Idaho Vehicle Sediment Control BMPs
Idaho is home to many beautiful natural landscapes and boasts some of the most rugged and untouched land in the nation. This wonderful land is dotted with streams of fresh mountain water and larger rivers, including the Snake, Salmon, Clearwater, and Boise Rivers. The Snake River flows from Yellowstone National Park and runs through Idaho on its course to join the Columbia River in Washington State. Local and Federal regulations are in place to protect the surface waters throughout the state.
Idaho Department of Environmental Quality (DEQ) NPDES Stormwater Permits
The Federal Water Pollution Control Act was amended in 1972 and became known as the Clean Water Act (CWA), establishing the National Pollutant Discharge Elimination System (NPDES) permit program. Among other things, the program ensures that all industrial and construction site operators have proper erosion and sedimentation controls in place to prevent excessive sedimentation, which is the most common source of water pollution. The Idaho Department of Environmental Quality (DEQ) administers a program under the Idaho Pollution Discharge Elimination System (IPDES). Construction activities that disturb one acre or more (or are part of a larger common plan of development) must obtain stormwater permit coverage.
Many construction projects in Idaho obtain coverage under the IPDES Construction Stormwater General Permit. To apply for coverage, contractors will submit a Notice of Intent (NOI) and create a Stormwater Pollution Prevention Plan (SWPPP), which outlines the sources of contaminants on the job site and the measures to reduce their effects on surface waters. Each measure in the SWPPP is called a Best Management Practice, or BMP, which can be either structural or procedural. Idaho DEQ publishes and references the Idaho Catalog of Storm Water Best Management Practices, which provides detailed descriptions of BMP design, applicability, and maintenance.
BMP 40: Vehicle Sediment Control BMP
Vehicle Sediment Control is a commonly required BMP on construction sites where there is a risk of sediment tracking onto public roadways. The Vehicle Sediment Control BMP is designed to minimize sediment deposited on roadways that is tracked out from construction vehicles at egress points. An effective stabilization technique is essential because any sediment that is tracked off-site is challenging to remove and effortlessly flows into storm drains. To reduce off-site tracking, entrance points are stabilized with a rough surface to remove sediment or with a water-based solution to clean tires.
To ensure this BMP effectively tracks all construction traffic throughout the site, all construction traffic must be diverted through a stabilized entrance. An effective construction entrance may use a combination of techniques and processes to maximize sediment capture. The SWPPP will include additional BMPs to capture residual sediment, including regular inspections and maintenance, ongoing street sweeping to capture residual track-out, proper grading of the entrance to reduce runoff, and installation of a sediment basin to capture drainage from the entrance area.
If possible, the Vehicle Sediment Control structure should be graded to allow drainage to flow back onto the site, reducing runoff during rain events.
Aggregate Pad Construction Entrance
An Aggregate Pad Construction Entrance is a traditional construction entrance technique that uses aggregate to create a rough surface that can dislodge and contain sediment from vehicle tires. The aggregate pad should use crushed stone, typically 2–6 inches in diameter, with a top layer of smaller stone as needed to improve performance. A depth of 9 inches is the minimum depth, but additional aggregate can be installed depending on soil type. The pad should be a minimum of 50’ in length, but a 100’ length is often used for increased performance. The pad should be at least 15’ wide and extended to the full width of the entrance. On high-volume sites, the width should be increased to allow two vehicles to exit over the aggregate. The entrance points are flared to provide a wide turn as cars turn onto the roadway.
The aggregate is installed over a nonwoven geotextile filter fabric, which prevents fine sediment from the substrate from entering the aggregate layer. The filter fabric should be resistant to rot and mildew to prevent breakdown.
The entrance should be inspected regularly and repaired when the majority of the sediment is no longer contained. When the aggregate becomes compacted or saturated with sediment, the performance is significantly reduced. Maintenance of an aggregate pad requires installing additional aggregate (2 inches or more) to topdress the pad and restore the rough surface. Maintenance frequency will increase during wet seasons as the aggregate can quickly become saturated with mud. Inspections should be conducted regularly and after storm events, with maintenance performed as needed to maintain effectiveness.
At the end of construction, all aggregate and the geotextile filter fabric must be excavated and removed. Prior stabilization will include restoring the original soil and may require reseeding with grasses and other native plants.
Construction Mat or Turf Reinforcement Mat BMP
Low-volume and smaller construction sites can use Construction Matting or Turf Reinforcement Matting as a construction entrance. These systems primarily stabilize the entrance surface and reduce rutting, but provide limited sediment removal compared to aggregate pads or mechanical systems. These are often used on soft substrates, in saturated soils, or in wetlands because they can immediately stabilize the substrate and provide some protection for existing vegetation.
When using Turf Reinforcement Mats, woven mats, or construction mats, include a 50-foot length of matting and a wide turn radius. The entrance should be 20’ or more to accommodate the largest vehicles using the exit. The entire width of the egress point should be stabilized, or vehicles can be restricted to exit over the mats.
Mats can be cleaned as needed to remove accumulated sediment.
Rumble Strips and Rattle Plates BMP
Rumble Strips and rattle plates are steel panels that have corrugated ridges or welded pipe or angled steel, which produce a rough surface that shakes vehicle axles as they drive over. The shaking motion can help dislodge additional sediment. Some rumble strips are raised to create a void below, while others are laid flat on the substrate. These structures are often constructed on-site, but many manufactured options exist. Dimensions vary by design and manufacturer and are selected based on site conditions and vehicle traffic.
Rumble Strips are often used in combination with aggregate construction entrances to remove additional sediment, and are placed in the center of the aggregate pad. Sediment builds between the ridges and must be swept, scraped, or washed to restore functionality.
Wheel Washing Station BMP
If other methods are not effectively cleaning sediment, a wheel washing station may be used. Wash Stations or wash racks are effective BMPs that clean vehicle tires with pressurized water. Wheel washing stations typically consist of a reinforced concrete rack or a cattle-guard-style structure, along with a pressurized water source. A wash can use a simple, manually operated sprayer or use a more advanced automated sprayer system.
When introducing water to the construction site entrance, proper drainage is required. All runoff should be diverted into a sediment trap to contain sediment on-site.
FODS Trackout Control System
The FODS Trackout Control System is an effective, reusable BMP that minimizes sediment trackout. Compared to traditional aggregate entrances, it has been shown to reduce street sweeping by 59% in high-traffic-volume applications. Because the system is highly effective, FODS are commonly specified at 35’ in length to provide equal performance to a 70’ aggregate-style construction entrance. The system is designed to be reused and relocated and can offer cost savings on linear projects and over its 10+ year lifespan.
The FODS Trackout Control System consists of a set of 12’ x 7’ HDPE mats, which are formed into pyramid-shaped structures on the surface. The pyramids deform and flex vehicle tires in multiple directions to dislodge sediment, rocks, and debris that is lodged between tire treads. The system can be installed over existing substrates, including dirt, concrete, and asphalt. Installation, maintenance, and removal can be completed without heavy equipment, and a standard installation can be completed in under 30 minutes.
The pyramids can hold up to 2.5” of sediment, as the tires only come into contact with the tips of the pyramids. When sediment builds between the pyramids, maintenance is required. Maintenance of a FODS construction entrance is typically completed using a street sweeper, or a skid steer with a broom attachment can be used to brush away sediment to restore performance. Cleaning can also be completed manually using a FODS Shovel, which is designed to fit between rows of pyramids. Contractors can integrate maintenance into regular street sweeping to ensure the entrance remains compliant with permit requirements.
Additional Resources
Idaho DEQ General Permit for Discharges from Construction Activities