China's sponge cities: soaking up water to reduce flood risks

From permeable roads to rooftop gardens, cities are investing in ways to harvest rainwater

Trillions of litres of free, fresh water drop from the sky onto the world's cities each year - yet most of it is channelled straight into gutters, drains and rivers. At best, this represents a waste of a valuable natural resource. At worst, it can lead to devastating urban floods like the one in the Japanese city of Joso this month that killed eight and destroyed hundreds of homes.

With cities getting bigger and climate change threatening to bring more extreme weather, some scientists and politicians are proposing "sponge cities", a reimagination of the urban environment where almost every raindrop is captured, controlled and reused.