In its ongoing quest to help rain water seep back into the ground -- and not gush into the sewer system, overwhelming it -- the city Water Department has enlisted a number of tactics, from special street tree tubs that retain water to a new fee structure that rewards businesses with "greener" landscaping practices.

Earlier today, officials unveiled another: They dedicated the 800 block of Percy Street as the city's first porous street.

Most streets are impervious asphalt, solid sheets of blacktop that allow no water to penetrate. This block of Percy is "permeable." Although it's said to be structurally strong as other paving materials, this asphalt has a network of spaces that allow rain water to pass through. There's also a layer of stone underneath, which acts like a temporary reservoir for the water, holding it until it gradually soaks in.

The city's new stormwater plan -- Green City, Clean Waters -- still to be approved by state and federal officials, envisions many such streets throughout the city. The plan is to wait until a street needs repaving, then replace the impervious asphalt with the new asphalt.

"While one porous asphalt street will only have a small impact today, our vision for the future included hundreds of miles of porously paved streets, which will significantly reduce stormwater overflow on our sewer system here in Philadelphia," said Philadelphia Water Department Commissioner Howard Neukrug.

Streets Commissioner Clarena I.W. Tolson offered another benefit: "The unique properties of porous asphalt may result in quicker melting of snow and ice from the street. This benefit could mean that less road salt will be needed to keep the street clear and safe in the winter months. Less road salt means less cost and less pollution of our rivers."