The 'spongy' cities of the future

Tangled mats of muddy vegetation line the footpaths of Underwood Park, a narrow stripe of green winding along a creek beneath the small volcanic cone of Ōwairaka (Mt Albert) in Auckland, New Zealand. In the water, clumps of sticks and the occasional plastic bag are marooned on protruding rocks and branches.

A winter storm swept through the city overnight, dropping heavy rain, and Te Auaunga (Oakley Creek), one of the city’s longest urban streams, has overflowed its banks.

"But that’s supposed to happen," says Julie Fairey, chair of the Puketāpapa local board, who is showing me around Underwood and the neighbouring Walmsley Park.

The connected parks are designed to collect excess stormwater, soak it up like a sponge, and slowly release it back into the creek. The debris left behind is evidence this "secret infrastructure" is working, Fairey says. The two parks are flanked on both sides by public housing developments. "This stuff is designed to flood so that the houses don’t," she says.

The researchers calculated that 50% of Auckland’s surface was green or blue, even after excluding its harbours

It wasn’t always this way, Fairey tells me, as we watch a black shag drying its wings on a rock. Less than a decade ago, the waterway was a concrete-lined culvert that ran through seldom-visited muddy fields. When it flooded, water sloshed into the surrounding suburbs. It collected engine oil, sediment and rubbish and sucked this unhealthy mixture out into the city’s famous harbour, rendering the beaches unsafe to swim.

But in 2016, work began to free Te Auaunga from rigid concrete, and restore it to a more natural, meandering shape. Its banks are now lush with native vegetation like harakeke (flax) and tī kouka (cabbage trees), as well as reeds, ferns and other filtering wetland plants.

The changes have increased this part of the city’s ability to absorb excess rainfall, an attribute sometimes called “sponginess”. Auckland was recently named the most spongy global city in a report by multinational architecture and design firm Arup, thanks to its geography, soil type, and urban design – but experts warn it may not lead the pack for long.

As climate change intensifies extreme weather events worldwide, what can other cities learn from Auckland's successes – and failures?

The godfather of the "sponge cities" concept is Peking University landscape architecture professor Yu Kongjian, who almost drowned in a flooded river as a child, but managed to pull himself to safety by grabbing hold of the willow branches and reeds that fringed its banks. In 2013, as an urban designer, he proposed that cities use nature rather than concrete to slow down heavy rain – an idea now being implemented in cities across China and gaining attention around the world.

As the planet warms, intense rainfall and flash flooding is predicted to significantly increase. The spongier a city is, the more resilient it will be in the face of those threats. "As our climate gets more extreme, we've got to deal with increasing hazards," says Mark Fletcher, global water leader at Arup and co-author of the company’s "spongy city" report.

Fletcher and his colleagues found that Auckland was the spongiest of seven global cities, marginally outscoring – in order of sponginess – Nairobi, Singapore, Mumbai, New York City, Shanghai and London.

A city that sits low on the ranking like London, it just means you have to try a bit harder to build some sponginess in – Mark Fletcher

The team mapped the proportion of grey (concrete and buildings), green (vegetation) and blue (ponds and streams) in the seven cities using Geographic Information Systems (GIS) technology, satellite imagery and machine learning. It then combined these maps with information about soil types and runoff potential to figure out each city’s natural absorbency.

Low-lying, coastal Auckland is New Zealand’s largest city, with a population of 1.4 million people and an annual average rainfall of 1210mm (48in) – slightly more than New York, and twice as much as London typically receives in a year.

The researchers calculated that 50% of Auckland’s surface was green or blue, even after excluding its harbours (London had the least of the seven cities, with 31%). In an intense rainfall event – 50mm falling in 24 hours – they estimated that 35% of the water falling on Auckland would be absorbed into these spongy blue and green parts, leaving 65% that would have to be dealt with by engineered stormwater systems – or else overflow and flood.

“It's a measure of how that city was developed, and its core underlying natural morphology,” says Fletcher.

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Nairobi came a close second, at 34% spongy. The Kenyan capital has even more green and blue areas than Auckland – mainly parks and urban backyards – but higher run-off potential due to its clay-dominant soils, which absorb less water than sand or gravel. In the lowest-ranked city, London, just 22% of the water in a similar rainfall event would be absorbed – a risk highlighted in real life in July 2021, when 47.8mm of rain fell in one hour, causing widespread flooding of roads, homes and Tube stations.

Kathy Waghorn, an urban researcher at the Auckland University of Technology’s School of Future Environments, is not surprised that her city was rated as relatively spongy. "We've got low urban density, we still have a lot of single-level dwellings, we still have gardens," she says.

Auckland’s geomorphology plays a role too, she says: the narrow strip of land enfolded in two huge harbours, the dozens of small dormant volcanoes dotting the city, the streams running down their green sides, and beneath them, the legacy of their lava – basalt and scoria caves and sinkholes. "The volcanic field has shaped some of that open space," Waghorn says. "Even our stone is sort of spongy."

But Waghorn and other researchers are concerned that Auckland will not keep its crown for long.

A bundle of long-term trends all point towards the city becoming significantly less absorbent, even as climate change increases the number of extreme rainfall events and the risk of flooding. (In Auckland, for every degree of global warming, the intensity of short-duration storms is projected to increase by 14%.)

For starters, there is strong political pressure for urban intensification in the city. New Zealand has among the least affordable housing in the OECD, and housing demand is concentrated in Auckland. The New Zealand government has recently signalled sweeping changes to planning rules that are likely to prompt "a change in density and height across a great deal of the isthmus", says Waghorn. "The spongy suburbs that we've got will disappear, because there'll be a lot more impermeable surface – more housing, more driveways and parking areas."

