When it rains in the City of Brotherly Love, problems soon follow because more than half the city has "combined" sewers - pipes that carry both storm water and sewage. When it rains, the system fills quickly. The surplus, which includes raw sewage and road oil, backs up into basements and gushes untreated into rivers through 164 overflow pipes.

Instead of going the route of many other cities and building miles-long, multibillion-dollar tunnels to hold storm-water overflows--and then pumping it back into the system when the rain stops--Philadelphia's 20-year stormwater management plan is based on "green infrastructure" and offers benefits that can be appreciated above the ground.

Philadelphia's plan envisions transforming the city into an oasis of rain gardens, green roofs, treescapes, and porous pavements, which advocates say is cheaper than tunnels and makes for a more liveable, prettier city with higher property values and better community health.