‘Sponge cities’ can help protect against flooding. Here’s how

This article was originally published in October 2022 and updated in November 2023.

Even the driest months in low-lying, coastal Auckland see a lot of rainfall. That’s three flood risk factors right there.

But the New Zealand city also has a lot of natural attributes that help excess water drain away quickly, making it more resilient to flooding and other hydrological events than many other urban centres.

And this “sponginess”, or ability to draw water away from the surface, needs to be a key consideration in urban planning, experts believe. Sponge cities will have a greater capacity to deal with more extreme weather and rising sea levels caused by climate change.

The concept is attributed to Chinese landscape architect Kongjian Yu and eventually became part of China's urbanism policy in 2014. He has since been awarded the 2023 Oberlander Prize for his work addressing climate change-accelerated urban flooding.

Having grown up in rural China, Yu developed an awareness of the seasonal movement of natural waterways and the role of vegetation in slowing their course. But as he explained to the Cultural Landscape Foundation behind the Oberlander Prize, it was witnessing the impacts of "grey infrastructure" and pollution on delicate local irrigation networks that drove him towards developing the sponge city philosophy.

Based on longstanding Chinese irrigation practices, the concept utilizes natural drainage systems in urban sites to reduce the likelihood of flooding, water shortages and the urban heat island effect, and it is now being used all over the world.

Auckland is top in a list of "spongy" cities, according to research by professional services firm Arup. Sponge cities work in tune with nature to quickly soak up heavy rainfall, rather than solely relying on grey infrastructure like pipes and pumps.

A city’s sponginess is affected by the balance between blue (ponds, lakes), green (grass, trees) and grey (buildings, hard surfaces) infrastructure. Soil types and vegetation, as well as the water runoff potential, also have a role to play.

Sandy soils are generally spongier than more clay-based soils, but the depth of the soil and the depth to the water table also have an impact. If the groundwater table is close to the surface, this reduces the sponge capacity of the soil.

Despite being New Zealand’s most populous urban area, Auckland is also rich in green space. Its houses often have decent-sized gardens, and there are a number of large parks.

Half of Auckland’s land is covered by green or blue. This was one of the highest percentages in Arup’s report, only marginally behind Nairobi in Kenya.

While Nairobi’s grasslands make it a comparatively green city, it has a higher runoff potential than the other cities analyzed, because of a high percentage of clay within the soil. This makes it a slightly less good sponge city.

The least spongy city that Arup analyzed was Sydney in Australia, which has just 24% green or blue space, with many parks located outside of the city. The centre of the city is largely built-up and has impermeable concrete surfaces as a result. In addition, Sydney has a moderately high runoff potential, with a fairly clay-rich soil.

Sponge cities are able to soak up excess water and release it more slowly back into rivers and water systems.

Climate change is already having an impact on our weather systems, bringing about more flooding and heavier rainfall, as well as droughts. The impact of this extreme weather will be greater the more the planet warms.

Forty-four percent of global weather-related disasters are linked to flooding, the World Meteorological Organization says.

We need to measure and place more value on green and blue infrastructure – trees, grass and ponds – Arup argues, saying that we even need to design cities with sponginess in mind. Nature-based solutions to climate change are on average 50% more cost effective than engineered alternatives, and deliver 28% more added value than grey infrastructure.

Demand for housing is placing increasing pressure on urban greenspaces, affecting cities’ sponginess. But urban centres from Shanghai in China to Cardiff in Wales are factoring floodwater management into their planning. Shanghai’s rivers are being recruited to help it better tackle its urban drainage problems, and in Cardiff, “rain gardens” have been introduced to help prevent rainwater from overwhelming sewage systems.

Around 4.4 billion people live in cities and mitigating the impacts of increased rainfall and flooding through innovative urban planning is therefore vital, as the World Economic Forum's Delivering Climate-Resilient Cities report points out.

"Cities can provide greater protection to residents and assets against the impacts of climate change and natural hazards by investing in climate-resilient infrastructure solutions," it says.