protect all of that infrastructure in one fell swoop envisioned a box culvert 40 feet wide by 100 feet, but Chacon says that in addition to all of the infrastructure at the location, the city of Frisco had been planning to create a natural amenity for the citizens called Grant Park. With bike paths, foot trails, and recreational opportunities potentially within view of a major box culvert, Chacon says, “It did not seem like a good thing to put that kind of geometry in a park.”
The city needed an armoring system sturdy enough to protect assets but that would also offer enough aesthetic appeal to avoid creating a visual no-man’s land on riverbanks that could otherwise prove an asset to the community.
Chacon says, “We worked with HTNB, who put the road plans together along with the drainage plan, and Freese, Freese, and Nichols did a hydrology study for the entire watershed.” They recommended armoring the creek with gabion systems, and HTNB suggested the results of their deliberations be presented to Maccaferri. “They worked out the armoring plan,” he notes.
According to Fernberg, Maccaferri is unusual among gabion providers in offering its clients such design assistance. “We work with the project from beginning to end. We go from the design side or the planning side with the engineer, with the owner, and then all the way through with the contractor to job site assistance, field support, and all of that.”
He adds, “One of the benefits of gabions is that it’s a relatively inexpensive solution when you cost out all the different alternatives for erosion control. It’s fairly easy to install, and the learning curve on gabions is very fast.”
However, he notes, “A lot of suppliers don’t do field support; they don’t go out to the job site when the contractor is working on it to give them pointers or techniques to build a better wall or better whatever they’re doing.”
The bank facing the outfall was reinforced with 3-foot by 3-foot gabion baskets filled with rock ranging from 4 to 8 inches in diameter. Fernberg says the length of the gabion baskets the company recommends for a job often corresponds to the “customer’s or the contractor’s familiarity with the product, or their expertise in building gabions. If they are not really all that experienced, we lean toward the smaller ones because they are easier to manage. If they don’t have much experience, we point them toward 6-foot sections. If they are experienced with them, we point them toward 12-foot baskets, because they are more economical and they can put them in faster and easier.”
Hole in the Wall
“To keep all the small fine particles from migrating out of the soil that you’re retaining and going through the basket and out,” he explains, a geotextile fabric is placed on the excavated vertical surfaces before stacking the gabion baskets. “The needle-punched fabric, placed behind the wall and underneath gabion mattresses, allows water to flow back and forth from the soil through the fabric and into the creek, or through the gabion and into the creek bed.”
However, he advises that the base of the gabion wall should rest in direct contact with the earth. “You don’t want to put fabric under a wall, because that creates a little bit of a slip surface.”
Chacon says crews also placed a layer of riprap on the bed of the creek to minimize the possibility of undercutting at the edges of the installation.
According to Chacon, the city began construction in November 2012, and the installation at the outfall was completed within four to five months, which he considers “a reasonable amount of time.”
A four-tiered gabion wall now protects the creek bank across from the new outfall; the opposite, more gently sloped bank adjacent to the school property is protected by Maccaferri’s 18-inch gabion mattresses. Technicians hydroseeded the apex of the gabion wall, and Fernberg says the void spaces between the rocks in the gabion will rather quickly recruit moisture-tolerant plants to give the entire installation a very natural look.
Chacon concedes that before this project, he “didn’t know a lot about gabions,” but he says the techniques became clearer under Maccaferri’s guidance. “What was intriguing about it was the amount of hand work that’s needed putting in the rocks. To get a good finished look, you’ve got to give it a flat face by hand placement of the rocks.”
Showing customers how to achieve a stable wall is key to the company’s approach, Fernberg says. “We have engineers on staff that run calculations taking into consideration the soil parameters, the flow of the creeks, and working all that into the design to make sure the walls or the mattresses are heavy enough to hold down that soil, given the water in the situation.”
Dual Purposes
Although creeks in heavily populated areas are often bordered, surrounded, and criss-crossed by sewer lines, roadways, and other infrastructure of a potentially sensitive nature, when the need for stabilizing creek banks in urban areas arises, hard armoring is not always the preferred choice. More and more in the urban environment, softer solutions that mimic nature are gaining popularity. Part of the reason is the realization by engineers and planners that having a healthy, functional riverine ecosystem as a neighbor can bring real value to city living.
With evolving techniques in bioengineering and new ways of thinking about what a stream can mean to a neighborhood, city engineers and designers are beginning to avail themselves of some of these softer approaches to streambank stabilization.
The banks were stabilized along a 3-mile reach of Pleasant Run Creek.
A Golf Course Runs Through It
Matt Kerkhof says his firm has just completed “the largest municipal stream corridor restoration project in the nation” for the state of Indiana, through a joint venture between the Marion County Parks Department and the Indiana Department of Transportation (INDOT). Along a 3-mile reach of Pleasant Run Creek in Indianapolis, IN, Hoosier Aquatic Management reestablished stable streambanks where Pleasant Run passes through the municipal golf course.
Pleasant Run Creek, a tributary to White River, is about 15 to 25 feet wide on the city-owned golf course property, Kerkhof says, draining a watershed that is 85% impervious surface. The creek “experiences major urban runoff, and after a storm, water levels can rise up 10 to 15 feet in a few hours.”
