Green Roofs and Green Infrastructure REDUCE the volume of stormwater runoff. The key is that they not only reduce the amount of stormwater flowing through the sewer system and into the water treatment plant and/or the nearby river, ocean or bay, but they delay the release of stormwater until after the peak of the storm. This is critical in older urban areas that have Combined Sewer Systems (SWS). During peak storm events, stormwater flows into the CSC, often flooding the pipes and the treatment plant resulting in raw sewage being dumped directly into nearby waters in violation of the Clean Water Act.
Extensive green roofs can retain 50-100% of the rain, and prevent the rain from overflowing the city sewer system. A study of a New York City townhouse found that during a storm delivering 1 inch of rainfall, the green roof retained 100% of the water that hit the roof. A City of Portland study found that green roofs stemmed stormwater peak flow by 96%.
Not only to green roofs and green instructure projects – like bioswales and vegetated retention basins – reduce, retain and detain stormwater that enters into the sewer system, they also clean the water that does flow off site. Green roofs can FILTER 95% of cadium, copper and lead, and 30% of nitrogen and phosphorous contained in rain.