Containing Sediment

Sept. 1, 2009

“According to the EPA, soil is the number-one pollutant in waterways,” says Louise Oyen, environmental specialist with Geographic Solutions Inc., an engineering, surveying and environmental services company in Bay Minette, AL.

Even if the soil is not contaminated, if it is transported offsite it can fill up small tributaries and streams and move on until it reaches the mouths of rivers, bays, and other waterways. Oyen can see the results as it filters into the nearby Mobile Bay and eventually the Gulf of Mexico. Sediment increases flooding and turbidity, which reduces the capacity of wildlife to reproduce all along its path. Protecting water quality is the goal of her work.

One of the culprits is silt and sediment from construction sites. Oyen’s work, which includes National Pollutant Discharge Elimination System (NPDES) permitting and inspecting on a number of construction projects, helps stem the flow. “When a developer can see the Gulf of Mexico from his kitchen window, the idea of protecting water quality can really get to be important,” she says. “At that point, it’s no longer just about maintaining compliance with regulations. It’s about leaving a legacy for our grandchildren.”

Mobile, AL
One of her projects is a golf course and residential subdivision under construction on the eastern shore of Mobile Bay in Alabama. It covers a couple of hundred acres of rolling hills and wetlands and has some very sandy soils throughout. In addition, Oyen says, “The amount of rainfall generally ties with Seattle.”

It’s important to control erosion first, she says, and then contain the silt and sediment that does erode. For this project, she recommended a number of best management practices (BMPs) for erosion control on the slopes, including blankets on some of the steeper ones. On others, she recommended wide, flat terraces with silt fence along the edges. Where this wasn’t possible, she recommended other methods to create “speed bumps” to reduce the velocity of the water, such as straw wattles and Terra-Tubes fiber filtration tubes from Profile Products. Hydroseed was applied to steep slopes, and topsoil was added to other more gentle sloping areas.

“Adding topsoil to disturbed areas is a pretty good method of erosion control,” she says. “Topsoil doesn’t wash away as easily as subsoil. It has more organic material, so physically it’s a little stickier. It also creates a great seedbed.”

She recommended seeding these areas with rye grass and other seasonally appropriate vegetation recommended by Pennington Seed. On this project, precautions had to be taken to not use plants that reseed, so that they won’t interfere with the vegetation the golf course will plant once it’s in operation. On projects where site-specific recommendations are not made, Oyen consults the Alabama Handbook for Erosion and Sediment Control.

For inlet protection, she recommended creating several small barriers above each inlet. This was done either with straw wattles set out in a U-shape and anchored with sandbags or created completely with sand bags. One barrier was placed at the upper end of the storm drain and one or two immediately upstream to slow the flow of water and allow the sediment to pool. Low-grade silt fencing was in place when she started on the project, but sediment was overtopping it. The posts were undermined, some of the fabric under the trench had degraded, some had torn where rings attached it to posts, and some had separated. Oyen decided to replace the existing fence with the Belted Silt Retention Fence (BSRF), which she’d seen demonstrated at a local erosion and sediment control workshop called Red Water Blues.

The BSRF, from Silt-Saver Inc. in Conyers, GA, was designed by Roger Singleton, the founder of Silt-Saver, who designs all the company’s products, including a number of inlet protection devices.

“My recommendation for this or any other product is to put it to work and monitor the results,” Oyen says. “If the result is increased performance, I want to see that product on the ground. If not, I want to find something that does work.”

She’s found that projects that use the Silt-Saver system are better able to contain sediment onsite for longer periods of time than those using other silt fences. In other situations, when one post goes down, the posts beside it go down, too, she says, and if there’s a heavy sediment load, sediment goes pouring over. With the Silt-Saver fence, though, if one post is damaged, even by a vehicle, the posts on either side of it hold up very well, so the integrity of the silt fence line is less likely to be compromised.

The BSRF is designed as a system in which the fabric and the method of attaching it to the posts rely on each other. This spreads the load to give the fence extra strength across its entire length. The material is made of two layers of spunbond polyester fabric and a fiberglass scrim, or net, sandwiched between them. All three layers are bonded together so the fabric won’t separate.

