Green Roofs and Storm Water Runoff

Green roofs contribute to urban sustainability planning by reducing heating and cooling costs for buildings but they also help to reduce the amount of untreated water that flows into local waterways. They reduce runoff by approximately 90 percent.  In addition to a host of other financial benefits green roofs offers a storm water mitigation benefit of $4.26 per square foot.”

As reviewed in a 2011 Triple Pundit article, a study on green roof conducted by ConEdison in partnership with the Columbia University Center for Climate Systems Research, reveals that green roofs help cities find cost effective ways to deal with excess storm water.

Storm water runoff from urban areas is is significant source of pollution in nearby waterways. Although some of this water is treated by municipalities, during periods of high precipitation or snow melt, the excess flow often goes directly into nearby waterways.

One study indicated that if 1 billion square feet of New York City’s roofs were greened, the city could reduce its annual storm water flow by one third or 10 billion gallons.

Green roofs trap excess storm water and the Con Edison green roof study reveals that this is a cost effective way of storm water management. The researchers determined that the roof was trapping about 22 times more storm water on an annual basis than was previously thought possible, making it the most cost-effective solution available.

The researchers point out that green roofs have a distinct advantage over holding tanks and other infrastructure. Holding tanks simply capture excess flow, which is eventually sent to a treatment plant. In contrast, the excess flow captured by green roofs eventually evaporates or is absorbed by vegetation avoiding the sewer system all together. This not only provides an environmental benefit to local waterways, it helps to reduce energy consumption and other costs related to treatment systems.

© 2012, Richard Matthews. All rights reserved.

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