Gone are the days when cementing a stream was the only solution to a tough slope stabilization problem.
Streams are systems, and the best ways to fight erosion on their slopes often involves multiple solutions, according to the US Army Corps of Engineers Waterways Experiment Station’s Environmental Impact Research Program on Bioengineering for Streambank Erosion Control.
These days, hard-armor options range from traditional to nontraditional retaining walls such as vinyl pilings, from interlocking concrete blocks to concrete gabions, and from plastic channel liners to engineered soil-filled bags. Retaining walls require a drainage system behind them.
Hard armor almost always is used in conjunction with erosion control blankets or turf reinforcement mats made of such materials as jute, coir, wood-fiber, or polypropylene. Mats and blankets protect slopes from splash erosion as well as provide tensile strength.
They also provide a stable material through which vegetation can grow. Seed can be sown on the ground and covered with a blanket or mat, or the blanket or mat may be placed on the soil first and seedlings or sod can be planted through holes in the fabric. The roots grow into the soil and protect the slope.
Hard armor increasingly includes vegetation, especially native vegetation. Natives already have proved that they can survive and even thrive in the climate and soil. In addition, they’re a known quantity. Exotics may become invasive-or a complete failure.
Exposed vegetation lessens the impact of rainfall and wind on the soil. Its root systems help hold soil particles together and thus increase overall slope stability. Roots also enhance water infiltration into the soil and improve water quality.
Vegetation also provides food and shelter for wildlife, including birds, waterfowl, fish and other aquatic creatures, and insects. On the other hand, it may not grow, either because too much is consumed as food by insects, rodents, deer, and other wildlife, or because of extreme wind, water, or temperatures. It should be maintained, especially in its early years.
According to the Corps of Engineers, vegetation aids in slope stabilization on all three zones of slopes: the toe zone, the portion of the slope between the bed and the average normal stage of the stream; the splash zone, the portion between normal high-water and normal low-water flow rates; and the terrace zone, the portion above the splash zone.
In all three zones, a mixture of grasses, herbs, shrubs, and trees should be used whenever possible to provide a diversity of wildlife habitats. They should be planted in the zone where they’re found in nature.
The toe zone undergoes the most erosive stress and undercutting and often requires hard armor, commonly reinforced with mats and blankets. Vegetation decreases the velocity of the water flow, which both reduces undercutting and allows sediments to drop to the bottom of the channel, consequently improving water quality. Vegetation also acts as a buffer between erosive materials in the water and the slope.
The splash zone frequently undergoes high erosive stress as well, due to flooding, currents, ice and debris, and freezing and thawing cycles, according to the Corps of Engineers. Hard armor may be used, possibly reinforced with blankets and mats. Vegetation forms underwater obstacles to currents as well as sediment.
Because floodwater rarely reaches the terrace zone, stabilization is less critical here, especially for flatter slopes. If it isn’t vegetated, however, it’s very susceptible to erosion and to any flooding that does occur. Vegetation intercepts floodwaters from over-bank flooding, reduces excessive saturation by evapotranspiration, and holds the slope together with its root systems. Depending on the conditions, including existing erosion, the slope, and the soils, mats or blankets may be necessary.
The following projects are two creative solutions that used two very different hard-armor and bioengineering techniques to solve two very different slope stabilization problems.
Drainage Channel Slope Stabilization
While eroding drainage ditches lined with vegetation might be picturesque, they don’t provide adequate drainage. This is the case with drainage ditches in the Harlingen Irrigation District in the lower Rio Grande Valley in Texas, which provides drainage, flood control, and water supply services to 88.3 square miles of Cameron County.
The bags were stacked to reinforce the slope beside the culvert. Coir netting was placed on the soil, and live stakes of native trees were planted in the channel itself as well as up the slope.
“The irrigation district has a lot of problems with hundreds of ditches,” says Trent Street, head of the US Department of Agriculture Natural Resources Conservation Service (USDA-NRCS) design section in Texas, who oversaw and approved the design of the slope stabilization project. “They all have steep slopes and heavy clay soils. Back in the day when the channels were constructed, the recommended slopes were much flatter, but they couldn’t get the land rights. The lower Rio Grande can’t handle all the water it gets from upstream. They’re constantly having problems, especially after major rainfalls.”
