Silt and Sediment Control Techniques

Sept. 1, 2008

Most construction in most states is now subject to the provisions of the National Pollutant Discharge Elimination System (NPDES). The initial phase involved sites of 5 acres and greater, and Phase II, implemented in 2003, affects projects as small as a single acre. The result is that in recent years, many more people have had to become aware of erosion and sediment control issues and how to effectively deal with them.

Sedimentation occurs naturally, both on large and small scales. A single raindrop striking the ground can cause tiny bits of soil to splash and move, and a sudden heavy downpour can quickly produce a strong torrent of moving groundwater that transports large amounts of soil.

According to the Tennessee Department of Environment and Conservation (TDEC), the natural geologic rate of soil erosion averages about 400 pounds per acre per year, which is offset by a roughly equivalent amount of soil creation from the weathering of bedrock and other natural material. However, construction activities can increase soil loss exponentially, with major projects leading to as much as 150 to 200 tons of lost soil per acre.

An astounding 5 tons is lost when one millimeter of soil is removed from 1 acre, according to TDEC. Moving soil not only can carry off important nutrients needed for revegetation after construction is complete, but also may necessitate trucking in replacement soil. In addition, an assortment of potential environmental hazards include:

  • Metal and pesticide pollutants sorbed to soil particles entering streams and wetlands
  • Siltation of aquatic habitats
  • Damage to sewers and ditches
  • Increased water treatment costs for municipalities

To combat these problems, a variety of solutions are available, from replacing lost ground cover to a host of best management practices (BMPs), which can significantly reduce sediment runoff.

Virginia Road Realignment
Lewis G. Manhart, environmental monitor for the Virginia Department of Transportation (VDOT), describes a complex road improvement project that involved both horizontal and vertical realignment. “Some sections went from two lanes to four lanes,” he says, “with cuts and fills on the project. The road is the major access to a mountain recreation resort with lots of traffic.”

Water volume and water velocity were two elements of concern with this project. “Water volume we could not do much about. We had few or no options to reduce the volume of water. We could affect water velocity and flow direction.” The solution that Manhart used in this case was the Erosion Eel sediment control barrier from Friendly Environment.

“We used the Erosion Eels in three applications,” he explains, “for curb inlet protection, ditch lines, and perimeter control. As inlet protection, we used it to slow water velocity as it came to the curb inlet. Slowing the water resulted in some ponding around the inlet out into the road way.” Not surprisingly, this was a matter of concern to inspectors because of potential interference with traffic.

Manhart notes that some Erosion Eels have higher flow rates, which likely would have minimized the ponding effect. He adds that they functioned well as sediment control barriers. He also found them easy to install, maintain, and remove.

“In ditch lines, the Eels were used to reduce water velocity,” Manhart explains. “Slowing the water allows sediment to settle out behind the Eels. Single Eels worked well in small, short ditches when installed correctly. There must be a good contact between the Eel and the soil surface to reduce the potential of water running under the Eel. Stones and clods prevent good soil contact. Longer, wider, and larger ditches required more Eels. In one case, we used multiple Eels to create a large check dam. In a ditch line, when installed according to manufacturer’s specifications, they worked well, were easy to install, easy to maintain-just clean out the trapped sediment-and somewhat easy to move.” He notes, though, that waterlogged Eels with trapped sediment can get quite heavy.

For perimeter control, Manhart primarily used a silt fence, but added Erosion Eels to close gaps in the fence. Some gaps in the silt fence were intentional, serving as temporary pass-throughs. “[Crews] pulled the Eels out of the way when they needed to pass through and returned them when they were through working,” he says. “In another case, some silt fence was torn up, and they used the Eels for immediate replacement in short sections of perimeter control. Eels worked well in both cases.”

Occasional intense rainstorms resulted in additional maintenance of the Eels, but Manhart notes that such storms are a problem for most erosion and sediment control products.

Whereas in some parts of the country regulatory inspection is a bit spotty, Manhart says, “We have good and consistent inspection and monitoring. Most of our projects are inspected and monitored regularly-weekly in most cases and more frequently when rainstorms pass through. Most of our contractors are pretty good at doing erosion control work needed after inspection and monitoring. Many needs are identified and the work repair is scheduled based on the contractor’s inspection. Contractors and inspectors are Virginia State erosion and sediment control certified.”

