Green infrastructure (blue-green infrastructure)

Green infrastructure is a community planning concept that seeks to combine natural and engineered elements to deliver a suite of ecosystem services to local residents. The umbrella term ecosystem services refers to resources and benefits generated or improved by nature, covering such factors as improved air quality, water purification, outdoor recreation, and climate mitigation and adaptation features.

By definition, green infrastructure makes heavy use of “green” elements such as forests and trees, gardens, open fields, and parklands. The related concept of blue-green infrastructure extends green infrastructure to include “blue” water-based elements. Blue-green infrastructure seeks to incorporate natural floodplains, rivers, ponds, and wetlands into community planning alongside purpose-built assets such as canals and water treatment plants.

As the Environmental Protection Agency (EPA) notes, planners can build green and blue-green infrastructure into their communities at multiple scales. These scales range from highly localized developments to infrastructure serving an entire city or region.

Background

Green infrastructure was developed as an alternative to legacy stormwater management systems, which are known in this context as gray infrastructure. Gray infrastructure takes its name from the fact that much of it is constructed from concrete and steel. It consists of a network of gutters, sewers, ditches, dams, sewage tunnels, permeable pavement, and pipes that divert stormwater away from inhabited areas and reroute it to natural bodies of water or to water treatment facilities. Gray infrastructure also collects and delivers household graywater to sewage treatment plants. The term “graywater” describes any type of wastewater not contaminated with human waste; it primarily includes drainage from fixtures and appliances like bathtubs, showers, dishwashers, and washing machines.

Gray infrastructure primarily uses a centralized approach to stormwater and wastewater management, capturing and collecting incoming water before strategically diverting it to a planned destination. It is designed to reproduce nature’s ability to absorb and retain water but offers limited utility beyond its primary purpose. The gray infrastructure installed in many urban centers is also aging, having handled large volumes of stormwater and wastewater for decades.

Green infrastructure initially evolved as a complementary system to supplement existing gray infrastructure in need of upgrading or replacement. These coupled systems are sometimes referred to as green-gray infrastructure, and they are effective at reducing the heavy water volumes that erode gray infrastructure. Unlike gray infrastructure, green infrastructure is also capable of harvesting urban rainwater and utilizing it as part of a wider water resource management strategy, which serves as a key example of its broader set of case-uses.

Research into the economic benefits of coupled systems have found that they offer significant long-term cost savings. A study published in the academic journal Resources, Conservation & Recycling (Xu, Tang, Haifeng, and Xu, 2019) reported that green-gray infrastructure systems have the potential to cut lifecycle costs by as much as 94 percent when compared to gray-only infrastructure. The study recommended investing in the optimization of coupled systems and encouraged urban planners to make increased use of green infrastructure technologies in their municipal stormwater and wastewater management plans. The study reflects a growing sentiment among policymakers and urban planners to make more conscientious use of green and blue-green approaches to local development.

Brief History

Experts often characterize the concept of green infrastructure as an outgrowth of the modern environmental movement, which rose to national prominence in the United States during the late 1960s and became a major topic of sociopolitical concern in the 1980s and 1990s. Scholarly analysis of the history of green infrastructure notes the concept’s interdisciplinary roots. Its underlying principles evolved over decades, leading to the first known usage of the term in 1994 when policymakers referenced “green infrastructure” in a land conservation report prepared for Florida’s then-governor Lawton Chiles. The report highlighted the importance and effectiveness of natural systems as complements to human-made infrastructural assets, noting that gray infrastructure requires thorough long-term planning and positing that policymakers therefore have ample opportunity to incorporate green infrastructure into their wastewater management strategies.

In 1995, author Charles E. Little published the book Greenways for America, which experts credit with popularizing the term “greenways.” The greenways movement, which gained momentum following the book’s publication, seeks to establish and maintain landscaped spaces in urban, semiurban, and rural areas. Greenways are viewed as a means of encouraging outdoor recreation and linking a region’s parks and nature preserves through an interconnected network of paths and trails. The greenways movement foretold the subsequent rise in interest surrounding green infrastructure by formally importing the concept of landscape ecology into urban and regional planning.

The EPA formally adopted the term “green infrastructure” in 2007, using it to describe a novel approach to low-impact stormwater management, which endeavors to apply natural hydrologic processes to engineered systems. During the 2000s, urban planners, civil engineers, and policymakers began to recognize the ecological and cost-efficiency potential of using ecological services to meet community-based needs. This touched off a concerted move toward scale planning that incorporated landscape and nature features into their designs at increasing rates. The trend reflected an underlying realization among localities that nature conservation and the restoration of natural landscapes were vitally important to building sustainable, healthier, and more livable communities.

