Sponge cities are designed to prevent flooding in cities, but they also prevent urban heat
The ‘sponge cities’ have been successful in mitigating urban flooding problems. But they also have an unintended benefit of reducing urban heat. So what kind of urban construction model is a sponge city?
A sponge city, according to Chinese guidelines, is one that has transformed hard surfaces, such as 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 on a city-wide scale.
Sponge city policies are a set of nature-based solutions that use natural landscapes to catch, store and clean water; the concept has been inspired by the ancient wisdom of adaptation to climate challenges, particularly in the monsoon world.
Therefore, sponge cities can alleviate urban flooding, water resources shortage, and the urban heat island effect and improve the ecological environment and biodiversity by absorbing and capturing rainwater and utilizing it to reduce floods. Rainwater harvesting can be repurposed for irrigation and home use. It is a form of a sustainable drainage system on an urban scale and beyond.
The concept was proposed by Chinese researchers in early 2000 and accepted by the CCP and the State Council as urbanism policy in 2014. 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 RMB in support, China pushed urban design developed to prevent flooding.
Instead of grey infrastructure, which is to centralize and accumulate water using big reservoirs, speed up the flow by pipes and drains, and fight against water at the end by higher and stronger flood walls and dams, a sponge city would typically include porous brick or concrete pavement, porous asphalt roads, green roofs, green verges, bio-retention basins, ponds, rainwater wetlands, grassy swales, and vegetation buffer zones.
A sponge city also has an unintended benefit of reducing urban heat. For example, a study in Guangzhou, China suggested the adoption of porous bricks and porous concrete could lower pavement surface temperature by 12 and 20 °C, respectively. The air temperature can be reduced by up to 1 °C.
A green roof – one with vegetation covering it – reduces rainfall running off, alleviating flooding, and can reduce ambient temperature and alleviate heat stress through evaporative cooling in summer. A case study in Hangzhou, China indicated that green roofs could generate moderate effects on pedestrian air temperature reduction (around 0.10–0.30 °C) while achieving a peak cooling performance of 0.82 °C.
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 of 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, formal responses have not been well implemented. In China, there are no dedicated plans, policies, and actions for 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.
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.
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.
In addition to reducing temperature and flooding, a sponge city can provide financial, institutional, and social benefits.
Sponge city construction is capable of connecting different government departments and enhancing 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 is required, along with a technical database that includes solutions to likely problems across disciplines and assesses the performance of the common techniques.
Sponge city works can also improve social equity by reducing ambient temperature, improving outdoor thermal comfort, reducing heat-induced morbidity and mortality, and alleviating energy poverty.
Other projects for urban heat mitigation can be realized through a sponge city approach, such as green buildings, low-carbon eco-cities, smart cities, forest cities, and haze treatment. Many challenges from technical, economic, social, and institutional perspectives remain, and local pilot projects are needed for performance assessment, project optimization, and reducing uncertainties and risks.