Carlo Becker arrives at the meeting a little late. The train is on strike again, to reach the Adlershof district on the south-eastern outskirts of Berlin, he has to drive his car in a traffic jam, “although I don’t like it”, as he says. Becker himself suggested the meeting point, at the corner of Newtonstrasse / Zum Großer Windkanal: It’s a place you won’t find in any travel guide, a place that looks anything but spectacular. Carlo Becker, 64, a landscape architect, knows him very well. In the mid-1990s he went to Adlershof almost every day; the district, which was used for military purposes during the GDR era and housed many research facilities, had to be renovated and redeveloped after the fall of the wall. Becker and his landscape architecture office were part of the planning team. 25 years have passed since then, but he comes back to it over and over again when he has to use a specific example to explain what the Sponge City really looks like.
How do we get to the first question: What is Sponge City anyway? To understand this concept of town planning, imagine a cleaning or bath sponge. A sponge does something very mundane and is still very effective: it absorbs water, holds it for a while, and then releases it again with a delay. For landscape architect Becker, this is exactly what a climate-friendly city should do: it shouldn’t drain rainwater quickly through sewers, but rather trap it where it falls and use it to irrigate. plants and shrubs and, in general, green spaces. Plants and green spaces cool the city by evaporation when temperatures rise too high, or protect against flooding. Not just: “If the water is managed locally, the sewage system is relieved,” says Becker. It doesn’t seem like much, but in reality it is a lot.
Open detailed view
“You should create green spaces off the streets,” says Berlin landscape architect Carlo Becker. He has been working on the concept of the sponge city for many years.
(Photo: Francesca Polistina)
Berlin city center has a mixed sewerage system, which means that wastewater and rainwater flow through the same pipes to the sewage treatment plants. This is not uncommon for large European cities, from Madrid to Moscow: in the 19th century, sewers were built in the same way, but at that time cities were not as densely populated and climate change was not not such a big deal yet. Today it’s different: If it rains a lot (and experts assume that extreme precipitation will increase), the sewer system can quickly become overloaded and overflow. The excess dirty water from toilets and showers – which is heavily diluted with the rain, but still contains harmful substances – then flows into one of the many reservoirs or into canals and rivers without passing through the stations purification.
For Stephan Natz, spokesperson for Berliner Wasserbetriebe, the effects for a city like Berlin are particularly serious because the Spree and the Landwehr canal flow very slowly. The water therefore takes a long time to become clean again, and the natural environment suffers. Outside the city center there is a separate sewage system, but here too the pipes are designed for normal rainy events.
There are hardly any gullies in Adlershof
This is why the motto is: Empty as little rainwater as possible in the pipes in order to free the sewer system. Collect as much water as possible to fight the drought which is already a reality in Berlin and which has already damaged half of the urban trees. In Adlershof, almost the whole district was built without any drainage, that is to say without gullies: between the streets and the sidewalks, there are green hollows in the shape of a bathtub which serve to infiltrate the water; the roofs of the new buildings are planted with grass and roof bushes, the green spaces are lower than the streets so that the rain can enter. And yet: if it weren’t for the green facade of the HU Berlin Physics Institute, made up of climbing plants, you probably wouldn’t notice that this district is different from many others. “The sponge city is often invisible,” explains Carlo Becker.
Open detailed view
The facade of the Institute of Physics of the Humboldt University in the Berlin district of Adlershof is built according to the principles of the sponge city.
(Photo: Schöning / Imago Images)
Adlershof is not the only district in Berlin where rainwater is managed locally. The Rummelsburger Bucht, Potsdamer Platz or Karow-Nord, all built in the 90s, are also part of it. Other examples include the 52 Degrés Nord district, which has just been completed, or the Schumacher district on the site of the former Tegel airport, which is to be built according to the principles of the sponge city. In any case, the city of sponges is a “combination of different measures,” as the director of the Berlin Rainwater Agency, Darla Nickel, explains. These are, for example, green roofs, artificial drainage basins and water bodies, wetlands or, in some cases, cisterns.
It is true that new construction projects offer great freedom in terms of town planning, but the crucial question remains: how can the Sponge City be established in the city center, where everything is already densely built and where margins for maneuver are reduced? ? For Carlo Becker, the concept of the sponge city is inconceivable without a change in mobility. “The streets should be transformed into green spaces”, explains the landscaper, even if the attempts in this direction have not always been well received by the inhabitants because, for example, the car parks have become green shoulders.
The state of Berlin and the Berliner Wasserbetriebe are trying to apply regulations: in the case of construction projects in the area of mixed sewer systems, i.e. inside the S-ring Bahn, rainwater can no longer flow into the sewage system. “It is not always possible to manage all the rainwater locally,” explains Nickel, because often there is simply not enough space to store or infiltrate the water. “But you can almost always get at least some of the rainwater and use it again.” According to Nickel, the goal is to reduce the area connected to the downtown sewer system by one percent each year.
There are models in the Netherlands and China
The name Sponge City comes from the English Sponge City, and Becker himself had the German translation “Sponge City” protected as a word mark. If you ask the experts, the models are mostly found in the Netherlands: cities like Rotterdam and Amsterdam have long been concerned about the issue of rainwater (and sustainability in general). But also in China, the country of origin of the term, many projects are being implemented. Hamburg is a striking example in Germany: the Hanseatic city also wants to become a sponge city, “with green roofs, soft soil, hollows, ditches, plants on the facades or grass cobblestones in place of the asphalt “can be read on the website of Hamburg Wasser, the local water supply company. In Hamburg, too, the heat is becoming a major burden on city dwellers, and the authorities are looking for practical solutions to cool the city.
Is the sponge city also the answer to natural disasters like the floods in North Rhine-Westphalia and Rhineland-Palatinate? “You cannot understand what happened in North Rhine-Westphalia, even with the sponge city,” says Nickel. Because in mountainous regions the effects of extreme precipitation are much stronger than in flat areas like the capital, where hills and mountains act as a funnel towards narrow river valleys. The principles of the Sponge City apply everywhere: “Whether in town or in the countryside: water needs more space,” explains Nickel.
Becker speaks of “sponge landscapes”: “If the landscapes worked like a sponge, then they could hold more water”, explains the landscape architect. In reality, however, more and more landscapes have been cleared, wet depressions have been drained, small bodies of water have been removed and wooded areas have disappeared, according to Becker. It is therefore necessary to combine the concept with other measures. Because: The only answer to climate change is not the sponge city – but does it even exist, the only answer to climate change?