Human activities can negatively impact hydrologic and chemical cycles, pollute air and water, degrade soil, and reduce biodiversity. Failure to maintain basic ecosystem functions places humans at risk because of our dependence on these functions. As human populations and resource consumption increase, it becomes even more important to preserve basic ecosystem functions. Sustainability is the principle and practice of creating and maintaining the conditions under which humans and nature can exist in productive harmony to support present and future generations. Green infrastructure is one tool or approach to creating sustainable urban environments.
Numerous studies attempt to place a dollar value on ecosystem services, though the value of these services is infinite in the sense that human survival depends on them. Costanza et al. (2014) place an annual value of 125 to 145 trillion dollars on ecosystem services, which is more than double the global GDP. The authors also estimate an annual loss of 4.3 to 20.2 trillion dollars, underscoring the negative impact humans are having on basic ecosystem functions necessary for our well-being.
A central tenet of sustainability is that we must preserve ecosystem services necessary for humans to survive and prosper. In 1987, a World Commission on Environment and Development report (UN, 1987) defined sustainable development as "development that meets the needs of the present without compromising the ability of future generations to meet their own needs".
A challenge is incorporating basic sustainability concepts into social and economic systems. These systems historically have not placed proper value on ecosystem services, resulting in the degradation of these services. For example, stormwater in urban areas was traditionally viewed as something that negatively impacted humans, primarily through flooding. Traditional stormwater systems were designed to discharge stormwater to the nearest receiving water as quickly as possibly. The result was a dramatic change in urban hydrology, increased flooding downstream, reduced water quality, and loss of habitat. Sustainability stresses using stormwater as a resource. Using stormwater as a resource restores some natural resource function and provides economic and social benefit. Sustainable practices achieve a balance between environmental, social, and economic factors.
Green infrastructure is an approach to managing urban wet weather impacts that mimics, restores, or maintains natural hydrology. Green infrastructure includes a wide array of practices, including infiltrating, evapotranspiring, or harvesting and using stormwater. On a regional scale, green infrastructure is the preservation or restoration of natural landscape features, such as forests, floodplains and wetlands. On the local scale, green infrastructure consists of site and neighborhood-specific practices, such as bioretention, trees, green roofs, permeable pavements and cisterns. Regional and local practices are coupled with policies such as infill and redevelopment that reduce overall imperviousness in a watershed.
Green infrastructure is an important component of sustainable urban communities. Green infrastructure helps maintain ecosystem services in the following ways.
The following section provides detailed information on each of these. For more information on stormwater green infrastructure practices, link here.
It is beyond the scope of this page to describe in detail the relationship between green infrastructure, sustainability, ecosystem services, and stormwater. Some suggested references on this topic are provided below.
An increasing focus on Green Infrastructure has brought an awareness that stormwater management can provide numerous benefits beyond improving water quality and urban hydrology. Trees, for example, provide a multitude of benefits beyond stormwater management, as discussed in this article. In particular, vegetated stormwater best management practices (BMPs), including tree-based systems and other bioretention systems, offer opportunities to achieve multiple benefits, ranging from aesthetics (see, for example, this presentation by Dr. Steven Rodie from the University of Nebraska - Omaha) to ecosystem friendly designs.
It is clear that pollinators, both vertebrates and invertebrates, are in decline (see , , , , ). Vegetated stormwater BMPs can be designed to be pollinator-friendly. The following sections provide numerous links to information that can be used in designing and implementing pollinator-friendly stormwater BMPs.
Although much of the information on these pages is general, many of the practices can be incorporated into vegetated stormwater BMPs.
Carbon sequestration is the capture and long-term storage of carbon from atmospheric carbon dioxide. Sequestration occurs in plants and soil. Carbon sequestration is often discussed in the context of climate mitigation, but there are numerous benefits to increased carbon (organic matter) content in soils, including improved soil structure, infiltration through soil, nutrient retention and cycling, and pollutant attenuation.
The primary mechanism of carbon sequestration is through enhanced vegetative growth. Nowak and Crane (2002) estimated that, in 2002, urban trees stored 700 million tonnes of carbon with a gross carbon sequestration rate of 22.8 million tonnes of carbon per year. They estimated the average urban forest carbon storage density was 25.1 tonnes of carbon per hectare, compared with 53.5 tonnes of carbon per hectare in forest stands. This illustrates that urban forests store considerable carbon, but also that there may be opportunities for further increases in carbon storage.
Management activities can promote carbon sequestration in urban soils. Examples include leaving grass clippings and growing perennial gardens (see , ). Practices that promote deep rooting of vegetation establish stable carbon forms that extend deeply into the soil.
Jo (2001) presents the following strategies for green infrastructure practices that promote carbon sequestration in urban settings.
Brown et al. (2011) present strategies for minimizing the carbon footprint of urban areas. An important stratgey is better utilizing organic wastes generated within the urban landscape, including land application of organic waste.
