Philly's First Porous Street Curbs Pollution

The new porous Percy Street in South Philadelphia was built to eliminate problematic eliminating storm water runoff. Photo: Mary Mazzoni, Earth911

The city of Philadelphia has engineered a new way to mitigate standing water in the streets, stop flooding in residents’ basements and combat storm water woes – replace standard asphalt streets with porous, sponge-like substitute.

Last week, the city’s mayor, Michael Nutter, Deputy Mayor Rina Cutler and City Councilman Frank DiCicco unveiled the city’s first porous green street – on Percy Street in the South Philadelphia neighborhood.

Porous materials, including porous asphalt, are designed to allow water to soak through an otherwise impenetrable surface, eliminating storm water runoff.

Porous asphalt is as structurally strong as conventional asphalt and looks almost identical, but it also includes voids or spaces that allow water to pass through the material. A layer of stone underneath provides temporary storage for water as it slowly soaks into the ground, which prevents polluted runoff from passing into nearby storm drains and into the sewer system.

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Residents of Percy Street complained to the city about crumbling curbs and sidewalks, small sinkholes and water in their basements – sometimes sewage, said Joel Palmer, vice president of Bella Vista Town Watch – a community group involved with the installation of the green street.

“The city stepped up,” Palmer said. “They not only wanted to fix it but fix it forward-thinking way.”

Like most older cities, parts of Philadelphia have a combined sewer system that includes a storm-water system and a sanitary system – water from showers, sinks and toilets.

During heavy rain, water treatment facilities can’t accommodate all the water, and water from both the sanitary and storm-water systems are diverted together into local rivers – called combined-sewer overflows (CSOs). The EPA recently mandated that the city correct the problem.

In response, the Philadelphia Water Department created the Green City, Clean Waters initiative– which will enact a variety of programs to decrease the city’s CSOs.

The porous green street is one of several pilot programs, and engineers and water department representatives think it looks promising.

“The storm water that would normally flow off of the sidewalks and street down the gutter and into our inlets will be diverted and infiltrated into the soils, completely removing the first inch of rainfall from our sewer system in each rain event and sufficiently detaining the next inch to allow the treatment plants and sewers to handle it,” said Pete Reilly, the primary design engineer for the porous asphalt used on Percy Street.

The project – in which the entire street was ripped up and all underlying infrastructure, including sewer systems and water mains, was replaced – took about five months.

The water department has installed porous surfaces on basketball courts, playgrounds and parking areas, but this is the first installation on a public street. Reilly said it will be the first of many, though no specific location has been identified for the next green street.

“Since Percy Street was our first roadway installation of porous asphalt, we do want to monitor its performance before continuing with installing similar systems,” Reilly said. “We would also like to pilot the use of porous concrete gutters to collect rainwater in a similar manner, and continue to expand our knowledge of porous asphalt by performing some off-street applications in the near future.”

Other cities, including Portland, have installed porous surfaces on residential streets, but Philadelphia hopes to be the first large city to install an abundance of them, Reilly said.

The residents of Percy Street couldn’t be happier with the result.

“The residents got a very nice street out of it,” Palmer said. “It went from a street in very terrible condition to being once of the nicest streets in the neighborhood and the city.”

Engineers and the water department will continue to monitor the performance of the green street while they plan future installations.

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