Stabilizing Channels and Streambanks

March 1, 2007

When municipal officials in North Vancouver, BC, Canada, engage in streambank stabilization, they focus on ways to do so in a more environmentally sustainable manner. That’s because North Vancouver is home to about 23 salmon-bearing streams.

“The watersheds are small-about 7 to 15 hectares-but are still supporting native salmon populations,” says Richard Boase, North Vancouver environmental protection officer. “Our community is built in the middle of these, so the streams are suffering with everything associated with urbanization and damage being caused by all the peak flow disturbance,” he points out.

North Vancouver’s challenge is similar to that of many municipalities throughout North America: the need to stabilize streambanks and channels not only to address a specific erosion problem but also in response to urban development or redevelopment.

Hastings Creek is one of North Vancouver’s salmon-bearing streams that called for attention in 2005. Erosion had been occurring in the stream, which was beginning to meander closer to the backyards of private properties.

“The stream was in a dedicated park, so there was no machine access for more traditional types of riprap or other sorts of hard-armoring approaches,” says Boase. Such methods would typically require bank preparation and removal of existing vegetation, he adds.

Boase researched his options and chose Deltalok in an effort to find a solution. The system provides a vegetated face for such applications as bank protection. It consists of GTX bags filled with sand and soil, held together with Deltalok connectors to create an interlocking soil mass to encourage and maintain vegetation.

“We looked at the site and came up with the idea that this particular site would be a great application for the product in an urban environment where the stream is highly susceptible to urban peak flow problems and all of that associated with a lot of impervious surface area,” says Boase.

Some 1,500 Deltalok GTX bags were placed at two sites on the stream itself-one an area where private property was being threatened and another area where there was a historical pond feature that had developed a long time ago. The stream was cutting into the berm supporting the pond.

“If we didn’t do anything in terms of intervention, sooner or later we were going to lose the pond and have a fairly significant problem there,” notes Boase. “We used the product to completely rebuild the creek bank and at the same time shore up the area of the berm supporting the pond.”

Workers were able to transport the materials down into the stream and rebuild the bank from the bottom up, retaining existing vegetation in addition to being able to use plantings integral with the product. District workers planted three types of willow, using some seedlings in pots the size of a coffee cup, some in half-gallon pots, and rooted stakes that came six to a 1-gallon pail.

The stabilization of the two sections involved a wall measuring 6 feet by 30 feet at the site where the stream made a 90-degree turn and where streambank erosion was threatening private property loss, as well as another wall section measuring 3 feet by 15 feet.

The project took two weeks in September 2005, with the majority of the work being site preparation because workers had to conduct a fish salvage at both sites.

One of the more interesting challenges, Boase notes, was that crews had to work around a large tree stump that had fallen in the stream but became a feature of the stream that the district wanted to maintain. “We spent a fair bit of time drilling some holes and using some cable duckbill anchors to secure that thing in prior to starting to lay the bags in and around it,” he says. “We built the wall in and around this structure that was sticking out of the bank.”

Boase explains that certain parts of North Vancouver can get close to 10 feet of rain between November and June. “We’ll have periods of prolonged and fairly intense storms that come through, so there are fairly significant flows. This has always made it a challenge for us in dealing with these urban streams.” But for most of the year, the streams are quiet, meandering, and beautiful, he says.

“And then the winter rainy season comes up and we’ve got 4 feet of water, full bank width rumbling down these sections, and we have to design for that,” he adds.

The project was treated as a pilot project and funded by Deltalok, which provided the product; Northwest Hydraulic Consultants, which drew up the report and provided the hydraulic design; and the district, which provided the soil and rock medium to fill the bags as well as installation labor.

It’s been more than a year since the project was completed, and to date Boase has been impressed. “It’s a great product to work with in difficult-access situations,” he says. “It preserves a lot of the existing important natural features of the stream.” He’s also impressed with its looks: The stream is in a park setting, and there was virtually a closed canopy over the creek in just a year’s time.

“We were able to get noticeable growth,” Boase says. “The willow is doing well in the lower-light conditions. The main test is going to be whether the section of bank holds out over the longer term. If we can get 20 years out of it, that would be a satisfactory installation, because a lot of other things are going to happen in these urban streams over that time period.”

