The Future City is Green!

by Dr. Anna Zakrisson on Wednesday, May 6, 2020

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A sustainable city is densely populated. This might appear counterintuitive, but the current urban sprawl weighs heavily on both environment and economy.
A more densely populated city will allow for more effective land use and reduce pressure on surrounding agricultural, recreational, and wild surrounding lands. Also, local infrastructure such as local transport and other community services will become more productive, and we can likely expect more work opportunities and a thriving culture.
That all sounds great, but what’s the rub?

How green infrastructure allows for densely populated cities!

To create a densely populated city, much of the green areas will become built land. As the city develops, green spaces will slowly turn impervious.
Impervious surfaces such as stone, concrete, and asphalt build up heat throughout the day exacerbating urban heat island effects and increase flooding risks as the natural hydrologic cycle is disturbed. Also, there are well-documented negative effects on human mental health and general welfare if cities cannot provide sufficient access to green areas.
One way to solve this problem is through green infrastructure. These nature-based solutions enable green areas to be built on top of the existing or new grey infrastructure, for example, a park on top of a parking garage, or an urban garden on top of a school.

There are many different types of green infrastructure tailored to solve a variety of different problems and provide distinct ecosystem services.
A traditional green roof, for example, cools the city through the vegetation, acts as a pollution filter, reduces the annual runoff that would otherwise end up in the city’s sewers (through Retention), and increases property and community values. However, a traditional green roof cannot act as a stormwater management tool on the project scale, which is why there are other solutions available to tackle that issue.

Green infrastructure detention solutions

Purple-Roof and Blue-Green roofs are concepts that offer Detention on top of Retention. This means that the roof system provides a delay in peak runoff that makes them functionally comparable with a stormwater tank or a cistern.


In a way, these concepts are perhaps better described as “stormwater solutions that also look and function like green roofs.” In a way, these systems are not competing with traditional green roofs at all, but with old and outdated grey stormwater infrastructure.

A blue-green roof generally stores more water and often includes some active outflow technology. The benefits are that previous rainfall can be stored and used as irrigation for the plants. However, the blue-green roof system is often heavy, can be costly, and require a dead flat roof.

The Purple-Roof is a light-weight concept that slows down runoff using friction and can thus also be installed on sloped roofs. If you are interested to learn more about the hydrology behind this concept, take a look at the video below:



Why not combine different green infrastructure in one project? Use the Purple-Roof concept for sloped roofs and where HVAC equipment is available (the detention capacity of Purple also remains without vegetation), and then use blue-green technology for the spacious second-floor patios and deck to save on irrigation? High-retention traditional green roofs with mineral wool “sponge roofs” are an excellent solution when stormwater management is not needed on the project level but on a city scale.

There are many other types of green infrastructure out there, ranging from rain gardens to living walls. All have a particular purpose and a role to fill in our future cityscape.

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