(Berlin) A gaping hole 20 meters deep is gutting the heart of Berlin. Within two years, this titanic project will give birth to the largest reservoir in the German capital, capable of collecting water that has become precious.
“Before, the goal was to drain rainwater so you can cross the city without rubber boots,” jokes Berlin water management spokesman Stephan Natz, a construction helmet screwed onto his head at the edge of the river. concrete crater.
But the metropolis has changed course and is now working to store rainwater where it falls to limit flooding and fight drought.
This is the concept of the “sponge city”, theorized in the 1970s and adopted in 2018 by Berlin. In the United States, China and Europe, many urban centers have converted to this approach which proposes to absorb, collect, drain and reuse runoff water.
Climate change is leading to “more patchy rainfall, with droughts followed by heavy rains, and increasing warming, which increases evaporation,” Natz says.
Symbol of the ongoing transformation: the giant water reserve under construction in the center of the capital, less than two kilometers from the iconic Brandenburg Gate.
The 40-meter diameter pool will collect nearly 17,000 m3 of water – five times the capacity of an Olympic swimming pool – store it and then send it to a treatment plant.
Because with a 150-year-old sewer system and more extreme weather than in the past, the city is no longer able to manage both wastewater and rainwater: “in the event of heavy rain, the waters mix and overflow into the Spree River, causing fish deaths and visual pollution,” explains Stephan Natz.
At the same time, Berlin, although built on former marshes, has been facing a severe lack of water for several years. After five years of drought, water tables have still not returned to their normal level, according to data from the Leibniz Institute for Freshwater Ecology and Fisheries.
“There has been a slow realization of the value of water in Berlin. It’s one of the driest places in Germany,” underlines Darla Nickel, director of the German capital’s Rainwater Management Agency, created to support the city’s transformation into a sponge.
Each new real estate project is now required to apply this strategy by developing rainwater harvesting techniques.
Built about 5 years ago, Quartier 52 Degrés Nord, in the southwest of the city, carefully applies the new precepts. Around three large pools in a row where the wind blows through the reeds, young parents with strollers, children and elderly people happily rub shoulders.
Rainwater is “collected on green roofs and in these basins. The water evaporates, creating nicer air,” says Darla Nickel. Green sidewalks are also inclined so that water infiltrates more easily into the soil.
“You see it can be really simple! » exclaims Ms. Nickel.
But the challenge is also to increase this type of measures in the heart of the city – even if Berlin is half as dense as Paris.
“We progressed much more slowly with the existing building than with the new construction,” concedes Darla Nickel.
In a historic district of Berlin, for example, a square is being renovated to collect rainwater and inject it into groundwater.
The rainwater management agency supports more than thirty projects linked to the “sponge city”. The municipality also encourages individuals to install water collectors or a green roof by exempting them from fees for the management and purification of rainwater.
Local officials, however, are aware that it will take several generations before Berlin truly becomes a “sponge.” “It remains to be seen whether climate change will give us time,” observes Stephan Natz.