Saving Creekside Property
In San Rafael, CA, streambank erosion in Mahon Creek was causing much consternation for a homeowner whose property abutted the creek. San Rafael officials got permission from the US Army Corps of Engineers to do emergency repairs, because the homeowner was about to lose a wall and much of the property.

Steven Zeiger, senior associate engineer and stormwater program manager for San Rafael, says the city needed to do something to stop the erosion, because Mahon Creek takes in a lot of water from an upstream watershed. But the city couldn’t do “just anything.”

“The new rules here with Phase II [of the National Pollutant Discharge Elimination System] are you can’t just put anything you want like riprap in there-you’ve got to bioengineer something,” says Zeiger. “We called Matterhorn to do both projects.”

Matterhorn California had done a previous streambank stabilization project for San Rafael, with which Zeiger was pleased. The company’s Gravity Living Retaining Walls are an embankment stabilization system made of interlocking precast concrete modules that provide pockets for stone, soil, and vegetation. Coated Helix Anchors are used to replace poured concrete piers in wall foundations. Precast blocks are connected by a synthetic cord.

For this project, San Rafael used Matterhorn’s Secura Slope. Matterhorn, which manufactures the precast concrete modules, engineers installations, and installs the product, was the prime contractor on the Mahon Creek project.

“Mahon Creek is tidal-the level rises and falls with the adjoining San Francisco Bay, 7 feet twice daily,” explains Matterhorn’s owner, Phil Zeidman. He says the bank was fenced and heavily landscaped with mature trees and shrubs for the parking lot of the adjacent building.

“We could not enter from the parking lot,” he says. “We had to build the retaining wall from the creek, which is about 60 feet wide.” The wall Matterhorn built is 100 feet long and 8 feet tall.

“We opted for a floating work platform of 4- by 8- by 3-foot-thick Styrofoam blocks with a plywood top,” Zeidman says. “We drove two half-inch rebar through each of 10 blocks so they could ride the tide. We lashed them loosely together and to the bank.”

To protect the creek environment, Matterhorn installed studded T-fence posts and plywood along the proposed foundation edge. The company sandbagged the outside for additional protection and placed plastic sheeting inside to prevent leakage of the concrete from the footing.

“We installed Helical Piers vertically at 5-foot centers and [Terratec] Helix tiebacks at 10-foot centers from the platform and from the bank at low tide,” Zeidman says. “Once they were in place with their caps, the reinforcing for the grade beam was installed and inspected.”

Because the project was being installed adjacent to a bird rescue center, the city engineer spearheaded numerous inspections during the process to ensure no damage was done to the creek environment or the ducks. The wall provides resting spots for the ducks as well as the potential for finding food as the tide changed over the face of the wall, Zeidman says.

“In order to cast the concrete footing, we had to wait for maximum low tide to expose the creek bottom,” he explains. “Also, since we had to pump the concrete, we could not use an accelerator to speed up the setting of the concrete-we had to rely on the cement to generate the heat.”

His company used seven sacks of cement to the cubic yard as opposed to the usual five sacks.

“An accelerator might cause the concrete to set up in the hose during pumping should we have to stop momentarily,” Zeidman explains. “The concrete generated so much heat that despite it being the end of November and 7 p.m., we were in polo shirts while the inspector wore a jacket.”

After installing the first row of modules in the wet concrete to bond the wall to the foundation, Zeidman’s company installed a Mirafi 140N filter fabric against the embankment to keep the silt from plugging the drainage.

“We backfilled the modules and the space behind to the fabric with crushed rock to provide for drainage as well as ballast to stabilize the embankment,” Zeidman says. “The front of the modules was topped off with soil to encourage vegetation growth along the face of the creek. Once we were able to work on the wall from the end, we removed the floating platform, stakes, and plywood.”

After three weeks of building the foundation, it took a week to erect the wall. “The city must have been pleased with our work and pricing as they have since had us repair another section of Mahon Creek a half-mile upstream to protect a residence. This one we were able to install from the bank,” Zeidman says.

Retaining Walls to Hold Back Water
In a project two years ago in Indio, CA, the circumstances called for a retaining wall system, says Nick Jansson, P.E., vice president of Soil Retention Products in Carlsbad, CA. A residential developer wanted to build up against the floodplain.

