Today, the City of Philadelphia unveils its first street paved with porous asphalt. It may only be a tiny Bella Vista alley that rarely sees auto traffic, but it’s one of hundreds of such side streets, and it marks a milestone in part of a larger plan.
As one of America’s first cities, Philadelphia is home to one of the nation’s oldest sewers. In the time since the stormwater system was built, modern development has blanketed the region with impervious concrete and pavement. During a heavy rain or snow melt (50-60 times each year), the drainage system overflows, mixing with wastewater and dumping directly into the Delaware and Schuylkill Rivers (from which the WaterWorks draws to provide our taps).
In 2009, the city finalized a plan to address the runoff overload, pledging $1.6 billion over the next 20 years. Instead of ripping up the streets to build a huge (and expensive) below-ground holding tank, the EPA approved a strategy of implementing a series of “green” stormwater infrastructure programs.
Municipal structures are being retrofitted with green roofs (and businesses and residents are encouraged to follow suit), basketball courts are being repaved with permeable concrete, parking lots are being designed to include vegetated strips and old asphalt lots are being broken up and turned into rain gardens. The idea is to turn the city’s surface into a sponge, instead of an impervious barrier. Most of these water-management techniques have been used successfully before, but not on the scale of an entire Northeastern metropolis.
Porous asphalt is much like regular paving material, but made without the finer grains in the mix. This creates a surface with much larger gaps, through which water can seep. Beneath the blacktop is a layer of stone that acts as a reservoir, slowing down the seeping liquid even more. The street surface is still relatively smooth (smooth enough to meet ADA requirements), and because it has room to expand and contract, is much less likely to form potholes.
The Percy Street transformation cost the city $330,000, reported the Philadelphia Inquirer. The next metamorphosis is set for Webster Street between Broad and 13th, later this year