Copenhagen is adapting to a warmer world with rain tunnels and 'sponge parks'

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This photo shows a broad tree-lined pathway at Enghaveparken, a park in Copenhagen, Denmark. Adults and children walk along the unpaved pathway, which is covered by shadows of branches and leaves.

Copenhagen, Denmark, is expected to receive 30% more rainfall by the end of the century. The city is responding with a massive long-term adaptation plan. Enghaveparken, pictured here, is part of that plan. The park was redesigned after a 2011 flood to be able to transform into a massive reservoir in the event of heavy rain. Claire Harbage/NPR hide caption

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Claire Harbage/NPR

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COPENHAGEN, Denmark — Sometimes, a single storm can change a city. For Copenhagen, that storm hit on July 2, 2011.

"It was over 100 millimeters of rain in a couple of hours. It fell directly on the city of Copenhagen," recalls Mark Payne, a climate researcher at the Danish Meteorological Institute.

In this 2011 photo, a truck and a van are partially submerged in water after a cloudburst of extremely heavy rain in Copenhagen, Denmark.

In July 2011, Copenhagen was hit by a "cloudburst" — an extreme rain event. The storm dumped more than 5 inches of rain on the city in a few hours. Martin Lehmann/Polfoto via AP hide caption

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Martin Lehmann/Polfoto via AP

Sewers overflowed, and the streets filled with raw sewage, causing more than $1 billion in damage to property. Video from the weather event shows city streets transformed into waterways and passing city buses generating waves that broke against storefronts, while the rain persisted, relentless and torrential.

In this photo, Mark Payne, a climate researcher at the Danish Meteorological Institute, stands by the railing of a building's terrace, overlooking a street lined with multistory buildings. He's wearing glasses, a brown T-shirt, light gray shorts and black flipflops.

As the atmosphere warms, it can hold more moisture. This can lead to more intense rainfall events. "The typical rule of thumb is that for 1 degree Celsius of warming, the atmosphere holds 7% more water," says Mark Payne, a climate researcher at the Danish Meteorological Institute.
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Claire Harbage/NPR

Climate scientists like Payne call these storms "cloudbursts," or extreme rain events. As the planet warms, the atmosphere can hold more moisture, which can lead to more intense rainfall events.

Amanda Devecka-Rinear and her young daughter stand at the edge of their front yard, which borders a bay. They are skipping gravel-like stones on the water.

"The typical rule of thumb is that for 1 degree Celsius of warming, the atmosphere holds 7% more water," Payne says.

Europe is the fastest-warming continent, according to scientists from the United Nations and the European Environment Agency. Temperatures are increasing at twice the global average rate, leading to more fires and flooding.

Top: Parts of Copenhagen were built centuries ago. Now the city is racing to adapt to a warmer and wetter future. Bottom: The Kastellet, or Citadel, one of the best-preserved fortresses in Northern Europe, was damaged in the 2011 cloudburst in Copenhagen. Claire Harbage/NPR hide caption

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Claire Harbage/NPR

That means cities built centuries ago are racing to adapt.

In Copenhagen, meteorologists predict the city will receive 30% more rainfall by the end of this century. Just a year after the flood, city officials adopted a "Cloudburst Management Plan," a $1.3 billion public works project to complete 300 flood-mitigation projects over two decades.

This photo shows a large-diameter tunnel — the Kalvebod Brygge tunnel project — under construction. A tall ladder extends upward to the bottom of the tunnel's circular opening. The bottom of the ladder is in an area with a concrete floor and tall concrete walls on the left and right.

Construction work on the Kalvebod Brygge tunnel project in Copenhagen. When completed, tunnels like this one will hold excess rainwater during extreme rain events like the 2011 cloudburst. Claire Harbage/NPR hide caption

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Claire Harbage/NPR

Tunnels beneath the city

One of the plan's most high-profile projects is nearly complete: the Kalvebod Brygge Cloudburst Tunnel.

In this photo, Jes Clauson-Kaas is photographed from about the chest upward. He's wearing a yellow fluorescent vest and hard hat, as well as safety goggles.

Jes Clauson-Kaas is chief consultant for the Copenhagen waterworks utility HOFOR, which is overseeing the construction of the Kalvebod Brygge Cloudburst Tunnel. Claire Harbage/NPR hide caption

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Claire Harbage/NPR

To show it off, engineer Jes Clauson-Kaas carefully descends down nine stories of rickety scaffolding along the edge of a massive concrete hole bored into the earth.

