Soft and full of holes, sponges absorb and retain water. With heavy rainfall and storms predicted to increase due to climate change, cities that are equipped with plenty of permeable surfaces such as parks, trees and lakes that soak up water like sponges, will be better able to mitigate floods.
To measure how well urban spaces can manage excess water, Arup, a global built environment consultancy, surveyed seven major cities at risk of heavy rainfall and severe floods using an artificial intelligence and land use analysis system called Terrain. In March 2022, it published a report on its findings called the “Global Sponge Cities Snapshot.”
“From region to region, you see cities being developed in a slightly different way,” explains Thomas Sagris, the digital water research lead at Arup. “For example, some of the cities in China, they tend to be more densely developed with high-rise buildings. In Auckland (New Zealand), you see more low-rise buildings, more spaces between them.” A city’s design impacts its ability to accommodate water, he says.
Each city was assessed on three key factors: the quantity of water-absorbing green and blue spaces such as grass, trees, lakes and ponds; types of soil and vegetation; and “water runoff potential” – a calculation of the amount of rainfall that will run off the land, rather than be absorbed by it.
Terrain analyzes high quality satellite images to locate and identify green and blue areas, and gray infrastructure, such as buildings and pavements, within a given urban space. By incorporating additional data sets on parameters such as soil quality, the cities’ natural absorbency was calculated and expressed as a “sponginess score.”
Ranked from highest to lowest in their overall “sponginess” rating, the seven cities surveyed were Auckland (35%), Nairobi (34%), Singapore (30%), New York (30%), Mumbai (30%), Shanghai (28%) and London (22%).
Auckland, in New Zealand, came out on top for its city-wide stormwater management systems and large parks. Not far behind is Nairobi, the Kenyan capital. With a national park on its outskirts, which is home to lions and giraffes, the city has the largest quantity of permeable land of all seven locations. However, densely populated areas in the city have little to no green infrastructure with floods often submerging its slums, according to the report.
Mumbai, in India, tied with Singapore and New York for third place in the “sponginess” rankings. Next in the list was Shanghai, and the last place went to London, largely because the city has clay-rich soil which is less water-absorbent, says Sagris.
According to the report, cities can improve their “sponginess” by adding more parklands, meadows, green roofs and other green infrastructure. The key lies in understanding where “flooding hotspots” are and strategically integrating nature-based solutions into existing gray infrastructure to improve the overall climate resilience of the city.
It can make economic sense too: another report, which Arup published with the World Economic Forum in January, suggests that investments in climate resilient, nature-based solutions are “on average 50% more cost-effective than man-made alternatives.” Yet only 0.3% of overall spending on urban infrastructure was allocated to nature-based solutions in 2021.
Sagris hopes that city planners and stakeholders will invest in green infrastructure which, he says, does much more than just act as a sponge.
Incorporating nature into cities helps to combat climate change by absorbing carbon, and brings health and social benefits for citizens, he says. “These (green) areas can become places where communities meet and engage.”