How “sponge cities” can reduce the risk of Spanish-style flooding

It’s rush hour in Rotterdam. The city hums with traffic as workers head home, but the Hofplein station platform is quiet except for a few parrots screeching overhead.

Trains no longer stop at this derelict terminus on the disused Hofplein line, which is being turned into a park reminiscent of New York’s High Line. Where trains once ran, trees now grow; gardens filled with wildflowers replaced the passengers. Rotterdam takeover of the High Line will – whisper it – surpass the original when completed in 2027. Architect Dirk van Peipe thinks so, but he’s biased – his firm De Urbanisten is designing it.

“I spoke to Robert Hammond (co-founder of the High Line) and he was jealous of our plans,” enthuses van Pepe. Jealous, he says, because Rotterdam’s Hofbogenpark will not just be something beautiful; it will also help the city cope with storms that are becoming more frequent and intense climate change.

In a week where thousands of homes were evacuated as thunderstorms and torrential rains hit coastal Spain again, the need for urban areas to be able to cope more effectively with excess water has never been more acute. The Sponge town The concept used at Hofbogenpark could be part of the solution. It originated in China but has since spread across the world from Canada to Mansfield, Nottinghamshire.

In Rotterdam, rainwater falling on the surrounding streets will be directed to the park, where it will be treated and stored in an aquifer. Recycled water will be pumped in on dry days to water plants, fill ponds and feed fountains for children to play in.

“It’s a combination of water management with the improvement of public spaces,” says van Peipe. “It’s really cool.” Rotterdam is known for its bold architecture – it’s called “Manhattan on the Meuse” for a reason – but Hofbogenpark operates on a modest scale.

“It’s designed so that the hedgehog can walk from one end to the other,” says van Peipe. “It is not only for humans, but also for other earthlings; frogs, bats, bees, butterflies. That was another thing that made Robert Hammond jealous.”

Only a pilot section of the Hofbogenpark is open, but when the €53 million park is complete, it will wind for 2km through some of Rotterdam’s most deprived areas of nature. Hedgehogs (“if they appear”) will gain access to it through plant-lined ramps.

It is one of the few projects aimed at absorbing flood water, increasing biodiversity and improving access to nature in Rotterdam. Others include wetlands, tidal parks and water plazas that have been submerged in neighborhoods across the city. These fringes of leaves hold millions of liters of rainwater and are used as skateparks, amphitheatres and basketball courts when dry. Among the cities that have copied this idea are Copenhagen and Montreal.

A movement to make cities spongier has begun Chinese landscape architect Konjian Y. As a child, he saved himself from drowning in a flooded river by grabbing a reed along the bank. Changed by this experience, in the mid-2010s he pioneered the use of natural solutions for flood control.

Many Chinese cities adopted his opinion, which is spreading throughout the world. This is necessary. The devastating floods in Spain are yet another reminder of how ill-prepared our cities are extreme weather. Recently, Germany, France and Italy have also been badly affected.

Last month, a year’s worth of rain fell in one day in and around Valencia. City parks and water plazas would do little to stop the devastation, admits van Peipe. “The problem did not start in Valencia; there was so much water coming in from outside,” he says. “We have to look much wider. We need to go upstream and see if we can trap the rainwater there.”

Like Rotterdam, Mansfield also tries to make drainage cool. Once dreary urban wastelands in the Nottinghamshire city have been transformed into flower-filled rain gardens. Concrete buildings and dirt sidewalks are among the spaces that have been reimagined. They are not just nice to look at. Underneath the flower displays are layers of soil and an absorbent medium consisting of natural materials that absorb rainwater.

Dozens of tanks – giant bowls covered with grass – have also been dug near flood-prone homes.

“They store water for days after it rains,” says Adam Boucher, who manages the £76m Severn Trent Mansfield Green Drainage Project. “I’ve seen dragonflies buzz around them, kestrels come down to bathe in them—they’re just a hive.”
An area totaling four hectares – or 5.6 football pitches – is being converted into what is known as Sustainable Urban Drainage Systems (or SuDS) in Mansfield. Together they will hold 30 million liters of surface water, equivalent to 12 Olympic-sized swimming pools, says Severn Trent.

The firm is the highest-performing public water company in England despite being fined £2m for illegally dumping 260m liters of sewage into rivers last winter. Preventing surface water from entering sewers leading to discharge is a key motivation for Severn Trent’s commitment to support SuDS.

The company’s work in Mansfield is the largest green drainage initiative in the UK. It’s a welcome move from an industry that can’t handle the nation’s plumbing. Other companies are taking note.

“We went to England, Yorkshire and the south,” says Boucher. “We’ve learned a lot from this project and we’re going to share it.”
Mansfield was chosen because of the risk of flooding and because, like Rotterdam, its soil holds water. Boucher admits that similar results will be “harder to achieve” in cities built on clay, such as in nearby Derbyshire, where the company will soon install such systems.

Mansfield’s SuDS did not go unchallenged. Some residents regretfully gave up parking spaces for rain gardens.
There are other forces working against the urban sponge movement, including urban densification and the fashion for cobblestones over front gardens. Bringing landowners and stakeholders together can also be difficult, says Mark Fletcher, global head of water at Arup, a British engineering consultancy.

“There is progress in London – all the key parties are coming to the negotiating table,” he says. “I think the sponge city will be created when that becomes a common goal.”

There’s no “silver bullet” for urban flooding, cautions Fletcher, but he says cities have plenty of room to increase variability. He said: “There is a lot of untapped potential, from rooftops to car parks.”

Sponge city first places

Hofbogenpark, Rotterdam
Rotterdam’s answer to New York’s High Line is designed so that a hedgehog can walk on it. The park will also store and recycle rainwater when it opens in 2027.

Bentemplein, Rotterdam
This square water complex holds around 1.7 million liters of rainwater (which is channeled to the Hofbogenpark) and is also a skate park, basketball court and amphitheater. It spawned imitators in Copenhagen and Montreal.

Te Auaunga Oakley Creek, Auckland
The project to renaturalise this altered stream has engaged residents and brought “huge biodiversity benefits”, says councilor Julie Fairey. There were still floods in 2023, but they were “less devastating”, she adds. Arup’s 2022 report named Auckland the spongiest of the 10 cities.