Public Roads - Summer 2023

Managing stormwater to protect the Nation’s waters is a common challenge of highway construction sites. Most water quality problems in lakes and rivers are the result, in part, of stormwater runoff. Typically, grading and earth-disturbing activities during construction require removal of vegetation that dissipates raindrop energy and filters and controls stormwater pollution. Construction grading and earth disturbance leave areas of land susceptible to rainfall-induced soil erosion caused when raindrops fall directly on the soil surface and dislodge soil particles. These dislodged soil particles are suspended in stormwater runoff and may carry fertilizer, pesticide residue, and other chemicals that are detrimental to aquatic life. While erosion is a naturally occurring process, construction exacerbates erosion rates. Erosion rates on construction sites can be as high as 500 tons (454 metric tons) per acre per year, rates which are 100 times greater than agricultural land uses, and 2,000 times greater than erosion of land protected by leaves and vegetation in naturally wooded areas.

Sediment—materials that are transported in water runoff from one area to another as the result of erosion (e.g., tiny particles of clay, rock, and minerals)—can have negative consequences on downstream receiving water bodies, including impacts on water clarity, disruptions to aquatic ecosystems, and repercussions on human, animal, and plant health. Installing a silt fence, also known as a sediment fence, on construction sites is a standard practice used for controlling runoff and mitigating the effects construction site runoff has on the surrounding environment, including lakes, streams, and habitats.

In addition to ecological issues, excessive amounts of sediment may change the characteristics of a streambed and impair streamflow through culverts. Flooding, increased culvert maintenance, increased drinking water treatment costs, and loss of recreational value are other impacts that can occur from excessive sedimentation.

First and Last Defense

One of the principal objectives of the Clean Water Act is to “maintain the chemical, physical and biological integrity of the Nation’s waters.” Per 23 CFR 650.203, it is also the policy of the Federal Highway Administration that all highways funded in whole or in part under Title 23 of the United States Code shall be located, designed, constructed, and operated according to standards that will minimize erosion and sediment damage to the highway and adjacent properties and abate pollution of surface and ground water resources. To meet these goals, contractors rely on erosion and sediment control practices and products. One of the first sediment control practices employed on a construction site, prior to major ground disturbances, includes sediment barriers (e.g., silt fences, fiber rolls, synthetic rolls, and compost socks). Sediment barriers often function as the last line of defense prior to stormwater runoff discharging from a site. As runoff flows across construction sites, sediment particles may become dislodged and suspended by the kinetic energy of runoff flow velocity. Sediment accumulates and will stay suspended until the velocity and kinetic energy reduce, providing favorable conditions for sedimentation to occur. Sediment barriers provide favorable settling conditions by creating areas of impoundment (pooling water behind the barrier), thereby reducing runoff velocity, and retaining sediment onsite.

Silt fences are one of the most common and recognizable practices used in construction stormwater management, but silt fences have issues of their own. Design guidelines typically stipulate the installation of silt fence sediment barriers on level ground along the construction site’s contour lines to intercept sheet flow to spread flow across the entire length of fence, thereby maximizing the impoundment area. It is often difficult to precisely follow a site’s contour lines during installation, and site constraints may not allow for proper placement. When these difficulties arise, the resulting installation of silt fences leads to points of concentrated stormwater impoundments at lower elevations—where deeper impoundments with a smaller surface area will decrease functionality of a silt fence and possibly lead to installation failure. Failure can be in the form of structural collapse or undermining leading to decreased performance, which may render the silt fence completely ineffective due to minimal impoundment capabilities. Excessive wetness, ponding water, or silt fence failure can result in a costly delay for the construction schedule.

Large-Scale Testing Enhances Defense

Recently, the Alabama Department of Transportation (ALDOT) sponsored a study with the Auburn University–Stormwater Research Facility (AU-SRF) to evaluate the installation effectiveness of their standard silt fence detail. AU-SRF is one of the university stormwater research facilities across the Nation that partner with State departments of transportation and regional, local, and Tribal transportation agencies and assist with research, product evaluation, nonproprietary innovations, and first-hand training to facilitate innovative and practical solutions for stormwater management. Large-scale testing of silt fence installations by AU-SRF used hydrologic conditions mimicking runoff from a 0.5-acre drainage basin spread across 100 feet (30 meters) of wire-backed reinforced silt fence, resulting in a testing flow rate of 0.22 cubic feet (0.006 cubic meters) per second across a 20-foot (6-meter) wide installation. This flow rate is indicative of the expected runoff from a 2-year, 24-hour storm event for construction conditions in central Alabama. Sediment loading of 1,128 pounds (512 kilograms) was introduced to the runoff across the 30-minute test duration to simulate sediment-laden conditions. This loading is also based upon typical construction conditions in Alabama and calculated using the modified Universal Soil Loss Equation. During testing, structural integrity, sediment retention, and water quality parameters were assessed to determine performance.

ALDOT used a silt fence consisting of 3.5 ounces (99 grams) per square yard of nonwoven geotextile fabric, installed on metal T-posts with wire backed reinforcement. ALDOT’s design guidance for a standard installation of silt fences specifies maximum T-post spacing of 10 feet (3 meters) and an installed height of 32 inches (81 centimeters). A silt fence may be positioned in a 6-inch by 6-inch (0.15-meter by 0.15-meter) trench or sliced in with a silt fence slicing machine. Although the American Association of State Highway and Transportation Officials require minimum steel T-post densities of 1.3 pounds (0.59 kilograms) per foot, silt fences installed in Alabama are often supported by 0.85 to 0.95 pounds (0.39 to 0.43 kilograms) per foot T-posts.

