Brushing up on erosion control
Thirty years ago, when residents of Wilmington, N.C., purchased homes in Long Leaf Hills, their back yards bordered a picturesque creek that could be crossed easily by jumping. By 1989, drainage problems and erosion had stretched the creek’s banks, creating a channel that was 40 feet wide and 10 feet deep at some points.
Today, Wilmington officials are using soil bioengineering to stabilize the creek banks, control flooding and restore the natural habitat of Long Leaf Creek. The process is based upon traditional engineering practices and incorporates vegetation (instead of only concrete and riprap) to create the structural systems for stabilization.
The environmental emphasis of soil bioengineering, as well as its permanence and aesthetic value, are among the benefits cited by city officials who have adopted the discipline. From North Carolina to Oregon and Alaska, it has been used successfully in an assortment of applications for streambank erosion control.
vital solution The Long Leaf drainage project encompasses nearly 2,000 lineal feet with a drainage area of 780 acres, says David Mayes, engineer and project director for the city of Wilmington. The drainage area contains residential, office, institutional and commercial properties. Approximately 25 homes line the Long Leaf segment of the creek.
At the upstream end of the project, the creek flows adjacent to a state-maintained right-of-way that includes a twin 6-foot-by-6-foot, concrete box culvert. The culvert drains to an 84-inch pipe positioned beneath another road that crosses the creek.
“That causes a hydraulic problem,” Mayes explains. “Water backs up into the creek and causes localized flooding.” He notes that downed trees and debris also block water flow, adding that, because the city does not own the property, the creek has not been maintained.
Solving the drainage problems would not be enough to alleviate concerns of Long Leaf homeowners, who watched helplessly as their property slowly dissolved; stabilizing the banks would be an essential part of the solution. “The banks are essentially vertical in some places,” Mayes says. “The cross-section of the creek is very inaccessible, and the erosion had gone as far as to creep up on storage buildings and fences.
“The major problem is property loss,” Mayes continues. “We’re in the coastal plain, and the soils in this area are sandy. Sandy soils and vertical creek banks just don’t mix.” In 1989, when the city undertook an assessment of the creek, engineers recommended that the Long Leaf banks be stabilized by installing concrete culverts or “piping” down the entire length of the site. In 1996, a city bond issue raised $1.95 million for the project.
“We proceeded to hire [an interdisciplinary team of consultants] to look at several different alternatives and give us cost opinions for each one,” Mayes says. In addition to piping the creek segment, options included: * acquiring right-of-way and maintaining the creek; * armoring the channel with riprap or fabric form (concrete); * sloping the channel and grassing it; and * soil bioengineering.
Through the bond issue, Wilmington citizens voted to fix the creek, thereby eliminating simple maintenance as a viable option. Armoring the channel was rejected primarily because it was not as aesthetically desirable as lining the channel with grass. However, creating the slopes for the grass lining would require more easement than the city wanted to purchase and more land than the homeowners wanted to lose.
Therefore, the city’s choices for stabilizing the creek bank were reduced to piping and soil bioengineering. Piping was the most expensive solution; in addition to installing the concrete culverts, the city would be required by environmental regulators to mitigate another creek site (i.e., if the city installed concrete in one natural setting, it would have to remove concrete from another setting).
Soil bioengineering would cost nearly $550,000 less than piping, Mayes says. Additionally, its ecological elements sold city officials and citizens.
At its root Soil bioengineering incorporates living plant materials to restore and protect streambanks while creating a natural habitat and a natural filtration system for water. It comprises a variety of techniques that are employed in concert according to the needs of a given site. Some of them include: * vegetated geogrids (live branch cuttings placed in layers with geotextile fabric wrapped around soil lifts); * live siltation construction (live branch cuttings placed in trenches at an angle from the bank to offer immediate overhang, trap sediment and protect the toe against erosion); * live fascines (sausage-like bundles of live branch cuttings that are placed in shallow trenches, partly covered with soil and staked in place); * brushmattresses (combinations of live cuttings and live fascines installed to provide cover and protection of streambanks); and * live cribwalls (woody cuttings inserted into a log or timber framework).
The woody vegetation used in soil bioengineering can have a significant effect on habitat benefits such as providing overhanging cover and shade for streams, nesting and foraging sites for birds, and cover and food for some land animals. Willow and dogwood are used most commonly because of their excellent rooting ability.
When construction begins on the Long Leaf project next fall, the creek banks will be cleared, and geotextile fabric will be installed to reconstruct and reinforce the banks. “They start building up the bank of the creek using soil and geotextile fabrics, and then they install live cuttings within the bank of the creek,” explains Mayes, adding that native vegetation will be selected locally and installed by the contractor.
Vegetated geogrids will be used to stabilize the sandy banks, provide overhanging cover for aquatic habitat and aesthetic benefits to the homeowners. The upper areas of the banks will be planted, and, where space allows, the banks will be cut back and installed with brushmattresses.
