‘On every roof something is possible’: how sponge cities could change the way we handle rain | Environment
You might visit Amsterdam for its canals and who can blame you, really. But the really interesting waterways aren’t under your feet—they’re over your head.
Beautiful green roofs have sprung up around the world: specially selected plants growing on structures designed to manage the extra weight of biomass. Amsterdam took another step forward son-green roofs specially designed to capture rainwater. One project, the Resilience Network of Smart, Innovative, Climate-Adaptive Roofs (Resilio), covered more than 9,000 square meters (100,000 square feet) of rooftops in Amsterdam, including 8,000 square meters on social housing estates. Across the city, the blue-green roof coverage is even greater, estimated at more than 45,000 sq m.
The “mushroom city” concept. is becoming more and more popular. Planners are rolling out more green spaces that absorb heavy rains are becoming more severe as the world warms. This simultaneously reduces flooding and recharges the bottom layer of absorbent rock, which can then be used in times of need. While cities were designed to divert stormwater as quickly as possible, they are increasingly exploiting this resource.
A big challenge with mushroom cities is that much of the urban areas are rooftops. Green roofs will capture some rainwater to hydrate the plants there, but blue-green roofs go a step further, with infrastructure that collects the liquid, stores it and distributes it to building residents to water plants and flush toilets.
The system works in layers. On the surface you have plants: some combination of mosses, shrubs, grasses, ferns, herbs, and sedum, a hardy genus that is a major part of green roofs. (While plants need sunlight to survive, on a roof they can be bombarded with too much light. It can get hot and windy up there.) Plants are rooted in the soil, providing nutrients and support.
Below that is a filter layer that keeps the soil from seeping into the next layer: a lightweight crate system that stores the water. Finally, underneath this are additional layers that keep water and plant roots from penetrating the actual roof. “You actually have an apartment rain barrel on top of your roof,” says Kasper Spaan, climate adaptation policy developer at Waternet, the public water management organization in Amsterdam that participates in Resilio.
Water levels in the blue-green roof are controlled by a smart valve. If the forecast says a storm is coming, the system will release stored water from the roof ahead of time. This way, when it rains, the roof refills, meaning less rainwater enters gutters and downspouts in the surrounding area. In other words, the roof becomes a sponge that can be squeezed out as needed. “In the squeezable sponge city, you make the whole city malleable,” says Spaan.
This makes the traditional stormwater management system more flexible, but also more complex. So the Resilio project used software from Autodesk to model the impact of blue-green roofs and flood risk in Amsterdam, while also adjusting for climate change.
“You can look at historical flood patterns and then you can do simulations to help you understand. If I can get that much capacity out of the drainage network when the storm comes, I’ll reduce flooding by 10, 15, 20 percent,” says Amy Bunzel, Autodesk’s executive vice president of architecture, engineering and construction design solutions. “So our software allows them to do simulations and play with different trade-offs.”
Beyond the benefits of urban mushrooms, blue-green roofs can cool the upper floor of a building by essentially “sweating” stored water. With the right types of native plants, they can help wildlife taking care of native pollinating insects. Going a step further, scientists are experimenting with growing crops on rooftops under solar panels, known as rooftop agrivoltaics. In theory, combining this with blue-green systems could improve the efficiency of solar panels by cooling them with evaporating water.
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Not every building can turn blue-green. The extra infrastructure isn’t very heavy, but the water it holds is. So while it’s relatively cheap and easy to fit the system into new construction, accounting for the extra weight, older buildings may need retrofits to accommodate it. In the long run, this can save the building money by reducing the volume of water purchased from a municipal system. Like any technology, its price will drop as it becomes more widely available.
The idea is for places subject to worsening droughts and floods to deploy not only mushroom city concepts on the ground—such as patches of soil with drought-tolerant plants to absorb rainwater into aquifers—but also on their buildings. “We think the concept is applicable to many urban areas around the world,” says Spaan. “South of Europe – Italy and Spain – where there are really drought-stricken areas, there is a new focus on rainwater harvesting.”
Cities could even incentivize blue-green roofs by providing tax breaks rewarding building owners for reducing their contribution of stormwater to overburdened sewer and water systems. American cities like Los Angeles and Pittsburgh release something similar: taxes on the amount of impervious area on a property, encouraging landowners to develop gardens and other green spaces.
The city of tomorrow is therefore not the concrete-smothered metropolis of science fiction, but an increasingly green and spongy landscape that can be squeezed out in times of need. “Our philosophy is ultimately not that on every roof, everything possible,” says Spaan, “but that on any roof, something is possible.”
