Instead, she and her family decided to go green and build a permeable surface driveway. After Carpenter's driveway was flattened, a base level was prepared, and on the surface, interlocking four-inch by six-inch cement pavers and sand were neatly arranged in a geometric pattern.
An eco-friendly alternative to a giant slab of concrete, green driveways are considered permeable surfaces that absorb rain water and car pollutants, and prevent them from running off into the storm-drain system -- and ultimately polluting the Bay.
With so many solid surfaces like roads, driveways or sidewalks in the community, green driveways are an excellent way to allow water to percolate back into the ground naturally, she said.
"They're better for your property and neighbor by not having all the water pool to one side of the house. It just makes more sense, and they look nice," she said. "We can't make roads permeable, but driveways, pathways and parks can be. The more permeable surfaces we can use, the easier it is on the Earth."
If a cement paver ever becomes loose, Carpenter said she simply scoops the dirt out and flattens the paver down again. She said she liked the idea so much that the patio and walkways in their house were converted to cement pavers and sand as well.
Previously, city ordinances prevented homeowners from building non-cement or asphalt driveways, but as of 2001, the Palo Alto City Council passed an ordinance that allowed permeable paving materials, such as grass or gravel, to be used for residential driveways.
Bob Morris of the Public Works Department in Palo Alto said that the city supports green driveways and encourages developers to prevent storm-water runoff. He added that green driveways are ideal for decreasing the amount of water and pollutants in the storm-drain system.
"We didn't need to accommodate large volumes of water before; but as more buildings got built, there was more water runoff, overwhelming capacity in the storm system," Morris said. "We want to make sure the water that goes into storm drains is as clean as possible. The cleaner it is, the better it is for the fish, the Bay and the environment."
Besides controlling water runoff, green driveways also mitigate the "urban heat island effect," according to the Environmental Protection Agency's Web site. As a city or neighborhood grows, more and more concrete is used to create parking lots, roads and buildings; as a result, solar heat reflects off the concrete, causing hotter air temperatures. Driveways that use grass can contribute to lowering air temperature by transpiring water back into the air, creating an air-conditioning effect.
Several methods and materials can be used to build green driveways. A common one is to arrange concrete pavers with two-inch to five-inch gaps set between them. The pavers are laid over a sub base and bedding layer; and on the surface, turf grows in the spaces, allowing water to seep into the ground.
Other approaches include grass and gravel pave. Grass pave, which looks like a regular lawn after it's been installed, is a honeycombed plastic grid that prevents root compaction. The grid is laid out over a base level, and a surface layer is spread out. The grass then grows up through the grid.
Maintained like a normal lawn, this method also prevents erosion. However, since there is a limit to how much weight and car-exhaust fumes the grass can handle, it's important to let the plants rest and regenerate from time to time.
Gravel pave works similarly with a plastic grid set above a porous base. Most gravel driveways eventually compact over time, but with most gravel pavers, the grid prevents gravel from lodging in the soil sub-base.
Mark Maricinek of Greenmeadow Architecture designed a green driveway for a restored 1954 Eichler in Palo Alto. To transform the concrete jungle in the front of the house to something a bit greener, Marcinek set 12-inch concrete pavers four inches apart. Grass now grows in the gaps.
He suggests that those interested in converting their driveway to a permeable surface consider hiring a landscape architect or designer. "It's important to look at the drainage and the base for car weight; otherwise, the car will be sinking in the mud from the weight," he said.
Owner Norman Adams said his driveway is occasionally mowed and that people often mistake it for a regular lawn. He now keeps orange safety cones handy just in case the neighborhood gets busy and people park in front of his driveway.
Magic, a Palo Alto nonprofit organization that aims to protect the community and environment, also has three green driveways, one consisting of mulch and small plants, and the others of loosely laid-out brick. Dave Schrom, a volunteer at Magic, said that since Palo Alto owns too much hardscape, green driveways are a win on all levels.
"I think most people remain oblivious to [green driveways], but I don't think people are aware how important they are to the community," he said. "We treat rain when it hits the ground as waste product, but we can use it as a resource."
Besides residential driveways, permeable pavers are used in parking lots, trails, fire lines, truck storage yards, boat ramps and high-traffic pedestrian areas. Although it's not mandatory that smaller developments, such as residential properties, have a green driveway, the Palo Alto's storm-water pollution-prevention ordinance requires commercial developments that create more than 10,000 square feet of impervious surface to somehow treat storm water on site.
Although more common in large developments, Morris noted that green driveways might catch on in the residential sector as well. "It seems more and more architects are proposing these types of things," he said. "I think it's the wave of the future." Ý
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