From its source in Colorado, the Rio Grande flows through New Mexico and marks the border between Texas and Mexico. At the southeastern tip of the state, it reaches Cameron County and the Gulf of Mexico.
The irrigation district gets water from three major reservoirs, two in Texas and one in Mexico. When water levels in the reservoirs reach a certain height during major rain events, water is released into the valley, Street says.
In the 1930s, two channels were carved into the flat farmland, and the adjacent land was reserved to serve as floodways during these rain events. According to the US Geological Survey, in order for land to be considered a floodway, water from a 100-year flood may be discharged without cumulatively increasing the water surface elevation more than 1 foot.
The drainage ditches were dug in the 1950s and 1960s to take the water to the floodways. The slopes were approximately 1.5:1 at the time. The soils are clays with high shrink-swell properties. When the land floods, the floodways fill up and the ditches stand full of water. The water saturates the slopes and, when the water recedes, the slopes fail.
In 2008, Hurricane Dolly, a Category 2 hurricane, caused multiple slope failures along four drainage ditch sites. The irrigation district applied for and received approximately $1.2 million for the repairs through the NRCS Emergency Watershed Protection (EWP) Program.
Gary Geraci, a design engineer with the USDA-NRCS design section in Texas, did the design work for the project, which involved four of the most severely affected sections of the channel, a total length of some 2,900 feet. The first challenges were the land rights constraints and the deep, narrow ditches. Geraci specified vinyl sheet pilings to stabilize the sides of the channel.
The wall was designed with the US Army Corps of Engineers’ design software, CWALSHT. The soil design parameters were determined by a geotechnical investigation and testing. The geotechnical site assessment consisted of 20 test borings.
“Another aspect of the design challenge was that we had some drain pipes that emptied into these channels,” Street says. “We had to do some modifications. We had to cut them off outside the work area and put in new installations over the wall.”
The contractor, Howard Pebley, president of McAllen Construction in McAllen, TX, choose ShoreGuard sheet piling from CMI (Crane Materials International) in Atlanta, GA. ShoreGuard is commonly used to stabilize and raise the height of levees and earthen dams. It also can be used for cantilevered floodwalls in levee repair projects. It meets Federal Emergency Management Agency standards for 100-year-flood design criteria and has been used by the Army Corps for more than 15 years.
CMI’s Jeff Holm explains, “The corrugated profile of sheet pile adds to the stiffness and bending moment capacity of the sheet. A standard flat sheet would have to be extremely thick, and thus would not be cost effective, to have the necessary bending moment capacity and stiffness required to drive the sheet pile in the ground or resist the lateral loads that may be exerted on the sheet in a retaining application.”
Once the design was completed, McAllen crews started the construction and excavated the channel slopes. The approximately 10- to 15-foot-deep channels were excavated from the top of the slopes to the bottom of the channels, forming slopes approximately 1.5:1 on both sides of each channel. The completed excavation provided a wedge with a 4-foot-width at the bottom of the slope. This wedge area was for the drainage system that was constructed behind the sheet-piling wall.
McAllen then began driving the first piling into the channel bottom. “The ditches had about one foot to 18 inches of water in them,” says Pebley. “We drilled through it.”
And almost immediately encountered rock-hard clay.
“The biggest challenge we had was that we had never installed vinyl sheet in as hard a clay,” Pebley says. “We did quite a bit of R&D to find out how to drive the piles. We talked with the manufacturer about the installation and they were very helpful. In the end, we had to build a mandrel to drive it in. It was quite costly, but we pride ourselves on doing what needs to be done.”
Crews drove the mandrel, a piece of metal shaped like the sheet piling, into the channel bottom with a drop hammer to create a path for the first panel, drove in the panel, and encountered their second problem, he says.
“It took more energy to pull the mandrel out than to drive it in. We had to use a pneumatic extractor to extract it.”