Numerous stormwater activities can promote carbon sequestration. These include use of bioretention, tree trench/tree box, green roofs, and constructed wetland practices to treat stormwater runoff, improving soils and turf, and disconnecting impervious surfaces. Note that some stormwater practices, such as bioretention and green roofs, may export carbon, particularly in the first years after establishment. In addition, construction and maintenance of stormwater control practices leads to carbon emissions that can offset carbon sequestration. Moore and Hunt (2013) state "Despite accounting for sequestration by vegetation in these systems, only stormwater wetlands and grassed swales were predicted to store more carbon than what was released through construction and maintenance".
Additional information on stormwater management and carbon sequestration can be found in the following articles and reports.
The National Wildlife Federation defines biodiversity as the variety of life. Biodiversity can be considered at many levels, ranging from individual species to communities, landscapes, and ecosystems.
Biodiversity is important to ecosystem health, which in turn allows ecosystems to deliver services that humans require. Some of the benefits of enhanced biodiversity include the following.
Biodiversity almost always decreases in human-impacted ecosystems, including urban landscapes. The primary reason for this is loss, fragmentation, or degradation of habitat. The loss of native vegetation and wildlife often promotes the spread of invasive species, which may further degrade or eliminate habitat. Aronson et al. (2014) studied bird and plant diversity in cities around the earth and observed the density of bird and plant species (the number of species per square kilometer) declined substantially: only 8% of native bird and 25% of native plant species were currently present compared with estimates of non-urban density of species. Additional information on impacts of urban landscapes on biodiversity can be found in these articles and reports.
Several strategies can minimize urban impacts on biodiversity loss, including the following.
Additional information on enhancing urban biodiversity can be found in the following articles and reports.
Stormwater practices can be designed to enhance bioversity while providing their basic function of treating stormwater. Examples of BMPs that directly impact biodiversity include bioretention, tree trenches and tree boxes, stormwater wetlands, green roofs, and disconnected impervious surfaces routed to pervious areas. Other BMPs can be designed to incorporate vegetative features that enhance biodiversity. For more information on the relationship between stormwater and green infrastructure, see Stormwater management, Green Infrastructure, and Low Impact Development.
Wikipedia defines climate resilience as the capacity for a socio-ecological system to: (1) absorb stresses and maintain function in the face of external stresses imposed upon it by climate change and (2) adapt, reorganize, and evolve into more desirable configurations that improve the sustainability of the system, leaving it better prepared for future climate change impacts. The difference between resilience and adaptation is subtle, but adaptation is often considered to be a response activity, often to a single issue, while resilience is a systems-based approach. Resilience and adaptation are responses to a changing climate, while mitigation involves attempts to reduce the magnitude or rate of climate change.
Effects of climate change on urban areas are well documented and beyond the scope of this article. For more information, see the following articles and reports.
Briefly, climate change effects in urban areas include the following.
The U.S. Environmental Protection Agency developed an excellent website, titled Green Infrastructure for Climate Resiliency, that describes green infrastructure practices that can help communities prepare for and manage these effects of climate change. These practices includes the following.
The role of green infrastructure in climate change mitigation in urban areas primarily consists of increasing storage of carbon. This is discussed in greater detail in the section on carbon sequestration.
One of the principal goals of green infrastructure is to mimic natural processes. This includes retaining rainwater as close to its source as possible and allowing it to infiltrate or using the water (e.g. as a gray water source). Retaining rainwater has several benefits summarized below and discussed in greater detail in an EPA report titled Benefits of Green Infrastructure.
Infiltration practices include bioinfiltration (rain gardens); tree trenches and tree boxes; infiltration trenches, basins, and underground infiltration; permeable pavement; and swales designed with check dams or bioretention bases. Rainwater/stormwater harvest systems used for irrigation, improved soil, and disconnecting impervious surfaces and routing runoff to pervious surfaces also utilize infiltration.
Stormwater practices that infiltrate runoff reduce pollutant loading to surface receiving water (lakes, streams, wetlands). Infiltration practices are among the most effective methods for maintaining or improving surface water quality. An EPA report titled Performance of Green Infrastructure provides a literature summary of green infrastructure effects on water quality.
Most infiltration practices provide significant pollutant removal as captured runoff infiltrates vertically. Exceptions include chloride, which is not attenuated; nitrate, unless denitrifying conditions occur within the vertical soil profile; and some organic pollutants. Soils with very high infiltration rates and low organic matter content may also be less effective at removing pollutants.
Green infrastructure practices that do not infiltrate stormwater or provide limited infiltration typically provide treatment of runoff. Most pollutants are effectively removed. However, chloride and nitrate are not effectively treated and phosphorus may be exported from some practices that utilize engineered media with a high organic matter content. Examples of BMPs that may export phosphorus include green roofs, swales, and bioretention practices.
