The long read: As we face increased flooding, China’s sponge cities are taking a new course. But can they steer the country away from concrete megadams?
were built from scratch. Builders paved floodplains and farmland, felled forests and channelised rivers, leaving stormwater that once filtered into the ground with nowhere to go but up and over levees. Then, one notable flood struck the national government where it lives.in 60 years. As much as 46cm of rain fell on some parts of the city, filling underpasses and flooding roads a metre deep. Landscape architect Yu Kongjian barely made it home from work. “I was lucky,” he says.
Globally, urban flooding has become particularly acute as the land area covered by cities worldwide has doubled since 1992. Researchers from Johns Hopkins Universityhow impervious surfaces increase flooding: every time a city increases coverage of absorbent soil with roads, pavements or car parks by 1%, runoff boosts the annual flood magnitude in nearby waterways by 3.3%. To counteract this trend, sponge cities seek places throughout urban areas for water to sink into the ground.
Yu and other urban water detectives are looking to manage water to a grander extent, seeking out connected routes for water to slow and flow across entire watersheds, which often extend beyond jurisdictional boundaries. Solving a city’s flooding problems requires coordination with communities and landowners upstream. Ideally urban designers could absorb water where it falls, reducing stormwater runoff at every rooftop and at every farm field upstream.
With this information, Slow Water practitioners can better understand how a particular variable affects the way water behaves. When their landscape maps are complete, they send test floods through the digital model they’ve created. These experiments allow them to identify pinch points where water is constrained and will flood first. Then they experiment with a topography adjustment or the addition of a wetland or pond to see how each affects stormwater behaviour.
The project was nearly complete when I saw it in April 2018. About 4km long and perhaps two city blocks wide, the park follows the river. Workers removed concrete along the river channel and excavated soil to widen the riverbed. That dirt was then moulded into a large berm running down the centre, creating two channels. The river flows on one side, while the other channel has large holes of varying depths that act as filtration pools and direct the water flow.
If China ignores this specificity, its broad ambition for sponge cities may falter, says Chris Zevenbergen, an expert in urban flood-risk management at the IHE Delft Institute for Water Education in the Netherlands and a visiting professor at China’s Southeast University. The rush to develop cities in the past 20 years did not allow builders time to understand imperfections in design and make changes.
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