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PROTECTION ON HIGH
New options for storm water and improving water quality are offered by increased installation of green roofs throughout the nation.
STORM WATER runoff has been identified by the U.S. EPA as one of the major sources of water quality impairment throughout the United States. Recent rules developed by EPA's National Pollutant Discharge Elimination System (NPDES) emphasize use of best management practices (BMPs) to improve runoff quality. Green roofs are a new option for storm water BMPs - offering the potential to convert thousands of square feet of idle space on rooftops to retention areas.
Green roofs provide peak flow reduction of rooftop runoff and roof surface temperatures. Because a green roof absorbs most of the ultraviolet light striking it, the roof lasts much longer than a standard roof. It also acts as providing an insulator, lowering building heating and cooling costs, as well as an added living space for residents in congested spaces.
Green roofs provide hydrologie control of small, frequently-occurring storms and their effects on ecosystems downstream. They also offer combined sewer overflow mitigation, watershed pollutant load management, and compliance with the NPDES Phase II Final Rule.
Research on the role of green roofs as a storm water BMP has been limited. The City of Portland, Oregon began its EcoRoof program in 1996, and collected data during a 15-month period during 2002 and 2003. The monitoring data found that the water retention of an extensive green roof with a 4- to 5-inch media depth was 69 percent of total rainfall. Peak flow reductions of 80 percent were observed. Research conducted at Michigan State University has shown that 66 percent of the precipitation was retained by an extensive green roof studied over an average of 24 rainfall events. (This data was included in a paper given by Bill Hunt (see below) and colleagues at a 2004 green roof conference.)
NORTH CAROLINA RESEARCH
Much green roof research on storm water retention and water quality in the United States is being conducted at North Carolina State University (NCSU). By discovering the extent to which green roofs can be used as nutrient reduction BMPs, the research engineers can determine what removal efficiency to assign to them. Since 2002, some NCSU professors and students...