Summary of Key Points
Following the start of the rainy season in 2026, severe rainfall in central and eastern China caused significant urban flooding, with insurance losses exceeding 11 billion yuan, highlighting the shortcomings in urban drainage systems. The root causes of flooding lie in the excessive hardening of surfaces (which prevent rainwater from infiltrating) and the frequent occurrence of extreme weather events that exceed the design capacity of drainage networks. Sponge cities, as a solution, have moved from pilot projects to a standard practice, with policy support and financial backing. Some cities have seen notable success, but they face challenges such as poor coordination in planning, lack of maintenance, and concerns from developers. In densely populated areas, the implementation of sponge city initiatives requires overcoming spatial limitations, with urban renewal being a priority. Sponge cities are not intended to replace traditional drainage systems; rather, they serve as a complement to them, providing a "soft expansion" of drainage capacity while also incorporating emergency drainage measures to cope with extreme weather.
I. Rainwater Cannot Be Retained: The Combined Impact of Hardened Surfaces and Extreme Rainfall
Why do cities flood during heavy rains? There are two main reasons:
- Surfaces Are Too Hard: Urbanization over the past few decades has led to the paving of many areas with concrete and asphalt, which are impermeable to water. In three major urban agglomerations, the area covered by impervious surfaces has increased from 22,500 square kilometers to 69,300 square kilometers, with over 300 cities having a hardening rate of more than 65%. Under natural conditions, 70-80% of rainwater would infiltrate into the ground; now, 70-80% becomes surface runoff, relying entirely on drainage networks. The water absorption capacity of asphalt roads is only 6-12% that of grasslands, quickly overwhelming the drainage systems.
- Excessive and Rapid Rainfall: Climate change has made extreme rainfall more frequent. What used to be a once-in-a-century storm now occurs every 20 years, far exceeding the design standards of drainage networks (many systems are designed for events occurring only once every 1-3 years). Even larger pipes would not be effective; these systems rely on gravity to drain water, and when rivers rise during heavy rains, they can back up into streets. Additionally, low flow rates during normal times can lead to clogging. The combination of these factors results in flooding.
II. Sponge Cities: Policy Support but Challenges in Implementation
Sponge cities aim to absorb rainwater like sponges, and progress is rapid, but there are many issues:
- Visible Policy Efforts: The State Council has issued guidelines (the "National 20 Measures") to promote their implementation, and Shenzhen has enacted laws to regulate their construction. The Ministry of Finance has provided funding to cities such as Quzhou (which received 90 million yuan). In Sanming, 42.49% of the main urban area is now permeable, and all seven flood-prone areas have been eliminated; Yangzhou has received an A rating in performance evaluations for three consecutive years; Beijing has achieved 40% of its urban area covered by sponge facilities.
- Challenges in Implementation:
- Dis disconnect between Design and Function: Engineering solutions may be functional but unsightly, while aesthetic designs (such as rain gardens built at high positions) prevent water from entering drainage systems.
- Lack of Maintenance: Many facilities are neglected after construction, leading to water accumulation and plant damage in green spaces.
- Developers' Concerns: An increase in costs by 3-10% is not a major concern for developers, but they worry about affecting the appearance of their properties, which could impact sales.
III. Can Dense Cities Accommodate Sponge Cities? Urban Renewal Is Key
Some argue that China's high population density makes it impossible to implement sponge city initiatives. However, this is not the case:
- Spatial Constraints Are Overcome: European and American cities have a population density of 3,000-4,000 people per square kilometer (with more low-rise housing and green spaces), while China has 10,000 people per square kilometer. Sponge city elements, such as permeable paving, green spaces in residential areas, and rooftop gardens, can have a significant impact when combined.
- Old Cities Need Sponge Cities More: Central urban areas, with the highest hardening rates and highest flood risks, require immediate renovation. Lu Qiuje believes that sponge cities will become a mandatory requirement for urban development, just like fire safety measures.
IV. Sponge Cities Are Not a Panacea: They Supplement Traditional Drainage Systems
Sponge cities are not a substitute for traditional drainage systems; they complement them:
- Soft Expansion of Capacity: They can store or infiltrate rainwater, reducing the amount and speed of water entering drainage networks. For example, a system designed to handle a one-year flood could, with sponge city additions, handle a five-to ten-year flood without the need for larger pipes.
- Limited Effect on Extreme Floods: Sponge cities are effective against moderate rainfall (20-30 millimeters), but extreme events still require additional measures such as storage ponds and emergency drainage systems. In 2021, the state proposed a comprehensive approach involving source reduction, network management, water storage, and emergency response.
- Effective Evaluation: The success of sponge cities should be measured by how much rainwater they retain and how many flood-prone areas they alleviate, not just by the appearance of the facilities.
Conclusion
China's urban water management is evolving from a situation where floods are inevitable to one where cities can "breathe" (i.e., effectively manage water flow). This transformation requires legal enforcement, professional collaboration (between designers and engineers), and support from developers. It also depends on public understanding: sponge cities are not a miracle solution, but when used properly, they can solve significant problems. After all, no one wants to see their city flooded again.