How urban landscapes can be adapted to meet increased flood risks using strategies that work with nature, instead of against it…
When architect and urban planner Kongjian Yu was a child, he almost drowned in a flood. Did he have any idea that as he pulled himself to safety, using the riverbank’s willow branches and reeds, he was planting the seed of a lifelong passion for protecting communities from future flood risk?
Years later, in 1997, Yu led a team studying urban water systems. They hooked on to the metaphor of a ‘sponge’ to describe how natural wetlands retain floodwater and release it during droughts. This is a notable contrast to manmade flood control strategies, which involve installing concrete walls along riverbanks, and removing natural features like bends, pools, and vegetation – an approach that tends to be expensive, and has a significant impact on the natural environment.
Yu’s team proposed that city infrastructure should switch focus from channelling water away, to, instead, absorbing excess water like a sponge. And so ‘sponge cities’ were born. Not only do sponge cities aim to prevent the effects of flooding, but they do so while maintaining the ecological benefits that healthy river systems and wetlands provide, such as air and water purification, carbon storage, and temperature regulation.
Why do we need them?
One in six properties in England is at risk of flooding, according to the Environment Agency, and it’s estimated that by 2050, coastal and river flooding will cost the UK between £1.6 billion and £6.8 billion annually. But climate change isn’t the only reason we’re experiencing more flooding. The UK government recognises that traditional flood defences, like concrete dams, embankments, and walls, will no longer be enough to protect us. Many cities worldwide, including Copenhagen and Rotterdam, have already adopted sponge city principles to protect against climate change.
How do they work?
A sponge city is made up of natural features that can absorb floodwater. One key component is permeable pavements, which, unlike traditional tarmac, allow rainwater to soak into the ground, reducing surface runoff and replenishing groundwater. They also lessen the ‘urban heat island effect’ (densely populated city areas can be up to 12°C warmer than the nearby countryside) which benefits buildings, humans, and animals, too.
Another common feature of sponge cities is green roofs. Just like it sounds, these essentially use vegetation to cover rooftops, which not only provides natural insulation, but also absorbs rainwater and releases it slowly. These aesthetically pleasing roofs reduce energy consumption, and offer valuable green spaces for city dwellers to enjoy (and also support their mental health). In Amsterdam, blue-green roofs go a step further, with systems that gather and store water, which is then recycled for the residents to use for watering plants and even flushing toilets.
Urban wetlands – such as rivers, lakes, swamps, and ponds – play a major role in flood prevention within sponge cities, but shockingly, more than 75% of the UK’s wetlands have been degraded or destroyed in the last 300 years, as noted in a 2023 study in the journal Nature. These wetlands absorb and store water flow by trapping and slowly releasing surface water, rain, and floodwaters. Not only do wetlands reduce the speed of floodwater, and distribute it gradually over the area, but they also offer much-needed recreational spaces for local residents and improve air quality. In Rotterdam, ‘water squares’ have been constructed for two purposes – to function as community spaces for socialising, skating, and theatre performances during dry periods, and as flood basins when rainfall is high.
Wuhan leads the way
Although there are sponge cities all over the world, from Auckland to Kenya, China’s national government pioneered the concept in 2013, with a multibillion-dollar national policy after substantial flash flooding. Wuhan became a pilot sponge city, and, according to a working paper prepared by the University of Leeds and the Coalition for Urban Transitions, it has proved that natural infrastructure “can be employed both quickly and cost-effectively to increase the resilience of urban areas to a changing climate”.
The paper notes that the project was almost $600 million cheaper than upgrading the city’s drainage system, and that benefits also include improvements in biodiversity and conservation. The city’s Yangtze River Beach Park has been known to register three degrees cooler than in the city, and the value of land in the surrounding areas has more than doubled since the changes were made.
Let that soak in
While sponge cities have many benefits, they can’t always keep up with accelerating climate change. Zhengzhou in China spent £6 billion on the sponge city programme and was still unable to cope when, in 2021, the city experienced its heaviest rainfall in history. It has been suggested that London should become a sponge city, as flash flooding has been described as the main environmental risk to people living in the capital, with the London Climate Resilience Review’s 2024 report noting the city was “underprepared” for climate change. While progress can be seen in some areas, such as rain gardens in Waltham Forest, and green roofs in Hackney, experts say that the implementation and long-term maintenance of projects in such a built-up area are difficult. However, smaller-scale initiatives – such as incentivising green roofs, restoring urban ponds, and implementing more permeable pavements – could open the floodgates to real change.
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