2023-09-30 13:37:37
Berlin, like many other cities around the world, is feeling the effects of climate change, with prolonged periods of drought followed by torrential rains. The German capital aspires to become a “sponge city”, an urban model designed to absorb and manage rainwater efficiently and minimize the risks associated with flooding.
On Emma-Ihrer Street, rows of six-story brick buildings are airy and surrounded by green spaces. This district, near Lake Rummelsburg in the southeast of Berlin, is one of the pioneers of the “sponge city” concept.
“There are no manholes. We created a ‘vegetated basin’. So, when the water is absorbed by the ground, it feeds the water table. When it evaporates through the trees, it causes cooling linked to evaporation”, describes the spokesperson for the company responsible for managing the city’s water, Berliner Wasser Betriebe, Stephan Natz.
The trunks of the trees have a circumference three times greater than that of their counterparts planted a little further away at the same time but whose roots are out of reach of this “green bowl”.
“The interior courtyards of the buildings are all green, everything is planted, the roofs are green… They act like ‘sponges’. When there is too much water, the surplus is diverted towards the lawns, where it can enter the earth”, explains Stephan Natz in the RTS show Tout un monde.
A series of measures
The “sponge city” project has various tools, such as cisterns, channels around trees, bowls and green roofs. “When we talk about a ‘sponge city’, there is no patented recipe,” says landscape architect and one of the initiators of the project Carlo Becker.
And added: “We’re talking about a cascade of different modules and elements to be superimposed on each other. We start with the green roof, but it’s the first step. It’s good if we add a cistern to collect water in anticipation of periods of drought, to water the trees. The next step is to install an evaporation bed, which retains water to cool the atmosphere in the event of extreme heat.”
At the end of June 2017, torrential rains fell on Berlin, flooding certain neighborhoods. [Stephanie Pilick – Keystone]
The results of this approach are convincing. Stephan Natz remembers the heavy rainfall of June 2017, when parts of Berlin received almost 200 liters of water in 24 hours. “Sponge city” neighborhoods have proven successful and these systems have worked effectively to prevent flooding.
Up to ten billion euros
In 2018, Berlin ended rainwater runoff into pipes for every new construction.
This ambitious project would cost between 5 and 10 billion euros, according to estimates by Berliner Wasser Betriebe. Costs vary depending on items. For example, a vegetated bowl costs around 9 euros per square meter, while transforming a paved sidewalk into a green surface costs around 60 euros per square meter.
To achieve the objective of 30% of the urban surface capturing water, the City must accelerate the transformation of its urban landscape. Although the city is relatively ahead in Germany in terms of “sponge city”, much remains to be done to achieve this ambitious goal.
Günter Müller-Czygan, professor at Hof University in Bavaria, emphasizes that the main difficulty lies in the need to make urban modernization coexist with heritage protection: “In the old, we often have a conflict with the protection heritage, he says. From a static point of view, it would be possible to make a green roof or a green facade. It’s a political question, not a technical one.”
Nathalie Versieux/vajo
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