Sediment and Erosion Control on Construction Sites
Extreme terrains require different approaches.
Friday, August 31, 2007
By Jennifer Pinkley
Phase II of the National Pollutant Discharge Elimination System (NPDES) went into effect in March 2003, causing professionals around the country to scramble to figure out how to meet the new regulations. Efforts to comply are quite different around the country, with various types of projects requiring different approaches to control erosion and sediment. Some parts of the country, with strict enforcement of the Phase II requirements, quickly have developed effective techniques to reduce onsite erosion and sedimentation. Other areas still are feeling their way through the new network of regulations.
There is one constant for all projects, according to Jerald Fifield, president of HydroDynamics Inc. in Parker, CO: “Contractors, developers, and builders will always save money in the long term by continually stabilizing the disturbed land while construction activities occur.”
Although there is a huge variety of projects that must deal with erosion and sediment issues, the following sections describe erosion and sediment control methods used for three different types of construction projects: Florida wetlands, flat residential sites, and severe slopes.
Florida Wetlands
Erosion control specialists in Florida deal with conditions not found in many parts of the country: the prevalence of wetlands and standing water bodies. According to Cheryl Moore of Cheryl L. Moore Consulting LLC, “Florida is a little unique in one sense that we have a lot of surface waters, a lot of wetlands, and we’re sitting right on top of an aquifer, too. Even though it’s relatively flat, there are still significant problems above and beyond what some other states are dealing with.”
Florida is becoming very aggressive in implementing erosion and sediment control programs. According to Moore, EPA inspections throughout the state have started to keep many contractors in more of a proactive rather than reactive mode when considering erosion and sediment control.
If projects result in sediment entering a wetland, fines certainly will result. In addition, regulatory agencies don’t allow any type of heavy equipment to retrieve sediment from a wetland. Moore says that if sediment washes into wetlands, “Companies must go out with hand labor, such as wheelbarrows and shovels, and slowly scrape off each layer of soil until you get back down to natural vegetation and soil in that wetland.” The process can be quite expensive and time consuming, providing a strong incentive to prevent erosion and conduct monitoring to make sure projects are in compliance.
Some Florida water bodies other than wetlands sometimes may be regulated heavily. “We have a list of impaired water bodies here in Florida that have zero tolerance for turbidity. I recommend to my clients if they’re working next to a protected area like a creek or a river that they do turbidity monitoring along the way to make sure there are no violations.
“Different water management districts have different requirements as far as setbacks and staying away from wetlands, and lot of times that’s a challenge,” Moore continues. “You can’t have any turbid discharge or sedimentation impacting the wetlands.”
Florida also has unique soils that dictate the type of erosion and sediment control methods that are most effective. With sandy soils, contractors are probably in good shape with properly installed and maintained silt fences. However, in many parts of Florida there are clay-type soils, which according to Moore “require more than just a silt fence. You’re going to have to use additional methods to keep from being in violation.”
“On an average job, I suspect that silt fence barriers might remove about 1% to 30% of the total sediment generated from a project—if we are lucky and everything works according to what appears in the textbooks and an effective plan is implemented,” explains Fifield.
To prevent sediment runoff, many projects that must contend with clay soil start with the standard silt fence but also use sediment traps and perhaps diversion berms to keep soil particles from going out into the wetland areas.
“The only effective sediment—not erosion—control method that is effective for removing suspended particles found in runoff water is a properly designed sediment containment system, such as sediment ponds, basins, or traps,” explains Fifield. ”However, these structures will only be effective for removing design-size particles.”
Another sediment control tool that works well near wetlands is anionic polyacrylamide (PAM). PAM is a polymer that binds up soil particles and forces the particles to settle out before they reach the wetlands. This method focuses on very fine sediment, such as silt and clay, that is difficult to control using more conventional methods.
According to Fifield, if a majority of sediments are much smaller than 0.02 millimeter in diameter, even sediment containment systems will not be very effective without adding flocculants such as PAM. “This means that the silt fences, fiber logs, inlet barriers, and so on are very ineffective in removing small suspended particles when runoff from construction sites occurs,” he says.
Sometimes contractors will set up a flow-through treatment system using PAM. For example, they’ll install gel blocks of flocculent in areas of active flow. As water flows over the blocks, soil particles settle out. Then sites can usually get permission to discharge the clean water.