Stability will be restored to the Long Leaf creek bank immediately upon completion of construction, which Mayes estimates will move at a rate of 15 to 16 feet per day. “Once it is installed, it is stable; it will survive a gully washer,” he says. “But the stability and the strength of the creek bank will [increase] as the plants grow.
“We will end up with a streambank that’s stable and alive; it will have plants growing out of it,” Mayes explains. “A natural creek system allows for habitat, which a piped, closed system does not. Also, plants along the bank will have the ability to absorb some of the water, and, as a result, they will absorb some of the pollutants.”
Of fish and fishermen As they did in Wilmington, officials in Portland, Ore., and in Soldotna, Alaska, have used soil bioengineering primarily because of its ecological benefits. While one city sought to restore water quality and preserve fisheries, the other needed to preserve grounds for local sport fishermen.
In Portland, Oregon Department of Transportation (ODOT) planned to relocate and shorten a 1,000-foot, “S”-shaped segment of Johnson Creek to accommodate a new bridge and interchange. During the permitting process, the Johnson Creek Corridor Committee insisted that ODOT ensure the stability of the relocated section and restore aquatic and riparian habitat.
“The local watershed council was concerned about disturbing this particular area of the creek because of the habitat values and because of the local fisheries,” says Eric Machorro, watershed manager for Portland’s Bureau of Environmental Services. “There is an active fishery there, and salmon are in the stream, so there was a large effort to restore the stream further and to provide more habitat.”
As a negotiated settlement to the permit, the state agreed to use soil bioengineering in the restoration of the streambanks. Live siltation constructions were installed along the channel margin on the inside of the bend to provide overhanging cover, and brushmattress was installed on the inside bank to provide erosion protection and riparian habitat. Additionally, outside banks were protected with low vegetated geogrids installed above a rock toe that extended to the ordinary high-water elevation.
The vegetation “grew very quickly; after the first year, they had a nice brush layer there,” Machorro says. “It takes a couple of years for the vegetation to mature, so you can’t just walk away; but, once the vegetation is established, it becomes self-perpetuating and self-maintaining.”
In addition to being aesthetically pleasing, the vegetation has helped restore water quality and insect species along the affected area of the creek, Machorro says. All those factors have combined to support the local salmon population.
“A major component of water quality is temperature, especially if you’re trying to manage a stream for coldwater fisheries,” he explains. “The simple act of shading has a profound impact on the quality of water supporting the fisheries.” The insects, he adds, are a natural food source for the fish.
Preserving Portland’s fisheries was a major concern in remediation of Johnson Creek, but in Soldotna, a town of 4,000 people, officials were forced to look at the link between fishing and erosion from another perspective. Soldotna Creek is a tributary to Kenai River, and the convergence of the two water bodies provides a popular spot for sport fishing. Years of heavy foot traffic, as well as seasonal flooding and outflow of ice, had caused enormous damage to the riverbank.
A stretch of bank measuring 650 lineal feet and starting at the mouth of the river was particularly degraded, says Soldotna Mayor Ken Lancaster. “Being a very popular fishing hole, the area took an extreme amount of damage to the banks,” he says. “Also, the ice goes out in the spring, when the water comes up underneath the ice and forces it to go downriver [causing additional trauma to the banks].”
>From 1991 to 1995, when remediation of the area was complete, “we’d lost >in excess of 20 feet of bank in that area,” Lancaster says. Without >action, the riverbank would have continued to erode “to the point that we >may or may not have been able to save it,” he notes.
The Kenai River solution comprised a variety of soil bioengineering techniques. Overhanging cover was provided with live siltation constructions and live cribwalls. In wet areas, native sod rolls were used to stabilize the bank line and to re-establish vegetation. To create additional cover for fish, large rocks were placed randomly in shallow water in front of the live cribwalls, and small rootwads were anchored in deeper waters.
At the upstream end of the project, the city installed a fishing platform and trails that weave through the newly vegetated riverbank. It also added three staircases leading into the river to prevent foot traffic from degrading the site again.
The project took three summer seasons to complete, Lancaster says, adding that ice prohibited continuous work. However, each portion of the system stabilized immediately upon installation.
“We have a tremendous growing season in summer, and the vegetation took root and bloomed immediately,” Lancaster notes. “In fact, we’ve had to trim it back every year because the plants just keep growing and growing. It’s really been phenomenal how it’s worked.”
In 1995, following completion of the soil bioengineering project, Kenai River flooded, raising the water nearly 20 feet above the project line. Lancaster reports that wooden poles anchoring the walkways had to be replaced with metal, but otherwise, the system withstood the flood waters.
The long-term results Aside from monitoring early plant growth and periodically trimming mature vegetation, soil bioengineering requires little maintenance, according to those who have used it. In Long Leaf, Mayes anticipates that, in addition to trimming the growth, the city will be responsible for removal of downed trees as well as sediment or debris. “Soil bioengineering is, in my mind, a permanent stabilization of the creek bank with only the possibility of some minor spot maintenance,” he says.