A test in 2006 by the University of Georgia’s Department of Biological and Agricultural Engineering used ASTM standard methods to compare the Silt-Saver fence with a standard type C silt fence. It found that the Silt-Saver fence provided more efficient sediment removal and lower turbidity and also could withstand extreme loads and overtopping without structural failure.

It’s important to place and install any silt fence properly, Oyen says: Don’t put it perpendicular to a concentrated flow of water, or the water will either dam up or bypass the fence. Keep the trench free of woody debris, which can provide a channel for water to go through and undermine the fence. Make sure the posts are in firm ground and the backfilling is done correctly.

“Techniques in installation have really improved,” she says. “Roger [Singleton] came onto the site during the initial installation. He recommended angling the posts just slightly toward the upstream side so you’re getting a little bit of an advantage.”

The BSRF is easy to install because it doesn’t need wire backing for support, she says. The company recommends driving the 2-inch-by-2-inch, 4-foot-long hardwood posts into the ground until only 28 inches of the post are above ground and spacing the posts 4 feet apart. It also recommends stretching the fabric tightly along the inside perimeter of the posts; pressing a 1.25-inch slat against the fabric so the fabric is sandwiched between the post and the slat; and stapling the post, the fabric, and the slat together. Stabilizing the fabric this way prevents teardown from the top and adds linear support.

Alabama regulations state that, at a minimum, all BMPs should be inspected once a month or immediately following a three-quarter-inch rainfall, and generally as necessary to ensure that maintenance is adequate. Silt and sediment must be removed when they reach half the height of the device.

“There’s no one device or practice for silt and sediment control that’s going to be your answer,” Oyen says. “But if you think there is likelihood for a heavy sediment load between maintenance periods, Silt-Saver is the fence that will hold up where others have not.”

The Conservatory, Aurora, CO
Managing silt and sediment in areas where space is limited, such as developments where homes are built closely together, is challenging enough. When the space also has to hold soil excavated from basements, it’s even harder.

This was the situation for Shane Prahl, the environmental program manager with developer DR Horton Homes (Denver Division), which builds residential communities in 27 states, during the construction of The Conservatory. The Conservatory is a development of about 1,500 homes in the rolling hills of southeastern Aurora in Colorado.

“It’s a challenging site, to say the least,” Prahl says. “It’s really close quarters, and we have a lot of infill. It’s hard to keep the dirt out of the streets.” This isn’t just dirt, either, he says. It can carry pollutants from sources like spilled paint buckets, dropped grease guns, and fuel and hydraulic leaks.

He follows the Colorado Discharge Permit System and Stormwater Construction Permit along with the stormwater management plan that is customized for each site, which encourages community-based protection and preventive management strategies to ensure that public drinking water resources are kept safe from contamination.

Prahl bases his choices of BMPs on a number of factors. Does the city allow the BMP? Does it effectively prevent erosion? Does it effectively stop sediment? How much does it cost? How resilient is it? It has to take a lot of wear and tear, he says. If it doesn’t last, he can’t use it. How easy is it to install? “A lot of times there’s high attrition with the laborers hired to install the product,” he says. “We want to be able to teach them easily.”

The company usually uses redundant or multiple BMPs, except on very flat lots. In the case of The Conservatory, which has a combination of flat and sloping lots, the developer used multiple BMPs: inlet protection to filter the water before it reached the storm drains; straw blankets for erosion control on the detached walks and tree lawns (sidewalk buffers, planting strips, and parkways); and Gator Guard, from Gator Guard Environmental Products Inc. in Idaho, to keep silt and sediment out of the gutters.

“Gator Guard is predominantly what we use at The Conservatory,” Prahl says. “It acts as a wattle, but it’s more durable than a wattle. It can take a lot more abuse.”

DR Horton submitted the erosion control plan and obtained a permit from the state and the city of Aurora. Pase Contracting Inc., based in Parker, CO, was contracted to install and maintain all the BMPs.

