‘Sponge City’: How Copenhagen Is Adapting to a Wetter Future

In just two hours on July 2, 2011, a torrential, once-in-a-millennium storm battered and flooded Copenhagen, pounding parts of Denmark’s capital with more than 5 inches of rain. Critical infrastructure at the city’s largest hospital was swamped, as were major roads, basements, and businesses. The city that had been engaged with advanced sustainability planning for decades, it turned out, was woefully unprepared for the fierce rainfall, which caused $1.8 billion in damages. 

Shaken by the calamity, the city and its citizens grasped that such climate disasters — and deluges even more severe — were inevitable and required a rapid and strong response. To that end, Copenhagen brought together its top urban planners, landscapers, consultants, and architects to turn the city, which stretches across two main islands in the Baltic Sea, into the world’s first full-fledged “sponge city.” This state-of-the-art defense system — which combines nature-based surface features, like wetlands and parks, with large underground structures, like storage pipes and retention basins — is expected to fortify the city against cloudbursts and sea level rise for 100 years. 

The intricately designed network manages stormwater and rising tides by soaking them up, storing them, and then slowly returning water to the water cycle. The sponge city build-out has been so successful that cities as disparate as Auckland, Nairobi, Singapore, New York, Rotterdam, and Berlin — and many others in the U.S. and Europe — now consider it an example to emulate. 

“Even though the Chinese were first and coined the term sponge city,” explains Maryam Naghibi, an urban landscape architect at Delft University of Technology, in the Netherlands, “Copenhagen is a model for cities everywhere with such dense urban fabric.” 

The Intergovernmental Panel on Climate Change predicts that urban centers in high latitudes will experience both heavier and more frequent rain events in future decades, with 100-year flood events at least doubling in frequency across 40 percent of the globe by 2050, according to a 2024 paper published in Nature. Moreover, the melting of polar ice sheets will cause sea levels to rise — an obvious threat to low-lying, sea-bound Denmark. The Danish Meteorological Institute predicts as much as 55 percent more precipitation in the winter months by 2100, with downpours ever more intense, should global temperatures rise by 2 to 3 degrees Celsius over preindustrial levels. Denmark’s adjacent seas — the North and the Baltic — could rise by up to 4 feet.

“We have projections on how the city is going to look and where the climate crisis will be,” says Christian Nyerup Nielsen, the global director for climate adaptation at Ramboll, a consulting firm that helped develop climate adaption masterplans for several areas of Copenhagen. “We have to anticipate the extreme weather events a century from now.” 

A year after Copenhagen’s 2011 flood, planners unveiled the Cloudburst Management Plan, a comprehensive citywide blueprint to revamp the city’s defenses against heavy rainfall and storm surge and offer some protection against drought as well. Today, hundreds of flood-mitigation projects blanket the city, with hundreds more in the works. Some are massive, like subterranean pipelines roughly 10 feet in diameter that convey stormwater to treatments plants and then to the harbor. Others are more unassuming, including bioswales (vegetated depressions that retain and filter stormwater), pocket gardens, and “sponge parks,” which combine green roofs, permeable pavement, and water-absorbing plants. 

Although the Cloudburst project, which was originally slated to be completed in 2032, is less than halfway to its goal, experts say that Copenhagen is already in significantly better shape to withstand torrential rains. According to Naghibi, the city’s flood risk has been reduced by 30 to 50 percent in high-priority areas.

The sprawling array of completed projects includes both “blue-green” and “gray” innovations. The former reflect a nature-based approach: Green spaces like parks and gardens filter and retain stormwater; trees and other plants suck up water; open spaces, like lawns and meadows, allow infiltration; streams with revegetated banks that have been freed from underground pipes (a process called daylighting) spread out and retain stormwater; lakes that have been enlarged and ringed with wetlands increase water storage capacity. 

Likewise, green roofs and façades absorb rainwater — while freshening city air. The city also replaced impermeable pavement on roads, parking lots, and in public squares with permeable materials that enable them to drain more efficiently.

Complementing the surface modifications, Copenhagen also employs gray, or engineered, solutions to receive the overflow from parks and streets, store it, and — in times of extreme rainfall — release it directly into Copenhagen Harbor. This network includes mile-long underground tunnels, subterranean basins, pumping stations, and enlarged sewage pipes, which convey both sewage and stormwater. 

