Though construction sites can contribute many types of pollutants, including heavy metals, chemicals and oils, sediment is the biggest culprit. As any contractor visiting a job site after a heavy rain can tell you, runoff wreaks havoc when it courses over raw ground or plows through piles of topsoil. What’s more, some areas can stay bare for months, leaving them vulnerable through several seasons.
Fortunately, common-sense measures can reduce, if not eliminate, this problem. Structural solutions include silt fences, sedimentation ponds, erosion control blankets and temporary or permanent seeding. Non-structural practices like picking up trash and debris, sweeping sidewalks and streets, maintaining equipment and training staff and subs can go a long way to keeping pollutants out of any stormwater that runs through the site.
In its guide for developing stormwater prevention pollution plans on construction sites, the EPA outlines 10 Best Management Practices, or BMPs, for effective erosion and sediment control. Everyone involved in a project—architects and designers, builders, subs and homeowners—should know about these strategies.
Erosion Control: Keeping the Dirt in Place
- Minimize disturbed areas and protect natural features and soil. Delineating and controlling the area to be disturbed by grading or construction can greatly reduce the potential for soil erosion and stormwater pollution problems. Limit disturbed areas, keeping in mind that natural vegetation is your best and cheapest erosion control measure. Protecting and preserving topsoil is also a BMP, as removing topsoil exposes underlying layers that are often more prone to erosion and have less infiltration capacity.
- Phase construction activity. Scheduling or sequencing your construction work and concentrating it in certain areas can minimize the amount of soil exposed to the elements at any given time. Limiting the disturbance to places where construction activities are underway and stabilizing them as quickly as possible can be one of your most effective BMPs.
- Control stormwater flowing onto and through the project. Plan for any potential stormwater flowing into the project from upstream locations, and divert and retard flows to prevent erosion. The volume and velocity of on-site stormwater runoff should also be controlled. Diversion ditches or berms direct runoff away from unprotected slopes, and may direct sediment-laden turnoff to a sediment trapping structure.
- Stabilize soils promptly. Once construction activity has stopped, stabilize soils by providing temporary or permanent cover. Temporary covers include temporary seeding, mulches, matrices, blankets and mats and the use of soil binders. Permanent cover strategies include permanent seeding and planting, sodding, channel stabilization and vegetative buffer strips.
- Protect slopes with appropriate erosion controls. Steep slopes, slopes with highly erodible soils, or long slopes require a more complex combination of controls. Erosion control blankets, bonded fiber matrices or turf reinforcement mats are effective options. Silt fence or fiber rolls may also help control erosion on moderate slopes, and should be spaced at 10- to 20-foot intervals. Diversion channels and berms can also direct stormwater away from slopes.
Sediment Controls: The second line of defense.
- Protect storm drain inlets that could receive stormwater from the project until final stabilization of the site has been achieved. Upon completion, storm drain protection should be removed. You may need to coordinate with land owners on adjacent properties, too. Several types of filters are commonly used for inlet protection, including silt fences, rock-filled bags and block and gravel.
- Establish perimeter controls with silt fences and fiber rolls, to help prevent erosion and stop sediment from leaving the site. These barriers protect stream buffers, riparian areas, wetlands and other waterways. A sediment fence is comprised of a geotextile attached to two posts, which is trenched into the ground. Fiber rolls are open mesh tubular sleeves filled with a fibrous material that traps sediment. Both are effective in small areas, and shouldn’t be used in areas of concentrated flow.
- Retain sediment on-site and control dewatering practices. When sediment retention from a larger area is required, consider using a temporary sediment trap or sediment basin. These detain sediment-laden runoff for a period of time, allowing it to settle before the runoff is discharged. Dewatering practices remove groundwater or rain water from excavated areas, usually by pumping muddy water to either a sediment basin, an area enclosed by a silt fence or a flat, vegetated area.
- Establish stabilized construction exits. Vehicles entering and leaving the site can track significant amounts of sediment onto streets. Identify and clearly mark one or two locations where all vehicles will enter and exit the site, and focus stabilizing measures at those locations. Construction entrances made from crushed rock can be further stabilized with stone pads or concrete, and steel wash racks and a hose-down system will remove mud and debris from vehicle tires. Finally, don’t forget to sweep your site regularly.
- Inspect and maintain control. Without adequate maintenance, erosion and sediment controls will quickly fail, sometimes after just one rainfall.