Green Infrastructure benefits of green roofs

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Vegetation on the Target Center Arena green roof. vegetation consisted of a pregrown Sedum mat supplemented with 22 species of plugs and 16 species of seed native to Minnesota’s bedrock bluff prairies. Image Courtesy of The Kestrel Design Group, Inc.

Green roofs occur at the beginning of treatment trains. Green roofs provide filtering of suspended solids and pollutants associated with those solids, although total suspended solid (TSS) concentrations from traditional roofs are generally low. Green roofs provide both volume and rate control, thus decreasing the stormwater volume being delivered to downstream best management practices (BMPs).

Green infrastructure and multiple benefits

Green infrastructure (GI) encompasses a wide array of practices, including stormwater management. Green stormwater infrastructure (GSI) encompasses a variety of practices primarily designed for managing stormwater runoff but that provide additional benefits such as habitat or aesthetic value.

There is no universal definition of GI or GSI (link here fore more information). Consequently, the terms are often interchanged, leading to confusion and misinterpretation. GSI practices are designed to function as stormwater practices first (e.g. flood control, treatment of runoff, volume control), but they can provide additional benefits. Though designed for stormwater function, GSI practices, where appropriate, should be designed to deliver multiple benefits (often termed "multiple stacked benefits". For more information on green infrastructure, ecosystem services, and sustainability, link to Multiple benefits of green infrastructure and role of green infrastructure in sustainability and ecosystem services.

Benefit	Effectiveness	Notes
Water quality	●	Benefits are maximized for bioinfiltration. Biofiltration may export phosphorus if not designed properly.
Water quantity/supply	◕	Bioinfiltration helps mimic natural hydrology. Some rate control benefit.
Energy savings	◯
Climate resiliency	◔	Provides some rate control. Impacts on carbon sequestration are uncertain.
Air quality	◯
Habitat improvement	◕	Use of perennial vegetation and certain media mixes promote invertebrate communities.
Community livability	◕	Aesthetically pleasing and can be incorporated into a wide range of land use settings.
Health benefits	◔
Economic savings	◔	Generally provide cost savings vs. conventional practices over the life of the practice.
Macroscale benefits	◑	Individual bioretention practices are typically microscale, but multiple bioretention practices, when incorporated into a landscape design, provide macroscale benefits such as wildlife corridors.
Level of benefit: ◯ - none; ◔; - small; ◑ - moderate; ◕ - large; ● - very high

Green Infrastructure benefits of green roofs

Because of their use of vegetation in conjunction with building design, green roofs provide multiple green infrastructure benefits.

• Water quality: Green roofs provide stormwater treatment benefits, but because pollutant concentrations are generally low, these benefits are limited. Pollutant removal mechanisms include filtering, evaporation, transpiration, biological and microbiological uptake, and soil adsorption.

Green roofs employ engineered media that is effective at removing solids, most metals, and most organic chemicals. Green roofs are generally not effective at retaining phosphorus because of the organic matter content in the media. They therefore are likely to lose phosphorus during the first years after establishment, but may gradually retain phosphorus over time.

• Water quantity and hydrology: Green roofs are effective at detaining and retaining water and provide excellent rate control, although on a small scale. The ability of a green roof to detain and retain water is a function of both the media thickness and the sorptive properties of the media.
• Climate resiliency:
• Habitat improvement:
• Community livability: Green roofs are aesthetically pleasing, though they are typically not visible to the general public. A variety of vegetation can also be used, including perennial plants, shrubs, and trees.
• Health benefits:
• Economic savings:

Design considerations

Maximizing specific green infrastructure (GI) benefits of green roofs requires design considerations prior to constructing the practice. While site limitations cannot always be overcome, the following recommendations maximize the GI benefit of green roofs.

• Water quality
• Water quantity/supply
• Climate resiliency
• Habitat
• Community livability
• Health benefits
• Economic benefits