'Sponge cities' might be the perfect solution for urban flooding

Because of their size, construction materials, and methods, cities significantly impact natural processes like the water cycle. This has led to a phenomenon called "urban flooding." This term refers to the inundation of water in built-up areas, such as cities and towns, due to the accumulation of water from heavy rainfall, rapid snowmelt, or other sources that runs off materials like concrete and overwhelms the local drainage systems.

In urban environments, flooding can cause significant disruptions to daily life, property damage, and, if severe enough, even the loss of life. The extent to which flooding is a problem in a city depends on various factors.

The first is the presence or abundance of relatively water-impervious structures like roads and buildings. As cities grow, natural land surfaces are replaced with concrete, asphalt, and other impervious materials. This limits the natural ability of the ground underneath to absorb rainwater, leading to increased surface runoff that can overwhelm the drainage systems, especially if they are not adequately designed and maintained (regular blocking, etc.).

Cities, or rather the large, heavy buildings constructed there, also put a lot of strain on the ground underneath them. Since they are weighty, urban areas can experience ground subsidence due to excessive groundwater extraction or the weight of built-up areas, making them more susceptible to flooding.

To reduce the impact of urban flooding, city planners and policymakers can invest in improving drainage systems, incorporating green infrastructure (like parks, green roofs, and permeable pavements), and implementing flood-resilient urban planning strategies. These efforts can help mitigate the risk of urban flooding and protect communities from its harmful effects.

One strategy is a concept known as "sponge cities."

What is a "sponge city" strategy?

As the name suggests, a "sponge city" is an urban design concept that aims to improve water management and resilience against flooding by mimicking the natural processes of water absorption, storage, and purification.

In 2013, Chinese researchers and urban designers pioneered the innovative concept of the sponge city, which was originally proposed by Professor Kongjian Yu.

As an urban ecological planner and landscape architect, Professor Yu is a distinguished professor of urban and regional planning at Peking University and the founder of Beijing's Turenscape, one of the first and largest private architecture, landscape architecture, and urbanism practices in China.

Yu's "sponge cities" seek to reduce urban flooding, water pollution, and rainwater collection by soaking up rainfall and slowing down surface run-off. This is in contrast to conventional methods, which tend to rely on building pipes or drains to carry away water or reinforce river banks with concrete to prevent them from overflowing.

It is an idea rooted in ancient Chinese farming techniques, such as storing rainwater in ponds for crops. It also draws inspiration from global integrated urban water management (IUWM) initiatives, such as sustainable drainage systems (SuDS) used in the U.K. or low-impact developments (LID) used in the U.S. The approach is designed to help reduce surface runoff, mitigate the risk of urban flooding, and increase the amount of water available for various purposes.

In other words, it works like a giant water "sponge," hence the name. This concept incorporates several elements and strategies that include but are not limited to:

• Creating green spaces like parks, green roofs, and green walls increases the porous surface areas of a city and promotes the natural absorption of rainwater into the ground.
• Using, where possible, more absorbent materials for roads, sidewalks, and parking lots allows rainwater to infiltrate the ground and reduce surface runoff.
• Constructing landscaped areas designed to capture, filter, and infiltrate stormwater, helping to remove pollutants and reduce the volume of water entering the drainage system.
• Building artificial wetlands and ponds to store and filter stormwater, providing a natural habitat for plants and animals while improving water quality.
• Installing systems to collect and store rainwater from rooftops, streets, or other surfaces for irrigation, toilet flushing, or other non-potable applications.
• Manage the flow of water so that, instead of trying to channel water away quickly in straight lines, it is slowed down through the use of meandering rivers planted with vegetation.
• The implementation of various techniques to sustainably manage stormwater, including infiltration trenches, retention ponds, and swales.
• Avoiding construction in low-lying areas and instead using these places as a water sink, where excess water and runoff is emptied into a river, lake, or sea.

By integrating these strategies, the theory is that "sponge cities" can help address urban water management challenges, improve water quality, enhance urban biodiversity, and increase resilience to climate change-related events such as floods and droughts. Sounds great, but have any ever been built?

