Why do we need Green Infrastructure, and What exactly is it?
Years of urban development have created a vast network of interconnected impervious surfaces that have effectively decreased the natural hydrologic capacity of the landscape and diminished nature’s ability to manage stormwater and water quality. As a result, even a moderate rainstorm in most urban and suburban areas can produce significant quantities of stormwater that severely stress and often overwhelm traditional stormwater management infrastructure. In areas with combined sewer systems, this excess runoff often results in direct, untreated discharges of combined sewage and stormwater to our urban waterbodies. In areas with separate storm sewer systems, similar volumes of runoff pick up surface pollutants and scour tributary streams. In both types of systems, the end result inevitably is a severe impact on water quality and aquatic habitat in urban waterbodies. These impacts are compounded by the trends of continuing development, aging of existing gray infrastructure, and global climate change that brings more frequent and more intense rainstorms.
Many urban municipalities across the country are taking a close look at using green infrastructure practices or retrofit concepts to help alleviate many of these stormwater issues by managing stormwater and runoff close to its source. The term “green infrastructure” includes a range of natural and built systems that can occur at the regional, community, and site scales. At the larger regional or watershed scale, green infrastructure is the interconnected network of preserved or restored natural lands and waters that provide essential environmental functions, including stormwater control. These larger scale green infrastructure practices may include habitat corridors and water resource protection. At the community and neighborhood scale, green infrastructure incorporates planning and design approaches such as compact, mixed-use development; parking reduction strategies; and urban forestry that reduce impervious surfaces. At the site scale, green infrastructure mimics natural processes to help infiltrate, evapotranspire, and capture stormwater to maintain and restore natural hydrology. Site scale green infrastructure practices include rain gardens, green roofs (or ecoroofs), permeable pavement, cisterns or rain barrels, and other techniques that represent decentralized alternatives to the traditional approach of capture, conveyance, and distant downstream discharge. Note: Some of the case studies refer to small-scale green infrastructure retrofits as low impact development (LID) controls. For the purposes of this report, these terms are synonymous.
Green infrastructure retrofit projects on small scales have been noted to be highly effective. In pilot studies and demonstration projects across the U.S., small-scale green infrastructure projects have repeatedly shown considerable potential to reduce runoff volumes, peak flow rates, and pollutant loads. To date, these small scale green infrastructure practices have yet to be implemented in sufficient numbers or densities for their aggregate benefits to be thoroughly documented. However, a number of municipalities have undertaken modeling efforts to help understand the stormwater management potential and limitations of green infrastructure retrofits.
* from Case Studies of Green Infrastructure Hydrologic and Hydraulic Modeling Efforts for CSO Mitigation Assessment,
prepared for the U.S. Environmental Protection Agency, September, 2009, by LimnoTech, Water/Environment/Scientists/Engineers