More reasons cities (NYC in particular, but Seattle too) should mandate green roof adoption

The flooding in NYC earlier this month emphasized the fact in my head that our cities are due for a Manhattan project type of effort to make them more efficient and sustainable.
While and extensive green roof and cistern system would always solve problems like this one, such a system could make these problems far more rare, and reduce the load on the overtaxed sewer and drainage systems.
Some Background from the NYT's city room blog