In 2006, a landmark study examined the financial value of strategically using tree cover as a means of improving air quality and managing stormwater volumes. Its findings were jointly published by researchers Edward McMahon and Mark A. Benedict in their book Green Infrastructure: Linking Landscapes and Communities. The researchers concluded that tree cover held the potential to save U.S. municipalities as much as $400 billion in air quality mitigation, water treatment, biofilter, and stormwater pond investments.

Also in 2006, multiple agencies of the US federal government produced a report titled Ecological: An Ecosystem Approach to Developing Infrastructure Projects. Though the work mainly focused on sustainable transportation and road design practices, it signaled growing government recognition of eco-conscious infrastructure development. The Green Infrastructure Center, a Virginia-based nonprofit organization that helps communities make more environmentally conscious planning decisions, was also established that year. The Trust for Public Land, another eco-protection nonprofit group, has since financed and published multiple academic studies into green infrastructure. These studies examine the social and financial benefits of green spaces in urban planning, noting their multifaceted potential as effective ways to bridge access gaps to outdoor recreation opportunities while making better use of natural resources.

As the concepts of green and blue-green infrastructure have developed, they have been taken up by policymakers, urban planners, and stakeholders in many countries around the world. The European Commission (EC)the executive authority of the European Union (EU)now maintains an official green infrastructure strategy. East Asian countries including China and Japan continue to deepen their investments in green infrastructure, while the Commonwealth Scientific and Industrial Research Organization (CSIRO), Australia’s national scientific agency, has created a “national agenda” for making increased use of green infrastructure principles and technologies in the nation’s urban areas.

In 2019, US lawmakers passed the Water Infrastructure Improvement Act. The legislation formally incorporated the EPA’s integrated planning (IP) principles into federal law, giving municipalities and regional governments new paths to planning and financing green infrastructure additions to their existing stormwater and wastewater management systems. Notably, the Water Infrastructure Improvement Act also contains a specific description of “green infrastructure,” which now serves as the standard definition used by US federal agencies. It describes green infrastructure as “the range of measures that use plant or soil systems, permeable pavement or other permeable surfaces or substrates, stormwater harvest and reuse, or landscaping to store, infiltrate, or evapotranspirate stormwater and reduce flows to sewer systems or to surface waters.”

Overview

Experts are careful to note distinctions between green infrastructure and green building, as the terms are often confused and inaccurately used interchangeably. Green building refers to the certifications offered by organizations such as Leadership in Environment, Energy, and Design (LEED) for structures that conform to elevated energy efficiency standards. LEED ratings are endorsed by agencies such as the US Green Building Council, and now also extend to neighborhood development. As understood by the EPA, urban planners, and ecodevelopment professionals, green infrastructure refers exclusively to individual, municipal, and regional resource management systems for dealing specifically with stormwater and wastewater.

As the EPA notes, green infrastructure can exist across multiple scales, ranging from what it calls individualized “urban scales” to neighborhood scales and larger citywide or regional scales. At the individual level, green infrastructure includes basic interventions that have a long history of established use; examples include using barrels to collect rainwater for household use, adding trees to streets, and remediating abandoned or underused properties with landscaped elements. Green infrastructure at the neighborhood scale involves engineered solutions with community-level impacts, such as creating a multi-acre green space within an urban district, planting low-elevation rain gardens to absorb rainwater runoff from building roofs and graded properties, and creating artificial wetlands to serve subdivisions or housing complexes by harvesting rainwater resources and supporting localized biodiversity. Scaled up to the citywide or regional level, green infrastructure includes networked groups of localized projects that combine to improve air quality, boost the resilience of water resources, and protect against flooding while making outdoor recreation options more plentiful and accessible.

The EPA lists multiple technologies and systems that can be used in green infrastructure, which can be grouped into broad categories including stormwater management, construction and landscape architecture, building materials, and land use strategies. Each category contains interventions with applications at the urban, neighborhood, citywide, and regional levels. Stormwater management techniques comprise downspout disconnection, rainwater harvesting, rain gardens, and planter boxes. Major examples of construction and landscape architecture-based approaches to green infrastructure include bioswales, green parking areas, green roofs, and green streets. Permeable pavements represent a widely used green infrastructure building material, while land use strategies incorporate urban tree canopies and various land management policies and practices.

