Courtesy of De Dakdokters
A filter layer extends underneath, preventing soil from passing through to the next layer, a lightweight box system that stores water. And finally, underneath you have additional layers to prevent water and plant roots from infiltrating the roof. “You actually have a flat rain barrel on top of the roof,” says Kasper Spaan, policy developer for climate adaptation at Waternet, Amsterdam’s public water management organization, which is involved in RESILIO.
The water levels in the blue-green roof are controlled by a smart valve. If the weather forecast says a storm is coming, the system will release the water stored in the roof in advance. This way, when there is a downpour, the roof fills up again, meaning less rainwater enters the surrounding gutters and sewers. In other words, the roof becomes a sponge that the operator can wring out as needed. “In the ‘squeezable’ sponge city, the entire city becomes malleable,” Spaan says.
This makes the traditional stormwater management system more flexible, but also more complicated. Therefore, the RESILIO project used Autodesk software to model the impact of blue-green roofs and the risk of flooding in Amsterdam, also adjusting it to climate change.
“You can take a look at historical flooding patterns and then do simulations that will help you understand: If I could remove this much capacity from the drainage network, when the storm hits, I will reduce flooding by 10. 15, 20 percent,” says Amy Bunszel, executive vice president of architecture, engineering and construction design solutions at Autodesk. “So our software allows them to do simulations and play with different offsets.”
Beyond the sponge city benefits, blue-green roofs can cool the top floor of a building, essentially “sweating” stored water. With the right types of native plants, they can also boost biodiversity by feeding native insect pollinators. Going a step further, scientists are also experimenting with rooftop crops under solar panels, known as rooftop agrivoltaics. In theory, combining it with blue-green systems could improve the efficiency of solar panels by cooling them with evaporating water.