This is what a sewage system does when it is overwhelmed.Photo: Christopher ZurcherCross-posted from the NRDC's Smarter Cities. Every year nearly 30 billion gallons of wastewater filled with untreated sewage and pollution overflows into New York City's waterways. Last week New York Mayor Michael Bloomberg and the Department of Environmental Protection unveiled a proposal for a new "NYC Green Infrastructure Plan" to help reduce the city's serious sewage overflow problem. The proposal has two key components: widespread use of green infrastructure in public spaces -- such as green roofs, parks, tree boxes, roadside plantings, porous pavement in parking lots, cisterns, and rain …