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Sponge Cities - Green over Gray

Figure 1. Sustainable Green Metropolis (Photo credit. Adobe)

A sponge city is a new urban planning model in China that emphasizes flood management via strengthening green infrastructures instead of purely relying on drainage systems, proposed by Chinese researchers in early 2000 and accepted as a nationwide urban construction policy in 2014.  The “sponge city” initiative was designed to make greater use of lower-impact “nature-based solutions” to better distribute water and improve drainage and storage. 

A Sponge City relies on the installation of additional green areas and replaces concrete with permeable surfaces. These surfaces absorb water during times of rainfall. During peak summer, the water evaporates and cools the city down. Those solutions included the use of permeable asphalt, the construction of new canals and ponds and also the restoration of wetlands, which would not only ease waterlogging but also improve the urban environment.

Many cities, all over the world, are dealing with two seemingly unrelated challenges: flooding and drought. These two issues, however, are not as disconnected as they appear, and California suffered from both simultaneously throughout 2022 and into 2023. Usually, cities suffer from times of high temperatures, exacerbated by urban heat island effects, while at other times they experience high levels of sudden rainfall, with stormwater runoff systems unable to handle the quantities of water. 

As cities expand traditional infrastructure, increased use of asphalt and concrete increases the impervious area, which reduces the space for water to run off or drain away into, and escalates urban heat. Breakneck urbanization has encased vast stretches of land in impermeable concrete, often along banks of major rivers that traditionally served as flood plains. With wetlands paved over and nowhere for surplus water to settle, waterlogging and flooding were commonplace. 

A recent Intergovernmental Panel on Climate Change (IPCC) report said 700 million people already live in areas where rainfall extremes have increased, a number expected to grow as global temperatures rise.

In 2016, flash floods in Nairobi left streets submerged, trees uprooted and buildings in heaps, while Tropical Storm Elsa flooded New York in 2021. Both storms disrupted livelihoods and killed dozens.

An equal benefit of sponge cities is that they can hold more water in rivers, greenery, and soil instead of losing it to evaporation, meaning they are more resilient to drought. sponge cities aim to control and improve urban flooding, water pollution, and microclimates, recycle rainwater resources, and re-instate degraded environments. 

The key features of the sponge city concept- to be environmentally adaptive, systematic and comprehensive, and environmentally friendly- envision urban developments as systems that absorb, store, infiltrate, and purify rainwater. 

However, the concept has its limitations but no real downsides. Even if sponge city measures had been implemented in full, they would have been unable to prevent this year’s disasters. Experts believe sponge city infrastructure can only handle no more than 200 millimeters (7.9 inches) of rain per day.

The Sponge City philosophy is to distribute and retain water at its source, slow down water as it flows away from its source, clean water naturally, and be adaptive to water at the sink when water accumulates – this is in stark contrast to the conventional solution of gray infrastructure, which is to centralize and accumulate water using big reservoirs, speed up the flow by pipes and channelized drains, and fight against water at the end by higher and stronger flood walls and dams. 

Sponge cities aim to promote positive interactions between socio-economic systems within the cityscape and with the urban water cycle to enhance local urban resilience, particularly in the face of increasingly volatile water-related disasters. The theory of Sponge City emphasizes the basic principles of 'based on nature', 'source control', 'local adaption', protecting nature, learning from nature, preserving urban ecological space as much as possible, restoring biodiversity, and creating a beautiful landscape environment. All of this can be realized by achieving natural absorption, natural infiltration, and natural purification. 

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