Sponge cities to absorb the effects of the climate crisis

Floods are becoming increasingly frequent, even in Italy, due to increasing climatic instability. Sometimes, however, an extreme event can be the cause of a turnaround. On 2 July 2011, a torrential storm hit Copenhagen, causing more than USD 2 billion in damage. The disaster was the wake-up call that prompted the Danish capital to transform itself into a 'sponge city', with an extensive programme of urban landscaping to better absorb cloudbursts. Some cities, such as London or Berlin, benefit from natural features, large parks and green spaces that help drain excess water quickly. Others, such as Copenhagen, must actively design solutions in response to climate threats, expanding green spaces and innovative infrastructure.

This 'sponginess', i.e., the ability to absorb rainwater without harming its inhabitants, is now a key indicator of the climate resilience of an urban area, in fact, even in the metropolitan city of Milan as many as 90 interventions are underway in 32 municipalities to prevent flooding. The project, carried out in cooperation with the Cap group, is financed with more than 50 million euro from the NRP and will be completed in March 2026. To date, 30 worksites have been completed and 19 are in progress, but they have not yet solved the problem of the Seveso and Lambro floods, as seen 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 important architecture prizes, which was created in honour of the famous Canadian landscape architect Cornelia Hahn Oberlander. Growing up in rural China, Yu studied the seasonal swelling of watercourses and the role of vegetation in slowing their rush. But, as he explained on receiving the award, it was his first-hand experience of the devastating effect of 'grey infrastructure' on the urban landscape that prompted him to develop the concept of the sponge city. For Yu, the turning point was the Beijing disaster of 2012: the worst flooding in the history of the metropolis with 80 deaths and more than eight thousand homes destroyed. Cities, according to Yu, must work in tune with nature, instead of relying solely on grey infrastructure such as pipes and pumps. His idea is to apply natural drainage systems in urban sites to retain water and slow down the flow of rain in rivers, thus reducing the likelihood of flooding and mitigating water shortages and heat islands during the summer.

The sponginess of a city is influenced by the balance of blue (watercourses), green (grass, trees) and grey (buildings, impermeable surfaces) infrastructure. The type of soil and vegetation, as well as the potential for water runoff, play an important role. Sandy soils are generally more spongy than clay soils, but the depth of the water table also has an impact. If the water table is close to the surface, such as in Milan where it continues to rise, this reduces the absorption capacity of the soil. Auckland tops the list of the most 'spongy' cities in the world, according to a survey of ten major cities by the engineering services company Arup. Half of Auckland's land area is covered by green or blue infrastructure, compared to 39 per cent in New York and 31 per cent in London. In this research, developed with the World Economic Forum, Arup showed that nature-based solutions are on average 50 per cent cheaper than purely man-made alternatives and provide 28 per cent more direct and environmental benefits.

Copenhagen offers an example of a city proactively turning itself into a sponge. Spurred on by the 2011 disaster, the Danish capital launched a 'Stormwater Management Plan' to protect itself from extreme rainfall and rising sea levels. Today, hundreds of projects have been implemented in and around the city, and hundreds more are being developed, starting with the smallest - grassy ditches that retain and filter rainwater (bioswales), pocket gardens, green roofs, permeable courtyards and car parks, water-absorbing plants - up to large parks and reservoirs. The plan combines green infrastructure - such as the Enghave Park, designed to slow down water runoff, or the permeable square named after Karen Blixen - with grey infrastructure, such as large underground tunnels to store and divert rainwater runoff. This hybrid approach has made Copenhagen a global model for climate adaptation.