Sponge cities to absorb the effects of the climate crisis

Floods are becoming increasingly frequent, even in Italy, due to increasing climate instability. Sometimes, however, an extreme event can be the catalyst for a turning point. On July 2, 2011, a torrential storm hit Copenhagen, causing over $2 billion in damage. The disaster was the wake-up call that prompted the Danish capital to transform itself into a "sponge city," with a vast urban landscape redevelopment program to better absorb stormwater. Some cities, like London or Berlin, benefit from natural features, large parks, and green spaces that help rapidly drain excess water. Others, like Copenhagen, must actively design solutions to address climate threats, expanding green spaces and implementing innovative infrastructure.

This "sponginess," or the ability to absorb rainwater without harming its residents, has become a key indicator of an urban area's climate resilience. Indeed, in the metropolitan area of ​​Milan, as many as 90 flood prevention projects are underway in 32 municipalities. The project, implemented in collaboration with the CAP group, is funded with over €50 million by the National Recovery and Resilience Plan (NRRP) and is scheduled for completion in March 2026. To date, 30 construction sites have been completed and 19 are underway, but they have yet to resolve the problem of flooding in the Seveso and Lambro rivers, as demonstrated last week.

The "sponge city" concept is attributed to Chinese landscape architect Kongjian Yu and became part of Chinese urban planning policy in 2014. Yu, founder of the Turenscape studio, won the latest Oberlander Prize, one of the most prestigious architecture awards, created specifically to stimulate adaptation to new climate conditions, in honor of the renowned Canadian landscape architect Cornelia Hahn Oberlander. Raised in rural China, Yu studied the seasonal swelling of waterways and the role of vegetation in slowing their flow. But, as he explained upon receiving the prize, it was his firsthand experience of the devastating effect of "gray infrastructure" on the urban landscape that drove him to develop the sponge city concept. For Yu, the turning point was the 2012 Beijing disaster: the worst flood in the metropolis's history, resulting in 80 deaths and over 8,000 homes destroyed. Cities, according to Yu, must work in harmony with nature, rather than relying solely on gray infrastructure like pipes and pumps. His idea is to implement natural drainage systems in urban areas to retain water and slow the flow of rainwater into rivers, thus reducing the likelihood of flooding and mitigating water shortages and summer heat islands.

A city's sponginess is influenced by the balance between blue (waterways), green (grass, trees), and gray (buildings, impervious surfaces) infrastructure. Soil type and vegetation, as well as the potential for water runoff, play an important role. Sandy soils are generally spongier than clay soils, but the depth of the water table also has an impact. If the water table is close to the surface, as in Milan, where it continues to rise, this reduces the soil's absorption capacity. Auckland tops the list of the world's spongiest cities, according to a study of ten major cities by engineering services firm Arup. Half of Auckland's land area is covered by green or blue infrastructure, compared to 39% in New York and 31% in London. In this research, conducted with the World Economic Forum, Arup demonstrated that nature-based solutions are on average 50% more cost-effective than purely artificial alternatives and provide 28% more benefits, both direct and environmental.

Copenhagen offers an example of a city proactively turning into a sponge. Spurred by the 2011 disaster, the Danish capital launched a "Cloudburst Management Plan" to protect itself from extreme rainfall and rising sea levels. Today, hundreds of projects have been implemented in and around the city, while hundreds more are under development, ranging from the smallest—grassy ditches that retain and filter rainwater (bioswales), pocket gardens, green roofs, permeable courtyards and parking lots, and water-absorbing plants—to large parks and reservoirs. The plan combines green infrastructure—such as Enghave Park, designed to slow water runoff, or the permeable Karen Blixen Square—with gray infrastructure, such as large underground tunnels to store and divert stormwater runoff. This hybrid approach has made Copenhagen a global model for climate adaptation.