When rain falls on our roofs, streets, and parking lots in cities and their suburbs, the water cannot soak into the ground as it should because of the type of infrastructure we now have.
From roads to sidewalks and driveways to parking lots – these areas are paved with asphalt or non-porous concrete. While single-purpose gray stormwater infrastructure—conventional piped drainage and water treatment systems—is designed to move urban storm water away from the built environment, green infrastructure reduces stormwater at its source while delivering environmental, social, and economic benefits.
Margaret Mayfield, an architect in Los Osos, California says the use of permeable paving “is much better for the environment because it helps cut down on storm runoff. Along with green roofs and landscaping, it’s one more tool in the tool chest and it can also be more beautiful than traditional asphalt.”
To aid in helping the environment and cutting down on storm run off, some interesting technologies have come into play, some low tech while other are considered high tech.
Porous options available today
Permeable or porous paving options come in a variety of styles, some high-tech and some considered old-school. For patios, walkways, and driveways, common permeable options include grass with 18-inch wide tire strips. In dry climates where the weather is mild, this method of creating a driveway has worked perfectly well for years.
Obviously, this works much better than a solid slab of concrete or asphalt. Using loose stone or gravel is another method that allows rainfall to soak into the ground rather than letting it run off and has been used for centuries.
Another option is the use of a concrete or plastic grid system. Basically, this is a system that uses concrete or plastic blocks, forming a solid surface, allowing water to flow freely through the spaces in the grids. The grids can be filled with sand, gravel, soil or turf, and are long-lasting and easy to install.
New types of concrete or asphalt
Unlike conventional pavements, porous asphalt pavements are typically built over an uncompacted subgrade to maximize infiltration through the soil. Basically, there are two common uses for porous open-graded asphalt pavements, according to the Environmental Protection Agency (EPA).
An open-graded friction course (OGFC) is a thin, open-graded asphalt mix placed atop a dense-graded pavement. Rainwater drains into the OGFC and then out the side of the pavement.
Porous asphalt pavements use open-graded mixes placed atop a stone reservoir. Rainwater flows down through the open-graded mix layers into the stone reservoir where it can then infiltrate into the subgrade. Using this type of asphalt allows for the paving of streets, parking lots and other driving surfaces.
Instead of using regular concrete, many places are switching to pervious concrete or porous pavement, a material that offers the inherent durability and low life-cycle costs of a typical concrete pavement while retaining stormwater runoff and replenishing local watershed systems.
Like conventional concrete, its made from a mixture of cement, coarse aggregates, and water. However, it contains little or no sand, which results in a porous open-cell structure that water passes through readily. It sort of a Rice Crispy Treat.
When this type of concrete is used, it can take in storm water at a rapid rate of 3 to 5 gallons per minute per square foot of surface area, which is the stormwater flow rate needed to prevent runoff in most rain events.
In addition to stormwater control, pervious concrete pavements aid in reducing the urban heat-island effect. Because they are light in color and have an open-cell structure, pervious concrete pavements don’t absorb and store heat and then radiate it back into the environment like a typical asphalt surface.
The nice thing about permeable pavements is they are not always pricier than conventional paving. Many communities across the country offer incentives like rebates or reduced stormwater utility fees to those who opt for permeable paving. EPA spokeswoman Enesta Jones also says that the newer permeable pavements are already being used successfully across the U.S. and Canada.
