Stormwater management practices that protect, restore, or mimic the natural water cycle are referred to as green infrastructure (GI). It is a collection of engineered systems that utilize natural or “green” approaches to manage stormwater locally. Stormwater is stored temporarily at or near where it falls to be used by trees and vegetation, stored and used later for irrigation, or allowed to soak into the ground through layers of soil, which remove pollutants from the stormwater through natural processes.
Traditionally, municipalities have managed their stormwater utilizing “gray” infrastructure practices made up of gutters, basins, and pipes that transport stormwater quickly to local streams, rivers, and lakes. Within the last decade, the value of using gray infrastructure and green infrastructure together for local stormwater management needs has been recognized.
Advantages of GI
Green infrastructure has many benefits:
- Creates a more pleasant environment by creating “green” features (collections of trees, bushes, and plants) distributed throughout developed areas
- Reduces the amount of dark surface available to collect and hold solar heat as well as creates new opportunities for trees and bushes to create shade; both of these reduce the “heat island effect” in cities and developed areas
- Reduces the overall stormwater volume that is conveyed to local streams and rivers which reduces the overall risk of flooding and erosion
- Provides treatment by filtering or removing stormwater pollutants such as heavy metals, nutrients, sediment, and pathogens from runoff, which helps protect local streams, and rivers
- Temporarily stores stormwater locally to be used by trees and vegetation, reducing the amount of potable water that is needed for watering and irrigation
- Creates pervious surfaces that absorb rain and runoff, allowing water to penetrate into the soil and replenish groundwater aquifers
- Increases community green space which encourages more outdoor recreation
- Contributes to urban renewal
- Creates new long term green jobs to perform construction, inspection and maintenance of the GI
Types of GI
Below are brief descriptions of the types of GI practices covered in the NGICP.
- Permeable pavements
- Rainwater harvesting
- Rooftop stormwater detention
- Dry wells
- Stormwater wetlands
Bioretention facilities are shallow basins that capture stormwater runoff and use trees and plants to help treat the captured flow. Filtration, infiltration, and evapotranspiration are all used to capture, treat, and temporarily store and locally use stormwater. Variations of bioretention include rain gardens, curb cuts/curb extension, stormwater planters/tree boxes, tree trenches, and bioswales/vegetated swales.
Permeable pavements consist of a permeable, structural surface course with very small “holes” or voids in it, which is installed over an engineered, uncompacted storage bed. Stormwater passes through the surface course and then is temporarily stored to allow infiltration into the soil beneath the storage bed. The permeable surface course can be made of porous asphalt, pervious concrete, permeable interlocking concrete pavers, or reinforced turf/gravel. Permeable pavements are available now for parking lots, parking lanes, low-speed residential streets, alleys, walking trails, sidewalks, playgrounds, etc.
Rainwater harvesting involves capturing stormwater from rooftops (or other nearby impervious surfaces) and storing the water for nonpotable uses such as irrigation, toilet flushing, clothes laundering, car washing, and firefighting. Variations include rain barrels (typically can hold 55 or 90 gallons of water) and cisterns (can hold hundreds or thousands of gallons of water).
Rooftop stormwater detention stores stormwater on roof surfaces, either releasing the water gradually to the stormwater or combined sewer system or allowing the plants growing on the roof to use the water. Variations include blue roofs and green roofs.
A blue roof is a nonvegetated system of rooftop storage structures installed over a waterproof roof membrane that temporarily detains water on the roof surface and slows the rate at which it reaches building downspouts.
A green roof is a roof of a building that is partially or completely covered with growing media and vegetation on top of a waterproof roof membrane. Rainwater falling on the rooftop is retained in the media and then used by the plants.
A dry well is an underground storage facility used to capture and temporarily hold runoff from downspouts or small impervious areas until the water can infiltrate into the soil. Dry wells are typically used by homeowners or other small-scale applications.
Stormwater wetlands generally are large, shallow, vegetated basins or regions designed to capture and treat stormwater runoff from nearby drainage areas, such as a parking lot or roadway. As stormwater runoff flows through the wetland, pollutants are removed when velocity is slowed and particles settle out of suspension. Nutrients are absorbed through plant roots.