In some situations, contractors can use PAM for temporary erosion control. “It also comes in a granular form that can be distributed over the surface of the soil to help bind up the soil particles and keep them from eroding away as water flows over it,” Moore says. “The sediments aren’t suspended; they fall out very quickly” and are trapped by erosion control barriers around the perimeter of the project. Wattles are sometimes used around the perimeter to trap sediment. Developers may use wattles in front of individual homes instead of trying to keep silt fences up. Some developers consider wattles a better option because they’re lower profile and a little easier to deal with.
Inlet protection devices also are being implemented widely in Florida. One type of device is placed inside the inlet. Other devices are placed in front of inlets to trap sediment. Silt-Saver manufactures one type of inlet protection device that can be used with grate and curb inlets. “Those can be a really good alternative to block and gravel filters in Florida, since Florida doesn’t have much rock,” Moore says.
She notes that “a lot of people are now being proactive” about erosion and sediment control in Florida. “They realize that they can spend the money up front versus having to spend it later on. It’s a little easier and less costly to do something in advance.”
Residential, Flat Sites
Compared to wetlands, projects in the Midwest face different erosion and sediment control challenges. Clark Dohr, with New Town Builders in Colorado, is working on the Stapleton Redevelopment Project, a 4,700-acre (7.5-square-mile) site located within 6 miles of downtown Denver. The project is redeveloping an old airport into a mix of residential properties, retail properties, and open spaces. The residential sites will have approximately 12 units per acre.
New Town Builders is one of several builders working on the Stapleton project. Dohr’s company begins work after all of the earthwork is done and installs devices to control erosion and sediment runoff during the homebuilding process.
Slopes are not an issue with the Stapleton project, Dohr notes: “It’s pretty flat out here.” In addition, a main feature of the project is how close homes are to one another. “We get a lot of traffic around the houses,” says Dohr. As a result, he needed an erosion control solution that could stand up well not only to foot and vehicle traffic but also to the snowy and icy conditions typical of Colorado winters.
Dohr says that instead of using silt fences or straw wattles, he uses Ertec’s Perimeter Guard, which performs well on high-traffic sites. According to Dohr, Perimeter Guard is easy to install and stands up well to abuse on closely packed construction sites. It’s not easily disturbed even when construction vehicles bump into or drive over it. During the winter, snow and ice can affect the shape of the product, but Dohr quickly can reshape or reinstall it.
Perimeter Guard typically is installed around the perimeter of each home site, allowing flow-through while reducing the velocity of water. Made from high-density polyethylene, it also filters particles out of the water. The fine mesh of the product reduces the impact of wind and water but traps particles of sand, soil, or silt. The mesh can be cleaned periodically to remove sediment buildup.
Erosion and sediment regulatory standards in Denver are quite strict. Sites are inspected once a week, and any compliance issues must be taken care of within seven days. “We just had an inspection, and we’re the only contractor that didn’t have any compliance issues,” Dohr says.
Greg Lacy of Glenn Stephenson Enterprises works on projects in the Midland, TX, area. He usually works on one lot at a time, providing stormwater pollution prevention plans for his clients, conducting inspections, installing erosion control products, and cleaning up after a project.
“Midland’s not very big; we usually do 200 homes a year,” says Lacy. “Last year a state inspector was assigned to the Midland area, which has increased awareness of the new regulations.”
He notes, “Sediment and erosion control regulation are new in Texas, and companies are working to find their own boundaries, to figure out what they can and can’t do.”
According to Lacy, commercial construction contractors in the area generally use silt fencing. However, Lacy didn’t like this method for residential construction because it seemed more difficult to install and to move onsite. He chose Perimeter Guard because he thought it would be easier to both install and move.
Lacy says that permitting authorities in his area allowed him to choose the product he thought would work the best on his sites. Fifield says this is a sign that the authorities are open to new products and methods. “Usually, when the permitting authority dictates the methods, it is a very good sign that these people may not be very cognizant of what works and does not work on a construction site. Those flexible authorities are a joy to work with because they usually understand the limitations of BMPs [best management practices] and are looking for new and improved methods to protect the environment.”
“What I like about the Ertec product, when I put it down and come back later to put my sidewalks in, I can easily reinstall that product,” Lacy says. “We get so windy that silt fencing just ends up tearing up in the wind. It won’t last very long.”
In addition, Lacy is able to recycle the product for multiple projects. “We’ll sell the product to the homebuilder and install it for him,” Lacy says. “When he’s ready to go to the next house, we’ll reinstall the product and just charge him for the installation at that point.” This lowers the overall cost for his clients.
