China’s “Sponge Cities” Mitigate Urban Flooding & Dangerous Heating

‘Sponge cities’ have been successful in mitigating urban flooding problems, but they also have an unintended benefit of reducing urban heat deaths …

Zhuhai is a tourism destination known as a “forest city”—a city of 2.4 million in the Guangdong-Hong Kong-Macao Greater Bay Area at the heart of the world’s most populous area in China. It boasts 708 urban parks, tree-lined waterfront promenades, and a connection between sea and mountain.

As Zhuhai was not always so lush, nor contained roads transformed into porous pavements, but at the end of 2014, the Central Government of China issued an edict promoting what it described as ‘sponge cities’. In 30 national pilots, with 1.2–1.8 billion “Renminbi” (RMB)(official currency of the People’s Republic of China), China pushed urban design developed to prevent flooding.

According to Chinese guidelines, a sponge city is one that has transformed roads and pavements into permeable surfaces that can absorb, seep, purify and store water, and later release stored water for use.

In other parts of the world, similar infrastructure projects are known as low-impact development, blue-green infrastructure, or water sensitive urban design. But only China has implemented them at a city-wide scale.

Zhuhai has constructed more than 115 square kilometers (71.5 square miles) of sponge city infrastructure since 2016, accounting for nearly 1/4 of the total urban built-up area. Zhuhai boasts porous brick or concrete pavement, porous asphalt roads, green roofs, green verges, bio-retention basins, ponds, rainwater wetlands, grassy swales and vegetation buffer zones.

Zhuhai’s urban flooding prevention measures also help beat urban heat

‘Sponge cities’ have been successful in mitigating urban flooding problems, but also have an unintended benefit of reducing urban heat deaths. This is important because extreme heat kills.

Across Europe, it is estimated that the extreme heat events of 2020 caused a total of 6,340 extra deaths in France, Belgium, the Netherlands, and the United Kingdom. Meanwhile in British Columbia, Canada, the heatwave summer 2021 caused 569 extra deaths within a few short weeks. Cities are often the most dangerous place in a heatwave, with studies showing they can be up to 10-12 °C hotter than surrounding rural areas.

Although urban heat has been identified as a silent killer and one of the deadliest weather-related disasters, in China, there are no dedicated plans, policies, nor actions towards urban heat mitigation and adaptation. Potential urban cooling strategies are also hindered by the changing built environment and urban expansion, especially in developing countries. But controlling urban flooding through sponge city systems can have huge benefits for urban heat mitigation. 

For example, a study in Guangzhou, China suggested the adoption of these urban changes could lower pavement surface temperature by 12 and 20 degree Celsius, respectively, air temperature reduced by up to 1 degree Celsius:
• Porous bricks
• Porous concrete
• Green roof—vegetation covering it—can reduce rainfall runoff, alleviate flooding, reduce ambient temperature, alleviate heat stress through evaporative cooling in summer, generate moderate effects on pedestrian air temperature reduction (0.10–0.30 °C), while achieving a peak cooling performance of 0.82 °C. 

A sponge city can …
• Realize synergies of urban flooding and urban heat island mitigation, and the co-benefits could be applied in countries that have already established low-impact water management practices
• Provide financial benefits
• Provide institutional benefits
• Provide social benefits—Improve social equity through reducing ambient temperature
• Improve outdoor thermal comfort, reducing heat-induced morbidity and mortality, thereby alleviating energy poverty
• Realize heat mitigation through green buildings
• Realize heat mitigation through low-carbon eco-cities, smart cities, forest cities, and haze treatment
• Connect different government departments and enhance synergies, but only if there is a clear division of responsibilities

Industry and the private sector can also play important roles in providing social and financial support. Policies that detail everyone’s role are required, along with a a technical database that includes solutions to likely problems across disciplines and assesses the performance of the common techniques. Many challenges from technical, economic, social and institutional perspectives remain, and local pilot projects are needed for performance assessment, project optimisation and reducing uncertainties and risks.

In China, the co-benefits approach is already working: investment in sponge cities at the national and provincial levels can be used to subsidize urban heat mitigation works.

Beyond the mountain-sea-city landscape, Zhuhai has rapid economic development and a thriving arts and cultural scene. The green-blue infrastructure development has been no impediment to the city’s continued growth. This world-class example shows the multiple benefits achieved through sponge cities — a glimpse into a future that looks safer and more equitable for all. 

For the Hindustan Times article titled “Sponge Cities Beating the Heat in China”, please go to: www.hindustantimes.com/environment/sponge-cities-beating-urban-heat-in-china-101645780097643.html

Photos:
1 -MnDOT “Pervious Concrete” research guide: http://www.dot.state.mn.us/mnroad/projects/pervious-concrete-pavement/index.html
2- article: “UDG China Breaks Ground on Spiraling Green Roofed Kindergarten in Wuxi”: https://inhabitat.com/udg-china-breaks-ground-on-spiraling-green-roofed-kindergarten-in-wuxi/
3 – “Features and main characteristics of porous bricks” webpage: https://en-n.decorexpro.com/kirpich/porizovannyj/