The demand for housing has also prompted authorities and developers to eye up other large green spaces like golf courses and racecourses, which tend to be in prime residential locations, Waghorn says. Some have already been sold, and plans made to build thousands of new houses on the once-green space.

Cities worldwide will need to become more spongy, not less, if they’re to adapt to our warmer, stormier planet

At the same time, Auckland’s urban trees are being lost at an alarming rate. As well as improving air quality and keeping cities cool – reducing the need to burn fossil fuels for air conditioning – trees make a city spongier. Their roots absorb some of the stormwater and slow down its movement, says Waghorn. They also can’t be planted in concrete, so areas with trees have at least some permeable surface.

The Arup report found that despite Auckland’s sponginess, it has a lower percentage of trees than New York, Singapore or Mumbai – and there is little to stop those that remain from being felled, Waghorn says.

In 2012, a planning law change removed automatic protection for large urban trees in New Zealand. Since then, according to some estimates, around 250,000 have been cut down in Auckland – nearly a thousand trees per week.

In the last few years, Waghorn says, it’s been difficult to get the Auckland Council to list any new trees as "notable", which is in any case a time-consuming and expensive process that research shows does "not really protect urban ngāhere (trees) in any effective way" – leaving almost all trees on private property without any legal safeguard.

"Now, everybody who wants to build more intensively on their section can, and a tree is no impediment," says Waghorn. "So the first thing they do is cut down the trees."

Taken together, these trends could rapidly demote Auckland from its sponginess top-spot.

Cities worldwide will need to become more spongy, not less, if they’re to adapt to our warmer, stormier planet. Solutions will involve politicians, planners, developers and individuals, says Fletcher – and the entire concept of sponge cities provides a new way to think about what needs to be done. "Sometimes quite innovative things can be quite simple in concept," he says.

Even the most impermeable cities can take action to improve their absorbency, Fletcher adds. "A city that sits low on the ranking like London, it just means you have to try a bit harder to build some sponginess in."

High-rise New York City, for example, has introduced thousands of vegetation-filled planter boxes to city sidewalks. Los Angeles plans to de-concrete and re-wild its neglected river. Adding rooftop gardens, planting trees in derelict areas or people’s own gardens, and changing planning regulations to encourage the use of gravel rather than concrete in car parks and driveways can all help to change the map from grey to green.

Building or restoring nature-based flood-prevention infrastructure like mangroves, swales and wetlands costs around 50% less than traditional infrastructure (such as concrete sea walls) while delivering the same – or better – outcomes in cities, according to research by the International Institute for Sustainable Development (IISD). And if well-designed, natural infrastructure can have repercussions which flow well beyond stormwater, like reducing air pollution, storing carbon dioxide, or boosting tourism.

There is also a hugely important social benefit to re-orienting people towards the waterways that surround them, says Waghorn. Across Auckland, grassroots groups have formed to take care of their local creeks and catchments, and in some places they are beginning to come together with schools, local and national authorities, and mana whenua (indigenous Māori who have historic and territorial rights over the land) to devise stormwater solutions with plenty of other advantages as well.

In South Auckland, for example, there’s a plan to regenerate the Puhunui Stream, which was rated the dirtiest of all the city’s 31 waterways in 2010, and flows through one of its most culturally diverse sectors. Community groups, local authorities and mana whenua local tribal authorities have signed a charter to guide a project that is likely to take generations.

Traditional engineering solutions for stormwater will still be needed though, Fletcher cautions. "We will still need some residual grey – tunnels for transferring and storing water, some pumps. But we can use the green to significantly offset and reduce the scale of those grey infrastructure works."

Fairey, the local Puketāpapa board chair, is part of the diverse group who worked to transform Te Auaunga. As we walk through the Walmsley and Underwood parks, she points out the Pasifika artwork, the boardwalks and bridges, the spot left deliberately wild so you can pick your way across the creek, the pump track for skateboards and scooters, the creekside outdoor classroom used by three neighbouring primary schools, and the māra hūpara – a traditional Māori playground made of logs and tree stumps arranged for climbing, clambering and balancing.

Community planting days involved residents of the nearby suburbs in the project. Gradually, people who had once avoided the park began making it part of their lives. And as the plants grew, the birds returned. During my visit, there are ducks swimming in the pools, and a bright red-and-blue pūkeko perches on a flax bush. Another constituency voted with their fins – tuna, a Māori word for the native freshwater eels, returned to the creek far earlier than anyone had expected.

[T]o have the eels back was like 'Wow, we did it. We got the tuna back' – Julie Fairey

Seeing the eels for herself for the first time was a watershed moment for Fairey. "Just walking through comparing what it was like before with now, you wouldn’t have any doubt that you’d done the right thing. But to have the eels back was like 'Wow, we did it. We got the tuna back. '"

It was a clear sign that Te Auaunga is an awa again, she says – a river, with a life force and a history, no longer just a concrete culvert. These biodiversity and amenity elements of what is, first and foremost, a stormwater project should not be seen as just "nice to have", says Waghorn. They bring people into relationship with their waterways again. "When rivers are in drains underneath the surface, you don't even know they're there," she says. "But people know about them when they flood. "

Auckland became a relatively spongy city by chance. To remain so, it’s likely to need more projects like Te Auaunga, and to address some of the trends paving over its green spaces and toppling its trees. Worldwide, cities will need to find similar ways to work with nature to prevent flooding. In the process, they might also find connection and community in waterways and greenery that is welcoming for people and other creatures – like Auckland’s eels – alike.