Properly selected and installed products can help reduce the cost of trapping sediment during and after soil-disturbing activities.
Designed and installed properly, erosion control practices such as establishing perennial vegetation or installing grade control structures, turf reinforcement mats, or gabions can reduce the need for sediment control practices by stabilizing the soil permanently. However, when construction activities prevent the use of permanent erosion control practices, sediment control measures can be installed around the perimeter of the site and around storm drain inlets to trap sediment in stormwater runoff until erosion control measures can be established.
Straw bales and silt fence made of non-degradable synthetic fabric and have been among the more popular, traditional tools for sediment control during site construction. However, these materials can pose challenges to project owners and contractors.
A high-profile silt fence can trap more water than it has the strength to hold, causing it to fail and creating more erosion as the retained water is released.
Consider the silt fence. "In a highly dynamic construction environment, maintenance of silt fence can be very costly," said erosion and sediment control consultant John McCullah, CPESC, Salix Applied Earthcare, Redding, Calif. "In some cases, it may be impossible to prevent construction activities from destroying them and trucks from running over them and damaging the products. In addition, there are the added costs of removing, transporting, and disposing of the silt fence fabric when the project is completed."
As a result of such problems, some contractors tend to leave silt fence in place after it has served its purpose. That’s the assessment of erosion and sediment control consultant Jerald Fifield, CPESC, CISEC, president of HydroDynamics, Inc., Parker, Colo. "If silt fence is visible from the road, contractors usually remove it at the end of the project," he said. "Otherwise, they often leave it standing after vegetation has been established."
"Some contractors are beginning to consider how difficult it is to go back and remove some types of sediment control devices," McCullah said. "For example, anyone who has tried to pick up and remove a wet, soggy straw bale encrusted with mud knows just how difficult that can be."
During the last few years, a variety of biodegradable sediment control products made from natural fibers, including coir (coconut fiber), have been developed. Unlike synthetic materials, such as silt fence, they can be left in place to degrade naturally. In the process, these organic materials help support establishment of vegetation, which provides permanent erosion control.
Wattles installed in drainage channels must extend up the side slopes to provide sufficient freeboard so runoff can discharge over a low point without flowing around the edges of the wattle, which can erode the channel side slopes.
The lower profile of some biodegradable sediment control products, such as sediment logs or wattles, offer another advantage. Typically, they protrude no more than about 12 to 18 inches when installed according to manufacturers’ specifications. That compares with a silt fence, which usually stands about 3 feet high. The higher profile can trap more water than the silt fence has the strength to hold, causing it to fail and creating more erosion as the retained water is released. Lower-profile wattles allow excess water to move freely and reduce the risk of additional erosion.
Additionally, combined with the greater strength and durability of coir, wattles can withstand damage from equipment traffic better than silt fence or rock barriers. This can reduce the costs of maintaining sediment control devices during construction activities. A growing number of project owners and contractors are recognizing the advantages of biodegradable sediment control products for controlling sediment along the perimeters and around storm drain inlets on active construction sites.
"In the correct applications, silt fence can be a very good sediment control practice," noted Jennifer Hildebrand, CPESC, CPSWQ, a stormwater compliance manager with Weis Builders, based in Minneapolis. However, she reports, on projects where removal of silt fence is difficult or where it is subject to traffic damage, contractors are turning to other solutions. "Perimeter sediment control practices have changed dramatically," she said. "Instead of just silt fence, more contractors are also using alternatives, including biodegradable products such as wattles."
Provide a breathing space when placing wattles in front of a curb inlet in a sump so the runoff does not shove the wattles into the inlet, plugging it up and causing the area around it to flood.
Alex Zimmerman, CPESC, with CSI Geosynthetics, an erosion control consulting firm based in Vancouver, Wash., also reports a decline in the amount of silt fence used for perimeter sediment control on his projects, as some project owners and contractors choose biodegradable products instead.
Yanking silt fence out of the ground not only involves extra labor expense, it may also disturb the soil and create risk of more erosion, requiring extra time, labor, and materials to repair the damage. "Unless vegetation is well-established, removing silt fence may disrupt fragile vegetation, leaving bare spots," said Fifield. "If not protected from rain and runoff, these exposed areas can erode. Sometimes, on a good silt fence installation, equipment may be required to pull up the fabric. The equipment, itself, can also tear up the soil surface."
Zimmerman noted another reason for the growing popularity of biodegradable sediment control products in his area: "More developers and builders are looking for softer, greener alternatives to synthetic materials," he said.
