A new survey published this week by Arup has found that the urban centre of London is the least “sponge-like”, or naturally able to absorb heavy rainfall, of seven major global cities. London emerges with a “sponge” rating of just 22%, way behind Auckland, which ranks top with 35%.

Arup’s first-of-its-kind Global Sponge Cities Snapshot aims to prompt cities to ask – “how spongy am I?” Authors of the survey are calling on city leaders to move beyond concrete interventions and instead look to nature for solutions to climate-related challenges, such as managing heavy rainfall.

Authors studied a sample of approximately 150 square kilometres in seven diverse global cities – Auckland, London, Mumbai, Nairobi, New York, Shanghai and Singapore – to assess how well their existing natural infrastructure helps them absorb rainfall. 

The new analysis comes as the IPCC predicts that water-related risks will increase with every degree of global warming, with around 700 million people currently living in regions where maximum daily rainfall has increased. In 2021 London was hit with rainfall levels much higher than usual – with 47.8 mm of rain falling in a 24-hour period, most of it in just one hour. 

In Auckland, half of the area studied was made up of green and blue spaces, which helped it achieve its high “sponge” rating. The only city snapshot to contain a greater proportion of green and blue spaces is Nairobi (52%), however Auckland’s more permeable soil helped to give it a narrowly higher overall “sponginess”.

By comparison, London and Shanghai align much more with the ‘concrete jungle’ stereotype, with a higher percentage of hard surface in their urban centres - 69% and 67% respectively. This places them as the two least “spongy” of the cities surveyed. Also, in comparison to Shanghai, the urban centre of London had lower proportions of tree coverage further impacting its sponge ranking. The capital, however, has a mid-ranking soil type, similar to Singapore, Shanghai and Auckland and more permeable, with less run-off potential than Nairobi and Mumbai.  

Arup’s global water leader Mark Fletcher said: “Our message is simple - all cities need to be asking: ‘how spongy am I?’ Given the challenges of climate adaptation, cities can’t go on being concrete jungles, in conflict with nature. Every city has unique natural assets – grasslands, forests, parklets, lakes and ponds – these need to be quantified, valued and utilised in the same way we treat other vital resources. Our survey is not intended as a score card, but to show cities how they can use digital tools to quickly establish a far better understanding of their natural assets. Even the “spongiest” cities need to enhance and work with nature to deliver maximum resilience.”

To create the calculations, Arup’s team used an advanced digital tool, Terrain, which applies machine learning and artificial intelligence techniques to accurately quantify the amount of green infrastructure (e.g. grass, trees) and blue (e.g. ponds, lakes), versus the amount of grey (e.g. buildings and hard surfaces). The survey was based on detailed satellite imagery covering a snapshot of approximately 150 square kilometres of each city’s main urban centre. Authors supplemented this analysis with insight on soil types and vegetation, enabling them to estimate how much rainwater would be absorbed in a defined heavy rainfall event. Based on these factors, the cities ranked as follows.

1. Auckland – 35% spongy

2. Nairobi – 34% spongy

3. Singapore – 30% spongy

3. Mumbai – 30% spongy

3. New York – 30% spongy

4. Shanghai – 28% spongy

5. London – 22% spongy

The term “Sponge City” was coined in 2013 by Professor Kongjian Yu of Peking University, describing cities that work with nature to absorb rainwater, instead of using concrete to channel it away. Natural infrastructure is not only extremely effective in managing flood water, but also on average 50% more cost-effective than man-made alternatives, delivering 28% more added value. 

Will Cavendish, Arup’s global digital services leader, said: “To flourish, cities need to learn quickly how to deploy nature-based solutions, which bring far wider benefits than traditional engineered ‘grey’ infrastructure and contribute positively to carbon reduction. Thankfully, powerful new digital tools can help city leaders make this transition with speed and impact. We hope that cities will use these tools to drive more informed, faster decision-making that enhances the positive impact of nature-based infrastructure.”  

Click here for further information and to download Arup's Global Sponge Cities Snapshot report.