He adds, “All of our road projects require SWPPP [stormwater pollution prevention plan] documents. The plans are followed, but they are modified as field conditions unfold during the construction process. Plans are based on the information available at the time they are prepared. During the actual construction, field conditions are sometimes different. We make changes regarding the erosion and sediment controls needed and used during the construction process based on the actual field conditions. Sometimes we use more, less, or different controls than what was specified in the initial plan based on our actual field-conditions assessment. Plans are always the starting point for erosion and sediment controls for the contractor, VDOT inspection staff, and environmental monitors.

“Based on my experience,” he continues, “all VDOT-approved controls work well when installed according to manufacturer
specifications. All erosion controls have problems during intense rain events. The heavier the rain event, the less likely controls are going to work as planned and designed. VDOT does not normally specify which controls a contractor must use, but controls used must be approved for that application if a contractor wants to get paid. The contractor can decide to use Eels, dikes, silt fence, filter barrier, or rock check dams for ditch line controls. We can require contractors to replace controls if they are not working.”

Fierce Desert Winds
Jim Anderson of BMP Solutions was recently involved with a pair of residential construction projects, the Old Stone and the Seville communities in metropolitan Phoenix. It may not rain often in the Arizona desert, but when it does, it can come fast and furious.

“This past January, we had two or three good rains, with about an inch and a half of rain each time,” Anderson says. Strong winds sometimes occur as well. Anderson prepared for both possibilities by combining the use of a silt fence with the GatorGuard sediment control device.

He chose a silt fence with a filter fabric and wire back. “This is as good as you can get,” he explains. He spaces the posts about 8 feet apart and buries them 5 to 6 inches into the ground. Depending on need, he may use the silt fence together with the GatorGuard. As he describes, “We’ve had winds up to 75 miles per hour, and the fence will not blow out.”

Anderson estimates that he has used more than 25,000 feet of the GatorGuard product. He has found that it stands up well to the occasional rain soakings he encounters and that trucks can drive over it without major problems. “It can take quite a bit of abuse,” he notes.

Proper sediment control is crucial, he explains, because in recent years, pollution limits have been exceeded in Arizona. He notes that county and state regulators have been stepping up to bring Arizona into compliance with federal laws, and the EPA has been very strict when it comes to regulatory enforcement. As a result, he says, he is seeing more and more BMP products used to comply with SWPPPs.

For Anderson, SWPPP plans are definitely not static. “It’s a living document-if “˜A’ doesn’t work, we go to “˜B,’ and if “˜B’ doesn’t work, we go to “˜C.'”

Surviving a Tornado
Robb Brown is the director of environmental services for Cornerstone Environmental Services in Florida and works with a variety of residential construction projects. “Like everyone, we use standard silt fence very regularly, and like everyone, we realize that it is far from a perfect solution for every problem,” he explains. “We have in the past suggested rock dams or rock pads as ways to filter sediment from the runoff in areas where replacing silt fence over and over is impractical.”

Brown explains that a major issue he faces is what to “do in areas where traffic volume is too high or the building site is too compact to keep vendors from running over and ruining silt fence on a regular basis. In Florida, we also have the problem of UV rays quickly deteriorating silt fence and high winds tearing fence from the stakes.”

To solve these problems, Brown turned to the SiltShield reinforced fence, which he found has succeeded in addressing these issues. “We have used it at curbsides in both single-family and multi-family building settings and have seen our maintenance drop to minimal levels in those areas. Where silt fence may only last a few minutes or a few days in high-traffic areas, SiltShield has been holding up for a few months.”

An interesting episode occurred in his first encounter with the product. “We did our test run of SiltShield just a few days before a tornado hit the exact site where we installed it,” Brown explains. “We went back to check on it, and there were pieces of vinyl fence scattered all over the community. One of the homes under construction had a block wall blow down. We spent a full day out there repairing and/or replacing nearly every foot of silt fence. And the SiltShield held up perfectly through all of that. I wish I had taken pictures. I was sold before that, but really sold after that!”

