Green Infrastructure Toolkit

Green Infrastructure Strategies and Techniques

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.^{See footnote 1}  Vegetation on green roofs shades the roof and cools the air through evapotranspiration.^{See footnote 2}  In this way, vegetation can cause a green roof to be 100 °F cooler than a traditional black roof,^{See footnote 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.^{See footnote 4}  In addition to managing stormwater, green roofs help decrease energy use, improve air quality, and reduce heat.^{See footnote 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.^{See footnote 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.^{See footnote 7}  Permeable pavements also have cooling properties due to evaporation and reduced heat storage.^{See footnote 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.^{See footnote 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.^{See footnote 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.^{See footnote 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.^{See footnote 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.^{See footnote 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.^{See footnote 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.^{See footnote 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.^{See footnote 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.