What Happens When the Water Rises? Design for a Wet Future

Water squares, sponge parks, sustainable reclamation: they’re all tools to deal with rising sea levels and worsening rain

Zolima CityMag01 Nov 2023

Hong Kong is a very wet city. Every year, it receives an average of 2,431 millimetres of rain, four times more than London and twice as much as New York. And since so much of that rainfall comes from torrential summertime downpours, it has developed one of the world’s most sophisticated drainage systems, with countless concrete channels across the city funnelling rainwater into vast underground reservoirs before it is discharged into the ocean. It’s one of the reasons Hong Kong now manages to avoid the devastating floods and landslides that plagued it for so many years in the past. 


But every system has its limits. Last month, on September 8, more than 158 millimetres of rain fell on Hong Kong in a single hour. By the end of the day, some parts of the city had been subjected to an astonishing 800 millimetres of rain. The unprecedented storm flooded MTR stations and shopping malls, killing four people and causing an estimated HK$100 million in damages. In the aftermath, some commentators reflected on a statement made by Chinese University of Hong Kong architecture professor Edward Ng when some of Hong Kong’s latest stormwater tunnels had opened in 2020. “The design standard is based on past history,” he said. “It can cope with what happened 30 years ago, but not with what will happen in the next 50 years.” 


There’s no way around it: waters are rising. Not just sea levels, but rain, too, as climate change makes extreme weather events more common. That means Hong Kong, like countless cities around the globe, will need to think of new ways to deal with the water in and around it.  


“Do you fight against the water or do you live with the water?” asks Koen Olthuis, co-founder of Dutch architecture firm Waterstudio.NL. He sees three options for cities in coastal areas. They can hold the water back through levees and other structures, as the Netherlands has done for centuries and New York is now considering with a US$119 billion seawall project. They can leave and move elsewhere, as the former president of the Maldives once suggested for his waterlogged homeland. Or they can “flow,” as Olthius puts it, and adapt to a more watery reality. “I think you need to have a combination of all three.” 


Left: Master plan of the Benthemplein Watersquare. (Image courtesy of De Urbanisten)
Middle: Flooding by design in the Benthemplein Watersquare. (Image Courtesy of De Urbanisten)
Right: Overview of the Benthemplein Watersquare. (Image courtesy of De Urbanisten)


Left: Aerial view of the Benthemplein Watersquare. (Image courtesy of De Urbanisten)
Middle: Watersquare drainage. (Image courtesy of De Urbanisten)
Right: Public seating at the Benthemplein Watersquare. (Image courtesy of De Urbanisten)



Drain city or sponge city? 


Hong Kong owes its existence to the ocean, but in recent decades it has sometimes seemed as if it’s at war with it. Ever since Catchick Paul Chater and James Johnstone Keswick hatched the first large-scale land reclamation scheme in 1890, Hong Kong has steadily expanded into the sea, creating more than 70 square kilometres of new land, equivalent to seven percent of its original land area. Ironically, even as the city has expanded into the sea, access to water has grown more limited, with around 60 percent of the harbourfront off-limits to the public. Natural beaches have disappeared from the city centre, and mangrove stands and other wetlands have been eaten away by development on the city’s fringes. Riverbeds were entombed in concrete to create high-volume drainage channels that could quickly send floodwaters out to sea.  


With the changing climate has come a change in approach. Dutch landscape designer Dirk van Peijpe, co-founder of Rotterdam-based firm De Urbanisten, describes the old paradigm as “the draining city,” in which water management is purely a technical issue solved by engineers and hydrologists. “That’s a 19th and 20th century concept,” he says. “The 21st century concept is a ‘sponge city.’ You start with soil, water, flora and fauna. And then you end with buildings.” 


A sponge city is exactly what it sounds like: an urban settlement that is able to naturally absorb water rather than simply repelling it or sending it elsewhere. The term was first coined in the early 2000s by Chinese researchers looking to make cities more flood-resistant, and was adopted as Chinese state policy in 2014. But it has also been echoed around the world by researchers and designers creating what is often called “blue-green” infrastructure that aims to create a symbiotic relationship between the built and natural environments.  


Water flow in the Watersquare. (Image courtesy of De Urbanisten)



And it is gaining momentum in many parts of the world. Earlier this month, the Canadian city of Montreal announced plans to build 30 new “sponge parks” and 400 “sponge sidewalks” over the next two years. (The timing of the announcement was propitious: a few days later, the city received a record-breaking 100 millimetres of rain in a single day.) Last year, Montreal opened its first so-called water square, the Place des Fleurs-de-Macadam, which replaced a petrol station with a lively public plaza that can retain and slowly absorb water when it rains. Van Peijpe says the new initiatives are the fruit of a trip Montreal officials made to Rotterdam a decade ago. “It started with a visit to our water square,” he says. “Designers and engineers came and really took away a lot of insights.”  


Completed in 2013, the Watersquare Benthemplein was the world’s first urban plaza designed to act as a retention pond in heavy rains. “It started with a great idea and a sense of urgency,” says van Peijpe. “The urgency came from the fact that extreme rainwater events would be happening more often, and in Rotterdam we’re below sea level. We had this sewer system from the 19th century that was reaching its limits. Do we start investing again in this system of pipes and pumps or do we look for adaptive solutions?” 