“As soon as you disturb the soil, you have to have it protected,” Prahl says.

If it isn’t abused too badly, Gator Guard is reusable, he notes. It’s made of a UV-resistant geotextile fabric and filled with a recycled polyethylene foam similar to packing peanuts. At 8 pounds for a 25-foot length, it’s very lightweight. It has a high strength-to-weight ratio and openings large enough to allow water to pass through but small enough to block all but the smallest soil particles. It has an apron to minimize washouts.

According to a study done at San Diego State University’s Soil Erosion Research Laboratory, 15 gallons per minute per square foot of clean water can filter through Gator Guard, while less than 1 gallon per minute per square foot of sediment-laden water does. It’s 98% effective on a 3:1 slope. With a UV strength of 80% after 2,400 hours, Gator Guard has a five-year expected life span, even with some abuse.

Pase installed Gator Guard around the perimeter of each lot, says Justin Livensparger, the project manager for Pase. According to Gator Guard, the product should be installed with the apron facing uphill and pinned to the ground every 3 feet or so with either the capnails that are included, landscape staples, or large nails with washers. In high-flow areas, use two or more can be used together for extra strength. The units can be connected end-to-end to make longer wattles or cut, with some of the stuffing removed, to fit a smaller area.

Once the BMPs were in place, the city of Aurora conducted a preliminary inspection. “Aurora is strict in the effort they put into erosion control management,” Prahl says, “especially with cleanliness, housekeeping, and making sure BMPs are in place and effective.”

For the most part, monitoring is done to keep in compliance with the health department and EPA regulations, he says. Although the state mandates inspections every 14 days and after runoff events, the city requires developers to inspect them every 7 days and after runoff events. The city does its own periodic inspections as well. Gator Guard Environmental Products recommends monitoring for tears and escaping foam nuggets every one to two weeks and cleaning out excess sediment and repairing washouts after each runoff event.

Most of the damage comes from vehicles. Curt Pase, owner of Pase Contracting, says, “Having it driven over by subcontractors who require access to the lot is a pretty common occurrence, but Gator Guard can be reused provided the damage is not too excessive.”

“If we have reusable Gator Guard, vendors like Pase store it,” Prahl says. “We moved Gator Guard from another site to The Conservatory.”

Pinedale Anticline, Wyoming
Since 1987, when the Water Quality Act was added to the Clean Water Act, oil and gas companies have been exempt from the requirement to obtain NPDES permits for stormwater discharges from a number of activities, as long as the stormwater isn’t contaminated-although the boundaries of what constitutes contamination are unclear. The Energy Policy Act of 2005 added a provision that included construction activities, on the reasoning that their impact on the environment was low.

Typically, the area of disturbance to build a drilling pad, for example, is only about 4 acres, says Todd Erickson, chief development officer with EnerCrest Construction, an oilfield services company with offices in Wyoming, North Dakota, Utah, Colorado, and Montana. But some projects include roads, pipeline rights of way, and other infrastructure facilities and can be as large as 12 acres. Add dozens of these together and the amount of disturbance can be quite significant.

The Act of 2005 also gave states the authority to regulate discharges, however, and some have begun to require oil and gas companies to obtain permits for stormwater control during construction activities.

“It was a big surprise to people in the industry when they were asked to comply,” Erickson says. “I saw it as an opportunity to show them how.”

He began researching silt fence technologies. He was looking for a product that was innovative and, because there was a lot of work, very efficient. He found Tom Carpenter, CPESC, the founder and owner of Devon Distributing Corp. and the inventor and manufacturer of the tommy Silt Fence Machine.

“All silt fence is not created equal,” Erickson says. Some is made of flimsy material that degrades quickly in UV light. Some is stapled to weak wood posts. Some is badly installed. “The disturbance might last a couple of years,” he notes-longer than the product. “Silt fence got a bad name because of that.”

The first summer performing this type of work, EnerCrest installed about 300,000 feet of silt fencing with the tommy Silt Fence Machine in just one field. By the summer of 2008, the company had 40 to 50 people installing erosion control BMPs across all five states.

