Chinese sponge cities continue their development
By 2035, 70% of the Chinese population is set to live in an urban area. To cope with this galloping urbanization and its share of disadvantages, in 2015 the Chinese government launched a construction program for 30 sponge cities nationwide. Designed to absorb and then release large amounts of water, sponge cities are made up of porous spaces and surfaces. Permeable roads and sidewalks, green roofs and wetlands absorb, filter, store, purify and drain rainwater. The 30 sponge cities chosen across the country have an annual precipitation ranging from 410 to 1,830 mm on a case-by-case basis. Their aim is to retain 70 to 90% of rainfall.
Wuhan, the capital of Hubei province, also known as “the city of a hundred lakes,” is today China’s leading sponge city. Located at the merging of the Yangtze and Han rivers, it is vulnerable to flooding, especially during the monsoon months. After the torrential downpours of 2016, which caused 14 deaths and flooded a large part of the city (blocked roads and metro stations), 228 projects were launched in the two “pilot” districts of Qingshan and Sixin. Over 38 km2 were retrofitted, representing a total investment of 11 billion yuan (1.4 billion euros). In Nanganqu Park in the city center, rainwater now infiltrates into the soil and is stored in underground reservoirs. The system only discharges water into the river when the water level returns to normal. 20% of this investment comes from the central government while the rest was funded by private investment.
Between 12.5 and 19 million euros per km2
However, building a sponge city is extremely expensive. The Chinese Ministry of Housing and Urban-Rural Development (MoHURD) estimates the required investments to be around 100 to 150 million yuan per km2, i.e. a range of between 12.5 and 19 million euros per km2. Since 2015, the MoHURD has subsidized 16 sponge cities, providing 400 million yuan (50.6 million euros) per municipality and estimates the total investment required at 86.5 billion yuan (10.9 billion euros). The central government will only cover 15 to 20% of the development costs for a total of 450 km2. The rest will have to be provided by local governments and the private sector.
In addition to funding, a report written in 2017 by Chinese researchers notes that developing sponge cities in the country raises two other major challenges: governance and design. The first sites showed that the municipalities concerned by these projects must be encouraged to adopt local regulations, in order to alleviate water quality problems and measure and fully recognize the economic and environmental benefits.