Friday, August 20, 2010
The benefits of permeable surfaces are finally starting to soak in!
Recently I met a fellow gardener who told me about all of the great eco-friendly features of her landscape. One of the key features she mentioned was her lack of lawn.
Since maintaining a green lush lawn can require a lot of water and chemicals, many people feel that cutting down on lawn is a good thing for the environment. But when I stopped by her house for a visit later in the week, I wasn’t really sure she was as environmentally friendly as she thought.
Instead of a lawn, almost the entire front yard of her small property was filled by a huge, concrete circular driveway.
I have heard many times that pervious surfaces, or surfaces that allow water to soak into the ground, are better for the environment. But I thought that the main reason for that was to allow any chemicals that you use in your landscape to soak into the ground rather than finding their way into the storm drains. Since she doesn’t use chemicals, was her impervious driveway still bad for the environment?
I decided to consult an expert: Jan-W. Briedé, PhD, the Stormwater Outreach Manager for the State of Virginia. Here is what I learned:
“An impervious surface that large is bad for the environment even if the homeowner doesn't use chemicals,” Dr. Briedé explained. “It is a well established fact that the total amount of stormwater runoff is increasing with increases in impervious areas such as roads, roofs, driveways, and parking lots. In summary, the infiltration of rain water into the soil decreases with increases in development.”
“In agriculture fields and forests typically 50% of the rainfall infiltrates, or soaks into the ground, while in our towns and suburbs only 15% to 35% of the rainfall infiltrates. The increase in runoff that results from increases in impervious areas has devastating impacts on creeks and streams. It is also one of the reasons why we saw the flooding in Nashville earlier this year and recently here in the Washington area. Increased runoff and decreased infiltration of rainwater is also one of the causes of the dropping groundwater tables and the reason why we need to drill deeper wells to get to the groundwater. These are some of many reasons why we need to try to keep all the rain that falls on our property where it belongs….in our yard.”
I asked Dr. Briedé what a better choice would have been for my well-intentioned friend.
He explained that constructing the driveway out of something that would allow water to soak through is always the best choice for a large driveway.
“In historic times we have always had some form of pervious pavement,” Dr. Briedé said. “ Dirt roads would let water infiltrate, while in the cities during long forgotten time, cobble stones were placed far enough apart so that water could infiltrate into the cracks between the stones. The father apart the stones were placed, the more water could infiltrate, or the faster the water could infiltrate. The same principle is used with pervious pavement today.”
“There are essentially three types of pervious pavement: permeable pavers, permeable concrete and permeable asphalt. All three methods are worth considering around the home when you are considering landscaping projects. While permeable asphalt and concrete are best used for driveways and parking areas around the home, pavers can be used for all those as well as for decks and walkways. Using some form of pervious pavement in your landscaping, you can reduce the runoff coming from your property while keeping the water where it belongs: your soil, where it will be available to your plants. Can you imagine the difference it could make to your water bill if you could infiltrate more water into the soil on your property, instead of letting it runoff into the creeks and streams? Your trees and shrubs in particular will thank you for it.”
“In addition to using permeable surfaces, homeowners may also help reduce stormwater runoff by methods such as green roofs, rain barrels and cisterns, rain gardens and swales.”
Dr. Briedé suggested the following websites for more information:
Pavers: Interlocking Concrete Paver Institute
Concrete: Pervious Concrete: When it Rains, it Drains
Asphalt: Porous Asphalt: National Asphalt Pavement Association