Regarding other BMP solutions, Brown says, “Silt fence of course works well in certain applications, but it has limitations. We have had success with filtering rock bags and filter-sock-covered perforated PVC for “˜last stop before the inlet’ type protection. We have tested a couple of other inlet protection devices with little success. We try not to get too caught up in the newest innovation, because our experience has been that the price point changes significantly while the effectiveness changes marginally, if at all-and sometimes not for the better. After testing SiltShield, I can honestly say that it is the first product that I have recommended to my clients as a new innovation product that is so different it is worth the money. I showed it to one county inspector and he loved it.”

Brown notes that local regulatory inspection and enforcement is less than ideal. “Enforcement in our area has been pretty lax in the past, but inspections have significantly picked up recently. Florida’s enforcement strategy leaves something to be desired, I think. It seems that only sites that file an NOI [Notice of Intent] with the state experience any enforcement at all. Sites that do not get the required permits tend to stay under the radar. It is ironic that the sites that take at least some of the right steps are virtually penalized for doing so, while companies who know about this are essentially incentivized to disregard permitting obligations.

“For onsite enforcement, we have had very positive experiences,” he continues. “I know of some fines levied by the state, but I don’t have any personal experience with sites that I work with being fined or any other action [taken] other than regulatory inspections. I think “˜inconsistent’ is probably the best way to describe inspections and enforcement here. Some sites are closely watched, others are never more than a name on a piece of paper. Not to mention that all municipalities and counties either have their own regulations or their own interpretations of the state regulations.”

He adds, “SWPPPs are required for all projects in Florida that are over 1 acre, or part of a larger common plan of development, and that have the potential to discharge to any water of the state or MS4 [municipal separate storm sewer system]. SWPPPs do need to be updated regularly as the project progresses. City and county inspectors rarely, if ever, look for a SWPPP. State inspectors go through them with a fine-tooth comb checking for all necessary documentation. I think that inspectors from reputable companies doing weekly and after-rain inspections see the SWPPP as the backbone of pollution control for the site and take it seriously. State inspectors also take them seriously. Some of the construction companies who need to implement the SWPPP on a day-to-day basis surely see them as a formality, but I think even that is changing for the better.”

Protecting a Pristine Mountain Creek
In the remote Wyoming Range, an oil and gas company needed to build a 1.7-mile access road to reach its drilling site. Much of this road was to run parallel to Fish Creek, a pristine trout stream in an area managed by the US Forest Service. In some portions, the creek was only a few feet from the proposed road.

EnerCrest, in Big Piney, WY, was assigned the task of ensuring proper protection of the creek during road construction. Chief operating officer Todd Erickson explains that although the oil and gas industry has been exempted from federal stormwater regulations, the states of Wyoming and Colorado passed legislation in 2005 to “unexempt” this industry. The result, Erickson says, is that “Companies in this field went from not having to comply at all to having to comply. Our company fills that niche, assisting companies in proper compliance.”

One of the challenges the project faced was that much of the site was very steep. In addition, the area receives perhaps 4 to 5 feet of snow in the winter, so when it melts in the spring, this causes a significant amount of runoff.

To deal with these issues, EnerCrest utilized a combination of solutions. Working ahead of the earthmoving equipment to protect the creek and surrounding wetlands, it installed more than half a mile of silt fence, some by hand and some with the use of a tommy Silt Fence Machine from Devon Distributing in Iowa. Erickson explains that his company presently has four of these machines, with a fifth on order.

Other BMPs employed include matted slopes, utilizing cut and fill, hydromulching and hydroseeding, and the SedimentSTOP biodegradable filtration system. In addition to hundreds of straw bales used in conjunction with the silt fence, EnerCrest also made use of riprap chutes, gabion baskets, retention ponds, and water diversion channels to slow the flow of water and to reduce sediment runoff.

Because of the delicate nature of the environment in the area, Erickson says, “There was a lot of scrutiny of our work, since we were essentially the project stewards. We were watched very carefully, by both the US Forest Service and the Army Corps of Engineers.”