When assessed at 5 and 10 feet (1.5 and 3 meters) post spacing, the 0.95 pounds (0.43 kilograms) per foot posts were severely deflected, resulting in structural failure after the third performance test to the same installation. For installations evaluated using 10 feet (3 meters) post spacing of 0.95 pounds (0.43 kilograms) per foot posts, the middle post deflected, on average, 2.3 feet (0.70 meters) after the third performance test on that installation. Through a variety of installation modification iterations, an enhanced installation was developed that consisted of 1.25 pounds (0.57 kilograms) per foot steel T-posts spaced 5-feet (1.5-meter) on-center and offset 6 inches (0.15 meters) downstream of the trench. This installation resulted in the least amount of post deflection, with the center middle T-post deflecting only 0.15 feet (0.05 meters) after the third performance test. Offsetting the location of the trench allowed the T-post to gain an additional 6 inches (15 centimeters) of ground support as it was no longer installed within the trench. Furthermore, the offset installation reduces the height of the fence and provides additional anchoring of the geotextile due to the force of impounded water bearing on the fabric. Offsetting the trench also enhances compaction, as it allows for a tamper or compaction plate to traverse over the back-filled trench without tearing or snagging the silt fence. A complete testing methodology is available in the 2021 publication of Journal of Irrigation and Drainage Engineering (https://ascelibrary.org/doi/10.1061/%28ASCE%29IR.1943-4774.0001521).

Silt fence sediment barriers that perform as intended result in large impoundments, especially when sediment clogs or blinds the geotextile, greatly reducing the amount of water allowed to flow through. Researchers at the AU-SRF also developed a dewatering weir that acts to control the discharge of impounded stormwater at a controlled rate. The weir is comprised of common plywood and includes a v-notch and a series of orifices. The weir relieves excessive impoundment, while the orifices dewater impoundment over the course of several hours.

Even though water passes through this dewatering mechanism and not through a “filtration medium,” sediment capture upstream of the sediment barrier was minimally affected as both installations, with and without the dewatering board, captured over 90 percent of the sediment introduced by volume. This high performance is due to the impoundment depth and length being minimally affected by the dewatering board. These installation concepts can help practitioners maximize silt fence performance by minimizing structural failure and downtime associated with large impoundments affecting work areas.

The design was accepted by the Alabama Soil and Water Conservation Committee (ALSWCC). Perry L. Oakes, a professional engineer (PE) and Erosion and Sediment Control Program coordinator for ALSWCC, acknowledges the scientific validity of the design, “ALSWCC maintains Alabama’s Erosion and Sediment Control Handbook. The ALDOT research at the Auburn University Stormwater Research Facility ensures that the practices in our handbook are innovative and scientifically based, not only for ALDOT sites, but for construction sites throughout the State.”

“The Alabama Department of Environmental Management (ADEM) greatly appreciates the continued collaborative efforts between ALDOT and Auburn University regarding enhancements of best management practices for erosion and sediment control,” says Shelane P. Bergquist, chief of the Construction Permits Section Stormwater Management Branch at the Water Division of ADEM. “These efforts will continue to pay dividends in the protection of our waterbodies.”

Promising Modifications for Silt Fences

Richard Klinger, an environmental construction engineer at ALDOT, embraced the innovative design and worked to develop a Standard Specification (665608, https://aub.ie/siltfence) released in 2021. Of the designs, Klinger states, “ALDOT strives to continuously improve erosion and sediment control practices to keep pace with the ever-evolving environmental regulations.” He further comments on the modified silt fence design saying that “reduced height requires less material which is more environmentally friendly while delivering increased performance. At times, field conditions may differ from the design or an issue with the grading operation may cause unintended impoundment of the silt fence. The increased post spacing and the dewatering weir are a maintenance measure for these areas of unintended impoundment. The additional posts and dewatering weir can be installed in the existing silt fence without jeopardizing the structural integrity of the fence or having to completely remove sections and re-install. The dewatering weir is simple to construct, easy to install, and gives the contractor the ability to get back to work in these areas much quicker than with previous practices.”

Wade Henry, assistant bureau chief of final design at ALDOT, declares that the modified design “is just one of many examples where ALDOT’s investment in Auburn’s research continues to pay off. ALDOT is committed to continue to learn and practically apply that knowledge to evolve our erosion prevention best management practices, so that we can address erosion in the most effective and efficient ways possible.”

J. Blake Whitman, Ph.D., PE, Certified Professional in Erosion and Sediment Control (CPESC), is an assistant professor at Middle Tennessee State University in the School of Concrete and Construction Management. Whitman completed his Ph.D. at Auburn University where he evaluated the performance of ALDOT sediment barrier practices used on construction sites.

Michael Perez, Ph.D., PE, CPESC, is an assistant professor in the department of Civil and Environmental Engineering at Auburn University. He teaches courses in construction and stormwater management and oversees the research program at AU-SRF.

Brian Smith, CPESC, is an ecologist for the FHWA Resource Center - Office of Innovation Implementation. He holds a B.S. degree in biological science from Illinois State University and an M.S. in geology from Northern Illinois University.

For more information, visit http://stormwater.auburn.edu or contact Michael Perez at 334-844-6267 or mike.perez@auburn.edu.