Late last year, The Chartered Institution of Water and Environmental Management (Ciwem) published its Sponge Cities briefing note advocating the widespread adoption of the sponge city concept across the UK.
“Hard surfaces in urban areas exacerbate water pollution, the urban heat island effect and flooding from rainwater (known as surface water flooding),” Ciwem states. “Yet in recent decades, urban areas have increased in size by nearly a third, while losing a city-sized amount of green space. To tackle these challenges, we need to make our urban areas more spongy.”
That urban growth has contributed to what is now around 4.6M properties being in areas at risk, a 43% increase on the Environment Agency’s previous assessment for England.
Making a city “spongy” is a neat metaphor conveying the need to boost the absorbency of urban communities by reducing hard surfaces and instating natural drainage that can gradually absorb and then release water.
The idea has been around in its modern form since around 2013, primarily initiated in China to combat urban flooding, having first been proposed by the late Chinese landscape architect and urban designer Kongjian Yu. As well as alleviating floods, storing water is a growing focus, as cities acknowledge the need to adapt as much to drought as to sudden rainfall.
The beneficial byproducts of sponge cities are also recognised; the creation of natural drainage brings nature itself into urban environments; parks, trees and green spaces reduce the urban heat island effect while supporting biodiversity.
Sponge city strategies have been rolling out in China – from Beijing to Shanghai – and elsewhere across Asia, including Singapore and Hong Kong.
There are sponge city interventions in New Zealand and Australia, parts of Africa, North America and Europe as well as across the UK. Engineers and urbanists are also implementing ‘cloudburst’ plans – which utilise both nature-based intervention and hard engineering – as rainfall events become more unpredictable and intense.
Cities versus water
Some of the world’s oldest cities have been contending with water since their earliest establishment. Rotterdam, The Netherlands’ second largest city and home to the country’s largest port, was one of the first in Europe to adopt and pioneer the “sponge city” concept, developing its own “water-sensitive” approach in the late 2000s, although its history with water goes back much further.
“Rotterdam is a delta city that has always been dominated by water,” says Dirk van Peijpe, co-founder of Rotterdam-based urban design firm De Urbanisten. He outlines how the Nieuwe Maas (New Meuse) river running through Rotterdam – a main distributary of Germany’s Rhine and Meuse rivers – is just one part of the Rhine-Meuse-Scheldt delta in Rotterdam, along with several main branches of the Rhine including the Waal (main channel), the Nederrijn (Lower Rhine) and the Lek to the east of the city.
“Water also has the potential to impact Rotterdam during storm surges from the North Sea on the western side of the city,” he says.
Responding to that ever-present influence and the exacerbating effect of climate change, De Urbanisten worked with the city on The Rotterdam Climate Adaptation Strategy, launched in 2013.
“The question was how to adapt our city and protect our urban environment, not just using technical systems but to see if we could make our cities more adaptive and ‘spongy’ although the word wasn’t widely used at that time. We just called it climate adaptation,” he says.
The strategy triggered multiple interventions across the city. These included the development of water storage, including 10,000m³ capacity storage under Museum Park, as well as over 130,000m² of green roofs – with an overall target of 1M.m² by 2030.
Innovative typologies such as water squares – blending functionality with local amenity – were developed, one of the more prominent being Benthemplein Watersquare, which opened in late 2013. The square provides a drainage system to take pressure off Rotterdam’s sewers by acting as a temporary public storage basin for rainwater during heavy storms.
Benthemplein Watersquare, Rotterdam, was designed by De Urbanisten to prevent sewage overflow during heavy rainfall by collecting runoff from surrounding streets and roofs. It features collection basins that work as recreational spaces when dry.
Another noteworthy scheme was Keilehaven Tidal Park in the Merwe-Vierhavens area, completed in 2024, transforming a former industrial harbour into a green recreational amenity. It is designed to act as a sustainable, climate-adaptive public space that reintroduces a tidal, estuarine system into an urban setting.
De Urbanisten is also leading the design of the Rotterdam High Line or Hofbogenpar. Construction on the 2km elevated park, located on a former railway viaduct, was scheduled to start in 2025, with opening for public use planned for 2027.
“There, rainwater is very much part of the system,” van Peijpe explains. The structure will function as a purifying system for rainwater using solutions such as reed bed filters, before storing it in aquifers. “The result is a circular system now being implemented in Rotterdam,” says van Peijpe.
He adds that De Urbanisten is also involved in “a long-term research and design initiative that asks the question ‘What if sea levels rise by around 2m in 2100?’ Because then you pass the threshold at which we can still rely on the water safety system as we have it now.”
Flooding and drought mitigation measures are also part of the history of Denmark’s capital. City of Copenhagen director of climate adaptation and the lead on Copenhagen’s Cloudburst Management Plan Jan Rasmussen says the city began engaging with climate adaptation in its modern form in 2007.
