What on earth are sponge cities?

Posted on: 5 June, 2024

The concept of ‘sponginess’ has emerged in the urban planning discourse in recent years, but what does it actually mean?

As the 2050 target of achieving net zero approaches, the built environment is working hard to address its impact on the environment. From implementing sustainable alternatives to concrete to adopting prefabrication and pursuing retrofitting, more and more sustainable innovations are being implemented into the building lifecycle.

There are some arguments, however, that the time for climate mitigation has passed. Global warming has breached the 1.5 degrees Celsius limit over the span of a year for the first time. What’s more, the world is already feeling the impacts of a changing climate. Instances of extreme weather are becoming both more common and more deadly, and with this in mind, many believe that the time for climate adaptation has come.

The question of how we deal with rising sea levels and increased flooding in particular has given rise to the concept of the sponge city. But what is sponginess? And how does it work?

What is a sponge city?

A sponge city is an urban area with an abundance of natural or ‘green’ features that, in the event of flooding, can absorb water. These areas range from trees and green spaces to parks, lakes and even roofs.

While it may bring to mind images of the famous children’s TV show, this nature-based solution to climate change is a part of green infrastructure and a crucial component in sustainable urban planning that has already been implemented internationally.

How do sponge cities work?

The sponge city concept was first proposed in 2013 by Professor Kongjian Yu of Peking University. He outlined this concept as a way that urban spaces could work alongside nature to absorb rainwater, as opposed to channelling it away.

“A sponge city absorbs and holds excess water that results from rainfall, storms or flooding in its green spaces – essentially acting like a sponge.”

Why are sponge cities important?

Flooding is one of the most significant consequences of global warming.

According to a 2022 study, 1.8 billion people are affected by flood risk, which will be exasperated by worsening climate change and continued urban sprawl.

Devastating floods come at a significant cost, with a recent disaster in southern Brazil claiming the lives of at least 95 people. They also have a disastrous impact on the lives of civilians, displacing populations, damaging crucial infrastructure and destroying property.

The sponge city concept is a way for urban planners to deal with rising sea levels. Adopting this approach is crucial to protecting urban infrastructure and the lives of the people that inhabit it, whilst helping the built environment to adapt to a future that presents increased risk of extreme weather events.

How is sponginess measured?

‘Sponginess’ can be measured, as demonstrated by researchers at Arup. They recorded the percentage of major cities covered with green and blue infrastructure, and compared this with the amount of ‘grey infrastructure’ – think buildings, towers, concrete and roads. They also looked into the ability of each location to retain water through urban soil, as well as the level of plant life.

4 examples of sponge cities

1. Shanghai, China

The sponge city concept has gained significant momentum among urban planners in China – a country with a profoundly high risk of flooding. 641 of its 654 cities are affected by regular flooding, leading to the Chinese government naming 30 pilot cities for its sponge city programme.

Along with the risk of flooding, China is also experiencing a serious water shortage. Due to how quickly it urbanised, its water facilities are poorly spread across the country. The result is that nearly half of all major Chinese cities are sinking because of increased water extraction.

Starry Sky, a new ‘sponge park’, has been opened in Shanghai, and forms part of its approach to the challenges of subsidence, flooding, and water management.

2. Berlin, Germany

Berlin is unique from many other municipalities in Germany in that it doesn’t use pipes to transport water from springs. Instead, the German capital sources its drinking water from the ground.

The principle of sponginess were adopted by Berlin after it experienced flash flooding in 2017, and from then became part of the law for new urban development.

3. Jakarta, Indonesia

Jakarta is sinking at such an alarming rate that it’s being abandoned. In 2020, 170,000 people were displaced from Jakarta by land subsidence, and 60 were killed. Indonesia have chosen to build an entirely new capital in its place, 1,000 kilometres away, as experts predict large parts of the metropolis will be entirely submerged by 2050.

Many see the sponge city model as the only way to preserve Jakarta. By incorporating natural flows of rivers, avoiding the overuse of concrete and essentially turning Jakarta into an enormous sponge, hope remains that it can be salvaged.

4. Auckland, New Zealand

In Arup’s research into the sponginess of several major cities, Auckland came on top, achieving a 35% sponginess rating. It owes this success partly due to its geography and urban design, with researchers finding that 50% of its surface was green or blue.

Auckland has historically suffered from flooding. Despite the positive work in implementing the sponginess concept, the city experienced what has been dubbed ‘the biggest climate event in New Zealand’s history’ in January 2023, killing four and causing over NZ$1.95 billion in property damage.

What are the benefits of sponginess?

1. Absorbs excess water from flooding and storms

The most obvious benefits of sponge cities are their ability to combat the kind of flooding, storms and heavy rains that have ravaged countries across the world in the last decade. Their ability to absorb excess water can help protect infrastructure, prevent the damage of property and, most importantly, save lives.

2. Supports water supply

Draughts are another impact of global warming that can be addressed with sponge cities. Not only will sponginess absorb excess water, but it will also filter, sanitise and store it, giving communities access to clean water in instances of drought.

3. Improves air quality

Another benefit of how sponge cities work is the impact their design has on air quality. Air pollution, resulting from vehicle usage, causes 8.43 million premature deaths a year, but can be combatted by the incorporation of green spaces into infrastructure.

4. Creates green spaces for communities

Sponge city construction works in tandem with the principles of urban greening and biophilic architecture in improving the wellbeing of residents. Integrating green infrastructure into urban planning encourages a closeness to nature, which has proven benefits for mental and physical health.

The challenges facing sponge cities

While the benefits of this urban planning model are clear, there are potential hurdles and challenges that could hinder its implementation:

Governance hurdles and financing

China’s approach to the implementation of sponge cities has faced obstacles due to the complexity of property rights and water use regulations. There’s also the question of financing, which will be challenging for many governments amid worldwide economic uncertainty. This is particularly the case for developing countries when, in many cases, they are most at risk from extreme weather events in the first place.

Unclear definitions and standards

As a concept that is still relatively new in the urban planning space, sponge city projects aren’t standardised, even on a national level in China. This makes implementing the model challenging and leads to mixed and varying results. Many of the technologies that enable sponginess and integrated urban water management (IUWM) practices are also so new that they aren’t readily available to all projects and locations yet.

Dependency on geography and existing infrastructure

A large part of what makes sponginess effective is the existing topography, design, climate and infrastructure of a location. For instance, in the Chinese city of Jinan, the location of its water plants has rendered green and blue infrastructure all but ineffective in being able to absorb and retain water.

Final thoughts

With sea levels rising and extreme weather events becoming increasingly commonplace, the sponge model could be a promising development in the pursuit of sustainable urban planning and climate adaptation. It may not alone be enough to protect all communities from urban flooding, but when applied alongside other practices and innovations, it could shape the future of infrastructure and urban environments.

Urban planning is an exciting field that has a pivotal role in the design and function of our cities and communities. If you want to have a part in helping the built environment realise a sustainable future, UCEM’s MSc Urban Planning will give you the knowledge, skills and technical understanding you need.

Find out more: MSc Urban Planning – University College of Estate Management