Unlike synthetic materials, sediment control products made from natural fibers add organic matter to the soil. As this organic matter breaks down over time, it releases carbon, improving the soil. "Often, construction activities remove the nutrient-rich topsoil, leaving the much less productive sub soils on site," said consultant McCullah. "The carbon in biodegradable fibers is often a source of food for soil microbes, which help break down these subsoils to start the process of re-building the soil."
Keys to success
Regardless of the type of product used to control sediment at construction sites, Fifield emphasizes the importance of proper application. "Correct installation and maintenance of these products are critical," he said. For example, a sediment control barrier placed in front of a storm drain inlet that is in a sump should be high enough to create a pond that allows for deposition of sediment in runoff waters.
"If runoff is flowing over the top or around the ends of the barrier—whether it’s rock, bagged compost, or a fiber roll—you’re discharging sediment into the inlet," Fifield said. "However, by allowing runoff to pond in front of the barrier, some sediment can be removed."
It’s a similar story when using wattles or sediment logs to control sediment in a drainage channel during construction activities. Most wattle products must be trenched in and extended up the side slope to perform properly, he noted. If the remaining freeboard isn’t high enough for runoff to discharge over a low point, water will flow around the edges, eroding the channel side slopes. Also, if the diameter of the wattle is too small, the sediment-controlling benefits are minimal. Wattles must also be properly secured, Fifield said. "I’ve seen cases where the contractor failed to provide breathing space when placing wattles in front of a curb inlet in a sump. Then, during a storm, the runoff shoved the wattles into the inlet, plugging it up and causing the area around it to flood."
Remember that the design criteria and expected performance standards are critical to the success of the project, Hildebrand said. "Along with these standards, keep in mind that maintenance and removal are integral parts of that process," she said. "As an industry, we are often looking for one type of solution, and the reality is that a combination of practices which complement one another is often our most effective solution."
Greg Northcutt is president of Northcutt Communications, Inc., an editorial services firm in Port Orchard, Wash., which has been covering environmental issues and the construction industry since 1988. He can be contacted at 360-895-1887 or via e-mail at firstname.lastname@example.org.
Logs and wattles
Biodegradable sediment control products made of coir are well suited for construction site applications, according to Lanka Santha, P.E., CEO of RoLanka International, Inc., which manufactures a variety of erosion and sediment control products made from coir. "Coir is an abundant, renewable, natural resource that is strong and durable" he said. "It does not absorb water and it traps sediment in runoff better than straw or wood fiber. Also, coir decays at a much slower rate, holds its shape and structure for a longer time, and wildlife won’t eat it."
One of the company’s biodegradable sediment control products, BioD-SiltCheck, functions as a check dam. Its coir fiber log body, which is placed perpendicular to the flow and filters and reduces the speed of flowing water, is connected to two coir filter aprons on either side. The upstream apron prevents flowing water from undercutting the filter body, while the downstream apron prevents erosion caused by water flowing over the device. "It can be installed in flow channels as an alternative to silt fence, conventional wattle, rock, and straw bale check dams." Santha said.
Another of the company’s products, BioD-Watl coir wattle, consists of a lightly packed roll of coir fiber, ranging in diameter from 6 inches to 20 inches, wrapped in a coir twine netting. It is designed to filter sediment along site perimeters and around catch basins. Coir twine netting with 2-inch by 2-inch openings are designed to avoid trapping or injuring wildlife. According to RoLanka, coir wattles do not require special equipment to install in catch basins and are flexible and easy to work, allowing installers to make loops of the flexible wattle to provide breathing space and prevent blocking of the catch basin.
RoLanka’s newest sediment control product is Super-Wattle, which features one closely woven coir apron on the upstream side and does not require an anchor trench. The apron prevents runoff from flowing under the wattle. In conventional wattle applications, runoff commonly flows beneath the wattle between anchoring stakes. The Super-Wattle is held in place by metal staples driven through the apron. This avoids the time and labor required to dig an anchor trench and to drive wooden stakes through the wattle to hold in it place. Often, these stakes can flatten the wattle, increasing the potential for overtopping. In addition, eliminating the anchor trench increases the effective height of the wattle. "It can be installed along slope contours to shorten slope length, reducing runoff velocity, and to trap sediment," Santha said. "It can also be installed at the base of slopes and around the perimeter of construction sites as an alternative to silt fence, conventional wattles, and straw bales."