Monday, June 27, 2011

Permeable Pavement

How to reduce runoff & improve water quality 

The trouble with "runoff" is...the story's a bore. There are neither villains involved, nor poster animals at risk nor priceless scenery at stake. Yet the polluted water that runs off paved surfaces is the greatest threat to water quality in many areas of America today. It also contributes to groundwater depletion.

Did you even realize we had a water quality problem in the United States? Well, surprise, surprise. Despite progress made under the Clean Water Act since 1970 in cleaning up our nation's waters, 40 percent of our rivers and 46 percent of our lakes are still too polluted for fishing, swimming or aquatic life. Runoff isn't the only cause, but it is an important one. And you can do something about it.

Runoff happens as a matter of course when a heavy downpour or prolonged rain unleashes more water than the ground can absorb, or when a build-up of snow melts too rapidly. Then the excess travels overland until it finds a river, lake or ocean to call home. Along the way, it picks up dirt and debris as well as pesticides, fertilizers and other contaminants, all of which end up in our waterways.

In towns and cities, the mechanism is somewhat different but the outcome is the same. There, even moderate precipitation can lead to runoff because so much land is paved over with impermeable materials. Since water can't seep through the pavement into the ground below (where it might recharge groundwater supplies), it runs off into storm sewers, carrying car oil, antifreeze, dog poop, trash and other contaminants from the pavement with it. This polluted mess ends up in our waterways—including many drinking water sources and beaches—because that's where the sewers discharge it.

If you're a property-owner, your driveway may be part of the problem. So may your walkway or patio—or your business's parking lot. But none of them have to be. By repaving these surfaces with alternative materials and/or methods that allow water to pass through to the ground below, you can help keep your local waters clean and restore groundwater reserves.

Permeable paving options include:

  1. Poured-in place pervious concrete. For a solid pavement, use pervious concrete. It looks similar to conventional concrete, but with a more pebbled surface. The material is made with a different recipe, which yields a material with small "voids" or spaces in it, through which the water can seep.
  2. Permeable pavers. For a paved brick or cobblestone look, use permeable paving blocks made of concrete or cut stone. The pavers are laid atop a basement layer of sand and gravel. Water seeps through the spaces between the pavers.
  3. Paving grid. For a driveway that looks like a lawn, install a paving grid. Made of recycled plastic or concrete in a honeycombed or lattice pattern, the grids have open cells in which grass can grow. The resulting driveway looks natural but is strong enough to support a vehicle. Paving grids can also be filled with decorative gravel. The grid structure holds the gravel in place.
These alternatives cost more than conventional concrete or asphalt, but can increase your home's value because they can lower the risk of flooding in heavy storms and can reduce ice build-up and the need to shovel snow in winter. Also, permeable pavers and paving grids can improve your home's aesthetic appeal.

Note that your climate and site conditions, including water table and soil type, may make some pavement options preferable to others, so investigate these issues when considering your choices.

If you need a less costly solution for your driveway, consider simply laying strips of bricks, stones or concrete blocks where vehicle tires will go and planting the rest of the driveway area with grass. For walkways, try mulch, a less durable, but also less expensive approach.

Whether or not you repave, you can reduce polluted runoff with the following practices:
As small and mundane as these steps may seem, they will help keep the water in your area cleaner.

—Sheryl Eisenberg 

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On This Topic

RUNOFF IN A NUTSHELL—AND HOW PHILLY IS TACKLING IT. The city of Philadelphia, which gets its drinking water from the rivers that flow through it, is combatting runoff with green infrastructure, including rain gardens, green roofs, treescapes, and porous pavements.

Grass driveway
Grass and paving block driveway
GREEN DRIVEWAY EXAMPLES. The driveway in the top photo, located in Cleveland, Texas, looks just like a lawn, but is created with a plastic paving grid to provide support for vehicles. The one in the bottom photo, located in Isthmus, Wisconsin, mixes a plastic paving grid for the center and side grassy areas with permeable paving blocks. (Photos courtesy ofInvisible Structures, maker of the plastic grid product featured here.)

Permeable paving blocks
PERMEABLE PAVING BLOCKS UP CLOSE. The cut corners of these blocks provide room for gravel, through which water can drain. Permeable paving blocks come in a wide variety of styles.


Mimicking Nature to Solve a Water-Pollution Problem 

Protecting Water from Urban Runoff

Mother Earth News
Green Patios, Walkways & Driveways of Porous Pavement and Pervious Concrete

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The Green Driveway Articles

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Permeable Driveways

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What Are Permeable Pavers

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Permeable Paving