Erosion and Sediment Control
Perimeter Controls: They May Not Be Effective, but at Least They Are Expensive
Construction site boundaries are usually encompassed by perimeter controls that include: silt fence, wattles, sediment retention devices, hay bales, aggregate berms, brush barriers, manufactured devices, and vegetative buffers. Perimeter controls often act as the final opportunity to capture and contain transported sediment prior to off-site discharge of sediment-laden stormwater. Environments surrounding a site can vary immensely, ranging from a high-quality lake or stream, to a roadside ditch that drains to an underground storm sewer system. Sediment-laden discharges from construction sites can have detrimental effects on the ecosystem if perimeter control products and/or practices fail to perform as intended. There are many parameters that practitioners must consider when developing the stormwater pollution prevention plan; and selecting the appropriate perimeter controls is an important element. Devices used as perimeter controls typically treat sheet flow by removing sediment primarily through sedimentation and, to a lesser degree, the filtering of soil particles. As ponding occurs, particles fall out of suspension and are retained on-site upstream of the perimeter control. Filtration through perimeter control material is limited by the void spaces and flow-through capacity within the filtering medium that has the potential to degrade over time as the material clogs with sediment. However, it is often unknown how much sediment is retained and how much passes through the material. Structural performance of perimeter controls is reliant upon installation and material properties of the practice or product. Although the primary goals of perimeter control practices are typically the same (i.e., sediment retention and water quality improvement), knowing when, where, and how to use various perimeter controls may not always be apparent. Often, designers rely on rules-of-thumb for the selection, application, installation, and maintenance of perimeter controls. Relying on these rules-of-thumb often limits a user’s understanding of in-field performance, which can hinder proper application. When failures occur in the field, it is often unknown if the material, design, application, installation, or lack of maintenance was the cause. This industry round table will provide an opportunity for attendees to collaborate, interact, and discuss open-ended topics pertaining to their experiences with various perimeter control practices in an effort to identify lessons learned in the successful applications of practices. Discussions will revolve around types of perimeter controls, design guidance, application, in-field performance, typical failure modes, maintenance requirements, and innovations. The involvement of industry practitioners to share lessons learned will aid in the dissemination of knowledge and in-field improvements in the application, installation, and overall performance of perimeter controls.