Sponge cities: what if cities could absorb rain instead of fighting it?

For most of the past century, the logic behind urban water management was simple: get rainwater out of the city as quickly as possible.

Engineers built gutters, pipes and underground sewer systems designed to move water away fast. The faster the rain disappeared, the safer streets and buildings were thought to be. And for a long time, that system worked.

But cities have changed. They have grown denser, sealed with asphalt, concrete and rooftops that leave little exposed soil. At the same time, rainfall is becoming heavier and less predictable. The result is a system that was designed for a different kind of city and a different kind of climate. Sponge cities propose a different approach: instead of sending rainwater away immediately, they slow it down and give it space to be absorbed.

Despite the name, sponge cities don’t rely on giant sponges tucked under the streets (unfortunately). The idea is both simpler and greener: rooftops that absorb rain, parks that temporarily hold stormwater, and rain gardens that capture runoff instead of sending it straight into the sewer.

Even pavements can play a role: permeable materials allow water to slowly seep into the soil rather than bouncing off solid asphalt. None of these interventions are spectacular on their own, but together they form a network that slows rainwater down and gives it space to be absorbed, helping cities cope with storms that would otherwise overwhelm their drainage systems.

The concept gained global attention when China launched its Sponge City Program in 2015 after a series of severe urban floods. Dozens of cities began experimenting with ways to redesign streets, parks and entire neighbourhoods so they could better handle heavy rainfall.

The ambition is striking: by 2030, Chinese cities aim for around 80 percent of their urban areas to absorb and reuse most of their rainwater. That means parks that double as temporary water basins, residential areas designed to collect runoff, and green infrastructure woven throughout the urban landscape.

While sponge cities are often discussed as a solution to flooding, their benefits go well beyond stormwater management.

Because they rely on plants, soil and open water instead of concrete and pipes, these interventions can also cool down cities during heatwaves. Green roofs and rain gardens reduce what urban planners call the heat island effect, where built-up areas trap warmth and push temperatures several degrees higher than surrounding countryside.

But perhaps the most visible effect of sponge cities is simpler: they make cities more pleasant.

Rain gardens become small green oases along busy streets, parks that temporarily store water double as recreational spaces the rest of the year and green roofs that capture run off turn grey skylines into patches of vegetation.