As president of Hoosier Aquatic Management and owner of Living Log Inc., Kerkhof says the project was initiated not only to combat massive erosion onsite, but also as an offsite mitigation project implemented to mitigate for wetland disturbances resulting from INDOT’s 465/69 Northeast highway project elsewhere in the watershed.
Because his firm specializes in some of the softer streambank stabilization techniques, Kerkhof says he “often gets called in on projects where you’re limited on what heavy equipment you can bring in.” Obviously, when working on a golf course, it’s important to avoid leaving too big of a mark.
Bob Barr, a research scientist at Indiana University-Purdue University Indianapolis, a partner in the project, says that in addition to the environmental goal of reducing sediment loads in the stream to improve water quality, the stabilization project could help address safety issues that had more to do with sporting than with flooding. As a scientific advisor on the project, he notes, “It’s an alluvial stream, and it’s real easy for them to move, and that’s what we were seeing.” In the process it was creating a mess on the links.
The project, however, was designed not so much in response to a crisis as it was a treatment for a chronic disorder. Because of the impervious landscape throughout the rest of its highly urbanized watershed, Pleasant Run Creek had become unstable and deeply incised.
According to Barr, even the golf course manager had become concerned about erosion on the creek. Down-cutting in some areas alongside the fairways had been so severe over time that the banks dropped off a sheer 10 or 11 feet to the creek bed, creating an obvious safety hazard for golfers.
Kerkhof observes, “The stream itself did not exhibit increased flows or changes in dynamics. We wanted to change the slope-to stabilize it. The objective was to establish an area where native vegetation can grow,” providing natural protection against incision and meandering.
Ironically, for a project planned to restore vegetation, one of the main factors that had contributed to the erosion problem was, in fact, excessive vegetation. Invasive plants had crowded out local varieties to such an extent that under the canopy of leaves there was no longer any rooting system sufficient to hold the sandy local soils together. One of the more voracious of these invaders, Asian bush honeysuckle, grew there in such dense thickets along streambanks that nothing could grow in its shadow, leaving large areas of streambank soil barren of protective vegetation and highly susceptible to erosion at the slightest disturbance.
That is not to say there was not infrastructure on the site that also demanded protection. Kerkhof notes, “There was a big manhole that was compromised-the creek had entirely worked its way around it.”
Barr says the solution was to look about the area to find a design that would bring the stream back into harmony. “The idea was to use natural channel design” and to reconnect the stream to the floodplain where possible, with native plants doing most of the erosion control work. He says the specific plantings and contours of the improved and restored streambank would be based on studies of reference reaches of the channel both upstream and downstream of the project site.
After removing invasive species and treating the area to discourage their return, Kerkhof says, crews performed a little surgery to give the streambank a new shape and look. “We peeled back the shore, reshaped the slope, and installed coir logs.”
Along with standard coir logs, Kerkhof offers a specialty product called Living Logs, which are prevegetated coir logs fabricated to meet individual project specifications. “The consulting engineer will dictate the type of vegetation,” he says, but the plantings can be custom selected for rooting on the logs in a pattern corresponding to the ecological contours of the streambank.
“The nice thing about the prevegetated log is we can actually manipulate the plant plugs we put into the log while we manufacture it. For example, let’s say you wanted three iris, four swamp milk weeds, then another four iris-we could do any pattern you come up with as an engineer. That’s huge.”
On the project, Hoosier Aquatic Management used a combination of coir logs, live stakes, and Living Logs, which “encapsulate the root mass to allow the plants to become established.”
“The Living Log can be used from the top of the slope of a streambank-where you would want to use dry upland plants-down to the very toe. Typically we’d be using wetland plants to stabilize those areas,” Kerkhof says.
To ensure the client gets the authentic local genotype of the species, Kerkhof says, his firm travels the extra mile. If a client had a project in the mountains of Colorado, he would purchase local plants from that area and “bring them back and use those in manufacturing so that we actually supply the customer with local native plants.”
Although the cost may vary depending on the intricacy of the plantings, Living Logs can be purchased for as little as $10 per foot with a two-year guarantee, and can be installed by anyone from skilled technicians to average homeowners. According to Kerkhof, a crew of five can install about 1,000 linear feet of Living Log per day, which would include a log and a 6-foot buffer. A 1,000-foot bank restoration project, he says, would typically take about a month. However, he notes that the golf course installation “got dragged out a little” because of the large swaths of invasive species that had to be eradicated to make room for the native plantings.
Three and a half years post-construction, Barr says, “The physical channel is doing very well. People who live near the park are very happy with how the project has turned out.”
Still Evolving
There are a variety of approaches to streambank stabilization, Williams says, but there is one caveat: “It’s only going to be as good as the next significant flood. You can only go as far as the budget will allow.” When it comes down to deciding on specific techniques, he says, “A lot of it is money driven and a lot of it is event driven. The more robust it is, the more it’s going to cost.”
Williams concedes there is always an element of uncertainty, too. “You might be protected to the next 25-year flood, but there’s always a chance that next year there could be a 100-year flood.” Tempering that sobering thought, he adds, “There’s lots of research being done on streambank stabilization. It’s a hot topic. People are taking climate change into consideration.” For example, “You may have different timing for snowmelt that may affect what you do.” But ultimately, he says, given the budgetary and other constraints, the desired goal of any stabilization project is to build in protection to be “as robust as possible.”
