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News & opinion

15 MAY 2019

Innovative new flood defence projects

Tackling the 'new normal' of flooding has never been more pressing. Robyn Wilson profiles three innovative anti-flood projects.

Tokyo’s “floodwater cathedral”

The vast underground network of tunnels and pillars that lie 50m below Tokyo’s surface are a flood control system like no other. Completed in 2006, this mammoth ¥221bn (£1.5bn) scheme protects the Japanese capital from being inundated during heavy rains, which is no easy task when you consider how vulnerable the low-lying city is to such events.

The Tokyo Metropolitan Government’s latest flood map risk assessment puts one-third of the city within a risk zone, which is why it went to great lengths to protect its citizens – 6.3km in fact, comprised of a network of tunnels and chambers 22m beneath their feet. Known officially as the Metropolitan Area Outer Underground Discharge Channel, the scheme works by diverting overflowing river water through ducts, which collect in five massive concrete silos – each big enough to fit the Statue of Liberty inside.

Tokyo riverside
More than £1.5bn has been spent to protect low-lying Tokyo from flooding

“These are huge reservoirs that can collect billions of gallons of storm water,” says Dr Cecilia Tortajada, a water management expert from Lee Kuan Yew School of Public Policy at the National University of Singapore. She explains how this water then flows through a 10m-diameter tunnel and into a cavernous water pressure regulation tank – an engineering feat of such scale, it has made the scheme world famous. Nicknamed “the floodwater cathedral” on account of its awe-inspiring 59 concrete pillars, each weighing 500 tonnes, the tank regulates the volume of the water and then safely releases it into the Edo river.

Critically, though, Tortajada explains how the regional government has also integrated flood risk training into people’s daily lives to complement the underground defences. “In Tokyo, for example, the government has published leaflets that tell people where to go in the event of a flood, as well as training people from a young age in school,” she says, adding that after years of experience of natural disasters the country has gone to great lengths to learn from its past.

“Japan’s approach is it tries to address what the problems were so next time they don’t happen, both in terms of the infrastructure but also in regard to teaching people,” she says. “When it comes to things like smart cities, we tend to focus on planning and infrastructure and ignore the people. What Japan has done very effectively is train its residents, because cities are built for people, and we forget this in the built environment.”

Although a vital piece of infrastructure today, the project had difficulties getting off the ground due to concerns over its cost, says Tortajada. “But from the moment it was built to now, the amount of floodwater that has come through the system has been such that if you compare the cost of the damages that would have been created within the city then it’s already paid.”

The Building Research Establishment’s (BRE) director for innovation and resilience, David Kelly, agrees, adding that countries around the world should study Japan’s investment track record. “Lessons can be learned from countries such as Japan, where greater levels of investment in infrastructure and resilience have been deployed for decades, thus delivering greater levels of confidence to the development and insurance industries.”

Wuhan China
Wuhan is the most populous city in central China and one of the most flood prone, lying at the confluence of the Yangtze and Han rivers

Wuhan’s "Sponge City"

China’s rate of urbanisation since 2000 has been the most rapid that any culture, or any society has ever experienced,” explains Arcadis’ global cities director, John Batten.

“But this urbanisation was carried out in such a way that buffers and green spaces were not incorporated to the extent that they should have. So, what you had was an extreme amount of asphalt, concrete and glass that didn’t allow water to drain naturally into the water table and a lot of Chinese cities, through intensified rain events, would flood.”

Realising the severity of the problem, the Chinese government launched a ¥20.7bn (£2.4bn) “Sponge City” programme in 2015 to undo many of the urbanisation practices that had been put in place. This ambitious project aims to have 80% of the chosen cities constructed to a sponge city standard by 2030. Among the 16 pilot cities selected was Wuhan in the central Hubei province.

The Chinese government launched a “Sponge City” programme to undo many of the urbanisation practices that had been put in place. This ambitious project aims to have 80% of the chosen cities constructed to a sponge city standard by 2030.

