How the Movement Towards Sponge Cities Can Help Soak up Extreme Rainfall
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Cities in China are being redesigned to use green infrastructure to manage flooding and improve the quality of life for residents. This is Wuhan Sponge Park Research and Visitor Center. Courtesy: archdaily.com
We’ve all seen the images of cars and other necessities of urban life being forced downstream during extreme storm events. It’s terrifying that this could and does happen in our well-planned cities. Urban areas that have been traditionally designed with lots of concrete and asphalt when an out of the ordinary rain event happens, can’t cope with so much water. The rain just runs off on the surface because these surfaces are incapable of absorbing water - they are impervious. Too much rain overwhelms urban stormwater systems and overflows onto city streets creating rivers capable of carrying off human life and all our possessions too.
Now, prominent urban designers are encouraging us to think differently and design our cities with nature in mind.
Bioswales have become more common approaches to Sponge City development in North America. This new bioswale in Chicago is designed with engineered growing media from Omni Ecosystems that holds significant amounts of water flowing from the parking lot through the curb cut. Photo: Steven Peck
Conceived by Chinese landscape architect and professor Kongjian Yu, the “Sponge City” concept was formally endorsed by the Chinese government in 2015. They turned the idea into a national initiative by targeting 16 of the worst flooding cities in the country. Wuhan, for instance, since the 1980s had covered up all but 30 of its 127 lakes, diminishing the city’s water retention capability substantially. Each of the 16 cities were given the equivalent of $50 million to invest in planning and infrastructure – to reimagine their cities without the flooding problems. In seven years, the number of Chinese cities using the Sponge City techniques has risen from 16 to 30, with even more being planned.
"Implementing sponge cities is key to securing urban resilience and ecological health. By allowing cities to naturally absorb and repurpose rainwater, we not only combat flooding but also transform our urban environments into lush, green havens that thrive in harmony with nature’s cycles, countering the inefficiencies of traditional un-resilient gray infrastructure", says Professor Yu.
This methodology is crucial as cities globally face increasing challenges from extreme weather events and urban sprawl. Yu’s Sponge City initiative offers a sustainable alternative, emphasizing the vital role of ecosystem services through the implementation of various green infrastructure.
This is an example of one Sponge City technique. Porous paving, which supports a car above plants, reduces the urban heat island effect and allows rainwater to infiltrate back into the ground, where it can recharge aquifers. Photo: Steven Peck
“The Sponge City proposes a paradigm where cities are not merely engineered to resist natural forces but are designed to work synergistically with them, enhancing urban biodiversity and sustainability”, says Professor Yu.
The idea is not entirely new – the US has promoted low impact development for decades, as has the UK and Australia with their Water Sensitive Urban Design and Sustainable Drainage Systems, respectively. All of these approaches champion the use of techniques designed to infiltrate, store and purify water naturally, including the use of green roofs, rain gardens, retention ponds, permeable surfaces/paving, bioswales and trees and shrubs.
Sponge cities take the concept one step further by ensuring the water is captured at source, the flow is slowed down and cleaned so it becomes usable water. Think about how a sponge works – while it absorbs water, it generally can’t soak up a spill all at once. It takes different tries to get it all. The urban concept includes the use of rain gardens and ponds which can temporarily hold excess water. Sponges can also effectively re-direct the flow of the water – thus the idea of using winding, meandering streams and surrounding those small water bodies with wetland plants to further absorb and clean the water. Many ocean-fronting cities, anticipating sea level rise, are also constructing wetlands on their waterfronts to deal with overflowing water.
If city planners were to embrace the inevitability of stormwater and create a welcoming floodable area, this could be used beneficially by the community. Building elevated walkways through constructed wetlands and installing permeable surfaces are three examples which could be enjoyed by city residents regardless of whether the stormwater is rising or receding. These re-naturalized spaces would also become home to an innumerable number of plant and wildlife species and help improve local air quality. More greening techniques could also help reduce the urban heat island effect most cities experience in the summer months. Many studies have shown that people of all ages need nature for their mental health and well-being.
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This constructed wetland in Toronto is designed to hold water and provide much needed habitat for many wetland species. Source: J. McLean.
Conventional engineering practices to manage stormwater, commonly called gray infrastructure, such as dams, pipes and water treatment facilities are not designed to capture and use stormwater effectively and efficiently. These systems often become overwhelmed especially during a 100-year storm event – a situation many cities are experiencing more frequently. There will always be a need for gray infrastructure to manage black water for example, but enhancing that expensive built form with natural approaches is the basis for a sponge city plan. Every urban area has unique soil and other geographical characteristics, and with the expectations of future severe storms, urban designers need to consider as many nature-based solutions as possible in a sponge city plan.
In North America, one of the leading examples is Philadelphia. In 2011, Philadelphia began its Green City, Clean Waters plan, a 25-year, $4.5 billion project to reduce and effectively channel an average of 13 billion gallons of stormwater annually. A combination of green infrastructure technologies is being used in this low-lying city, which is anticipating a significant sea level rise over the next couple of decades.
Auckland, New Zealand is considered one of the best examples of a sponge city. It is naturally blessed with ponds and lakes, lots of parks, gardens and green space and has sandy soils, which absorb water more readily than clay soils. On the other hand, Sydney, Australia has a much higher percentage of hard, impermeable surfaces, making flood prevention and management much more challenging there.
A conceptual diagram of the elements and ecosystem services that result from sponge city development. Courtesy: re-thinkingthefuture.com
Conclusion
Spending billions on managing stormwater might seem wasteful but when you consider the physical and financial damages that severe storms can inflict on communities, it’s akin to a hedge against future climate changes. According to the World Meteorological Organization (2022), 44 percent of global weather-related disasters are linked to flooding. In Toronto in 2013, one extreme storm caused flash flooding and close to $1 billion in property damage. Insurers are starting to back away from insuring flood-prone areas as the liabilities continue to climb. As more people migrate to cities and climate change threats continue, future-proofing our cities just makes fiscal sense.
Remember Benjamin Franklin’s 1736 adage when commenting on fire-threatened Philadelphia – an ounce of prevention is worth a pound of cure. Bring on the spongy cities!
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Joyce McLean is the assistant editor of the Living Architecture Monitor and a freelance writer.
For more information on sponge cities, please visit https://www.turenscape.com/en/home/index.html, or
https://www.sciencedirect.com/science/article/pii/S2666916121000153