“It’s very wide and very flat out there, so unless you build up some type of wall or embankment, you can’t build,” Jansson explains.

The developer did build a high wall in combination with a screen, so if the area did receive high flooding, it would not overtop the wall but instead siphon off over the roadway to the other side of the development and settle in farmland.

“In that particular project, they chose our product because we do a segmental retaining wall system that is not a poured-in-place wall system like you see with most floodwalls,” Jansson says. A retaining wall system and an articulating concrete block system are two products offered by Soil Retention Products. The company manufactures and installs both products.

The benefit of using the retaining wall system for such an application is its installation ease, Jansson says. “It’s 10 times faster than a cast-in-place or masonry retaining wall structure.

“Another benefit is all these units are precast, and the field grid, which stabilizes the wall, is a noncorrosive material, as are the concrete block and the connection device between the block and the grid. In the long term, no corrosion is likely to occur, but more importantly when you are building these walls and you are adjacent to streambanks or waterways, you’re not going to have any concrete or other material spill in that area,” Jansson says.

“If you are building up against a running river, most of the time these walls [are built above] that water elevation,” he says. “Maybe the turf is protected by riprap and then as you get out, you have to build up these walls.”

Jansson says it’s been typical in the past to utilize articulating concrete block (ACB) products for embankments. “We don’t strongly endorse putting these mats on slopes for embankments,” he says. “We’d rather see those on the channel bottom.

“When you hang mats on a sideslope-and sometimes they do them as steep as 1.5:1-the soil particles this embankment is retaining or holding back are already almost at the natural angle of repose, meaning you are putting a mat on something that is not very stable.

“By building a retaining wall, you are really stabilizing the bank. You are gaining property, and in most cases, they don’t want people going into that channel anyway, so creating slideslopes out of these ACB products is something we are trying to get away from.”

On this project, Jansson’s company built 3,500 linear feet of wall at an average height of 12 feet, varying between 5 and 20 feet high.

“In that particular project, it might have been 30,000 square feet we built in about 15 working days,” he says. “If that 30,000 square feet was a cast-in-place wall, the estimated construction time would have been about 150 days.”

Jansson says on a large channel job, earthwork is one of a contractor’s biggest challenges. “A cast-in-place wall system doesn’t allow you to backfill the walls,” Jansson says. “You need to build it independently, and then you backfill it. We’re building up with the grader, so it’s a very rapid installation.”

In another situation, the company installed Enviroflex, an ACB product, in a creek that’s a tributary to the San Diego River in Santee, CA. Jansson says the city’s stormwater and the regional water-quality control board are heavily involved in the project. The regional board instructed developers and the engineer to use no hard armoring-meaning poured concrete or nonpervious material-in this channel.

“They needed some type of armoring to accommodate some of the sheer stresses and velocities associated with the channel,” he explains. “The benefit of using an ACB product is that it is not cast in place, so there are no spill consequences.”

Additionally, the holes in the blocks-approximately 2 feet by 2 feet-allow for filtration as well as vegetation.

A concrete channel from an adjacent city drains into the creek, creating flooding problems. “It’s a dump site behind downtown Santee, and it’s part of the city’s redevelopment effort to enhance the area with walking trails and a natural-looking channel. They are widening the channel in some areas to allow for more vegetation, less flooding, and more recreational opportunities,” Jansson says.

“Our retaining wall products can be built butted up or have a 9-inch spacing between the blocks, allowing for a planter pocket in each cell,” he notes. “As the erosive forces get higher, you need to close the block so the soil doesn’t erode out of the block. For the heavily erosive forces, we close the space in the block, and as you get into the free board or the lower-velocity area, you can open up the space to have full plantings.”

He points out that one cannot narrow out a channel without having some type of armoring to some degree, so an ACB product is an effective solution because it allows for vegetation, water infiltration, and groundwater recharge. He believes the industry is moving away from traditional methods such as riprap.

“The problem with riprap is you can never control it perfectly. What usually happens over time with riprap is it slowly makes its way down the slope, so you never have a consistent channel dimension. It will change over time.”

Jansson sees ACB products being used mainly for channel bottoms, spillways, or culvert outlets that are on relatively flat ground. He says Riverside County, CA, engineers are turning to soft-bottom solutions. “They want sediment transport,” he says. “They want things to be able to erode and supply sediment downstream all the way down to the ocean. I live in a community where our beach is rapidly going away and that is because there is no more supply.” He notes that this is a challenge in areas where much of the coastline is developed.

