In Amsterdam, there are 45,000 square metres of blue-green roofs that ‘absorb’ rainwater, allowing building occupants to use it to water plants and flush toilets, rather than immediately flushing it down the sewage system.
You might visit Amsterdam for its canals, and really, who can blame you? But the really interesting waterways are not under your feet – they’re above your head.
Beautiful green roofs have been popping up all over the world: specially selected plants grow on structures designed to handle the extra weight of biomass. But Amsterdam has gone one step further with blue-green roofs specifically designed to absorb rainwater. One of these projects – the Resilient Network of Smart, Innovative, Climate Resilient Roofs (Resilio) – has covered more than 9,000 square metres of Amsterdam’s roofs, including 8,000 square metres on social housing estates. Citywide, the coverage of blue-green roofs is even higher, estimated at more than 45,000 square metres.
The “sponge city” concept is becoming increasingly popular. Urban developers are increasingly planting green spaces that act as sponges to absorb rainfall from the increasing intensity of downpours caused by global warming. This both reduces the impact of flash floods and replenishes the absorbent rock layer. While cities used to be designed to drain rainwater as quickly as possible, they are increasingly designed to keep it in place, using water as a resource.
A big challenge for sponge cities is that most urban areas are rooftops. Green roofs soak up rainwater to hydrate plants, but blue-green roofs go one step further, with infrastructure that collects rainfall, stores it and makes it available to building occupants for watering plants and flushing toilets.
The system requires a layered design. On the surface are plants such as mosses, shrubs, grasses, ferns, herbs and crowflies, a hardy genus that is a staple of green roofs. (Although plants need sunlight to survive, they can be exposed to too much light, heat and wind on a roof, so choosing the right plants is key). Plants take root in the soil, providing nutrients and support.
Beneath the plant layer is a filter layer that prevents the soil from moving into the next layer: a lightweight crate system that stores water. Underneath are additional layers to prevent water and plant roots from damaging the actual roof. “It’s basically a flat rainwater tank on the roof,” says Kasper Spaan, a climate adaptation policy developer at the public water management organisation Waternet in Amsterdam, which is part of the Resilio programme.
A smart valve controls the water level in the blue-green roof. If a storm is forecast, the system will release the water stored on the roof ahead of time. So when the downpour comes, the “leachate” fills up again, which means less rainwater is released into the gutters in the area. In other words, the roof becomes a sponge that can be unscrewed as needed. “In the ‘sponge city’ we are making the whole city malleable,” says Spaan.
This makes the traditional system of stormwater management more flexible but also more complex. Therefore, the Resilio project used Autodesk software to model the impact of blue-green roofs and flood risk in Amsterdam, taking into account the climatic distribution.
“We can look at historical flood patterns and then run simulations to help us understand the processes. As much capacity as I can take out of the drainage network, I can reduce the impact of flash flooding when a storm occurs by as much as 10-20%,” says Amy Bunszel, executive vice president of architectural, engineering and construction design solutions at Autodesk.
In addition to the sponge-city benefits, the blue-green roofs also cool the top floor of the building, essentially “sweating out” or evaporating some of the stored water. With the right native plant species, you can help wildlife by providing for native pollinating insects. Scientists are going one step further and are also experimenting with growing plants on roofs under solar panels. This is called agrivoltaics. Theoretically, if this is coupled with blue-green systems, the efficiency of solar panels can also be improved by cooling with evaporating water.
However, not all buildings can be blue-green. The infrastructure itself is not very heavy, but the water in it is. So while the system can be installed relatively cheaply and easily in new builds, older buildings may need to retrofit the system to take account of the extra weight. In the long term, however, money can be saved by reducing the amount of water purchased from the municipal system. Like all technologies, it is expected to become less expensive as its use becomes more widespread.
The idea is to apply the sponge city concept not only on the ground – for example on patches of land covered with drought-tolerant plants that absorb rainwater into aquifers – but also on the roofs of buildings in places facing worsening droughts and floods. “We believe the concept is applicable to many urban areas around the world,” says Spaan. “In the south of Europe – Italy and Spain – where there are really drought-prone areas, there is a new focus on rainwater harvesting.”
Cities could even offer tax breaks to encourage the construction of blue-green roofs, rewarding building owners for reducing the amount of rainwater entering overloaded drains. In the United States, cities such as Los Angeles and Pittsburgh have already introduced something similar: a tax on the area of property that drains into sewers, encouraging landowners to develop gardens and other green spaces.
So the city of the future is not the concrete metropolis of science fiction, but an increasingly green and spongy landscape that can be “squeezed out” if necessary. “Our philosophy is ultimately not that everything is possible on every roof”, says Spaan, “but that something is possible on every roof”.
Source : https://www.wired.com/
Photos: https://resilio.amsterdam/
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