He says most sites in the area are flat and fairly simple to work with. “We just install the Perimeter Guard on the downhill slope of sites.” Lacy inspects his customer’s sites weekly. State regulations also require inspections each time there is 1 inch of rainfall, but Lacy conducts weekly inspections, even if there is no rain, to ensure compliance.
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Photo: Profile Products |
| Fiber wattles and flexible growth medium on a slope beneath Highway 154 in California. The original slope failure is shown at the beginning of the article. |
Fifield says that contractors onsite should conduct daily inspections to ensure that erosion and sediment control products are working properly. “If they do this, they will save money,” he says. However, contractors throughout the country should conduct formal, written inspections at least once a week. “Regulatory personnel should try to evaluate a site at least once every seven days but more than likely will not accomplish this task due to budget constraints. At no time should regulatory inspections occur less than once every 14 days,” he explains.
An erosion and sediment control issue possibly on the horizon for Texas builders is dealing with windblown soil. “We don’t have that much rain here in Midland, but it does get pretty windy, which can cause a lot of dirt to get up in the air,” says Lacy. “That issue will be difficult to address.”
Severe Slopes
The final type of site to explore is severe slopes—those so steep that it’s difficult to get onto the slope or so rocky that contractors can’t install anchors for erosion blankets.
Many contractors use hydraulically applied products to deal with slopes. However, there are two categories of hydraulically applied products primarily used to deal with severe slopes, according to Marc Theisen of Profile Products. The first type is bonded fiber matrix (BFM) products, which have been in use since the mid-1990s and are widely accepted as a cost-effective material for steep slope projects. “BFM consists of thermally processed wood fibers and a cross linked hydro-colloid tackifier that provides a chemical bond with a higher bond strength than traditional tackifier,” Theisen explains.
Contractors can adjust BFM application rates to match various site conditions. For example, Theisen says, “For very steep slopes that are somewhat rough graded, in order to achieve 99% ground cover, which is what you want for total coverage, you may need to go with rate of 4,000 pounds per acre.” For less severe slopes, such as 3:1, the application rate could be reduced to 3,000 pounds per acre.
“The Achilles heel of BFMs is that they have to cure, or dry out, so that the chemical can become active,” Theisen says. “If you receive precipitation within 24 to 48 hours after application, the materials are vulnerable.” After BFMs cure, they are very high performing.
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Photo: Profile Products |
| Revegetation at an open-pit quarry in Ontario, Canada |
The second product category is flexible growth medium (FGM). FGM consists of the same thermally processed wood fibers found in BFM but with the addition of half-inch-long, crimped, manmade fibers. These manmade fibers create a mechanical bond in addition to the chemical bond, increasing the loft of the hydraulically applied matrix. It dramatically increases the resistance to both rainfall impact and overland flow.
“[FGMs] don’t have to cure, allowing them to be effective upon application, which is a big breakthrough,” says Theisen. For example, contractors can use FGMs on severe slopes in parts of the country subject to frequent afternoon showers. Theisen says that BFMs are typically 90% effective, whereas FGMs are 99% effective.
For high-end hydraulically applied materials, very little site preparation work is required. This is a benefit because contractors don’t want to spend a lot of money trying to smooth out slopes that are difficult to access and grade. In fact, rough surfaces are preferable to smooth surfaces. Although slopes cannot have large rocks or boulders, rough areas are naturally more resistant to erosion and slow down the flow of water. Small pockets on the surface also trap moisture and help promote plant growth.
These products typically are applied in two steps. The first step is to put down seed. Theisen says, “It’s very important to select plants that are locally adapted, particularly on slopes where there will be low-moisture conditions. Slopes won’t retain moisture, so it’s very tough to get vegetation on steep slopes.” A small amount of the BFM or FGM is added to the first seed layer, mostly as a visual tracer to ensure uniform seed distribution. (Most BFM and FGM products are green when applied to soil.) Contractors also can add fertilizers and other organic soil amendments to help seeds germinate and grow. The second step is to cap the seed with another layer of BFM or FGM at a typical rate of 3,000 or 4,000 pounds per acre, depending on the severity of the slope.
Fifield believes that the key to successful erosion and sediment control on any site is “ensuring development of effective and practical sediment and erosion control plans to implement throughout the many different phases of a project.” The following projects highlight effective planning and implementation for very challenging severe slope conditions.
The first severe slope example is an emergency highway repair project in California. In early January 2005, Santa Barbara County in California received 23.7 inches of rain over a three-day period. One section of California Highway 154 known as Painted Cave is located on top of a steep mountain wall with a 1.5H:1V slope. With so much rain over such a short time period, trapped water that had built up on the uphill side undermined the highway. The trapped water blew a hole beneath the highway, washing one entire lane of the two-lane highway down the mountain.
