Welcome to Sponge City
Infrastructure that soaks up heavy rainfall could put a stop to catastrophic flooding in some of the world’s most climate-vulnerable major cities. By Ute Junker
If you have walked the streets of central Sydney recently, you will have noticed the bioswales. Better known as rain gardens, these small plots of kerbside plantings — featuring carefully chosen natives such as swamp banksia, basket grass and gymea lilies — capture and filter stormwater to remove oils, chemicals and other pollutants before it flows into the city’s waterways and harbour.
The rain gardens, planted by the City of Sydney, may be small, but they total more than 2,300 square metres across the city. They also demonstrate that there is room for natural interventions, which are especially important when it comes to the increasingly common problem of severe weather events.
When violent storms and heavy downpours hit the concrete canyons of our modern cities, the existing wastewater systems, which were designed to flush water out of the city as quickly as possible, are unable to cope with the huge amounts of rain. The result can be dramatic and sometimes devastating floods.
Enter the “sponge city”, which flips traditional urban design on its head. Pioneered by Kongjian Yu, founder of the Beijing design firm Turenscape, sponge cities use green spaces and adaptable infrastructure to absorb excess water, preventing drainage systems from being overwhelmed.
Across China, cities are incorporating Yu’s concept into their planning. In Haikou, a city of two million on the north coast of Hainan, concrete flood walls have been dismantled and replaced with restored mangroves and terraced wetlands.
Metropolises across the world are following suit. New York is investing in green roofs and porous pavements. In Copenhagen, parks are being redesigned to double as stormwater catchments. Even Albania is getting in on the action, planting a ring of forest — the Orbital Forest — around its capital, Tirana.
“The people who began [designing sponge cities] are landscape architects. They use nature-based solutions and a more holistic approach compared to Australia,” says Dr Elisa Palazzo, senior lecturer in urban design and landscape architecture at the University of New South Wales. “Here, we look at engineering to back up and improve our water drainage system. It’s a very technology-oriented approach.”
“We have seen in Brisbane how hard it is to stop the power of the river [during heavy rainfall],” says Dr Rob Roggema, distinguished professor of regenerative culture at Tecnológico de Monterrey, Mexico, referring to the city’s 2022 floods. “You want to keep your water in places where it can slowly sink into the soil and become part of the natural system again.”
Improving the permeability of cities by reducing concrete and upping greenery brings a host of other benefits, too. “It’s about microclimatic conditions, about making places more liveable,” Palazzo says. Increased tree plantings provide shade and create cooler conditions than built-up areas, and time spent in nature is proven to help decrease stress and anxiety levels.
Still, achieving buy-in from governments remains a challenge, and even in high-risk flood zones, economic considerations can get in the way.
Many of the jurisdictions readily embracing sponge city principles regularly experience extreme weather events. “Southern China is an excellent example, as it experiences a substantial amount of rainfall and violent storms. The area is also low-lying, in a massive delta, and thus vulnerable to floods upstream as well as sea-level rise,” says Dr Kris Hartley, who will take up the post of assistant professor at the Arizona State University School of Sustainability in August. “Hong Kong, especially, has a housing cost crisis and is addressing this by reclaiming land to build tower blocks.”
Roggema is currently leading a team of graduate urbanists and students on a city-wide plan called Green Metropolis Monterrey, which he hopes will act as a template for other jurisdictions. He says he sees a gap between the theory and the practical reality of sponge cities. “There is an awareness that we are in a new era for city planning, that we need to climate-proof our cities,” Roggema says. “Everyone seems to be interested in the principle, but in practice there’s a bit of a problem, with cities doing it on a small scale.”
Cities also exist within a wider context, of course — events in surrounding regions also impact their infrastructure. That is particularly true of river cities, which are affected by their watershed.
According to Palazzo, Europe and China have an advantage in this area because their political boundaries are often closely aligned to the topography, unlike countries such as the United States and Australia, where colonising powers drew arbitrary borders. “In Europe and Asia, the boundaries are geographical because those very old societies developed in that nature,” she says. “The watershed often provides a boundary. [It’s] not a straight line on the map, as happens in the United States a lot of the time, which gives you huge problems managing natural resources.”
In some parts of Europe, planning already happens at the catchment level. “In Italy, each big river has one single administering body,” Palazzo says. “There is an understanding that nature must be part of the equation.”
Some see digital mapping as a useful tool for larger-scale planning, but Roggema says it has inherent problems. “We think we have the data, and that leads to a fixed digital version” that is unable to deal with nature’s inherent unpredictability, he says. Instead, he advocates stepping back and using nature-based solutions. “If you allow the system to find its shape by itself, an equilibrium emerges out of that,” he says.
The Netherlands, a country where one-third of the land lies below sea level, embraces this approach. A series of dykes has long kept the sea at bay, but as many of them age, the government has found a very different way to shore up the country’s defences.
A pilot project in 2020 reinforced the 32-kilometre Houtribdijk dam, which connects the provinces of Noord-Holland and Flevoland and separates two bodies of water, the Markermeer and the IJseelmeer. Rather than shoring it up with rocks — which would have needed to be imported given the country’s short supply — the authorities turned to a different material: sand.
Ten million cubic metres of sand were pumped from beneath the thick mud on the bottom of the Markermeer and dumped against the dyke. Hydraulic engineers predicted waves and currents would disperse the sand and it would eventually settle in a form that would provide a natural barrier against storm surges. “Sand always moves — you’ll find a little bit less here, a little bit more there,” Roggema says. “Nature is always searching for the best balance.”
The innovation proved a huge success, improving water quality and increasing natural diversity in the area. It has informed the country’s infrastructure renewal program, which aims to have at least 1,100 kilometres of dykes reinforced in the next 30 years. The technique will also likely be introduced internationally as more regions try to cope with rising sea levels and coastal erosion.
Most importantly, working with nature, rather than against it, may provide solutions for a whole spectrum of problems. As Roggema says, “Every time we try to control nature, we create our own disasters.”
This is an extract from an article that appears in print in our twelfth edition, Page 114 of Winning Magazine with the headline: “Welcome to Sponge City”. Subscribe to Winning Magazine today.
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