What are sponge cities and can London become one?

Flooding in London three years ago caused widespread damage and disruption. One way to avoid a repeat is for the capital to become more… spongy. 

Sponge cities use green spaces like parklands and blue spaces like lakes and rivers to absorb rainwater and allow it to drain away safely. Given that flash flooding has been named a "lethal risk" to people living in London, the park-rich capital seems a natural fit for the concept. But it might not be that simple.

What are sponge cities?

The concrete jungles of the 21st century are proving inadequate in the face of increased rainfall and surface flooding. This type of flooding happens when large amounts of rain fall on "hard, impervious surfaces such as concrete, overwhelming the local drainage system", said the BBC. As more rain falls the system backs up, "leaving water to pool on roads and pavements".

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In the UK, sewage and rainwater are often carried in the same pipes, so heavy rainfall can lead to overflows of polluting wastewater into our waterways. A sponge city is designed to absorb rainwater by using green and blue spaces to lower the amount of excess rainwater entering the sewerage system.

Sponge cities "mimic natural processes", said The Rivers Trust, allowing urban areas to "absorb, store, and purify rainwater, much like a sponge soaks up water". They deploy various green infrastructure techniques, including permeable pavements, green roofs and urban wetlands, to slow down the flow of rainwater and gradually release it into rivers and streams. 

The natural systems of sponge cities "not only catch the water", said CPRE London, but "store and purify it, alleviate water shortage, support habitat and biodiversity, and keep cities cool rather than asphalt-heavy cities which insulate heat".

Cities such as Shanghai, New York and Cardiff are already "embracing their 'sponginess' through inner-city gardens, improved river drainage and plant-edged sidewalks", said Climate Champions.

But some researchers have criticised the model, said Jenny Knowles in a blog on MDPI. She argued that it is "not suitable for extreme, large-scale flooding" and that in denser areas, there may be "insufficient space to implement sponge city features".

Could London become a sponge city?

London has "an array of green spaces" with an estimated 8.4 million trees, so you might imagine it is spongy already. But in 2022 a development firm used computer modelling to compare London's sponginess with seven other cities: Auckland, Mumbai, Nairobi, New York, Shanghai, Singapore and Sydney. It found that London ranked sixth, ahead of only Sydney. 

In 2021, torrential rain "battered" London, said the BBC, damaging thousands of properties, flooding dozens of Tube stations and causing patients to be evacuated from hospital wards. Following that chaos, City Hall created the Surface Water Strategic Group to draw up a new flooding strategy for the capital.

A spongy feature being considered is the use of sustainable drainage systems (SuDS) but sceptics say that SuDS take up valuable space for parking and they need regular maintenance, which would need to be carried out by local residents. London is also very built-up so creating new green spaces is difficult and can be unpopular.

"As a concept, sponge cities is great; the issue is with its implementation," Susanne Charlesworth, professor of urban geography at Coventry University, told the BBC. But Alastair Chisholm, director of policy at the Chartered Institution of Water and Environmental Management, said it is "one of the most deliverable solutions we can do". He called on "all of our political leaders to really push the concept of sponge cities".