When vegetated landscapes are replaced by impervious (non-permeable) surfaces such as
rooftops, driveways, and parking lots, precipitation is no longer intercepted by leaves, evapotranspired by plants, and/or infiltrated into the soil. Instead, a much higher percentage of precipitation becomes stormwater runoff- a "waste product" of conventional urban development.
One inch of rainfall over one acre of impervious parking lot will generate more than 27,000 gallons of runoff. When stormwater runoff is discharged directly to local waterbodies, it negatively impacts water quality and can erode and undermine natural stream channels. When it is discharged to urban combined sewer system, it consumes conveyance capacity resulting in overflows of a mix of untreated sewage and stormwater to regional waterbodies, creating potential public health risks and water quality impairment.
However, stormwater runoff is not an inevitable by-product of development. By giving special attention to the path and fate of water as it moves through and across developed areas, the adverse impact of those areas on watersheds can be lowered. Green Infrastructure, or GI, is the term used to describe infrastructure designed specifically to minimize the generation of stormwater runoff. This is accomplished by incorporation of a wide range of site design practices, including permeable pavements, green roofs, rain gardens, bioswales, and constructed wetlands, and by directly reducing project footprints.
GI is a means of simultaneously incorporating economic and environmental considerations into the land development process. In urban watersheds, GI technologies can be used to divert and delay stormwater from combined sewers to reduce the frequency and volume of overflows. In addition, stormwater diverted from sewers in the upper portions of a sewer system
can increase the capacity of downstream sewer conduits, potentially leading to reduced combined sewer overflows (CSOs) even in portions of the sewer service area distant from the actual installations. A recent eDesign Dynamics study in Brooklyn, New York suggests that significant reductions in the frequency of CSOs could be achieved cost effectively with GI (Montalto et al., 2007).
