Just under a decade ago, China launched an ambitious project to revolutionize its cities. The idea was to protect its populations from floods, which would become worse due to climate change.
In 2015, China's Ministries of Finance, Housing and Urban-Rural Development and Water Resources identified 16 cities to implement sponge city pilot projects. In these locations, the control of rainwater runoff is bigger, increasing the capacity to absorb, retain and release rainwater when necessary.
That is achieved through measures ranging from building roads and sidewalks with permeable materials to green roofs and vegetated buffer zones. Permeable concrete can reduce surface runoff of rainwater, which can then be stored in underground reservoirs to be purified or discharged into rivers.
Green roofs reduce and purify rainwater. Rainwater can also be collected through pipes. Rain gardens, on the other hand, are areas that have a greater capacity to retain rainwater due to both being lowered and the composition of the soil and vegetation.
A study published in the scientific journal Elsevier analyzed sponge city projects in different Chinese cities such as Shanghai, Zhoushan, Suzhou and Xi'an. The research shows that rain gardens improve runoff by around 70%.
Limitations
In 2021, the World Bank calculated that 67% of the Chinese population lived in flood-prone areas in 640 cities. The country has determined that, by 2030, 80% of urban areas should be "sponges", absorbing and reusing 70% of the water from heavy rains.
The thirty cities chosen to implement the projects will receive annual subsidies of 400 million to 600 million yuan (about US$55,3 mi and US$83 mi, respectively).
In the government work report presented by Premier Li Keqiang to the National People's Assembly in 2017, the sponge city idea was included as a priority. The report proposed “starting building over 2,000 kilometers of urban underground pipe corridors, initiating a three-year action to eliminate major flood-prone sections in urban areas and promoting the construction of sponge cities,” PM Li Keqiang highlighted in the report.
Architect and landscape designer Yu Kongjian, who is also a professor at Peking University and one of the leading researchers on the subject, told People's Daily that the idea is for designed drainage systems and natural ecosystems to complement each other. “We propose building a set of green infrastructures and using natural sponge systems to solve problems that engineering pipe networks can't solve. This nature-based solution is a multi-objective system thinking, of which solving water problems is just one of the goals.”
Edited by: Rodrigo Durão Coelho
