The simulation took between 10 to 60 min, depending of the amount of runoff that was generated.

UNESCO: AQUATACAMA

As populations swelled, however, and more green spaces turned into concrete, the amount of runoff pouring into rivers dramatically increased.

CNN: Polluted rivers stain Portland's image

The system already copes with a tremendous amount of groundwater and runoff, especially at stations at the tip of Manhattan, where the water table has risen in recent years.

WSJ: Salt Water Puts City's Subway System 'in Jeopardy'