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What are sponge cities and how do they work?

Aditi Rajagopal in Copenhagen
March 1, 2024

Copenhagen is just one city among many around the world taking a novel approach to prevent repeated flooding. It is becoming a sponge.

Boats and houses in Nyhavn, Copenhagen
Copenhagen, a harbor city, is working to climate-proof its urban infrastructureImage: Steffen Trumpf/dpa/picture alliance

Though it is one of the busiest roundabouts in the east of Copenhagen, the air at Sankt Kjelds Plads isn't heavy with the smell and texture of exhaust fumes. And rather than the roar of engines, the soundscape is characterized by the sputtering notes of long-tailed tits.

The traffic circle, which is covered with shrubs and trees, is part of a large-scale experiment to transform public spaces in the Danish capital. The idea is to make Copenhagen more "livable" by creating places for citizens to meet and habitat for biodiversity, while simultaneously creating cogs in a flood-control machine. 

This transformation was sparked by the events of July 2, 2011, when Copenhagen was struck by what was dubbed "a once in a 1,000-year rain."

The massive downpour caused streets and homes to flood. With nowhere to go, the water stayed for days. Dead rats were seen floating around the city, and research later revealed that a quarter of sanitation workers had fallen sick with blood infections such as leptospirosis in the cleanup. One even died.

A tree-filled roundabout see from above, surrounded by mid-rise apartments
Sankt Kjelds Plads is one of over 250 spaces in the city that have been redesignedImage: City of Copenhagen

Over the next seven years, this kind of "cloudburst" started to become increasingly common, with four "once in a 100-year" rainfall events recorded in that period. Costing the city at least €800 million ($865 million) in damages, it was clear to policymakers that it was time to rethink the Danish capital.

Time to adapt to a more sponge-like urban design

Over the last centuries, the focus of urban development in places such as Copenhagen has been on creating "machine cities" that could be built up at speed and were efficient for housing, industry and the economy. But many of these cities, especially those that straightened rivers or built over floodplains, ended up meddling with the water cycle. 

With concrete and asphalt covering areas once given over to grass and soil, the water from heavy rains has nowhere to go. Too often, that results in flooding, and cities around the world are now exploring ways to reverse this kind of urban development. And they are doing it by turning themselves into urban "sponges."


In other words, they are creating spaces and infrastructure to absorb, hold and release water in a way that allows it to flow back into the water cycle.

With over 60 of its cities under redevelopment and now incorporating climate inlets, green reefs and rain gardens to hold water, China is leading the way. Jan Rasmussen, head of Copenhagen's "Cloudburst Master Plan," also saw potential for Denmark.

"Our politicians decided that there's really a need to get the water of the city very fast," said Rasmussen. "They asked if we could do this in a clever way, could we expand the sewer system? Could we handle rainfall at the surface?"

Soaking up the rainwater

Having studied sponge city projects around the world, Rasmussen's team conceptualized the redesign of some 250 public spaces that could help in the retention or redirection of floodwaters, including parks, playgrounds and the Sankt Kjelds Plads roundabout. The idea is to use the ability of trees, shrubs and soil to retain water naturally and let it flow to places where it is not destructive. 

A dozen ponds bordering the roundabout are designed to retain excess rainwater in the event of a cloudburst. Like other similar ponds around the city and wide openings on the sides of low-lying streets, they serve to funnel floodwater into a network of tunnels being laid 20 meters (about 65 feet) below the surface.

Artist's impression of an area that can be used as a pond during heavy rain
The idea is to create spaces that can be used by the public but also serve to retain water, should the need ariseImage: Tredje Natur
Artitst's impression of a playground flooded with water, surrounded by trees
How the above space could look after sudden heavy rainfallImage: Tredje Natur

During a "normal" downpour, rainwater is directed through this drainage system to the harbor. But when there is an excess like in a cloudburst scenario, a pumping station at the harbor will kick into action, forcing the water collecting in the tunnels out to sea, thereby creating space for more rainwater and preventing the streets from flooding. This is currently under construction and will be ready by 2026. 

"There will still be water in the streets. I mean, it's not going to be completely dry. But we'll go from 1 meter [of floodwater] down to 20 centimeters maximum," said Jes Clauson-Kaas, an engineer at HOFOR, the waterworks department responsible for the tunnel construction.

Long-term benefits make 'sound financial sense'

Part of the challenge is to get locals on board. And when it comes to closing kids playgrounds or city parks for extended periods to turn them into flood zones, or financing the adaptation plans via a levy on water bills, that is not always easy. 

But Clouson-Kaas said fitting a flood-prone city for the future makes sound financial sense. "We lost around a billion on this one event [in 2011], but we are expecting there will be quite a few events for the next 100 years. They're saying the potential loss could be at least €4 or €5 billion. So if we invest €2 billion, it still works out," he said.

Copenhagen is in the position — financially and political — to invest in such infrastructure now, rather than dealing with potential damages in the future. It's become a place that other cities are looking to learn about the benefits of creating an urban sponge.

Edited by: Jennifer Collins and Tamsin Walker