Among Cloudburst’s flagship projects, perhaps none is more eye-popping than Karen Blixens Square, one of the city’s largest public spaces, on the University of Copenhagen’s southern campus. The nearly five-acre expanse of undulating cast-concrete domes and oval pocket gardens functions as a gathering space for students and the public, an event arena, and a catchment for rainwater. In the event of extreme precipitation, the depressions under the mogul-like “bicycle hills” — which shelter more than 2,000 bikes — retain stormwater, thus taking pressure off the nearby canal. Throughout the square, planted garden beds also absorb water, promote evapotranspiration, and provide habitat for insects, birds, and wildflowers. 

“Copenhagen’s adaption efforts aren’t just technical and functional, but they’re social too,” says Naghibi. “The infrastructure is aesthetically pleasing and experiential, like collection basins that are also skate parks and amphitheaters.” Moreover, she says, the city’s design solutions “offer co-benefits like shade, biodiversity, and urban cooling.” In contrast, she says, China’s sponge city initiative is a broader, national program with a stronger emphasis on large-scale infrastructure. It aims not only to manage runoff, but also to conserve water and improve water quality across numerous cities.

Across the harbor from Karen Blixens Square lies historic Enghave Park, a roughly 11-acre swath divided into lawns, gardens, playgrounds, and ball fields. It took three years to redesign and physically lower the park, which now includes an integrated underground reservoir with a capacity of close to 6 million gallons. Concrete retaining walls guide rainwater into the park and increase its water storage capacity by an additional 3.7 million gallons. When the walls aren’t in the service of flood prevention, park visitors can sit on them.

Copenhagen’s water utility, HOFOR, is responsible for the four subterranean tunnels, or “water highways,” that form the backbone of the city’s new underground network. The two completed tunnels were expensive — a total of $98 million — but they were designed to handle the torrential downpours expected over the next century. Another tunnel is in progress, and the final one is scheduled to begin construction next year.

Copenhagen’s 2012 blueprint was significantly bluer and greener than gray, says Ramboll’s Nyerup Nielsen. But this has gradually shifted. “The plan was to have almost everything aboveground, but now there’s more beneath-the-earth construction than we envisioned. This is partly because when you really need to move fast you tend to fall back on what you have done before.” Gray infrastructure can handle significantly greater volumes of water than smaller-scale green modifications, he says.  

Copenhagen’s learning curve has been steep. “Because many pieces of the city are interlocking, when you change one thing it affects something else,” says city planner Jonathan Reghev, referring to the existing underground systems for energy, drinking water, and communications. For example, the plan to regrade city streets, turning them into “cloudburst roads” that funnel water into city parks, faltered because of the immovable water mains and electrical cabling beneath them. “It’s a massive engineering challenge, and there’s no end date now,” he says, referring to the original projection for completion. 

The quality of the water in the tunnels has presented another aggravation. There’s disagreement about how clean the runoff must be to flow into the harbor and freshwater ponds, says Reghev. “Some runoff will have to go back to the sewage treatment plants first. We didn’t know about microplastics and forever chemicals back when we began this. This has caused clashes between environmental protection norms and the adaptation planning.”

Northern Europe is currently suffering from severe drought, another consequence of the climate crisis. Denmark’s drought index has been above 9 (on a scale of 1 to 10) since mid-May. Although Copenhagen’s architects say that urban flooding is the city’s foremost concern, the sponge city model offers advantages in times of aridity, too. The city’s green spaces, which detain water, help to replenish the city’s aquifers, and its tunnels, which hold rainwater from moderate-size storms, function as reservoirs that can be tapped during dry spells. “This is one reason the tunnels are so big,” says Liam Blunt, a sustainable urban planning expert at Lund University in Sweden. “The extra size of the pipes is for storage.”

Mostly, though, the broader, long-term benefits of Copenhagen’s adaptation initiatives have yet to be realized. At the onset of the Cloudburst program in 2012, the city had stated plainly: “Until the full system is in place, Copenhagen remains vulnerable.” Experts and developers say that the city’s progress thus far is impressive, and worthy of emulation. But they also acknowledge that Copenhagen still couldn’t handle a tempest like the one that hit it in 2011.