He vividly recalls reading an Intergovernmental Panel on a Climate Change report which at that time predicted a 30% rise in rainfall and more frequent cloudburst events for Denmark.
“In 2008, we made the first analysis about the impact of climate change in Copenhagen and in 2010, we prepared the Climate Adaptation Plan, which was launched in 2011,” he says.
As fate would have it, the plan proved eerily timely. As it was being launched, what Rasmussen describes as “a monster rainfall” hit Copenhagen on 2 July 2011, causing $US1bn (£750M) of damage in two hours and triggering over 90,000 insurance claims. The estimated cost to insurers was between DKK4.9bn and DKK6.2bn (£567M-718M), which significantly impacted local insurers and global reinsurers.
“That’s why we prepared the Cloudburst Plan in 2012. The flood event had been so massive that we quickly understood that it was not possible to expand the existing sewer system to handle that amount of water. So, we wondered how we could climate-proof the city so we would not experience that again.
“We developed some typologies to build a plan for the entire city using the roads and green spaces to store the rainwater, and that led to a plan including more than 200 surface projects for streets, parks and boulevards.”
One intervention is at St. Anna’s Square in Copenhagen’s historic centre, which was transformed into a multifunctional cloudburst management site, designed to handle extreme rainfall and prevent flooding. It features sunken green areas – or cloudburst boulevards – that manage water on the surface, diverting it away from vulnerable buildings while also creating recreational spaces.
Karens Minde Park was also transformed into a sponge city area, capable of catching and storing 15,000m³ of rainwater to prevent floods. A soccer field at a higher corner of the park is the first of many ‘stages’ within it, capturing piped water that runs from an uphill area down to the soccer field, Rasmussen explains. “As rainfall continues, the water runs down to fill a lake and if that lake is filled, it runs down towards the park’s entrance. We built a wall around the entire park and a gate so that if that area is filled with water, the gate automatically closes.”
Drought and water shortages were also part of the strategy. “While drought has not historically been a problem due to an abundance of groundwater in the city, it is likely to be a challenge in the future. Maybe in 10 to 50 years, there’ll be a lack of groundwater, so we have to do something about it,” says Rasmussen. “The first priority is to avoid damage from rainwater, but number two is to figure out – can we reuse some rainwater?”
Enghaveparken is another Copenhagen “sponge” park, featuring 22,600m³ of water storage capacity to manage cloudbursts. A specific 2,000m³
retention chamber collects daily rain to clean streets and water plants, integrated with recreational areas.
Financing the DKK 11bn (£1.3bn) Cloudburst Management Plan was so significant that it required updated legislation to enable water tariffs.
Copenhagen lobbied for a change of national legislation to fund new infrastructure adaptation measures, with technical adaptations financed by utilities charges or water taxes by Greater Copenhagen Utility, which supplies drinking water, district heating, city gas and cooling. Meanwhile, aspects of the programme relating to urban space improvements would be financed through the city’s municipal budget.
Enghaveparken is a Copenhagen “sponge” park, featuring 22,600m³ of water storage capacity to manage cloudbursts. A specific 2,000m³ retention chamber collects daily rain to clean streets and water plants, integrated with recreational areas. Credit: City of Copenhagen
Full engagement
Arup global water business leader Mark Fletcher notes that sponge city interventions – particularly given the complexities of older, established cities – require broad-based thinking and planning.
This proved critical when Arup won an international competition to develop a stormwater management and drainage masterplan for Shanghai, China in 2019. The aim was to develop a “blue/green/grey” approach to manage stormwater and improve urban drainage across 640km² to address urban flooding, pollution and climate change. Arup partnered with the Shanghai Urban Construction Design & Research Institute to create the winning proposal.
“When we first looked at Shanghai, governance was a real issue, and one of the real strengths of that project was bringing everyone together. We got all the parties together and did workshops where we said ‘Okay, who’s responsible for this? Who’s responsible for that?’ We got them starting to talk to each other.”
Arup won an international competition to develop a stormwater management and drainage masterplan for Shanghai in 2019, to address urban flooding, pollution and climate change. Credit: Derek Lee
In the UK context, a move towards broader, system- or catchment-level approaches is helping drive wider implementation of water management interventions, he says. And he notes that the government’s launch of updated national standards for Sustainable Drainage Systems (SuDS) last June signalled a major shift away from traditional underground, pipe-based drainage towards surface-level, nature-based solutions.
“That’s about – is there something that you can do in the catchment or the city so that we get less water that goes into the pipe? If we take everybody on board across the catchment or in the city and everyone’s working together on it, all those pieces start to come together.”
Flood Ready London (formerly the London Surface Water Strategic Group), which encompasses the Greater London Authority (GLA), Thames Water, the Environment Agency and London Councils, published the London Surface Water Strategy last May.