Believe it or not, yes. But we'll need to travel to the Far East to find one.

What is an example of a "sponge city"?

And that city is Wuhan. Yes, the very same that has become infamous since the COVID-19 outbreak of 2020. Thankfully, in this case, it is for very different reasons. Wuhan has been at the forefront of the "sponge city" initiative since the Chinese government launched the “Sponge City Program” in 2013.

The program encourages cities to adopt green and blue rather than grey infrastructure. As a pilot city, Wuhan has implemented various strategies to improve water management and increase resilience against flooding.

Some of the "sponge city" concepts adopted in the city include, but are not limited to:

• Green infrastructure: Wuhan has invested in creating more green spaces, including parks, roofs, and walls. These spaces help absorb and store rainwater while providing recreational areas and enhancing urban biodiversity.
• Permeable pavements: The city has been using porous materials for sidewalks, parking lots, and other public spaces to reduce surface runoff and promote rainwater infiltration into the ground.

• Rain gardens and bioswales: Wuhan has constructed numerous rain gardens and bioswales to capture and filter stormwater, helping to remove pollutants and reduce the burden on the city's drainage system.
• Constructed wetlands: The city has built artificial wetlands that act as natural water treatment systems, filtering stormwater and providing habitat for wildlife.
• Rainwater harvesting: Wuhan has promoted rainwater harvesting systems in residential and commercial buildings to collect and store rainwater for irrigation, toilet flushing, or other non-potable applications.
• Sustainable urban drainage systems (SUDS): Wuhan has been implementing various SUDS techniques, such as infiltration trenches, retention ponds, and swales, to manage stormwater sustainably and reduce the risk of flooding.

These efforts have improved water management in Wuhan and contributed to the city's overall sustainability, resilience to climate change, and quality of life for its residents. Wuhan's success in implementing sponge city initiatives is an example for other cities worldwide seeking to adopt similar strategies.

What is the best "sponge city" in the world?

Even though the concept was invented in China, according to a study conducted by Arup, Auckland has been recognized as one of the world's top "sponge cities" among the eight urban centers profiled, according to research by professional services firm Arup (they did not look at Wuhan in the study). As we've seen, "sponge cities" can quickly absorb heavy rainfall by working harmoniously with nature rather than relying solely on grey infrastructure like pipes and pumps.

The study determined the level of "sponginess in a city by calculating the balance between blue (ponds, lakes), green (grass, trees), and grey (buildings, hard surfaces, pipes) infrastructure, as well as factoring in the impact of soil types and vegetation, and the potential for water runoff.

Auckland boasts a high percentage of green and blue space, with roughly half of its land covered by these elements, making it one of the best "sponge cities" in the world of the eight profiled, with a sponginess score of 35%. In contrast, Sydney, Australia, was found to be the least spongy city in the study conducted by Arup, with a sponginess score of just 18%, owing to its low percentage of green or blue space and high percentage of impervious concrete surfaces in the city center.

What other cities are being turned into "sponge cities"?

Most cities will have some degree of "sponginess" because they will have some form of drainage strategy of green space, but again according to Arup, some of the best-performing cities worldwide are as follows.

According to the "sponginess" factor, Nairobi takes second place with a score of 34%. New York City, Toronto, Mumbai, and Singapore all share third place with a score of 30%. Montreal secures fourth place with a score of 29%, while Shanghai follows closely in fifth place at 28%. Finally, as we mentioned previously, Sydney comes in sixth place with a score of 18% on the sponginess factor.

According to Arup, the factors or ratings were created by "calculating the amount of green and blue areas in the urban centers of each city. We then factored in the impact of soil types and vegetation and calculated the rainfall-runoff potential. We used this to produce our sponge snapshot."

So, now you know. And that is your lot for today.

Cities that adopt "sponge" technology present a viable solution to the current water management issues, especially amidst the mounting effects of climate change. These cities use natural methods to conserve and recycle rainfall, promoting eco-friendliness and sustainability in urban development. These "sponge cities" can pave the way for a more robust and comfortable future for city dwellers worldwide through creative planning, design, and community participation.