Sponge Cities: Urban Transformation Explained!

Copenhagen is embracing a unique approach to combat frequent flooding, distinguishing itself among other cities worldwide. It is evolving into a sponge.

Despite being a bustling roundabout in eastern Copenhagen, Sankt Kjelds Plads is not filled with the usual heavy scent and texture of exhaust fumes. Instead of the deafening noise of engines, the atmosphere is filled with the delightful chirping of long-tailed tits.

This traffic circle, adorned with lush shrubs and trees, is part of an ambitious project to transform public spaces in the Danish capital. The aim is to create a more “livable” Copenhagen by providing meeting places for citizens and fostering biodiversity, all while serving as a crucial component in flood prevention.

The catalyst for this transformation was the events of July 2, 2011, when Copenhagen experienced an unprecedented rainfall that occurs only once in a millennium.

The torrential downpour resulted in severe flooding of streets and homes. Trapped with nowhere to drain, the water lingered for days. The city witnessed the unsettling sight of dead rats floating around, and subsequent investigations revealed that a quarter of sanitation workers had contracted blood infections like leptospirosis during the cleanup. Tragically, one worker even lost their life.

In the following seven years, these intense “cloudbursts” became more frequent, with four “once in a century” rainfall events occurring within that period. The damages incurred by the city amounted to at least €800 million ($865 million). It became evident to policymakers that a reevaluation of Copenhagen’s approach was imperative.

Embrace a sponge-like urban design for adaptability and resilience

It’s time to embrace a more sponge-like approach to urban design. In the past, cities like Copenhagen focused on creating efficient “machine cities” that prioritized speed and functionality. However, these cities often disrupted the natural water cycle by altering rivers or building over floodplains.

By replacing grass and soil with concrete and asphalt, heavy rains have nowhere to drain, leading to frequent flooding. Thankfully, cities worldwide are now seeking ways to undo these effects and are transforming themselves into urban “sponges.”

In simpler terms, they are constructing areas and structures that can absorb, retain, and release water in a manner that enables it to reenter the water cycle.

China is at the forefront of this movement, with more than 60 of its cities undergoing redevelopment and incorporating climate inlets, green reefs, and rain gardens to retain water. Jan Rasmussen, the head of Copenhagen’s “Cloudburst Master Plan,” also recognized the potential for Denmark.

Absorbing rainwater

"Sponge Cities: Urban Transformation Explained!"

After studying sponge city projects worldwide, Rasmussen’s team came up with a plan to redesign 250 public spaces to manage floodwaters better. This includes parks, playgrounds, and the Sankt Kjelds Plads roundabout. The concept involves using trees, shrubs, and soil to naturally retain water and guide it away from areas where it could cause damage.

A dozen ponds surround the roundabout, designed to retain excess rainwater during heavy storms. These ponds, along with others in the city and openings in low-lying streets, help channel floodwater into a network of tunnels buried 20 meters underground.

Under normal circumstances, rainwater flows through the drainage system to the harbor. However, during intense storms, a pumping station at the harbor will activate, pushing water from the tunnels out to sea. This process creates more space for additional rainwater, preventing flooding in the streets. The construction of this system is ongoing and is expected to be completed by 2026.

According to Jes Clauson-Kaas, an engineer at HOFOR, there will still be some water on the streets, but the flooding will be significantly reduced. Instead of 1 meter of floodwater, the maximum depth will be 20 centimeters.

Long-term advantages align with sound financial reasoning

Long-term benefits are financially prudent. It can be challenging to gain local support when it involves closing children’s playgrounds or city parks for extended periods to convert them into flood zones, or funding adaptation plans through a water bill levy.

However, Clouson-Kaas emphasized that preparing a flood-prone city for the future is a wise financial decision. “We suffered a loss of approximately one billion from a single event in 2011, but we anticipate numerous events over the next century. Experts estimate potential losses to be at least €4 or €5 billion. Therefore, even with a €2 billion investment, it remains a viable option,” he explained. Copenhagen is currently in a strong financial and political position to invest in such infrastructure now, rather than facing significant damages in the future. It has become a model for other cities seeking to understand the advantages of developing an urban sponge.