When he arrives at the bottom, he lifts his head and brings two fingers to his lips to produce a loud whistle, its screech echoing along the curved concrete walls above.

"An opera singer's dream," he says, smiling.

Clauson-Kaas is chief consultant for the Copenhagen waterworks utility HOFOR, which is overseeing the construction of the tunnel. As he walks the perimeter of a cathedral-sized vault that will soon be the pump room, he motions to a hole in the wall just above his head: It's the terminus of a tunnel that's nearly a mile long and 10 feet in diameter and can hold 10,000 cubic meters of water, equal to the volume of four Olympic swimming pools. It's one of seven new tunnels dug under Copenhagen to hold excess rainwater during cloudburst events.

Clauson-Kaas explains how it works: Once the tunnel is full, he says, the pumps are switched on, "and they can simply suck out this water into the harbor."

A retention pond with a couple of "islands" with foliage and ducks.

Enghaveparken, in the Vesterbro district of Copenhagen, has been designed to help mitigate flooding in the city. Different areas of the park are lower and have retaining walls to keep water inside the park and away from the residential area nearby. Claire Harbage/NPR hide caption

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Claire Harbage/NPR

Collecting rain in "sponge parks"

Just blocks away from the Kalvebod Brygge tunnel is Enghaveparken, a century-old park that was redesigned after the 2011 flood to transform into a massive reservoir in the event of heavy rain.

This photo shows Jan Rasmussen, photographed from about the shoulders up in Enghaveparken. He's wearing a dark shirt and glasses. Blurry foliage is in the background.

Jan Rasmussen, director of climate adaptation for the city of Copenhagen, pictured in Enghaveparken. Claire Harbage/NPR hide caption

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Claire Harbage/NPR

Today, neighborhood children play on a soccer field carved deep into the ground. The park itself is enclosed by a 3-foot-high concrete wall with gates that automatically emerge from the ground, sealing the park so that it can hold floodwater.

Storing rainwater on the surface makes more sense than expanding the sewer system, says Jan Rasmussen, director of climate adaptation for the city of Copenhagen.

"You can't expand a pipe as easily as this area can be expanded," he says. "[It's] also much cheaper if we can use areas like this to store rainwater."

Rasmussen says Copenhagen has transformed 20 green areas like Enghaveparken into what the city nicknames "sponge parks." When full, he says, Enghaveparken can hold 25,000 cubic meters of water. When it's not raining, a 2,000-cubic-meter storage tank underneath a rose garden in the park holds rainwater collected from the neighborhood's storm drains, used to irrigate the park's lawns, trees and flowers.

In this photo, a man and woman walk on one of several raised walkways at Karens Minde Park in Copenhagen. The sunken area underneath the walkways is filled with foliage.

Karens Minde Park in Copenhagen has built-in areas for rainwater retention. It's one of the hundreds of projects that Copenhagen is taking on to prevent damage from flooding. Claire Harbage/NPR hide caption

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Claire Harbage/NPR

A cluster of people stand in the shallow waters of the Virgin River in St. George, Utah. An open cooler sits on a sand bar in between the grassy banks. The river provides the desert community with water, but climate change and a growing population threaten that supply. Local leaders are looking toward recycled sewage as a solution.

In addition to the sponge parks and cloudburst tunnels, Copenhagen has also just broken ground on a human-made island off its coast, named Lynetteholm. The island will house 35,000 people and protect the city against storm surges, another weather phenomenon happening more often here as the climate warms and sea levels rise. The island is expected to be finished by 2070.

The scale of the city's long-term climate adaptation planning has gained admirers among urban planners throughout the world, says Rasmussen. In addition to penning a collaboration with New York City on climate adaptation, he says, Copenhagen has also consulted with cities in South Africa, China, Germany and beyond on how they can build similar infrastructure to mitigate the increased flooding that comes with climate change.

"The trick to make all of this a success," says Rasmussen, "is intense planning."

This photo shows a raised walkway above a catchment basin in a green space in Copenhagen. Tall grasses rise up from the catchment basin, and some trees rise up near the walkway. Seen through the trees are some city buildings.

Evidence of Copenhagen's plan to fight flooding can be seen all over the city — like this catchment basin in a green space along Scandiagade street. Claire Harbage/NPR hide caption

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Claire Harbage/NPR

Esme Nicholson contributed reporting to this piece from Berlin. Edited by Rachel Waldholz

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