Urban futures, sponge cities and infrastructure resilience  

As global populations moving increasingly into cities, the risks of climate change increasingly threaten the socially vulnerable. Arup's Peter Holt discusses why blue-green infrastructure should be weaved into the fabric of urban landscape

It’s anticipated that by 2050, almost 70% of the world’s population will live in urban centres. There’s no doubt that this is an exciting prospect for the global economy – if innovation and productivity is prioritised, an increased capacity for sustainable, equitable growth could be unlocked.

Yet we can’t shy away from the consequences of climate change that we know come hand in hand with rapid urbanisation. Major global disfunction is already being experienced because of extreme weather events such as flash flooding and extended periods of heat and drought.

With 44% of all ‘disaster events’ related to flooding, a city’s ability to cope with heavy rainfall speaks to its broader climate change resilience. Stormwater can either be managed proactively or it will determine its own course, often resulting in damages to buildings or infrastructure and having a detrimental effect in local areas. We know that extreme weather events most negatively impact those most vulnerable in communities, requiring those in the built environment to rethink how resilience is embedded to create better outcomes for more people.

Resilience will only be unlocked by focussing efforts on adaptation and mitigation hand in hand, for example by working with nature and prioritising the integration of blue-green solutions, such as the reintroduction of wildlife, as well as investing in physical infrastructure like protective flood walls.

Though economic damages are easy to measure and analyse in the wake of natural disasters, assessing the lasting human and community impact is more complex and increasingly crucial. In short, the right interventions need to be mapped and actioned ahead of time in the knowledge that extreme weather events are only going to become more frequent and intense. Alongside this, we will continue to see the cost to rebuild towns and repair infrastructure damages increase – further displacing those most socially vulnerable.

This gives way to a perfect storm of contrasting issues and pressures that must be addressed with an inclusive and two-pronged approach, prioritising both mitigation and resilience to enable real global progress. By considering measures for climate impact mitigation and resilience hand in hand, holistic benefits can be unlocked so that our urban and natural environments can thrive together, enabling better lasting outcomes.

Understanding a city’s sponginess

Coined by Professor Kongjian Yu of Peking University in 2013, a “Sponge City” works with nature to absorb rainfall, indicative of its capacity to naturally cope with urban flooding. Understanding a city’s “sponginess” reflects the blue-green infrastructure embedded in its fabric such as public spaces, building developments, road networks and other urban spaces. Permeable surfaces like grass, plants and trees absorb rainfall in their soils and vegetation – helping to reduce flooding by natural removal or slow release of water back into the local environment. A reduction of impermeable surfaces also means these areas more resilient to the impacts of extreme heat too.

By contrast, areas largely dominated by grey infrastructure – including concrete, pavement and buildings – are not as resilient and may rely on manmade measures to respond to climate shocks, for example attempting to channel floodwater away and out of cities. We have seen this during recent urban flooding crises – in 2023, New York City entered a state of emergency due to flash flooding. Tropical Storm Ophelia claimed the lives of 13 people, stranded children in flooded school buildings and force the shutdown of major rail and highway infrastructure, leaving people trapped in vehicles needing to be rescued. Much of the surrounding conversation was related to the city’s preparedness as the impacts of climate change become more prevalent and detrimental. It reinforced the increased understanding that the intensity and impact of climate shocks like urban flooding need to be addressed proactively. To do this, we must invest in longer-term solutions rather than just reactive interventions during crisis.

Using the firm’s land use analysis tool Terrain, Arup developed the Global Sponge Cities Snapshot to assess the urban centres of ten global cities. With the understanding that the detrimental impact of flooding will increase by 4-5% if global warming reaches 4oC, the initiative was published as a public resource to encourage local governments, city actors and landowners to begin thinking about enhancing natural infrastructure as a multifaceted solution.

By analysing the amount of blue and green areas in and around city centres, our team of digital and climate experts factored in the individual vegetation and soil types identified in each area to calculate the rainfall runoff and absorbency potential. Though our findings must be considered in the context of the frequency of individual cities’ heavy rainfall and the likelihood of damage severity, the Snapshot lays a foundation of understanding for interventions to be prioritised.

We found that Auckland has the “spongiest” urban centre of 35% when compared to cities such as Mumbai (30%), New York (30%), London (22%) and Sydney (18%). Despite being New Zealand’s most densely populated area, Auckland’s rich green spaces, including large parks located across the city rather than being concentrated in one area, enable it to cope with urban climate shocks. Cities like London and Sydney have less green and blue spaces and tree coverage. Many of Sydney’s parks are located at one side of the city, leaving much of the in-land areas mostly covered in impermeable surfaces such as concrete.

A spectrum of nature-based solutions

Nature-based solutions are increasingly at the fore of conversations around designing interventions – ranging from the use of regenerative materials in construction to planning for nature recovery through development. Enabling us to adopt a blend of grey and green infrastructure, incorporating nature-based solutions allows us to develop solutions with longevity – working with nature, not against it.

Sustainable drainage systems (SuDS) are an example of this thinking in action. Arup is working with Severn Trent on the largest retrofit SuDS scheme in the UK, located in Mansfield, to reduce local flood risk through nature-based solutions. With an original aim of capturing up to 58,000m3 of surface water, the project will see the implementation of hundreds of raingardens, bioswales and basins across the town. Functional in providing flood resilience for the town ahead of climate change predictions, the project also offers the local community additional green spaces. This provides benefits to wildlife is and transforms streets in the area. Severn Trent is monitoring and plans to publicly share the project’s long-term performance so that the industry can better understand how similar blue-green infrastructure solutions can be optimised in the future.

Recognising their contribution to a city’s adaptability to population growth, urban drainage is being rethought globally. The optimal blend of blue, green and grey solutions depends on each city’s unique makeup – one size never fits all when designing next generation infrastructure. This is especially evident in Shanghai’s Urban Drainage Masterplan. Due to the city’s rapid urbanisation and development in recent years, Shanghai has been exposed to increased stormwater runoff – a result of reduced green space and impermeable surfaces. Part of Arup’s Global Sponge City Snapshot, Shanghai scored 28%, meaning it is naturally less resilient to flooding than cities including New York and Mumbai.

Shanghai is one of the world's best examples of a sponge city. Credit: Shutterstock

In partnership with Shanghai Urban Construction Design & Research Institute, Arup supported with the development of a plan that covers 640km² with a population of 15 million. The plan embeds the firm’s Design with Water framework – enabling a water-orientated urban planning approach for ecological development and accessibility. The plan outlines an integrated water cycle within the city to provide additional benefits to the population – namely economic growth and public health. Our plan also proposed changes to drainage governance structures to better prepare the city’s future development.

The future is nature-based

No solution can singlehandedly embed resilience to issues like urban flooding: different interventions need to work cohesively. For example, developing infrastructure in upper catchment areas and surrounding communities can help to relieve the pressure on towns. An example of this in action is Brighouse Flood Alleviation Scheme in West Yorkshire. The project will primarily use landscape features to create off-line storage basins to control the flood water within parks in the town. The six multi-use flood storage areas are designed to store approximately 10,000 m3 of water in the landscape, before allowing its slow release back into the river once the storm has receded. In dry weather the basin areas will be able to be used for recreation and biodiversity.

As the world’s urban centres evolve and expand in the face of climate change, traditional methods for managing stormwater can’t continue to provide all the answers. City actors, from local authorities to utility companies, need to accelerate a focus on nature-based solutions for sponginess and resilience, in turn helping to secure the next generation’s urban futures.

Author: Peter Holt, associate, Arup