Green infrastructure and Grey infrastructure

Runoff from stormwater continues to be a major cause of water pollution in urban areas. It carries trash, bacteria, heavy metals, and other pollutants through storm sewers into local waterways. Heavy rainstorms can cause flooding that damages property and infrastructure.

Historically, communities have used gray infrastructure—systems of gutters, pipes, and tunnels—to move stormwater away from where we live to treatment plants or straight to local water bodies. The gray infrastructure in many areas is aging, and its existing capacity to manage large volumes of stormwater is decreasing in areas across the country. To meet this challenge, many communities are installing green infrastructure systems to bolster their capacity to manage stormwater. By doing so, communities are becoming more resilient and achieving environmental, social and economic benefits.

What is green infrastructure?

Green infrastructure is the “strategic use of networks of natural lands, working landscapes, and other open spaces to conserve ecosystem values and functions and provide associated benefits to human populations” (Allen, 2012). Green infrastructure mimics nature and captures rainwater where it falls, and includes permeable pavement, rain gardens, bioretention cells (or bioswales), vegetative swales, infiltration trenches, green roofs, planter boxes, rainwater harvesting (rain barrels or cisterns), rooftop (downspout) disconnection, and urban tree canopies.

Green infrastructure examples include: - Urban forests and woodlots - Bioswales, engineered wetlands and stormwater ponds - Green roofs and green walls - Parks, gardens and grassed areas - Natural heritage systems (interconnected meadows, wetlands, ravines, waterways and riparian zones) - Urban agriculture

It also includes soil in volumes and qualities adequate to sustain green infrastructure and absorb water, as well as technologies like porous pavements, rain barrels and cisterns, which are typically part of green infrastructure support systems. The green technologies in this definition replicate the functions of ecosystems, such as stormwater storage and filtration.

There are many other terms used to talk about green infrastructure, including: natural infrastructure, nature-based solutions, nature-based climate solutions, low impact development, and more. Sometimes these terms are used interchangeably, but there are some key differences.

What is grey infrastructure?

Gray infrastructure is traditional stormwater infrastructure in the built environment such as gutters, drains, pipes, and retention basins. Green infrastructure examples include:

- Bridges

- Roads

- Parking lots

- Culverts

- Pipes

Challenges of grey and green infrastructure

Compared to green infrastructure, grey infrastructure currently has a clearer asset life, depreciation, and return on investment.

Challenges surrounding grey infrastructure include funding and public investment, maintenance, and increased urbanization. Urbanization presents a water management challenge because the introduction of more hard surfaces, like concrete or asphalt, contributes to higher volumes of stormwater runoff due to a reduction of infiltration. Due to its relative size, construction requirements, and finite life, grey infrastructure can also be seen as inflexible.

Green infrastructure presents challenges in terms of measuring return on investment, risk management, and effectiveness in urban areas. Current regulation—or absence of regulation—at the federal, provincial, and local levels also presents obstacles, as many green infrastructure projects don’t fit traditional wastewater treatment construction models, so they may not be standards or building/urban codes to govern how the projects should be implemented.

As a largely untested concept, green infrastructure also faces scientific uncertainty, socio-political uncertainty/acceptance, and decision making uncertainty.

Knowledge and experience for people making decisions and designing and operating green infrastructure presents challenges for traditional approaches. For instance, green infrastructure is thought of more as an urban design, due to the scale and dispersed nature of the works, compared to a large, engineering-focused infrastructure project. Green infrastructure is not a replacement for grey infrastructure and vice versa

There will always be a requirement for grey infrastructure to guarantee water quality for drinking, treatment of high volumes in small areas, and water transportation. However, depending on the specific localized conditions and objectives, green infrastructure may complement grey infrastructure to help reduce energy costs and create more livable cities for the future.

Canada is recognizing the value of a hybrid approach to water issues such as stormwater management. For example, green infrastructure can reduce the pressure on grey infrastructure through naturally filtering out non-point source pollutants. Awareness of the various options available to tackle individual scenarios and localized environments is growing throughout the water industry, watershed and environmental organizations, and all levels of government.