That led to the idea of a water square. “We thought of squares that could temporarily flood,” he says. “We disconnected the public space from the sewer system so the rainwater could freely flow into these depressions we made in the square. When we have rain events they’re normally short but heavy, so after a day the rainwater drains out of the basins into groundwater or aquifers.” 


Left: Arkup floating house on stilts. (Photo courtesy of Waterstudio.NL)
Right: Arkup floating stilt house in motion. (Photo courtesy of Waterstudio.NL)


Left: Arkup floating stilt house moored. (Photo courtesy of Waterstudio.NL)
Right: Arkup floating stilt house at dock. (Photo courtesy of Waterstudio.NL)



Good for humans, good for the environment 


De Urbanisten has since developed a host of related typologies. The studio is currently working on converting an old elevated railway in Rotterdam into an aerial park that will capture and filter rainwater; it has also designed a “tidal park” that will restore the natural ecology of a shoreline that had been encased in concrete; and in Cophenagen, it has designed a so-called “rain street” — essentially the same as Montreal’s sponge sidewalks — that includes an abundance of permeable planted areas that can naturally absorb rainwater. 


Hong Kong has been gradually adopting such an approach, too. “It’s something I see in Hong Kong’s new development areas,” says Fredrick Leong, executive director of environment and planning at the Hong Kong office of Aurecon, an international engineering firm. In recent years, the Drainage Services Department has adopted a number of sponge city policies, including the naturalisation of rivers and nullahs by replacing concrete with permeable surfaces and vegetation. “It’s not just for beautification, it’s to increase the capacity of stormwater management,” says Leong. And it makes the waterways more hospitable to insects, fish and birds, he notes. “It increases the ecological value of the nullah.” 


Van Peijpe says that’s a particularly important point, because “it’s not only solving rainwater issues or flooding issues, but also more and more an ecological agenda,” he says. In Hong Kong, a new reclamation area near Tung Chung will include 3.2 kilometres of “eco-shoreline.” That includes mangrove stands, which boost biodiversity while also protecting against coastal erosion and flooding, and which have fallen victim to urban development over the years. The eco-shoreline also includes mudflats, rocky areas beneficial to shellfish, and seawalls with ridges and pockets meant for nesting birds.  


[Eco-shorelines] allow shelter and accumulation of organic rich material along the roots of the mangrove trees, and other animals or plants or biota will colonise the habitat,” explains Leong. By contrast, older reclamation areas usually have hard shorelines that offer little opportunity for marine life to thrive.  


The same is true for artificial landscapes the world over. “It’s like living in a machine,” says Olthuis. Despite all the canals and retention ponds that are an integral part of the Netherlands’ historic water strategy, he says it’s easy to ignore the fact that much of the country lies under sea level. That’s the risk of an engineering-based approach that keeps everything out of sight and out of mind – until it isn’t. “I grew up never even thinking about different water levels. I always felt safe,” says Olthuis. “But this machine is travelling into climate change and it’s not actually that safe.” 


By contrast, a blue-green approach makes it clear that we are part of an water-based ecosystem with a natural ebb and flow. That’s a key part of the concept behind the original Rotterdam water square, which was developed through a participatory design process that involved working with people who lived in the surrounding area. “We always involve people – and more recently animals,” says van Peijpe. (The first is easy enough, but the latter requires extra effort. “Try to envision what a hedgehog needs to use the park, or butterflies or bees. It’s more difficult to ask them” — he laughs — “but if you think about their needs you can start to design for them.)  


Traditional land development works by envisioning the end use and working backwards from there, which often leads to natural spaces behind shoehorned into whatever space is left over. But van Peijpe says any development process needs to start with the land and its needs first.  


Left: Masterplan for Waterstudio.NL's floating city in the Maldives. (Image courtesy of Waterstudio.NL)
Right: Rendering of the Maldives floating city. (Image courtesy of Waterstudio.NL)



There’s potential beyond land, too. For two decades, Olthuis has been designing floating buildings and communities, and Waterstudio.NL has recently expanded into academic research in collaboration with Delft University. The studio has designed and built a floating house that can anchor itself with stilts, making it typhoon-resistant, and it is currently in the process of building a floating city in the Maldives that will welcome its first residents next year.  


Hong Kong has a long history of floating settlements; tens of thousands of people once lived in its typhoon shelters. But Olthuis — who has visited the city a number of times, including visits to speak at Business of Design Week — says the particular dynamics of land development in Hong Kong make it unlikely to see any significant amounts of floating housing in the near future. “But there are functions that take up a lot of space on land that you can put on water,” he says. Imagine a floating park, library or cultural facility that could move from one neighbourhood to the next. Waterstudio.NL is also working on enormous floating mangrove stands — ”We can build them on floating geo-textiles, these mattresses with soil where mangroves can grow roots,” says Olthuis — that can support marine life and keep water cool in ever-hotter summers.  


It all comes down to one thing: “You have to think about water,” says van Peijpe. Because there will soon be more of it than ever before, and the future for any coastal city means finding ways to embrace it. 



Writer: Christopher DeWolf


This column is produced in partnership with Zolima CityMag, an online magazine that explores Hong Kong’s arts, design, history and culture.