Green infrastructure techniques for managing stormwater come in a variety of forms, and several techniques can often be combined in one project. All provide stormwater management and water quality benefits, but each provides a different variety of co-benefits (social or public health, for example) and different approaches are more appropriate based upon site-specific conditions. The following describes the range of green infrastructure interventions, how each works, the benefits each brings, and the type of sites where the technique can be deployed.
Green roofs: Traditional roofs absorb sunlight and radiate heat into the surrounding air.1 Vegetation on green roofs shades the roof and cools the air through evapotranspiration.2 In this way, vegetation can cause a green roof to be 100 °F cooler than a traditional black roof,3 and these cooler roofs transfer less heat to the ambient air. Green roofs do not have as great a cooling effect on air temperatures as ground-level vegetation does, but they have the advantage of not taking up additional land and of keeping building occupants cooler.4 In addition to managing stormwater, green roofs help decrease energy use, improve air quality, and reduce heat.5 Green roofs, however, are not without challenges: They require greater structural support than cool roofs and are expensive to install.
Permeable pavements: Permeable pavements have spaces for air and water to pass through; the spaces allow water to infiltrate into the ground, reducing runoff. Asphalt and concrete can both be made porous by omitting the smaller aggregates that are usual components.6 More specialized forms of porous pavements include interlocking concrete pavers, in which water drains through the gaps between precast blocks, and grass or gravel pavers, in which fill materials are laid on top of a plastic grid.7 Permeable pavements also have cooling properties due to evaporation and reduced heat storage.8 Permeable pavements are appropriate for sidewalks, parking lots, alleys, and streets; some concerns about whether permeable pavements are appropriate for cold climates or high-traffic areas are being monitored and evaluated now in cities like Chicago and Washington, DC, with positive results to date.9
Bioretention and Bioswales: Bioswales are a type of stormwater retention that use an open-channel shape and vegetation to slow runoff and filter pollutants, reducing strain on stormwater infrastructure and improving water quality.10 Often integrated into streetscapes or used to convey stormwater away from critical infrastructure, bioswales can also reduce the need for gray stormwater systems to be installed by capturing and storing some of the stormwater.11 Bioswales can also reduce temperatures, increase habitat for urban wildlife, and improve air quality. As an added benefit, they are often aesthetically pleasing and potentially increase property values.
Green Streets, Alleys, and Parking Lots: Green streets, alleys, and parking lots can combine all of the above strategies (except perhaps green roofs) into a coherent package. By combining the strategies, green streets can provide multiple benefits, including runoff and pollutant reduction, air quality improvement, and urban heat island mitigation.12 Local governments primarily install green streets in the public right-of-way, but green alleys and parking lots can be installed on both public and private land. For all three, a critical element can be to minimize pavement in the first place.
Rain Gardens: Rain gardens are small gardens that are designed to survive extremes in precipitation, and help retain or reduce stormwater runoff through infiltration or storage.13 The gardens are often small and placed strategically in areas where stormwater currently overwhelms drainage capacity. They can be incorporated as part of general landscape design or as part of a larger streetscape (see Green Streets, Alleys, and Parking Lots, just below). In addition to managing stormwater and reducing nutrient pollution, rain gardens can also reduce temperatures, provide wildlife habitat, and improve aesthetics.14 Rain gardens can be installed in many different areas and do not need to take up much space.
Urban Forestry: Urban forestry is suitable for both public and private properties, including rights-of-way and near existing buildings and homes for shade. Urban trees provide air quality and heat reduction benefits, along with mental health and other social benefits.15 Urban forestry policies can include not only increasing existing canopy (many local governments are setting percentage targets) and planting new trees, but also ordinances to preserve existing mature trees, which provide greater benefits for stormwater and public health than young trees.16 Ongoing maintenance and care can be a concern for urban forestry, as well as balancing canopy goals with power utility concerns, particularly during extreme weather events.
1. Katie Moisse, Over the Top: Data Show “Green” Roofs Could Cool Urban Heat Islands and Boost Water Conservation (Feb. 2, 2010), available at http://www.scientificamerican.com/article.cfm?id=green-roof-climate-change-mitigation.
2. Evapotranspiration is a process by which plants absorb and emit water and use heat from the air to evaporate it, cooling the surrounding air. U.S. EPA, Reducing Urban Heat Islands: Compendium of Strategies, Green Roofs at 3 (2008), available at http://www.epa. gov/heatisld/resources/pdf/GreenRoofsCompendium.pdf.
5. U.S. EPA, Heat Island Effect: Green Roofs, http://www.epa.gov/heatislands/mitigation/greenroofs.htm.
6. Houston Advanced Research Ctr. (HARC), Cool Houston! A Plan for Cooling the Region at 9 (2004), available at http://files.harc.edu
7. Id.; Cambridge Systematics, Inc., Cool Pavement Report 9 (2005), available at http://www.epa.gov/heatisld/resources/pdf/ CoolPavementReport_Former%20Guide_complete.pdf.
8. Cool Communities, Cool Pavements, http://www.coolcommunities.org/cool_pavements.htm.
9. See, e.g., Chesapeake Bay Stormwater Training Partnership webinar, Monitoring Results and Lessons Learned from the BUBBA Award Winning Restoration Projects, available at http://chesapeakestormwater.net/wp-content/uploads/dlm_uploads/2015/12/Monitoring-Results-and-Lessons-Learned_v3.pdf
10. University of Florida IFAS Extension, Bioswales/ Vegetated Swales, http://buildgreen.ufl.edu/Fact_sheet_bioswales_Vegetated_Swales.pdf.
12. Low Impact Development Center, Green Streets, available at http://www.lowimpactdevelopment.org/greenstreets/background.htm.
13. Low Impact Development Center, Rain Garden Design Templates, available at: http://www.lowimpactdevelopment.org/raingarden_design/whatisaraingarden.htm.
14. See, generally, Center for Neighborhood Technology, The Value of Green Infrastructure, http://www.cnt.org/sites/default/files/publications/CNT_Value-of-Green-Infrastructure.pdf
15. U.S. EPA, Reducing Urban Heat Islands: A compendium of strategies, Trees and Vegetation, https://www.epa.gov/sites/production/files/2014-06/documents/treesandvegcompendium.pdf. Kathleen Wolf, Urban Nature Benefits: Psycho-Social Dimensions of People and Plants, Center for Urban Horticulture, http://www.naturewithin.info/UF/PsychBens-FS1.pdf.
16. U.S. EPA, Reducing Urban Heat Islands: A compendium of strategies, Urban Forestry and Vegetation, https://www.epa.gov/sites/production/files/2014-06/documents/treesandvegcompendium.pdf.