Between 4 and 8 feet high, the tops of the pilings became the new bottom of the slope. The pilings beneath the channel bottom extend approximately three times deeper than the height above. For example, Street says, “If they drove the pilings 15 feet deep, there would be a 5-foot wall above the channel bottom.”
Once the sheet piling was installed, McAllen crews started backfilling the wedge behind the wall. “We put a drainage system on the back side of the wall,” Street says.
The drainage system consisted of C-33 sand encompassing a pocket of coarse gravel with a weep hole, a flush mount wick drain filter system manufactured by JET Filter System in Casey, IL. The drainage system provides a means to relieve hydrostatic pressure behind the wall. The sand collects water from the surrounding soil and provides a passage for the water to the gravel and then to the weep hole, which allows the water to pass through the vinyl sheet piling into the channel.
According to JET Filter System, the media in the product is Mirafi Filterweave 300, which maintains long-term flow rates in high-gradient and dynamic conditions. The key component of the system is that the inner filter media cartridge can be removed from the front of the wall and either be cleaned or be replaced. The weep holes were placed every 6 feet along the wall.
After drainage system was built, McAllen crews screwed anchors through some of the panels and into the sand fill. “Where the wall extended 5 to 8 feet above the channel bottom, we had to put anchors for extra strength,” says Geraci, the designer. “We specified helical screw anchors.”
The anchors, from Helical Concepts Inc. in Wylie, TX, were 20 to 25 feet long, Pebley says. “It was a function of the torque. They have to develop a certain amount of strength, with the back of the wall trying to push them out.”
McAllen crews installed new utility pipes on top of the walls to replace the ones that had been cut at the beginning of the project. Crews placed compacted earth fill, from the excavations as well as a borrow area provided by the irrigation district, over the drainage system and the pipes. The completed slopes ranged from their original 1.5:1 to 3:1.
Crews planted native grass seed on the earth fill and covered the seed with a fiber mulch. The irrigation district is maintaining the vegetation.
Now stormwater runoff infiltrates through the grass, which holds the soil in place, and through the soil and the sand. Excess runoff flows through the coarse gravel and the weep hole filters into the channel, sediment-free.
West Fork Mill Creek Headwaters Restoration Project
The use of hard armor also has allowed a once overgrown, silt-filled ditch at the headwaters of Mill Creek in the city of Forest Park, OH, a densely populated suburb of Cincinnati, to become a beautiful, functional, vegetated swale.
What appears to be an eroded ditch is a drainage channel built in the 1950s or ’60s. Steep slopes and landrights limitations were the biggest challenges on the project.
“This is the first area you can see it as a stream,” says Terry Lavy, owner of TJ Sales & Consulting LLC in Covington, OH, a distributor for Envirolok Vegetated Environmental Solutions, a subsidiary of Agrecol LLC, both based in Evansville, WI.
Surface water flows from a culvert, collects into a small stream behind the Forest Park Senior Center, and disappears into another culvert, this one under a very busy stretch of Winton Road. A fair amount of water drains to the channel, which was cemented about 20 years ago, says Pat Madl, an engineer with CDS Associates, who designed the project. “It had silt buildup from over the years and was overgrown with cattails and honeysuckle.”
According to the Mill Creek Watershed Council of Communities, a pipe in the city’s stormwater sewer system discharges very high-velocity stormwater runoff from the impervious surfaces directly into the channel, causing erosion. Pollutants included sediment from the ditch as well as hydrocarbons, heavy metals, and nutrients from the street runoff. These pollutants were carried throughout the Mill Creek watershed and into the Ohio River.
The city of Forest Park applied for the funds through the Ohio EPA, Madl says, and received close to $50,000. Using Envirolok was a test project; the city wanted to try it to see how the modular units worked on steep slopes and as a stilling basin for water quality.
In 2012, the project restored the first 200 feet of the ditch, which is 5 feet at its lowest point. It removed most of the stream’s concrete channel armoring and a low stone wall, stabilized the channel bottom and walls with Envirolok vegetated bags, and revegetated with native trees and shrubs.
