Slope and runoff conditions determine what’s right for the site, but inspection and maintenance are always key.
It’s an environmental problem as well as a regulatory one: Silt and sediment from highway and building construction travel off-site, through the storm drain system, and into rivers and streams, altering habitat, reducing hydraulic capacity, and delivering pollutants picked up along the way. Besides affecting water quality, failure to control sedimentation may result in violations of a developer’s or municipality’s National Pollutant Discharge Elimination System (NPDES) permit and may generate lawsuits from concerned citizens and environmental groups.Satisfying the Guidelines
The challenge, then, is to find products that satisfy guidelines set forth by EPA and various state agencies and also meet the budget and time constraints of the development project.
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| The Triangular Silt Dike acts as an effective barrier in rocky soils. |
"We have to meet the criteria set forth by the Department of Natural Resources in our area," stresses Barry Carter, project engineer of highways for the J.H. Berra Construction Company in St. Louis, MO. He explains that the Missouri Department of Transportation (MDOT) tried everything–including straw bales, rock ditch checks, and combinations of silt fencing and rock ditch checks–before settling on the Triangular Silt Dike manufactured by the Triangular Silt Dike Company of Midwest City, OK. The dike is a barrier system constructed of lightweight and durable materials. Each 7-ft. section weighs just 7-9 lb. Protective aprons on either side of the barrier prevent both erosion and failure of the structure. These aprons are attached to the ground and held in place with U-shaped wire staples that help the dike conform to rough terrain. "This is the only thing that even comes close to meeting the criteria of the Department of Natural Resources," maintains Carter.
MDOT ran into similar problems on two different projects: a $6.6 million highway project at Route 60 that began in March 2000 and wrapped up later that summer, and a $10 million highway project at Route 19 that started in September 1999 and ended in the summer of 2000. Grades at both sites were very steep–between 4% and 5%. "MDOT had gone through almost every type of product out there and had been experimenting to find something that would work effectively. But whatever we installed blew out all the way to the bottom," says Carter. "Sheer velocity was getting us."
Confronting the Challenges
Carter explains that the dike was installed above and immediately around the inlets, creating stilling basins. "The water ponded, so as it ran over again, it picked up velocity from zero, so the volume of silt at the inlet was minimal. The dike did not fail us, so we put it in everywhere." MDOT used approximately 500 sections of the dike on the Route 60 project and about 600 on the Route 19 project.
"When we first used them, we spaced them about every 50 feet," Carter continues, "but we’re now experimenting with spacing them out a little farther because we think they will still hold." He says that MDOT still specifies straw bales when the grade is below 1% but believes the dike will be effective at grades steeper than 5%. "These products are a little higher in price than others, but they’re worth it," he asserts. "When you finally find something that works, you latch onto it."
According to Carter, the only challenge confronting MDOT was having to switch from the U-shaped pins to large nails to anchor the product because of the rocky soil surrounding the projects. Triangular Silt Dike sent representatives to the project sites to show the project managers how to install the dike.
Protecting Catch Basins
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| Slowing runoff on steep grades reduces sediment loss. |
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Another product designed for ease of installation is the Verti-Pro, a reusable device manufactured by Alpine Stormwater Management Company of Gahanna, OH. The device, which fits over a catch basin, requires no grate removal. The company’s president, Dan Strawser, explains that a metal frame clamps onto the grate. Filter fabric is stretched across the frame to prevent sediment from entering the catch basin or storm drain. At many sites, contractors install stakes at the four corners of the grate and stretch silt fence around the stakes to create a barrier. Verti-Pro works on a similar principle but is quicker to install and more uniform in performance.
"They’re already preboxed, so all you have to do is set them down and tighten them up," explains John Hill, who works for Complete General Construction Company’s dirt and fine-grade operations in Columbus, OH. "When the job’s done, you just clean them up and use them on the next job. You can get two or three jobs out of them before you have to replace the fabric."
Hill says he decided to try the Verti-Pro product after talking to Strawser at a few sites and seeing similar products demonstrated at erosion control seminars. Hill explains that workers on road projects sometimes had trouble lifting the catch-basin insert bags the company used to collect silt and debris. "The bags plugged up, were heavy, and created a big mess," he observes. Because catch-basin inserts must be emptied regularly to maintain their effectiveness, many that are sized to fit larger catch basins have features that allow them to be handled by heavy-lifting equipment. For example, the SiltSack geotextile insert from ACF Environmental in Richmond, VA, has pockets through which lifting bars can be inserted temporarily.
When pollutants other than sand and silt are a known problem–for example, around parking lots where oil, grease, or petroleum hydrocarbons are likely to be present in runoff–catch-basin inserts have the added benefit of accommodating filter media. Often contained in pads or "pillows" placed into the catch basin, the filter media absorb and retain pollutants. The Fossil Filter from KriStar Enterprises of Santa Rosa, CA, has an adsorbent medium, Fossil Rock, that removes oil and a percentage of dissolved metals from runoff.
