How Sponge Cities Work?

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.

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.

Aditi Rajagopal is a freelance journalist. This article was edited by: Jennifer Collins and Tamsin Walker, and it is published courtesy of Deutsche Welle (DW).