Because Envirolok dissipates the energy of stormwater runoff, the new swale reduces erosion and sedimentation in the channel, eliminating a potentially larger erosion control project down the road, Madl says. The native aquatic vegetation filters pollutants from street runoff. The project also enhances the habitat for birds, butterflies, and aquatic organisms, adds rain gardens and a viewing platform, and requires less maintenance from the city.
“It was a big job to be cleaning up the ditch,” Lavy says, “and because Winton Road is so busy, it’s almost inaccessible.”
Because Envirolok requires little excavation, Triton Services in Mason, OH, excavated the heavy clay soil from the ditch just 1 to 2 feet beyond the existing slopes, so there was little disturbance of the site. Crews removed the concrete floor, excavated a level foundation trough 12 inches wide and 3 inches deep as toe stabilization, and tamped it down. They also removed all the invasive vegetation, especially honeysuckle and cattails.
“They certainly cleaned it up,” Lavy says. “It’s the same depth, but they regraded the slopes. Some of the angles got more gradual, others were made steeper. Where they’re steeper, the flow capacity improved.”
Triton then filled some 900 Envirolok bags, which are made of a UV-resistant polypropylene nonwoven geotextile.
Because the job was fairly small, crews used a table jig, which held 20 bags in position at a time, seam-side out for strength. They trucked in the medium-75% sand for structural stability and 25% shredded topsoil for the vegetation-from a nearby gravel pit. They placed the medium on the table with a skid-loader or track-hoe, he says.
They used two systems to install the bags in the channel. “It’s real simple,” Lavy says. “Anybody can do it with a bit of training.”
In the bottom of the channel, where the most turbulence occurs and where logs get stuck, crews placed three rows of connector pins into the soil. With Kevlar geogrid, they used a system called clinching and twining to wrap and tie the bags together in one unit and placed three rows of bags side by side onto the pins.
The flatter the slope, the more surface is available for vegetation and wall stability, according to Envirolok. Triton placed coir erosion control blankets on both slopes and inserted two connector pins into each of the outer bags in the bottom of the channel, placing them to begin a 3:1 slope.
Crews then placed the first row of bags on the pins, tamped down each bag to flatten it slightly and ensure a solid, uniform structure, and inserted two connector pins into the top of each bag.
They then backfilled and compacted with well-draining soil.
They staggered the bags in each subsequent row for additional strength and inserted two pins in such a way that each pin would pierce a different bag above, connecting both rows. They used one connector pin for half- bags at the ends of rows. At the end of each row, they placed a bag at a 90-degree angle, extending it toward the back of the wall as a tie-back, then backfilled and compacted with well-draining soil after each row.
Above the channel, on the more gradual slopes, crews spread 3 inches of topsoil, planted a specialty seed mix, and covered the seed mix with coir matting for reinforcement.
“The biggest challenge was the tight working quarters,” Lavy says. “The community center was on one side and a park was on another, and we weren’t allowed to use the land. Envirolok was a help because it doesn’t take up much space.”
Another challenge was the timing. Although the project began in early October and it wasn’t a big job, the contractor had a scheduling conflict.
“We were looking at an Ohio winter coming at us,” Lavy says. “I was anxious to get the seed in the ground.” A variety of native shrubs in 1-gallon pots were used, as well as trees, including burr, red and white oaks, willows, sweet gums, and viburnums.
At first, the dormant bare-root seedlings specified were unavailable, so some trees in 3-gallon pots were used. Bare-root plants cost much less than potted ones, and they tend to grow stronger as they grow into the substrate.
By November 8, crews were planting grass seed, Lavy says, and they’d planted most of the trees and shrubs. “The black bags they’re planted in kept the soil warm, and we had a mild winter, so those extended our window a little.”
Says Madl, “I think we’ve had good success with the Envirolok. I might have kept some of the concrete in the channel, but it seems to have performed well for the toes of the slopes and the walls. The critical point is to establish the vegetation. After that, it should entail much less maintenance.”