This was clearly an emergency situation. The highway had to be closed, disrupting traffic on an important road. In addition, more rainfall was forecast, requiring engineers to come up with a fast and effective solution.
First, California Department of Transportation (Caltrans) engineers had to fix the road while avoiding future blowouts from trapped water. Contractors installed a pipe underneath the road extending down the 4-acre mountain slope to drain rainwater coming from the uphill side of the road. They also rebuilt the slope but then had to figure out the best way to revegetate and stabilize the slope.
Bruce Berlin, erosion control product manager with S&S Seeds in Carpinteria, CA, proposed using Profile Products’ Flexterra FGM product. He chose this option for several reasons. First, no curing or drying time was required (an important point since more rainfall was expected). Also, the product could use native seed to revegetate the slope. In addition, if the seed didn’t germinate, Flexterra still would provide adequate erosion protection for the severe slope.
The first step of the project involved installing fiber wattles. The wattles reduced the slope length, prevented rilling, and also provided firm footing for workers rappelling down the mountain to apply the FGM.
Peter New, landscape architect for Caltrans’s District 5, reports that the work has stabilized the slope. “So far the Flexterra has performed adequately,” says New. “It has remained well-knitted together and the soil bonding is evident. We haven’t seen a high percentage of germination, which we suspect is due to inadequate rainfall following application.”
Another severe slope project involves a Woodstock, ON, Canada, quarry operated by the company Lafarge. Although this project is located in Canada and is not subject to Phase II of NPDES, the project is a good example of applying BMPs to severe slopes. The quarry is an open-pit operation that supplies limestone for Lafarge’s cement production. One side of the quarry with a slope gradient of approximately 2.5H:1V, about 300 feet from top to bottom and totaling about 10 acres, began showing signs of erosion. Without corrective action, sediment would begin to affect nearby streams. To complicate matters, this area consisted of “overburden,” a mixture of clay, sand, and rock material with no topsoil—a surface not only susceptible to erosion but also extremely difficult to revegetate.
Lafarge first tried conventional hydroseeding, after hauling in dirt and grading the surface. “We tried this twice, with little success,” says Michael Bart, environmental engineer for the Lafarge Woodstock operation. “The combination of rain, snow, freezing, and thawing would wash out the topsoil we’d graded in and create large gullies. The only evidence of growth was in a few areas where the soil hadn’t eroded away.
“It was not only discouraging; it was costly,” Bart continues. “As a result, we were unwilling to make further investments until someone could demonstrate an effective technique on this site.”
In the spring of 2003, John Reynolds of Mulchit, located in Putnam, ON, Canada, and Dwight Johnson, regional manager for Profile Products, met with Lafarge officials and proposed trying Flexterra FGM on a 30-foot-wide test strip. Contractors graded the area to smooth out the rills and track-packed the area with a bulldozer with the cleat marks running horizontally across the slope to act as a basic erosion retardant. Finally, Future Green, an erosion control contractor located in Schomberg, ON, Canada, applied the FGM at a rate of 3,500 pounds per acre using a mixture of seeds and legumes, plus a slow-release nitrogen fertilizer, that would grow in the difficult conditions.
Application occurred in the fall of 2003, at the very edge of the area’s growing season. However, the following spring, after a long winter of snow, freezing, and thawing, inspectors found few signs of erosion on the slope. As a result, Lafarge decided to use Flexterra FGM on the remainder of the slope.
After two years, minor rilling appeared. To ensure permanent results on the slope, Profile Products’ Terra-Tubes were incorporated as a slope interruption device. Terra-Tubes trap, filter, and treat sediment-laden runoff and reduce hydraulic energy. They use an engineered composite of thermally processed wood fibers, manmade fibers, and polymers encased in heavy-duty knitted tubes.
Contractors again graded the slope and track-packed vertically. Then, four rows of 6-inch-diameter Terra-Tubes were placed along the face of the 2.5H:1V slope. Each row was spaced about 75 feet apart. Finally, the same mixture of Flexterra, seed, and fertilizer was applied hydraulically at the rate of 3,500 pounds per acre.
Site inspections during the spring of 2006 revealed good, stable cover on the site. The seeds germinated in late spring, covering the ground with new vegetation. Only one small rill was apparent, but by August it had fully grown in.
The end result of this especially challenging project is a secure, established slope plus well-established BMPs for future trouble spots at the quarry.
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