Complete General uses Verti-Pro primarily on county projects involving reconstruction, new construction, and road-widening projects in which much runoff is expected, states Hill. "And where we know there is a lot of ditch work," he adds. "This product holds up to all kinds of conditions. It helps us abide by standards set forth regarding inlet protection. Plus, the state and county engineers make certain we have the proper protection in place for inlets and drains or we won’t get paid for the job. If you want your money, you have to do what it takes."
Dale Horstebler, open-ditch crew superintendent at Madison County Ohio Engineers (MCOE), notes that Verti-Pro does the job of keeping the dirt away while his crew installs tile storm sewers. "We use the Verti-Pro until the grass gets established to keep down erosion around the catch basins."
For a while, MCOE tried silt fencing, but Horstebler says the loose dirt around older homes kept the crew from driving in stakes securely. "The Verti-Pro keeps its shape, and all you have to do is pressure wash it after a job to clean it up. We’ve got some we’ve been using now for three years."
Turning to Reusable Products
Reusable products are popular with many site managers, maintains Mark Converse, owner of Site Supply in Columbus, OH. He says the manager of a construction site in Forest Park, OH, a suburb of Cincinnati, recently purchased about 20 Dandy Bags, which fit over flat inlet grates of various sizes and shapes. "These bags did the job of bringing the site into compliance," reports Converse.
In Boise, ID, a carnival owner who sets up annually at a parking lot rented out by the Boise State University called B.A. Fisher Sales Company in Boise in a panic in June. He needed a product that would help him comply with federal ordinances regarding storm drains. When he washed his equipment at the end of each day, the water would wash carnival debris into a river 100 yd. from the carnival site.
"He called around until he found us, and we told him about the Dandy Bags," recalls Larry Fisher, owner of the Fisher Sales Company. "Because Boise’s population is over 100,000 people now, they are strict about the federal regulations concerning drains and inlets. The ‘mud police’ tell people they can’t wash their equipment without protection, but they often don’t know what kinds of products to tell them to try. The carnival owner bought eight Dandy Bags, and they collected everything you could possibly imagine that gets thrown on the ground after a carnival."
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| Dandy Bags help ensure NPDES compliance during construction. |
According to Fisher, the bags work to prevent solids from entering the storm drains, but not pollutants such as soapy water. "[The carnival owner] just took a shovel, cleaned off the bags, put the collected debris into a garbage bag, and was able to take the Dandy Bags to his next location," he says. Since learning what the carnival owner used, university officials, who rent the parking lot to other organizations for various events, have recommended the same product to other renters.
When quick removal or replacement is not a priority, an alternative to reusable products is to install 100% biodegradable products as inlet protectors. Nedia Enterprises of Jamaica, NY, makes a product called KoirWattle, a cylindrical module of coconut fiber encased in a coir outer netting. For many locations, the shape makes it easier to install than straw bales.
No matter what product is used directly at the inlet or drain, often silt fencing is also used to help slow down the volume of water and sediment coming off of a project before the runoff reaches the inlets and drains. Hill, for example, points out that he always uses silt fencing installed by his own workers. "That’s mandatory on all our projects. We just install the basic silt fencing with stakes."
"If built correctly, the silt fence creates a pond behind it, and the heavier sediments settle out," explains Thomas Carpenter, president of Carpenter Erosion Control of Ankeny, IA. Thus, other measures have a better chance of doing their job. Carpenter stresses that proper installation and timing are key. "The biggest problem is that a project’s erosion control plan involves silt fence being in place at the end of a project instead of at the beginning, middle, and end. The other problem is that not enough silt fence is used. The rule of thumb is 100 feet of silt fence per 10,000 square feet. But only 2,000 feet of silt fence may be used on a 20-acre site. Improper quantities is a major problem."
Installing Properly
As with any sediment-control measure, silt fence must be installed properly to work as intended. Improper installation involving, for example, fence that collapses or is undermined by runoff, is a common problem on construction sites and is one reason why many contractors believe that silt fence is largely ineffective. Carpenter, whose company sells the tommy Silt Fence Machine for automatic installation of silt fence, agrees that unless it’s installed well, silt fence is simply a waste of money and labor. He favors the static slicing method of installation.
Recent studies by the Environmental Technology Evaluation Center, established by the Civil Engineering Research Foundation through an agreement with EPA, tend to agree. A May 2001 study found that the static slicing method provided stormwater retention as good as or better than the "best" trenched installation and was far superior to common installations. In the slicing method, a soil disrupter thinly slices through the soil 8-12 in. deep, and an apparatus inserts the silt fence into the trench. The soil disrupter utilizes a chisel-type horizontal point to slightly disrupt soil upward and minimize horizontal compaction, thus creating an optimum soil condition for future mechanical compaction. The study also found that the static slicing method offered practical advantages over trenching. Maneuverability, minimal soil-handling and hand labor, consistent depth and compaction, and ease of installation–in windy conditions, on steep sideslopes, through rocky soils, and in saturated soils–were among the benefits.
Once installed, of course, all inlet-protection measures need regular inspection and maintenance, and accumulated silt and debris require removal to keep water flowing through whatever filter medium is used to prevent ponding and flooding.