Erickson comments that the Wyoming Department of Environmental Quality has not always closely enforced regulations, but he says that the Bureau of Land Management, which is the surface rights owner in many areas, has been very diligent in the area of enforcement. “They regulate all aspects of energy development, and they have a high interest in seeing that erosion control is done properly. They have the ability to withhold permits and to slow down the development process. You have to prove that you’re a good actor.”

A post-construction site tour demonstrated that the silt fence installed adjacent to Fish Creek, in addition to the other control measures, had effectively prevented sediment from entering the stream. Erickson proudly announces, “We’ve changed the way people think about BMPs.”

On Both Sides of the Regulatory Process
Jonathan Hunt spent several years working as a government inspector for a local conservation district in Pennsylvania. Such conservation districts have been delegated by the state Department of Environmental Protection to act as agents overseeing regulatory compliance.

Now, Hunt finds himself on the other side of the desk, working as the business development manager for erosion control products for River Valley Organics in south-central Pennsylvania. His firm works with many new construction projects that must put controls in place to prevent runoff.

One recent project, which he termed a “linear” job, involved an extensive amount of piping for a sanitary sewer. “It runs along a low-lying area,” he says, “where no one wants to build, and it includes some wetlands.”

Hunt explains that because of difficult weather and steep slopes, “the contractor was struggling with silt counts, having difficulty keeping silt and sediment runoff under control. I advised that they consider the use of Filtrexx FilterSoxx, and the contractor elected to install around 4,500 feet. It very much impresses me.”

On the other hand, Hunt has a dim view of the traditional silt fence. “It is very difficult to work with,” he says. “One regulator with over 16 years of experience once told me that he had seen silt fence correctly installed throughout a project just twice in his entire professional experience. When silt fence fails-and it does-it fails dramatically with a “˜whoosh’ of sediment. I joined River Valley after personal in-the-field experience with FilterSoxx, because I wanted to promote a BMP that actually does work quite well and is forgiving.”

He explains that the product “works much better than a silt fence, because it lies on the ground rather than standing up like a silt fence. It can handle up to 50% more water flow than a silt fence. If a silt fence fails and goes flat, you can have a sudden torrent of water and sediment flowing downhill, whereas a partial failure of a FilterSoxx won’t cause nearly the same damage.”

As an example, Hunt related an incident at a retirement home construction project in which an astounding 3.5 inches of rain fell overnight, the majority of it over a brief 45-minute period. According to residents of the retirement home, there was mulch coming in from the front door. The filter sock that had been laid down was subjected to approximately 24 times its design limit, yet still held reasonably well. A 56-foot-long section had its stakes pulled out, and the sock had been pushed about 13 feet but was still essentially doing its job. Had a silt fence been in place, Hunt is quite confident that it would have been flat and useless.

Although the product Hunt uses is not approved for NPDES control, he says that it has been accepted for this purpose, and it is expected that when the next NPDES manual comes out, the filter sock will be one of the approved BMPs. “It performs,” he says emphatically, “and helps the contractor stay in compliance much better than other products.”

He does advise, though, that when placed for perimeter control, the product not be driven over. “If this happens once, you can probably kick it back in shape and it will be all right. You likely have not destroyed its integrity. But if it’s driven over a second time, or if a vehicle spins its tires over the sock, then there will be problems. Of course, if you drive over a silt fence, it’s over. If you cause a small gash in a silt fence, it can become virtually useless, whereas a small gash in the FilterSoxx, because it is three-dimensional, may not be a problem at all.”

People in his firm must attend training sessions annually to obtain and retain certification to use the Filtrexx product, because it is sold only to certified installers. Properly used, he claims, it traps up to 99.99% of all particulate solids, and 50 to 75% of all suspended solids-“the goo that may not consist of particulate matter, but turns a stream brown.”

