Copenhagen hopes cloudburst tunnel will prevent future flooding

Workers in Copenhagen are constructing an ambitious underground tunnel aimed at protecting the Danish capital from future flooding. Situated 20 meters below the city, the Kalvebod Brygge Tunnel spans 1.3 kilometers, with a capacity to store 10,000 cubic meters of water during heavy rainfall, also known as a cloudburst.

"If we have an extreme situation where the sewage systems can't keep up, we can divert the water into this tunnel," explained Ditte Reinholdt Jensen, a climate change adaptation expert at Copenhagen's water utility company, HOFOR. The tunnel can be completely drained within 10 minutes using a powerful pump, ensuring it remains ready for subsequent rain events.

Cloudbursts - intense rainfall exceeding 10 cm (3.94 inches) over a 10 square-kilometer (3.86 square miles) area within an hour - are becoming more frequent worldwide, a trend linked to climate change. Such weather events can lead to severe flooding, as seen in 2011 when a "once-in-a-thousand-year" flood caused over $1 billion in damages to Copenhagen.

The Kalvebod Brygge Tunnel is designed to handle extreme rainfall statistically expected once every 100 years. Construction began in 2020, with completion anticipated in 2027. While expensive, proponents argue the infrastructure is essential. "Doing nothing also comes with significant costs," noted Reinholdt Jensen, emphasizing the potential for damage prevention.

Beyond this tunnel, Copenhagen has launched around 300 cloudburst projects over two decades to better manage rainwater. These efforts include localized solutions such as Sankt Kjelds Plads, part of the city's 'Climate-Resilient Neighborhood' initiative. Previously a large roundabout dominated by concrete and asphalt, the area has been transformed into green spaces designed to absorb rainwater, acting like a giant sponge.

Jan Rasmussen, project director for Copenhagen's climate adaptation planning, highlighted the importance of such projects. "Over the next century, we expect up to 30 percent more rain and more frequent cloudbursts," Rasmussen said. The city's strategies aim to safeguard infrastructure, basements, and public spaces from severe water damage.

Copenhagen's approach aligns with global trends in adapting to rising sea levels and intensified rainfall. The Danish Architecture Centre (DAC) is showcasing an exhibition titled Water is Coming, which explores innovative solutions for living with water in the future. "It's a global challenge requiring local solutions," said Pernille Stockmarr, a senior curator at DAC.

These efforts exemplify Copenhagen's proactive stance on climate resilience, offering potential models for cities worldwide facing similar challenges.