What is Green Infrastructure?
While conventional stormwater management directs water away from the site in pipes, the goal of green infrastructure is to capture stormwater where it falls and allow it to percolate into the ground. Green infrastructure mimics natural systems such as forests and meadows, using vegetation and soil as sponges to soak up stormwater. The parking lot and pavilion complex at Welkinweir show several examples of green infrastructure.
How does Green Infrastructure help?
Conventional pipes and impervious surfaces are ineffective at allowing precipitation to infiltrate into the ground and restore groundwater. Groundwater is the source of our drinking water for wells and provides the base flow for our springs, streams, and rivers. Because the goal of green infrastructure is to infiltrate stormwater, less runoff is generated, mitigating flooding and erosion. The plants, paving, and soil used in green infrastructure not only capture but filter pollutants out of stormwater. The precipitation that enters the groundwater is therefore cleaner than that which comes from conventional stormwater infrastructure.
Types of Green Infrastructure
Planted
These techniques use plants and soil exclusively to infiltrate and evapotranspirate stormwater. Plants – especially shrubs and trees – have been proven to improve air quality as well, by sequestering carbon dioxide and filtering out airborne impurities. Native plants offer additional benefits such as increased hardiness and providing habitat for beneficial wildlife.
Naturalized Stormwater Detention Basins
Stormwater detention basins are man-made depressions that collect rain water from development after a storm. Most detention basins are covered with lawn. In a naturalized basin, the lawn on the basin slopes and bottom is replaced with a variety of meadow plants to emulate a wetland system. These plants have deeper roots that are more efficient at rainwater infiltration and pollution removal than turf grass. The addition of trees and flowering shrubs further increases rainwater infiltration. Naturalized basins do not require fertilizers or toxic pesticides, are drought tolerant once established, and need mowing only once a year to control weeds. The city of Chicago estimates that naturalized basins have the potential to prevent 40-90% of stormwater contaminants which enter them from polluting our waterways.
Vegetated Swales
Vegetated Swales are broad, shallow channels that are planted with a variety of woody plants, perennials and/or grasses. They are designed to direct stormwater away from adjacent impervious surfaces, reducing its velocity and allowing some pollutants to settle out in the process. On steep slopes, check dams made of stone or earth may be used to further slow runoff and enhance its infiltration.
Green Roofs
Otherwise known as living roofs, green roofs are roofs that are planted with vegetation. These systems are made of multiple layers to protect the building’s roof, from waterproofing materials, to layers that prevent plant roots from invading the building, to lightweight planting media that lightens the weight burden on the building. Green roofs range in thickness from three inches to three feet and up, but all capture precipitation that falls onto the building, insulate it, mitigate the urban heat island effect by reflecting solar radiation, and even extend the lifetime of the building’s roof.
Rain Gardens
Rain gardens are gardens that collect runoff from the surrounding area. They are usually designed as depressions in the ground, and are filled with highly permeable soils with a bottom layer of stone to allow as much water to enter the ground as possible. The plants in these gardens are specially selected to withstand both drought and flooding. Rain gardens can be designed to fit any design scheme, from formal to natural, and are easily incorporated into a site’s landscape.
Not Planted
While they might not be “green” in color, these stormwater management Best Management Practices (BMP’s) are very effective at capturing and infiltrating stormwater runoff. They are designed to blend in with the built environment, while preventing the stormwater management problems that are caused by impervious surfaces like concrete and asphalt.
Infiltration Trench
Among the most unobtrusive BMPs, infiltration trenches can be easily hidden under gravel walkways, gardens, and lawns. They can be as simple as a stone-filled trench, leveled and lined with landscaping fabric and covered with soil or gravel; sections of perforated pipe can be added to carry excess stormwater away from the site.
Porous Pavement
Also known as permeable or pervious pavement. This paving system allows water to sink through and into the infiltration zone beneath. Layers of uniformly-sized stone and gravel underneath the pavement allow water to percolate into the soil as groundwater recharge. This special paving system may take the form of porous asphalt, porous concrete, or permeable pavers.