Ninety-five percent of EnerCrest’s work is in the oil-and-gas industry and 5% in the growing wind energy field. EnerCrest’s biggest project is the Pinedale Anticline, a natural gas field in the Greater Green River Basin in southwestern Wyoming. It covers approximately 120,000 acres of federal, state, and private lands. “It’s probably the largest concentration of gas wells in the US,” Erickson says.

EnerCrest began installing erosion control measures for Ultra Petroleum Corp., which has leases in about half the Pinedale field and Questar Gas, which has leases in about a quarter of it. EnerCrest also working for Shell Energy North America, which has leases in the remaining quarter.

An environmental engineering firm, KC Harvey, which is owned by EnerCrest, writes the stormwater plans, calling for a variety of BMPs including silt fences, sediment logs, wattles, hydroseeding, blankets and mats, sediment basins, and netting on roads “by the semi-load.”

The most difficult terrain in which to install silt fence is mountainous, Erickson says, and although the Anticline is mostly flat, it has a mesa that is quite hilly and can be rocky. The tommy is stable on slopes and in tight areas.

A small percentage of the time, the slopes are too steep for a tractor, and crews have to install the fence by hand. If the terrain is too rocky, they use wattles. Some of the other advantages of wattles are that animals can walk over them, they have less visual impact, they don’t tear or fall down, and they’re good for revegetation, Erickson adds.

The tommy turns easily-90 degrees within 10 feet-which allows it to maneuver around contours and to create J-hooks (smiles), which store silt, sediment, and water.

“A lot of silt fencing is used improperly. We call it the “˜burrito wrap,'” Erickson says. One problem is that the engineers who design the fencing haven’t always seen the sites, so they don’t know the contours of the hill, and instead of holding silt and sediment back, their designs can channel the water down the slope. It’s up to the company installing the fencing to create contours to hold the water back.

EnerCrest uses three-person crews, which it has found is the safest and most efficient. “In our industry, safety is more important than anything else,” he says. One worker drives a small 50-horsepower utility tractor with a bucket on the front and the tommy on the back. Two workers are on the ground, one to check on the installation and one to put the posts out.

The tommy uses the static slicing method, slicing 8-12 inches through the soil. The tip of the slicer pushes a small amount of soil upward and aside, which minimizes compaction. The tommy can slice through obstacles as large as 8-inch cobble and concrete waste. This method, and the fact that it’s done mechanically, is much more efficient than manually excavating and backfilling, especially with sod, sand, or saturated soil. “It might take 15 minutes to trench in 1,000 feet,” Erickson says.

The system also makes it possible to space the posts closer together for extra stability where water concentrates. “Traditional silt fence rolls are already stapled to the fabric,” Erickson says. “You buy them in 10-yard lengths. With the tommy, the fabric runs continuously through the machine, so it can’t have posts. Ninety per cent of it comes in thousand-foot rolls.”

As the tractor moves forward, the fabric is pulled off the roll and inserted into the slit in the soil. The tractor wheels roll over the soil that has been pushed up from the slit, compacting it so well that washouts are nearly eliminated.

Once the fabric has been installed and the soil compacted, the tractor returns to install the posts. On the ground, one worker holds the post in position and the second pounds it in with a hydraulic post pounder. Crews use metal T-posts attached with zip ties. Workers can drive them in deeper than wood posts and they don’t break or fall over, Erickson says, even in frozen ground.

KC Harvey monitors and maintains the sites. Depending on the stage of construction and the state, monitoring intervals can be between 14 and 30 days.  In some areas of Wyoming, some companies have applied for and gotten modifications to the inspection schedule; in some cases inspections need only happen on an annual basis.

Once a job is over, crews remove the posts with the tractor. “We have four tommys,” Erickson says. “We use them in multiple states. I’ve seen this stuff hold back literally tons of sediment.”

About the Author

Janet Aird

Janet Aird is a writer specializing in agricultural and landscaping topics.