Climate change and storm protection: Berlin is becoming a sponge city – economy

Carlo Becker comes to the appointment with a slight delay. The train is on strike again, in order to reach the Adlershof district in southeast Berlin, he has to drive his car in a traffic jam, “although I don’t like doing it,” as he says. Becker himself suggested the meeting point at the corner of Newtonstrasse / Zum Großer Windkanal: It is a place that you won’t find in any travel guide, a place that seems anything but spectacular. Carlo Becker, 64, landscape architect, knows him very well. In the mid-1990s he drove 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 redesigned 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 goes there again and again when he has to use a specific example to explain what the sponge city actually looks like.

Whereby we come to the first question: Sponge City, what is it anyway? To understand this concept of urban planning, imagine a cleaning or bath sponge. A sponge does something very mundane and is still highly effective: it absorbs the water, holds it for a while and then releases the water again with a delay. For landscape architect Becker, this is exactly what a climate-adapted city should do: it should not drain the rainwater away quickly using sewers, but rather hold it back where it falls and use it to irrigate plants and bushes and, in general, the green spaces. Plants and green spaces cool the city through evaporation when temperatures rise too high, or protect against flooding. Not only: “If the water is managed locally, the sewage system is relieved,” says Becker. Sounds like little, but in reality it is a lot.

The inner city of Berlin has a mixed water sewer system, which means that wastewater and rainwater flow through the same pipes to the sewage treatment plants. This is not uncommon for major European cities, from Madrid to Moscow: in the 19th century, sewers were built in the same way, but at that time the cities were not so densely populated and climate change was not yet such a big problem. Today it looks different: when it rains heavily (and experts assume that extreme rainfall 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 the canals and rivers without running through the sewage treatment plants.

For Stephan Natz, spokesman for Berliner Wasserbetriebe, the effects for a city like Berlin are particularly serious because the Spree and Landwehr Canal flow very slowly. So the water there takes a long time to get clean again, and the natural environment suffers as a result. Outside the city center there is a separate sewer system, but there, too, the lines are designed for normal rain events.

There are almost no gullies in Adlershof

That is why the motto is: Drain as little rainwater as possible into the pipes in order to free the sewer system. Collect as much water as possible to combat the drought that is already a reality in Berlin and has already damaged half of the urban trees. In Adlershof, almost the entire quarter has been built without any drainage – that is, without gullies: between the streets and sidewalks there are green, tub-shaped hollows that serve to infiltrate the water; the roofs of the new buildings are planted with grass and roof bushes, the green areas are lower than the streets so that the rain can flow in. And yet: if there wasn’t the green facade of the Institute for Physics at HU Berlin, which consists of climbing plants, one would probably not notice that this district is different from many others. “The sponge city is often invisible,” says Carlo Becker.

Open detailed view

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 nineties, are also part of it. Other examples include the 52 Degrees North 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 all cases, the sponge city consists of a “combination of different measures”, as the head of the Berlin Rainwater Agency, Darla Nickel, explains. These include, for example, green roofs, drainage basins and artificial bodies of water, wetlands or, in some cases, cisterns.

Now it is the case that new building projects offer great freedom in terms of urban planning, but the crucial question remains: How can the Sponge City be implemented in the city center, where everything is already densely built up and there is little room for maneuver? For Carlo Becker, the concept of the sponge city is inconceivable without a change in mobility. “One should create green spaces out of the streets,” says the landscape architect, although attempts in this direction were not always welcomed by the residents because, for example, parking lots became green verges.

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. within the S-Bahn ring, rainwater may no longer flow into the sewer system. “It is not always possible to manage all of the rainwater locally,” says Nickel, because often there is simply not enough space to store or infiltrate the water. “But you can almost always collect at least part of the rainwater and use it again.” According to Nickel, the aim is to reduce the area connected to the sewer system in the city center by one percent every year.

There are role models in the Netherlands and China

The name sponge city comes from the English Sponge City, Becker himself had the German translation “Sponge City” protected as a word mark. If you ask the experts, the role models can be found mainly in the Netherlands: There cities like Rotterdam and Amsterdam have long been concerned with the issue of rainwater (and with sustainability in general). But also in China, where the term originated, many projects are being implemented. Hamburg is a prominent example in Germany: The Hanseatic city also wants to become a sponge city, “with green roofs, loose soils, hollows, ditches, plants on facades or grass pavers instead of 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 for 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 can’t get a handle on what happened in North Rhine-Westphalia, even with the Sponge City,” explains Nickel. Because in mountainous regions the effects of extreme rainfall are much stronger than in flat areas such as the capital. There the hills and mountains act like a funnel towards the narrow river valleys. The principles of the Sponge City apply everywhere: “Whether in the city or in the country: the water needs more space,” says Nickel.

Becker speaks of “sponge landscapes”: “If landscapes worked like a sponge, then they could hold back more water,” says the landscape architect. In reality, however, more and more landscapes were cleared, humid depressions were drained, small bodies of water were removed, and wooded areas 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?