Downspout disconnection can be carried out at the homeowner level. It involves reconfiguring rooftop drainage systems to empty stormwater into storage vessels, gardens, or other permeable areas. When applied at the neighborhood or citywide level, it can dramatically reduce the amount of water handled by existing gray infrastructure, thus helping extend its lifespan and reduce maintenance and replacement costs.

Rainwater harvesting takes a broader, upscaled approach to stormwater collection. In addition to the localized techniques that apply to downspout disconnection, rainwater harvesting also includes constructing pits and aquifers capable of storing large fluid volumes. Some systems also use nets and other technologies to capture and extract moisture from fog and dew.

Rain gardens, also known as bioretention cells, are small, localized plant collections created in low-lying areas that are surrounded by graded slopes or other drainage systems. They route stormwater into the garden, hydrating plants and helping prevent water from pooling on driveways, sidewalks, and other surfaces. When rain gardens are situated in densely developed urban areas and surrounded by vertical walls, they are called planter boxes.

Bioswales are features built into road infrastructure, such as gutters, curbs, and parking lots. They use dense volumes of plants and/or mulch, which reduce the speed at which stormwater can flow while also filtering incoming water. Bioswales reduce gray infrastructure workloads and help prevent systems from becoming inundated when major storms occur. Green parking strategies incorporate bioswales, rain gardens, planter boxes, and other green infrastructure technologies into surface parking lots, creating walkability improvements while mitigating the so-called “heat island effect” that links human activity with higher localized ambient temperatures.

Green roofs continue to emerge as an increasingly prominent feature of urban spaces. These architectural features reimagine conventional rooftops as green spaces, covering them with vegetation that supports the evapotranspiration of collected stormwater and the infiltration of incoming rainwater. Green roofs can also be configured as produce-yielding gardens, and they have become particularly popular in major metropolitan centers where the cost of remediating damage caused by excess stormwater is likely to be very high. Similar concepts can be applied in cities at ground level, creating what are widely known as green streets.

Permeable pavement represents a critically important green infrastructure material, as it has mass-scale applications. It includes any type of concrete, asphalt, or interlocking bricks capable of allowing water to pass through, and can be used to finish any flat surface in a developed area. Urban planners make extensive use of permeable pavement materials in high-value areas where flooding, ice, and meltwater volumes pose significant property damage risks.

Developers can engineer urban tree canopies to improve localized vegetation density and dramatically accelerate the surrounding environment’s ability to absorb rainwater in their branches and leaves. Because trees filter carbon dioxide and other impurities out of the air, they can also yield significant air quality improvements when constructed on a mass scale. Conservationist approaches to land management and urban development have similar impacts, especially in cities surrounded by hills, natural wetlands, flood plains, and streambanks.

Blue-green infrastructure extends the concept of green infrastructure to engineered systems that connect with natural landscapes and bodies of water. It can also include artificially constructed supplementary resources such as canals and water treatment facilities to create a municipal water management system that closely replicates the structure and function of natural water cycles. Blue-green infrastructure represents a more complete and integrated approach and holds the potential to improve ecosystem service delivery beyond what green infrastructure or green-gray infrastructure could achieve independently. Its potential benefits include natural reductions in localized air pollution, turnkey irrigation systems for parks and outdoor recreation areas, and more plentiful and resilient access to drinking water supplies.

Cities in the United States and around the world are adopting green and blue-green infrastructure strategies at increasing rates. US municipalities with major green and blue-green infrastructure developments include San Francisco, New York, and Los Angeles. Internationally, ten EU cities launched a joint blue-green infrastructure pilot project in 2021, with municipalities in Belgium, Germany, the Netherlands, Norway, Sweden, and the United Kingdom launching a combined thirty-four programs with a total financial value estimated at €347 million (US $392 million).

Green blue infrastructure enhancements do not have to be limited to buildings or solid infrastructure. There can be a significant return on investment when these efforts are turned to urban water sources. For example, many urban areas have rivers and lakes within their confines, as well as wetlands and engineered greening. When projects are undertaken to improve water quality, availability, and biodiversity, human populations benefit from the increased green spaces and reduced waterborne diseases. These projects can also reduce ever-rising heat indexes in cities that are increasingly making urban areas less habitable. The most efficient air cooling projects of this type include botanical gardens, wetlands, green walls, street trees, and vegetated balconies.

Bibliography

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