When It Rains, It's Porous: Permeable Paving

Permeable pavement lets rain drain through your driveway or patio, minimizing flooding and toxic runoff.


| March/April 2008



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This custom driveway incorporates Grasspave2 sections to absorb rainwater runoff.

The next time it rains, look at your driveway. Much of the water washes off and into storm drains, carrying oil, grease and chemical residue from cars. This runoff can cause erosion, flooding and contamination of public waterways.

Fortunately, permeable paving materials allow stormwater to soak into the earth, where naturally occurring bacteria help digest contaminants before they reach the water table. Called pervious or porous pavement, these systems keep groundwater clean, help tree roots breathe, reduce the severity of flash-flooding and reduce the urban heat-island effect because they absorb water into the ecosystem and don’t reflect heat back into the environment.

Two-thirds of the rain in urban areas falls on a paved surface, and some local governments encourage pervious paving to improve groundwater. In Chicago, new construction projects are now required to capture the first half-inch of rainfall on site.

Other locales are limiting the amount of a building lot’s land that can be covered by buildings or pavement. Build a house near Maryland’s environmentally sensitive Chesapeake Bay, for example, and no more than 10 percent of your land can be covered by anything that prevents rainwater from reaching the ground directly—a regulation that restricts house size.

If you already have a driveway, does it make sense to rip it up and replace it with something porous? Probably not, if it’s in good shape. However, if your driveway is due to be repaved, or if you’re building a new house, it might be time to look into a pervious system.

Surfaces that allow seepage

The simplest and cheapest way to let water run through your driveway is to build it with gravel, also known as aggregate. Single-size, angular particles—thoroughly washed to remove potentially clogging dust and dirt—create a driveway with as much as 40 percent open space between the stones, allowing practically any amount of rainfall to soak through.





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