Asked about regulatory consistency, Hunt has quite a bit to say about his local regulatory environment. “There are significant variations in regulatory methods across Conservation District lines due to a variety of factors, including district challenges and priorities, district personnel experience, training and tenure, as well as personality, culture, and regulatory approach among district, state, and individual personnel,” he acknowledges. “These things all lend themselves to the perception of regulatory inconsistency. Another aspect of this is due to the inherent nature of people and politics. In the mid-Atlantic area, the tendency is the closer you get to the Chesapeake Bay, the higher the bar is held. In the district where I worked, we were known for being “˜tight’ on environmental regulations. At the same time, my observations are that different approaches, even very diverse approaches, can be effective-or not!-in achieving the environmental objectives. There is probably no single “˜correct’ approach, although if you get five regulators in a room together, you might get six “˜correct’ approaches.”

Regarding how carefully SWPPP documents are followed, Hunt says, “It depends. Wal-Mart has a formal SWPPP project document requiring SWPPP training for all subcontractors and daily inspection logs.” On the other hand, other projects he has encountered get by with self-inspections just weekly and after significant rainfall.

“When I inspected projects without a formalized SWPPP document,” he recalls, “I requested to see and flip through their maintenance log looking for date gaps. If there were missing dates, they received two citations. One was for failure to follow the plan and permit, and the second citation was for potential for pollution.”

Hunt adds, though, “most contractors try to do the right thing. If a project is overwhelmed by an extreme storm event and runoff occurs, you’re not considered out of compliance if you had a proper plan in place. Of course, you still have to clean it up. But the contractors who try to get by without proper controls in place can be shut down completely.”

Roadside Ditch Runoff
Missouri has a lot of miles in its highway system, and most of those highways are accompanied by roadside ditches that require sediment runoff control. Countless streams and rivers crisscross the state, and it is critical that these waters remain free of silt and sediment buildup.

Part of this task falls to Randy Stiers, vice president of operations for Wehmeyer Farms. His choice of BMP is largely determined by the nature of the ditch he is working on. For relatively shallow, flat ditches, he prefers the Triangular Silt Dike, a 12-inch-high foam impediment to sediment runoff. “I think I use more of this than anybody,” Stiers says.

He says that the Missouri Department of Transportation (MODOT) has approved the Triangular Silt Dike as a type II ditch check. More commonly, a ditch check dam is composed of rocks placed 18 inches high and the width of the ditch, but Stiers has found that the job can be accomplished more economically by lining a ditch with several of the dikes. However, if a ditch is too steep, he’ll resort to the rock check dam.

“The Triangular Silt Dike has open ends,’ he says, “so it is possible to slip the foam into the sleeves of the dike next to it-you can make this as long as you want.” When appropriate, he’ll often use them as a long continuous barrier. At Wal-Mart construction sites, he may combine enough of these dikes to create a barrier as long as 100 to 200 feet. “These work much better than placing straw or hay bales back to back,” he explains.

However, when there is a tremendous amount of flow, Stiers has found that a silt fence works better, because a standard silt fence stands 3 feet high. He rarely uses the dikes on residential work, but has used “hundreds and hundreds” with MODOT projects on ditch lines.

One problem he has encountered with the dikes, however, is that with excessive rain, they can come loose from the adjacent pieces. “In the case of a 3-inch rain, the pieces will all blow out together,” Stiers says.

He says matter-of-factly that MODOT doesn’t particularly care that a barrier fell apart when it was constructed properly; they just want it to work. So Stiers had to come up with a solution. “We used to zip-tie them together, but then we found that these tended to freeze in the winter. Now, we use small hog rings and tie the pieces together after slipping one dike inside another, and this has worked well. It’s not part of any written maintenance policy, but it’s something we came up with that works.”

Stiers handles stormwater reports for all of his commercial and residential projects, and this involves a weekly inspection and walk-through, checking all BMPs. In addition, inspections are required after a significant rainfall. How much rain triggers a required inspection that varies by county. In St. Louis County, an inspection must be done after even a 0.25-inch rain, while in other surrounding counties, a 0.5-inch requirement is more the norm. MODOT inspectors are careful to sit in on each job to ensure proper installation and effectiveness, but Stiers has found them to be reasonable in their expectations from contractors. 
About the Author

Steve Goldberg

Steve Goldberg writes on issues related to erosion control and the environment.