Fletcher notes that it includes an initiative that incentivises utility companies to install SuDS such as rain gardens when refilling the approximately 165,000 holes dug annually in London.
“It’s essentially saying – when we dig up a road or when we dig up a footpath, why do we always put impermeable materials back? Why don’t we put things back that might help us?”
He points out that the amenity created by “sponginess” provides a useful pathway towards positive community engagement.
“The sponge city concept is not a silver bullet. It doesn’t solve everything, but it does improve resilience to regular flooding, and you get the added benefit of better biodiversity and public green space. The point is that there are multiple benefits, not just flooding, that come out of this solution rather than just sticking pipes and pumps in.”
WSP fellow, landscape advisory Ludo Pittie describes strategies such as those adopted by Rotterdam and Copenhagen – along with many international examples – as “foundational” blueprints for “the international understanding that the way we design our cities has to change”.
Successfully implementing that change lies, at least partly, in adhering to an overarching framework, something that has increasingly been a point of difference in London’s latest efforts.
“In the UK context, we have been leading on the SuDS Master Plan for Westminster City Council [developed 2024-2025] with Thames Water and we’ve also worked on the London Surface Water Strategy, which was the first time such a plan was developed across the entirety of London at a strategic level.
“Up until recently, what you’d had is a lot of London boroughs doing their individual surface water management plans. And this was the first time there was an aggregated London-wide perspective on bringing all of that together across catchments.”
Progressive developers are also influencing the picture, he says, citing Barking Riverside in East London, where a flood-prone brownfield site has been used to build a resilient, sustainable community using advanced SuDS.
Key features include storing 100% of surface water runoff in parks to prevent Thames flooding, rising ground levels per Thames Estuary 2100 guidelines, and using permeable paving, green roofs and swales to improve water quality.
“We’ve seen a lot of developments that we’ve worked on that include those principles, where private developers will take the lead and adopt those green infrastructure networks from the outset.”
Thinking big
Beyond London and initiatives such as the SuDS Master Plan, Pittie believes sponge cities would benefit from a nationwide approach – citing the way SuDS are now mandated for new developments in Wales.
He questions whether England’s delay to the implementation of Schedule 3 of the Flood and Water Management Act 2010 – the legislative framework designed to mandate the use of SuDS in new developments across England and Wales – is where the problem lies. Despite being enacted in Wales in 2019, delays to implementation have continued elsewhere, partly due to concerns from developers regarding increased costs and regulatory burdens.
“I wouldn’t necessarily say Schedule 3 delay is the reason we don’t have sponge cities. But I think sponge cities should be a top-down national policy.
When it becomes a national policy framework, moving forward with implementation becomes a lot easier because there’s a mandate for it.
“I think we have an opportunity to bring everything together in terms of policy and implementation. I think especially with AMP8 [Asset Management Period 8] and the next period too, we’re going to see even more action towards bringing that to bear through the catchment management plans.”
The government’s New Towns programme is also “a huge opportunity”, Pittie says. “If you’re designing something new, you can do it in a much more streamlined way. It’s possible “to really incorporate those green infrastructures from the outset, and that translates from principles at the masterplan stage to the concept drawings, to how you draw the cross sections with engineers, to how you then do the detailed design.”
With flood risk bringing direct implications for mortgage lending, property valuation and insurance, Pittie argues that the financial sector’s continued engagement is also critical, pointing to “valuable work” by the Flood Action Coalition. Established last November by insurer Aviva and change-making organisation The Conduit – WSP is also a member – the aim of the coalition is to scale finance with the goal of creating a billion-pound bond for flooding.
“I think we’re okay,” Pittie says of the UK’s overall score card. “There’s been a lot of work done since The Flood and Water Management Act in 2010. More can be done for sure, but a lot of the plans we’re seeing are continuing to move that dial. It’s about what policy changes are required regionally, but also how we start to enact and finance those and bring the right partners together to start to deliver some of these schemes at scale.”
Kongjian Yu
12 May 1963 – 23 September 2025
The pioneer of Sponge Cities Kongjian Yu – pictured at left with friend and colleague global water business leader Mark Fletcher in Beijing last June – died aged 62 on 23 September 2025. Media reports confirmed that he was killed in a plane crash in Mato Grosso do Sul state, Brazil, while reportedly filming a documentary about his work. The urban designer, landscape architect and Peking University professor’s untimely death followed his inclusion in the opening programmme of the São Paulo International Architecture Biennale, which aligned with COP30 in Belém in November 2025 through its focus on climate-resilient urbanism. ArchDaily described Yu’s passing as “a profound loss to the fields of landscape architecture and environmental sustainability”, adding that “his legacy and transformative ideas will continue to shape cities for generations to come”.
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