Mats and Liners
Streambank and channel stabilization projects span the gamut. For instance, Agviq in Virginia Beach, VA, was engaged in the latter part of 2006 in a federally funded channel stabilization project at theNaval Amphibious Base, Little Creek, in Virginia Beach.

The channel is close to a landfill, which had been capped 10 years ago. Contaminated sediment was found at the bottom of the canal, which Agviq removed. During the removal process, the company had to remove the trees along the canal that had provided a buffer zone.

International Erosion Control Systems provided Cable Concrete CC-35 open-cell mats to prevent future erosion. The open cells were filled with topsoil and planted with native species.

Cable Concrete integrates flexible stainless steel cable into high-strength concrete to promote durability and flexibility. Maximum protection is provided when the mat is paired with a polyester geotextile base cloth. The needle-punched geotextile allows moisture in the subsoil to drain, preventing hydraulic pressure buildup beneath the protective concrete mat. The subgrade material is held intact by the weight of the Cable Concrete and separating ability of the geotextile.

The high-strength mat also shields subgrade material from high water velocities and wave action.

In Lost Hills, CA, a feeder canal from the California Aqueduct servicing several thousands of acres of farmland was experiencing a 5% leakage rate. The Lost Hills Water District had lined nearly a mile of canal using high-density polyethylene (HDPE).

The district put out bids for work for lining additional sites along the canal. The winning bid went to Firestone Specialty Products, which had promoted its EPDM (ethylene propylene diene terpolymer) GeoMembrane synthetic rubber as an acceptable alternative. Firestone’s products are typically used as seepage control to keep stream water from infiltrating soil.

Colorado Lining was responsible for installing 1,200 linear feet of membrane, mechanically attaching it to shotcrete along 4 miles of the canal. Two reaches of canals were lined in what is essentially a lining pilot project for other state irrigation districts, notes Andre Harvey, a Colorado Lining regional manager based in California.

“Part of the funding conditions was that the Lost Hills Water District would allow other irrigation districts to visit their lined sites to help them determine if it would be a viable option in their locations,” he says.

The EPDM was installed in December 2005 and January 2006. Two terminal reservoirs at the end of a few reaches of canals were also lined.

Harvey says although the primary purpose of the work was to address the seepage challenge and maintain the required water volume with minimum waste, there also had been significant erosion in the channel.

“Some of the canal prism had to be reshaped before we could line portions because of erosion,” he says. “A lot of the embankments had slipped off and fallen into the bottom of the prism after heavy rains.”

Twenty feet above and to the west of the Iron Horse Inn-a 150-room motel in Durango, CO-flows an irrigation ditch. Beyond the ditch is a hillside.

“The problem I was having was seepage of the ditch underneath the building and the pavement, creating sinkholes in my parking lot along with extra settling the buildings have done over the past 30 years,” says Mike McCardell, the inn’s general manager.

Additionally, the ditch, known as the Animas Valley Consolidated Ditch, had a 2-foot drop, so the water didn’t flow very fast, accounting for the seepage problem.

To address the problem, McCardell chose Penda Corp.’s SmartDitch after researching options on the Internet. The product is made of corrugated HDPE plastic designed to fit into an existing ditch’s meandering course. It can be installed with hand tools.

Previously, McCardell had tried mixing bentonite clay in with the water, hoping it would flow into the cracks and holes and stop the leaks and using a large tamper to pack in the ground.

“It didn’t solve the problem,” he says.

McCardell considered an option being utilized south of his motel’s property, where the owners embedded their ditch in concrete. He considered installing a culvert but was told the ditch had to remain open because of debris that flows down it.

Nine hundred linear feet of SmartDitch was installed in the spring of 2005 after workers corrected the grade. Workers installed the ditch liner and anchors and backfilled alongside the liner with sand over a period of a week.

Natural vegetation surrounds the ditch. The liner goes under a driveway on the south end and a highway on the north end where the ditch connects with two culverts. The transition pieces were fashioned from steel.

“This product basically saved our property,” McCardell says.

About the Author

Carol Brzozowski

Carol Brzozowski specializes in topics related to resource management and technology.