Gray infrastructure is the system of pipes and ditches that channel storm water. Green infrastructure is the harnessing of the natural processes of trees and other vegetation — so-called ecosystem services — to carry out the functions of the built systems. Green infrastructure often intercepts the water before it can run into streets and become polluted and stores the water for gradual release through percolation or evapotranspiration. Trees also clean dirty water through natural filtering functions.
Advocates say green infrastructure isn’t just about being green — it makes financial sense, as well. Its cost-effectiveness depends on how benefits are assigned and valued, and over how long a time scale, but green has been shown to be cheaper than gray.
A 2012 study by American Rivers, ECONorthwest, and other groups examined 479 projects around the country. About a quarter of the projects were more expensive, they concluded, and 31 percent cost the same; more than 44 percent brought the costs down, in some cases substantially. New York City, for example, expects to save $1.5 billion over the next 20 years by using green infrastructure.
The concept of green infrastructure is hardly new. New York City has long preserved watersheds in the Catskills Mountains and Hudson Valley for the city’s drinking water supplies. Since the 1990s, in order to comply with federal regulations, New York City has committed $1.5 billion dollars for protection of the forests that blanket much of the Catskills rather than build and operate a water filtration and purification plant costing $10 billion.