Coffee isn’t the only thing that needs to percolate. Water needs to perk through the soil to slow down runoff and trap certain pollutants. However, in urban areas, where there is twice as much pavement as buildings, about two-thirds of rainfall hits an impervious surface, where it runs directly into streams, rivers and lakes — unfiltered, or without percolation — carrying with it pollutants like oil, salts and metals that contaminate our water.
We can’t do much about the weather, but there are options for patios, driveways, sidewalks, parking lots and other hard surfaces in the use of porous, or permeable materials.
The move to permeable surfaces is driven by two things, said Drew Bender, regional manager of J.F. New, a natural-resources and ecological consulting firm headquartered in Walkerton, Ind. The first is regulations to require the control of flooding and to improve water quality. Second, there is a cost benefit to developers as space becomes a premium.
"The opportunity to create green space and attractive features in sustainable development is catching on from a marketability standpoint," he said. Because porous concrete or asphalt is a relatively new process, it may seem costly. But that should improve as contractors become more experienced in the process.
Engineers and scientists have made improvements in the design of storm-water filtration systems and have been successful with pervious surfaces. In Chesterton, Ind., the Coffee Creek Center is an example. The effort combines native plants, whose roots filter pollutants, and underground pipes that put wastewater to work, said Steve Barker, director of the Coffee Creek Watershed Conservancy. A series of pipes, called level spreaders, move water captured in an underground pond downhill to nourish the plants in an expansive prairie.
Permeable paving is most appropriate for sidewalks, patios, driveways or alleys. It is not for high-traffic or high-speed areas because it has lower load-bearing capacity than conventional pavement.
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Pervious materials have more voids, or air spaces, which allow water to pass through. Traditional concrete has from 2- to 6-percent void structure, while pervious concrete contains around 25 percent
The method that is used for pervious concrete application is based partially on a soil’s percolation rate. The soil percolation rate is the amount of time water takes to move through soil, measured in minutes per inch. Finer-textured clay soils have slower percolation rates than sandy soils.
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