GREEN INFRASTRUCTURE RESOURCE GUIDE | 5 GREEN STORMWATER INFRASTRUCTURE GREEN ROOFSuse soil and plants in place of traditional roof material to enable rainfall infiltration and the evapotranspiration of stored water. Green roofs can be cost-effective in dense urban areas where land values are high. There are two common types of green roofs: extensive and intensive. Extensive green roofs typically have a growing medium of 3 to 4 inches, are commonly planted with sedum (a hardy groundcover), require less irrigation, and have low maintenance requirements. Intensive green roofs have up to 12 inches of growing medium and can support shrubs and trees. The ability to maintain larger plant material also introduces a need for constant irrigation and a more regular maintenance schedule.7 Green roofs are easily accommodated during building design. Retrofitting an existing roof requires structural evaluation and calculations. PERMEABLE PAVEMENTS infiltrate, treat, and/or store rainwater where it falls. Permeable pavements provide an alternative to conventional pavement systems and can be made of pervious concrete, porous asphalt, or permeable interlocking pavers. Permeable pavement can be used in a variety of settings and is good for walking, biking, parking areas, and low-trafficked roads without heavy truck use. Permeable pavements typically include an underdrain system that prevents the pavement from flooding when rainfall exceeds infiltration. This practice can be particularly cost-effective where land values are high or flooding or icing is a problem. Permeable pavements must be swept and vacuumed regularly to prevent the pores from clogging, which prevents infiltration. RAINWATER HARVESTING involves collecting or storing rainwater for later use, such as for irrigating lawns or gardens.8 A typical rain barrel is sized to hold between 40-75 gallons of rainwater stored above grade; cisterns can hold up to 10,000 gallons or more of rainwater and can be stored above or below grade. Both systems can help reduce a building’s overall potable water usage. To be most effective, their water should be used up between rain events to maximize water capture with each storm. STREAM RESTORATION involves restoring or “daylighting” streams and channels by removing artificial barriers (such as pipes, channelization, or steep grades) and returning them to natural conditions, creating wetlands, providing appropriate vegetation along banks, and restoring natural habitat of streams and non-tidal wetlands.9 Roger Foley Low Impact Development Center, Inc. Low Impact Development Center, Inc. Arlington County, VA
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