Rain, seepage, evaporation

10 April 2025

As climate change takes hold, extreme weather events are increasing: Heat waves are proving particularly problematic for the very young and the old; heavy rain is flooding streets and basements and pushing sewage systems to their limits. This is why municipalities like Berlin are working to ensure that their cities can retain and store their precious water more effectively – rather like a sponge.

Where pharmaceuticals and pesticides once used to be produced in the VEB Berlin-Chemie plant back in the days of the GDR, people now live. Not all of the approximately 1,000 apartments in the 52 Grad Nord district in Berlin-Köpenick have yet been completed – there is also a lack of playgrounds, and green spaces are yet to materialise. For several years now, however, the sustainable residential complex has boasted an eye-catching development at its heart: An artificial lake consisting of several water basins that extend over a total of 6,000 square metres. However, it isn’t there just for the residents to splash around in or to chill out in the evenings. The basins are fed via underground pipes from the rainwater that falls on the surrounding houses. This arrangement ensures that this precious liquid commodity is not flushed into the sewers but remains on hand to offer the neighbourhood some welcome cooling respite during the heat of the summer.

In the summer months, the water is purified by a filter system. In winter, the natural sewage treatment plant made up of the marsh plants and grasses that grow at the fringes of the ponds is sufficient for the task. What has arisen is an urban biotope that serves as a habitat for fish and amphibians and also as a landing station for waterfowl. Houses that cannot feed their rainwater into the basins divert it into infiltration ditches. These consist of packed masses of gravel in the subsoil through which the rain can slowly seep into the groundwater.

“Schwammstadt” (Sponge City) is the name of the principle to which the district and the city of Berlin have committed themselves. The idea is to restore the natural cycle of rain, seepage and evaporation that has been broken in urban areas.

Concrete and asphalt prevent rain from seeping into the city’s subsoil. To avoid flooding, the principle which has been in use since the late 19th century is the complete removal of all water. Rainwater is directed out of the city via the sewers. In the age of climate change, however, this principle is coming up against its limits more and more frequently and to more obvious effect.

Extreme weather such as heavy rainfall is becoming more frequent and severe, flooding basements and underground stations and causing millions of euros in damage. And such events are also increasingly causing the old combined sewer system, which in Berlin, as in almost all major cities, extends under the historic urban area, to overflow: Wastewater from showers and toilets has to share a pipeline to the sewage treatment plants with rainwater runoff. If the pipes fill up during heavy rainfall, some of the untreated water gets discharged into rivers – often with fatal consequences for fish and other aquatic animals.

The human inhabitants of cities are being affected by increasingly hot summers. This is because concrete and asphalt heat up more and for longer than soil. During the day, urban areas become up to three degrees warmer than the surrounding countryside, with the nighttime difference increasing by as much as twelve degrees. This puts a particular strain on older people and those with chronic cardiovascular diseases. 15,600 premature deaths were reported in Germany in the hot summers of 2018 and 2019. A trend that is becoming even more acute as global warming progresses.

To catch the rain and keep it where it falls, thus reducing urban heat stress while also preventing the sewers from overflowing, experts have developed various concepts and measures. These include greened roofs, such as those which can be seen on most of the houses in the 52 Grad Nord district. According to figures from BUND (German Federation for the Environment and Nature Conservation), green roofs alone can collect 50 to 100 percent of the rainwater that falls on these buildings over the year. Rainwater can also be stored in cisterns and used to flush toilets or irrigate gardens and green spaces, as is planned in a residential district in Berlin-Spandau. This both saves precious drinking water and takes the pressure of the bank accounts of the residents.

Green façades, such as those at the Institute of Physics at Humboldt University in Berlin-Adlershof, also absorb water and evaporate it in the summer, thereby improving the local microclimate. As natural thermal insulators, they also save energy and costs for heating and air conditioning.

In Rummelsburger Bucht, the first urban district in Berlin to have been planned along “sponge city” lines back in the 1990s, trees stand in green hollows in which rainwater can collect and slowly seep away. In the Buckower Felder development area, infiltration ditches are also being created in the ground under the trees, which can store even more water. As in Rummelsburger Bucht, there are no more gullies. If possible, every drop of rain is used on site and directed over the slightly sloping terrain into the hollows. Excess water feeds the ponds of an artificially created wetland on the edge of the district.

The Berlin Rainwater Agency even maintains that the residential area’s sponge-city-friendly facility is cheaper than the construction of a new sewer system. Founded in 2018 by the state of Berlin and the water companies, the Rainwater Agency has since then been advising administrations, housing companies and property owners on how to use their rainwater sensibly.

Cities such as Amsterdam or Copenhagen, which set out to become “sponge cities” back in the early 2010s, are role models. In Berlin, rainwater management has been mandatory for newly built houses and residential areas since 2018. Since 2021, new buildings within the S-Bahn ring have only been allowed to discharge their rainwater into the sewers in exceptional cases.

What is usually easy to achieve in new builds through targeted planning is much more challenging in districts made up of old building stock. If homeowners opt to retrospectively green their roofs or facades, they receive subsidies from the state of Berlin. Even more funding is available if they combine the planting with a solar PV system.

Hamburg has been pursuing a similar funding policy with its green roof strategy since 2014. The Hanseatic city is set to go even further by making solar green roofs mandatory for newly built or renovated flat roofs from 2027.

But rain doesn’t just neatly fall on houses, of course. This why the district of Berlin-Mitte is looking to unseal a total of 150,000 square metres in the coming years – the area of 21 football fields. One example of an associated measure is the green gully. Green gullies bear their name because the asphalt surface gets chiselled open around them and grass and shrubs are planted. Below the Earth’s surface, a substrate ensures that rain can seep away easily and only gets as far as the gully during particularly heavy downpours.

Even the 15,000-square-metre paved area of the freshly renovated Gendarmenmarkt now conceals a sponge. Rain is collected in an underground ditch system before seeping onward into the groundwater.

When the skies absolutely open, even the concerted “sponge city” measures are not enough, at least at the moment. The water companies have therefore built huge underground wastewater storage facilities, where the water can be temporarily “parked” during heavy rainfall until the sewage treatment plants have capacity again. A total of around 300,000 cubic metres of underground storage space has already been created under the inner-city districts, plus 90,000 cubic metres at the sewage treatment plants.

The largest wastewater “parking lot” extends under the entire length of the Mauerpark. The 654-metre-long pipe can hold 7,611 cubic metres of wastewater. That would fill about 42,000 bathtubs. The idea is to ensure that the kind of flooding witnessed in 2016, when the sheer mass of water ended up stacking cars on top of each other like toys in the adjacent Gleim Tunnel, never happens again – and to protect the aquatic inhabitants of the Spree and Panke rivers from premature death.