Lancaster agrees. “I’m of the opinion that [this solution] is permanent,” he says. “I anticipate that my kids and grandkids can fish [on Kenai River] long after I’m gone.”
Because soil bioengineering is a new discipline for many cities and counties, and because it is labor-intensive, the initial costs are high, Mochorro says. “I think it’s important to remember it’s a newer technology, so it’s more expensive at first. But it does more than just stabilize the bank; it provides habitat and aesthetic values. I believe over the long term it’s cheaper because you’re not having to go back and pour more riprap or repair concrete work.”
Mayes says that, although cost was a consideration when planning a solution for Long Leaf, residents of the community were swayed by the aesthetic value of soil bioengineering. “We held two public meetings in which we invited our team of consultants, city staff and residents that owned property along the creek,” Mayes says. “The residents were overwhelmingly in favor of soil bioengineering. They liked the fact that there was a creek behind their houses, and they wanted to see something [that would preserve the natural aspects].
“They understand that, when you have a construction project, there’s going to be some ‘pain and suffering.’ There’s equipment behind your house, trees are being removed, there’s noise back there, and there are people back there,” Mayes adds. “But they also understand that they will have an aesthetically pleasing, natural creek system that’s stable and will not erode their property anymore. I was pleasantly aware that, with some minor exceptions, the citizens really ate this solution up.”
Mayes believes his city’s experience represents of a trend in erosion control. “Engineers can size a pipe, they can size a channel, and they can line it with something that will not erode – like riprap or concrete. But engineers need to be looking for more comprehensive solutions.
“Soil bioengineering is one of them,” he adds. “I’m anxious to see this project work because I think it could work in a lot more places than just Wilmington.”
Robbin Sotir is president of Robbin B. Sotir & Associates, a soil bioengineering firm based in Marietta, Ga.
The Colorado Department of Transportation recently built the Goddard Avenue bypass to improve traffic flow in Trinidad. A 3 acre-foot stormwater detention basin was incorporated to control erosion and capture sediment.
Using four culverts, the detention basin intercepts stormwater runoff from the Goddard Avenue drainage system and from watersheds located north and west of the interchange. In addition to reducing sediment, the basin was designed to improve the water quality of runoff into a picketwire ditch irrigation canal and to reduce the frequency and severity of damage to the ditch resulting from excessive runoff.
The basin was constructed by excavation, and an earthen dam embankment comprised of silty and sand clay was placed in 2-foot lifts. The dam has a top width of 12 feet, a crest length of 150 feet and 4:1 slopes. The crest elevation is 8.5 feet above the bottom of the basin.
Flexible lining that could accommodate predicted settlement and provide spillway protection for waterflows was included in the basin design. Conventional gabions, as well as articulating concrete block (ACB) revetment, were selected as suitable materials for the outfall.
The general contractor for the project, Littleton, Colo.-based Kiewit-Western, opted to install the ACB materials, supplied by American Excelsior, Arlington, Texas. A five-man crew hand-laced the 6-inch thick, precast concrete blocks at a rate of 2,500 square feet per day.
A thin layer of roadbase material was spread over the spillway and compacted to fill voids in the subgrade. A geotextile was then placed loosely on the subgrade and “shingled” with upstream strips overlapping downstream strips.
The blocks were installed from the spillway crest into the upstream termination trench and then downstream from the initial point of installation. Open areas between the blocks were backfilled with soil to protect the geotextile from ultraviolet radiation and to support future revegetation.
A native grass seed mix was applied to the site to improve aesthetics and increase hydraulic stability. Additionally, trees and shrubbery were planted within the basin and on its slopes to enhance appearance.
Last summer, officials in Rayne, La., were literally watching their city’s new wastewater treatment plant disappear. High winds produced constant wave action that pummeled the shorelines of two settling ponds, causing severe erosion.
For help, the city called upon Lafayette, La.-based Mader-Miers Engineering, which built the plant. “We had used 5-inch stone along the waterline in previous installations,” explains Fred Trahan, chief engineer for the company. “But hurricanes [in Rayne] had kicked up a lot of wind action and created waves 4 or 5 feet high, which just pounded the rocks right out of there.”
Although the primary pond covers 58 acres and the secondary pond just 12 acres, the solution to the plant’s erosion problem was the same for both sites. To re-establish stability along the shorelines, engineers installed a layer of grout cement around the perimeters of the ponds. It was anchored by geomatrix matting supplied by Akzo Nobel Geosynthetics, Enka, N.C.
Krielow Brother Construction, Norfolk, La., fashioned a 9-foot-wide area around the perimeters, and the matting was anchored in trenches along the top and bottom edges. A 2-inch layer of pea gravel grout mix was poured onto the matting.
The installation took fewer than three weeks to complete. The settling ponds were restored successfully, and neither wind nor waves has caused additional erosion.