Permeable Pavements Prevent Water Runoff
It happens every time it rains. Water runs down driveways and sidewalks, cascades across parking lots, courses through streets, all flowing toward the nearest gutter, retention pond or river. As the rain persists and reservoirs back up, homes, businesses and roads take up the slack. The result? Hydroplaning cars, flooded basements, and sometimes—as in the case of Europe last summer—staggering economic losses.
Most flooding, after rainstorms and snow melts, could be avoided—if the water was allowed to seep into the ground as nature intended. But the ever-sprawling pavement of our increasingly urban and auto-dependent society blocks that natural absorption. And runoff doesn’t just swamp our homes and streets. As rainwater sweeps across roads and driveways, it carries with it oil, fertilizers, soaps, pesticides, even bacteria-laden pet waste. It tears loose soil and silt, causing erosion. All these contaminants wind up in storm drains, where they usually travel untreated into the nearest river or lake.
Polluted runoff has serious consequences for the health of our waterways, wildlife, and ourselves. The Wisconsin Department of Natural Resources links it to habitat destruction, the death of fish and other aquatic creatures, diminished drinking water quality, and river and lake siltation. The U.S. Global Change Research Program blames coastal runoff for beach closings, coral reef die-offs and "dead zones," including the New Jersey-sized expanse choking out life in the Gulf of Mexico.
According to an estimate by Barbara Johnson of the Kansas State University Home Assessment System (Home*A *Syst), one inch of rain falling on an average home’s 1,500 square feet of pavement produces approximately 900 gallons of runoff. Fortunately, homeowners can take steps to reduce their property’s impact. Permeable pavements allow water to drain through a driveway, path or patio into the soil below. "Then, as the water moves down into the ground, many of the pollutants traveling in that runoff can be filtered out," explains Alyson McCann of the University of Rhode Island’s Home*A*Syst. And as the filtered runoff enters the groundwater supply, she adds, it increases the amount of drinking water available.
Letting Water Flow
Permeable pavements come in a wide variety of materials and prices, requiring different amounts of maintenance. Gravel, wood mulch and crushed seashells are cheap and easy to install, though McCann warns that they must be periodically raked back into place, and must be replaced altogether every few years. "But you also need to resurface a traditional driveway occasionally," she points out.
Reasonably hard surfaces can be created for a relatively low cost by putting down a bed of sand, adding a layer of pea gravel, and topping it with crushed limestone. Placing widely spaced bricks or concrete slabs, and filling in the gaps with sand or grass, is another simple solution. Paving blocks, such as Uni-Eco Stone, look like traditional cobblestones or bricks but have channels that funnel water in between each block into underlying sand and gravel.
For a truly green driveway, why not park on the grass? Several companies, such as Invisible Structures, make plastic grid systems that allow grass to grow through or to be planted on top. The grids (sometimes made of recycled materials) prevent erosion and bear the weight of vehicles so grass roots aren’t crushed. Lattice-shaped paving blocks made of stone or brick create a similar effect.
Porous asphalt and concrete look just like the real thing, but fine materials are left out of the manufacturing process, leaving spaces in the pavement through which water can pass. Porous asphalt also uses less tar (a petroleum product that can vaporize in the sun and pollute the air) than regular asphalt. Rebecca Winer of the Center for Watershed Protection (CWP) in Maryland warns, however, that these materials are quite costly and require regular maintenance to keep the surface free of clogs.
Permeable pavements cannot be used in all situations. CWP recommends against using them on soils with high clay content that would limit water absorption, or on sites that are close to the groundwater supply, which could be polluted if too little soil lies between the pavement and the water table. Some looser pavement materials would also limit wheelchair accessibility.
Installing a permeable driveway, walkway or patio may be more expensive than traditional materials and may be more work. But as McCann puts it, "When you consider the cumulative environmental total of all homes, one home does make a difference."
PHOEBE HALL is an E intern.