Additional information can be found in this manual on the following pages.
Green infrastructure practices help mitigate air pollution in urban areas. The primary mechanism is sequestration of pollutants by vegetation. Trees, in particular, are effective at sequestering pollutants. Vegetation may also reduce energy demand by providing shade and adsorbing solar energy, which in turn results in reduced emission of pollutants. The cumulative air quality improvement obtained from green infrastructure therefore includes direct and indirect benefits in resource units or monetary value. Stormwater practices that promote vegetation include bioretention, tree trench/tree box, green roofs, vegetated swales, and improved turf.
Some articles providing information on pollutant and energy reductions associated with green infrastructure include the following, with primary conclusions from the studies shown in quotations..
There are additional benefits to green infrastructure beyond environmental benefits. These are described briefly below.
Sustainable communities are places that balance their economic assets, natural resources, and social priorities so that residents' diverse needs can be met now and in the future.
Communities want to protect their water quality while also getting the greatest possible benefit out of every investment they make. Many are conserving, restoring, or enhancing natural areas while incorporating green infrastructure practices, such as trees, rain gardens, green roofs and other practices. These green infrastructure practices mimic natural systems into developed areas to manage rainwater where it falls. Green infrastructure practices are an integral component of sustainable communities because they can help communities protect the environment and human health while providing other social and economic benefits, allowing communities to achieve more for their money. Using green infrastructure practices strategies to reduce stormwater runoff can strengthen efforts to preserve open space and natural areas and encourage development in existing communities. These practices help make neighborhood streets and greenways pleasant and safe for walking and biking and reinforce a sense of place. Integrating green infrastructure and sustainable communities encourages collaboration in development decision and promotes green building practices.
Sustainable communities that fully integrate green infrastructure approaches use community design to help simultaneously achieve environmental, economic, and social goals. These goals include improving water quality, revitalizing neighborhoods, reducing flood risk, and providing recreational areas that encourage physical activity. Community planners can enhance these and other benefits by selecting the types and locations of green infrastructure practices that best support their goals.
To learn more about sustainable communities and green infrastructure, including how to develop a Sustainable Communities and Green Infrastructure Plan, read the EPA's report on Enhancing Sustainable Communities with Green Infrastructure.
A living street is new type of street that is narrower and has less pavement than existing streets. Reducing the width of existing streets reduces construction costs and allows room for the installation of trees and rainwater gardens to treat stormwater. Where there is a need, bike trails and sidewalks are installed to provide for safe pedestrian and bike movement. Living streets are designed for cars, people and the environment. Rainwater gardens and street trees remove pollutants from stormwater before the water enters area lakes, helping to improve lake quality. Narrower streets and street trees also slow traffic, creating a safe environment for everyone. In 2012, the City of Maplewood partnered with the Ramsey Washington Metro Watershed District to install a living streets demonstration project. For more detailed information about the project, go to the Ramsey Washington Metro Watershed District web site.
A green street is a stormwater management approach that incorporates vegetation, soil and engineered systems such as pavement to slow, filter and cleanse stormwater runoff from impervious surfaces such as streets and sidewalks. Green streets are designed to capture rainwater at its source, where rain falls.
Green streets protect water quality in rivers and streams by removing up to 90 percent of pollutants. They replenish groundwater supplies, absorb carbon, improve air quality and neighborhood aethestics, and provide green connections between parks and open space. Vegetated curb extensions improve pedestrian and bicycle safety and calm traffic. For information on green streets can be found on the EPA's web site
One principle of green infrastructure involves reducing and treating stormwater close to its source. Green streets provide a source control for a main contributor of stormwater runoff and pollutant load. In addition, green infrastructure practices complement street facility upgrades, street aesthetic improvements, and urban tree canopy efforts that also make use of the right-of-way and allow it to achieve multiple goals and benefits.
Green streets can incorporate a wide variety of design elements including street trees, permeable pavements, bioretention and swales. Successful application of green techniques will encourage soil and vegetation contact and infiltration and retention of stormwater. Bioretention is a versatile green street strategy. Bioretention practices can be tree boxes taking runoff from the street, as well as planter boxes or curb extensions. Permeable Pavement systems have an aggregate base which provides structural support, runoff storage and pollutant removal through filtering and adsorption. Tree trenches and tree boxes reduce stormwater runoff, help to reduce the urban heat island effect, improves air quality and urban aesthetics.
Watch a video, produced by the EPA on the benefits of Green Streets
Healthy ecosystems provide important health benefits to humans, as discussed above (e.g. clean air and water, pollination, etc.). Several studies indicate additional health benefits to humanns, including increased physical activity, reduced stress, increased social capital, reduced incidence of disease, and climate regulation. The following articles and reports provide information on human health effects of green infrastructure.