When rainfall hits impervious pavements such as conventional concrete and asphalt, the water runs off, collecting pollutants along the way and ends up in stormdrains and waterways. Pervious pavements allow water to pass through the surface and infiltrate into the soil below rather than running off impervious surfaces and into surface water.
Pervious pavements include pervious asphalt, pervious concrete, pervious interlocking concrete pavers, plastic grid systems and amended soils. These pavements have the dual benefit of serving as a parking or drive surface and a stormwater management BMP. This fact sheet provides an overview of the benefits associated with pervious pavements. In addition, examples of municipal ordinances and programs that incorporate pervious pavement into development and roadway reconstruction projects are provided.
Whether soil conditions favor infiltration or filtration, peak runoff rates are reduced and water quality treatment is afforded by using pervious pavements. Runoff volumes are reduced and ground water is recharged where underlying soils are suitable for infiltration. Numerous analyses of discharge from pervious pavement under-drains show that concentrations of suspended solids, total solids, phosphorus and nitrogen were lower than typical discharges from impervious surfaces, and that concentrations of copper, zinc and lead were also reduced.
Two long-term monitoring studies conducted in Rockville, MD, and Prince William, VA indicate removal efficiencies of 82 to 95 percent for sediment, 65 percent for total phosphorus, and 80 to 85 percent of total nitrogen (EPA, 1999). Removal efficiency is a function of introduced load/concentration, depth of filtration and materials used to filter.
When installed over a drainage storage bed, pervious pavements allow runoff to be stored and infiltrated into the surrounding soils or filtered, and collected by an under-drain system that discharges to the storm sewer system or directly to receiving waters. Pervious paving can reduce the size of engineered stormwater treatment facilities by reducing the amount of runoff needing treatment. Currently, the General Construction Permit will allow site designers to reduce the water quality volume sizing required when using pervious pavement, up to a maximum of ½ acre of new impervious surface.
During the winter months, water does not accumulate on the surface of pervious pavements unlike traditional impervious pavements preventing ice build-up. This decreases the need for using deicing agents such as sand and salt saving a municipality time and money. In addition, there is a decreased chance of a public safety hazard in pedestrian areas.
Pervious asphalt consists of fine and course aggregate stone bound by a bituminous-based binder. The amount of fine aggregate is reduced to allow for a larger void space of typically 15 to 20 percent. The installation at the Ramsey Washington Metro Watershed District offices in Little Canada, MN installed pervious asphalt in their parking lot.
Pervious concrete is a mixture of Portland cement, fly ash, washed gravel, and water. Unlike conventional concrete, pervious concrete usually contains a void content of 15 to 25 percent which is achieved by the addition of a fine, washed gravel.