In the course of the just-concluded flood season, 19 of the 30 pilot "sponge cities" in China witnessed flooding, which has triggered doubts about the efficacy of the whole sponge city project.

The idea behind sponge cities is simple: collect and store excess water during periods of heavy rainfall, and then parcel it out during subsequent dry periods. The Ministry of Housing and Urban-Rural Development defended the plan on Sept. 12, saying that the sponge cities will not become effective immediately, and that their construction is still accelerating.

According to guidelines for the sponge city project, by 2020, 20 percent of urban areas will be able to absorb excess rainwater through upgraded sewer systems, filtration pools, wetlands and permeable public spaces; by 2030, that figure will rise to 30 percent, an official with the ministry said.

Xie Yingxia, deputy director of the Water Affairs Institute under the China Academy of Urban Planning and Design, noted that the public should not deny the utility of sponge cities just because of flooding caused by extremely heavy rainfall. Xie added that even sponge cities are only effective at offsetting flooding within a certain range.

In 2015 and 2016, the Ministry of Finance, the Ministry of Housing and Urban-Rural Development and the Ministry of Water Resources selected 30 cities, including Beijing, Shanghai and Shenzhen, in which to pilot the sponge city project.