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Can China’s sponge cities really prevent flooding?

With the rapid development of urbanization in China, the construction of urban facilities has caused the underlying surface to harden. In 2014, the construction of sponge cities was proposed to solve the problems of urban waterlogging and urban water pollution. The concept of urban rainwater management was first proposed in the United States in the mid-1980s, called Low-impact development, which emphasized the protection and use of local natural landscape features to ensure the quality of runoff rainwater.

In 2015, China identified 16 first batches of sponge cities for pilot projects and invested 30 billion yuan in construction funds in three years. According to public information, Zhengzhou was selected as a provincial pilot for the construction of sponge cities in Henan Province in 2016. By 2020, Zhengzhou would invest 53.48 billion yuan to build a sponge city project.

Severe flood disasters still happen even with sponge city construction

In July of this year, China's Henan Province encountered a rare and extremely heavy rain, and many cities experienced waterlogging, causing casualties and serious economic losses. Zhengzhou, Henan, as a sponge city construction site, has had a huge investment in this project, but why was the flood disaster still so severe?

This extreme rainstorm has been sorted out and here are statistics: the three-day rainfall reached 617.1 mm from 20 o'clock on the 17th of July to 20 o'clock on the 20th, which is close to the average annual rainfall, 640.8 mm. Both hourly precipitation and single-day precipitation have broken through the 60-year historical record since Zhengzhou was established in 1951. One hour of precipitation of 201.9 mm is equivalent to 150 West Lake. This torrential rain in Zhengzhou has occurred once in more than a thousand years.

However, as a northern inland city, Zhengzhou does not have a high water capacity. The highest flood control standard for the construction of Zhengzhou Sponge City is designed and constructed in accordance with the 200-year standard. Zhengzhou is located in the transition zone from the middle and lower reaches of the Yellow River to the Huanghuai Plain, where the terrain is relatively low. Also, Zhengzhou has developed into super-large cities, land development is relatively saturated. Due to the concentration of population and resource concentration, exposure to major disasters is higher and losses are greater.

Practical usage of sponge city

Sponge City cannot play a role before such a huge amount of precipitation, so what's exactly its usage?

The original intention of sponge city construction is to manage urban water resources, purify water quality, and promote the underground infiltration of urban surface runoff, and the sustainable circulation of rainwater, not to solve the problem of waterlogging. Therefore, the sponge city cannot solve the waterlogging caused by high-intensity rainstorms. The problem it solves is the runoff retention of medium and light rain, and it plays an auxiliary role in solving urban water pollution and drainage problems.

Based on the idea of low-impact development, the construction of Sponge City also includes pipe network construction, regulation and storage facilities, and water conservancy and flood control facilities. But the most significant concept is the coexistence with nature. The basic principle of large-scale urban planning and construction is to maintain the hydrological process as much as possible, reserve space for rainwater storage and discharge, instead of digging lakes for landscaping, causing the original wetland ponds to be squeezed as well as not much green corridor.

The Lynbrook Estate development project in Victoria demonstrates effective implementation of Water Sensitive Urban Design(WSUD, Low-impact Development in Australia). The project combines conventional drainage systems with WSUD measures at the streetscape and sub-catchment level, intending to attenuate and treat stormwater flows to protect receiving waters within the development. Primary treatment of the stormwater is carried out by grass swales and an underground gravel trench system, which collects, infiltrates, and conveys runoff. The main boulevard acts as a bioretention system with an underground gravel-filled trench to allow for infiltration and conveyance of stormwater. The catchment runoff then undergoes secondary treatment through a wetland system before discharge into an ornamental lake.

The Mangrove Ecological park in Futian, Shenzhen is built by the sea. An environment without freshwater makes it difficult for animals to survive. The construction of the Sponge City project can collect rainwater and store it in the Blue Cloud Lake in the park, turning it into a freshwater lake. With the help of Tencent Cloud, the park re-planned the whereabouts of rainwater: repairing green roof rainwater gardens, vegetation buffers, green roofs, laying permeable concrete, permeable bricks, and other facilities. Rainwater flows along the planned path. In the freshwater lake, the rainwater that exceeds the standard overflows into the sea. The ecological water supply for the park is as much as 400,000 cubic meters each year.

In addition to the urban construction project, the concept of the sponge city has also been applied to many buildings.

For example, Tencent Binhai Mansion is laying sponge bricks made of ceramic permeable material to slowly release rainwater from the ground to underground pipelines. Sensors such as flow meters and water level gauges are installed inside the sponge brick. In the "cloud", it is easy to judge whether its water seepage function is normal or not. There is also a sky garden full of green plants in the building. In addition to flower viewing, the rainwater collection system at the bottom can store, and release water intelligently for watering flowers and plants.

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