Sponge City | Convert Berlin into a sponge with ease

This is what a sponge city-friendly neighborhood could look like: with green spaces, street trees, green facades and roof gardens Photo: Rainwater Agency A yellow sponge with a face greets users of the new mobile phone game “Sponge Tasty” on the display. During the intro, a cloud appears at the top of the display and pours thick raindrops onto the sponge, which laughs and swells to twice its size. “Imagine if Berlin was a sponge,” it says underneath. After the rain, the animated figure gets some sun and shrinks again - then a gray veil covers the display. The sponge looks like it's getting sick. »Berlins Schwamm ist versiegt«, ist nun zu lesen.

With the browser game from the Water Competence Center Berlin, Berliners can find out how this can be changed again. First they select a district and end up with a photo of a gray, sealed residential area. Digital puzzle pieces can now be used to add meadows, trees, rain barrels and similar measures that prevent rainwater from flowing into the sewage system. That is exactly the idea of ​​the urban planning concept of the sponge city, also known as decentralized rainwater management: to ensure that water is collected or stored by plants and soil during heavy rainfall and that it evaporates with a delay in heat and drought, thus providing cooling. The city then functions like a natural sponge.

“Every raindrop that doesn’t flow into a canal is a good raindrop,” says Stephan Natz to “nd”. He is the spokesman for the Berliner Wasserbetriebe, which was involved in the game development together with the Berlin Rainwater Agency, the Senate Departments for the Environment and Urban Development and other partners as part of the “Smart Water” project. The reason was a grant from the Senate Chancellery, which was intended to be used to promote the necessary adaptation to the climate crisis, says Natz.

“Every raindrop that doesn’t flow into a canal is a good raindrop.”

The aim of the entertaining online game, which has been played almost 1,500 times so far, is to “popularize the sponge city idea”. Natz believes that it is aimed primarily at children and young people who are helping to shape the Berlin of the future and who are already "annoying their parents about it." In addition to the game, a flood information portal and the blue-green infrastructure planner (BGI planner) were also designed as part of the “Smart Water” project. Blue infrastructure means bodies of water and green infrastructure means plants.

The BGI planner is a map-based web application that is primarily intended for administration. According to Natz, this is “doubly important” because it is the largest land owner in the city and at the same time the approval authority for other owners. The planner should show which places are affected and how badly by heavy rain, heat and sealing. It contains potential maps, for example for green roofs, and evaluates the effectiveness of various measures. The open source application makes it visible where an infiltration trough or an unsealed area has a particularly significant impact.

Such planning is urgently needed for the capital because it is something like “the Sahel zone of Germany,” says Natz. On a long-term average, around 580 liters of rain fell per square meter per year in Berlin, significantly less than in many other places. In recent years, the amount of precipitation has usually been even lower, he regrets. But when the rain comes, “you are almost washed away.” In June 2017, almost 200 liters of water per square meter fell from the sky on a single day.

In a heavily sealed city, these amounts have nowhere to seep away, which is why numerous streets, basements and subway stations were flooded that day. The combined sewer system in Berlin's city center also overflows during heavy rain, causing a mixture of rain and wastewater to be channeled into the Spree, Havel and Landwehrkanal, making the water extremely polluted. Since such extreme weather events and longer heat periods are becoming more frequent due to the climate crisis, the management, i.e. storage or use of rainwater on site, has been mandatory in the Water Resources Act for newly developed properties since 2018. The owners must therefore keep the areas that are paved with asphalt or concrete to a minimum and prevent rainwater from flowing into the sewage system. This is also of great importance for the production of drinking water.

Specific measures in the area of ​​green infrastructure include creating meadows and beds, planting bushes and trees, and greening roofs and building facades. An example of blue infrastructure is the Piano Lake at Potsdamer Platz, which is fed exclusively from the rainwater collected from the roofs of the district and cools the surrounding area when it is hot. But natural ponds, lakes and rivers also fall under blue infrastructure.

»Beauty is also built.«

As part of the “Blue Pearls for Berlin” program, around 30 small bodies of water such as ponds and ponds are to be revitalized. Berliner Wasserbetriebe is planning to revive the dried-out Beerenpfuhl in Hellersdorf through the controlled discharge of purified rainwater from the adjacent residential area. Many bodies of water are currently suffering from the fact that a lot of nutrients and pollutants are washed in with rainwater when it runs off streets.

A mix of green and blue infrastructure is, for example, meadows where water can collect after a heavy downpour and gradually seep away. In Berlin there are two corresponding hollows in Dahlem and Rummelsburg, where the Technical University of Berlin is researching which plants other than grasses are suitable for additionally promoting biodiversity. Deeper roots could improve water absorption and evaporation performance, according to the website of the German Federal Environmental Foundation. To do this, however, a plant must be found “that can cope with wet feet just as well as with prolonged dryness,” says Natz.

Ultimately, the sponge city has “multi-use,” as he calls it, not only for groundwater protection, but also for microclimate and biodiversity. The principle is already being implemented well in new development areas such as Adlershof, Rummelsburg, Karow-Nord, Blankenburg-Süd and in the Tegel Schumacher district. The Buckower fields in Neukölln are also a good example. In addition to green roofs, there is also a park that can be used as a flood area in the event of heavy rain to protect the adjacent residential area. In addition, tree trenches, i.e. underground rainwater storage tanks, which are directly combined with the root area of ​​street trees.

There are also rigolents under the Gendarmenmarkt, which is often decried as the city's bald spot, says Natz. He therefore finds the criticism of the square in Mitte, which reopened completely sealed in 2025 after two years of renovation, unjustified. Its purpose is events ranging from classical open-air concerts to the Christmas market. “So you couldn’t plant a forest there,” he explains. Nevertheless, the square was rebuilt to suit the sponge city. Large trenches are installed under the pavement, i.e. hollow bodies into which the square's rainwater runs and then seeps into the groundwater.

Worse are places like Alexanderplatz, where this is not possible, among other things, because of the subway underneath. In general, the conversion of the existing building is the biggest hurdle on the way to becoming a sponge city. There is a lack of legal requirements, funding, and in some cases space - a groundwater cistern, for example, takes up the space of two to three underground parking spaces - and often solutions across properties. Here Berlin depends on the goodwill of the owners.

This is also why it is necessary to “keep explaining to what extent the sponge city benefits us,” says Natz. Last but not least, “beauty is also built.” That's exactly what the new mobile phone game shows: As soon as the gray areas are supplemented with meadows, flowers and trees, the picture becomes colorful and happy characters run through it. In reality, it doesn't happen that quickly. The conversion of the whole of Berlin into a sponge city will take generations, predicts Natz.

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