With 11 million residents, Wuhan is the most populous city in central China. It’s also one of the most flood prone, lying at the confluence of the Yangtze and Han rivers. Flooding from the two waterways has been reduced by the construction of levees and upstream reservoirs. However, urbanisation had reduced the retention capacity of the city, resulting in a lack of surface water and green space. So severe was the issue that in 2016 during summer monsoon, torrential downpours killed 14 people in Wuhan, and a further 22 in neighbouring provinces.

That same year Arcadis was appointed to redevelop parts of the city using a series of green infrastructure techniques, with a goal to manage 60% of the city’s rainwater. This involved creating new garden cities, bioswales, artificial ponds and wetlands, as well as vegetative buffers, which increased water re-use and gave space back to the rivers, rather than attempting to fight against the overflow. Permeable pavements, which allow water to flow through them rather than across the surface, were also installed, as were upgrades to the city’s urban drainage system, water storage and purification facilities.

The programme has also yielded several side benefits, says Batten, the most interesting of which is a drop in temperature during the city’s extremely hot summers. “By providing green canopies and reusing storm water, the sponge city approach helps to cool the city down.” It has also spurred on developers to start incorporating green vegetation in their buildings as part of “closed-loop water management”, which retains and reuses rain fall from storms.

Batten adds that property values have risen in the sponge city areas of Wuhan as a result of the improvements in green infrastructure, something that he believes can be easily transferred to other flood-prone cities.

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Stratford upon Avon flood defences
In Stratford-upon-Avon, new homes are elevated on piles to allow flood water to pass underneath them

Stratford-upon-Avon: embracing floodwater

Numerous developers had tried – and failed – over the past 15 years to make new development viable at 61 Shipston Road in Stratford-upon-Avon. The brownfield site, which backs on to a floodplain for the nearby river, regularly finds itself under water at times of heavy rain fall. The land is classified as flood zone 3, which means it has a greater than 1 in 100 probability of flooding every year.

Still, given that Stratford-on-Avon District Council’s Local Plan requires the construction of 3,500 new homes in the town by 2031, the developer, Holloway Property Development, felt it worthwhile to find a solution. The group appointed Baca Architects, which specialises in building on or near water, to come up with anti-flood designs for 11 new homes. “We design with water in mind from the very beginning,” says Richard Coutts, Baca co-founder and director. “So rather than try to shoehorn water and push it around the development, we embrace that as a constraint from the beginning.”

In practice, this takes the form of elevating the homes on piles to allow flood water to pass underneath them. “If this was to be on a solid mound, then you wouldn’t be able to provide the water storage on site,” explains Coutts. “The way we’ve managed to do that is to take a very gently sloped road from the highway and raise it at a very gentle gradient, into the site. And most of the houses are accessed off that spur.” This enables safe movement in and out of the main properties in the event of a flood and provides level access to most of the plots, which is required by building control.

The team has also connected a raised walkway to a footpath on a historic elevated tramway at the back of the site to provide a through route from Shipston Road, which gives two means of escape where there’s flood risk. Coutts says this will also enable residents to continue with their working day regardless of the conditions.

In addition to the piled construction, removable louvred screens around the base of each dwelling allow water to flow unobstructed, while preventing not only rubbish and debris from washing up under the homes during floods, but also vermin infestations when the waters recede. Permeable paving will then be used on all hard-standing surfaces, with large below-ground storm water tanks that will help reduce pressure on the existing sewerage system.

The designs are the perfect example of how, as the BRE’s Kelly puts it, “new developments should not only reflect the immediate risk but demonstrate creative solutions that mitigate the effects of climate change”. And with the latter becoming an ever-more pressing issue, Coutts believes other flood-prone parts of the UK could use similar techniques to unlock land.

“This is a clear strategy to unlock those brownfield sites located within floodplains or next to rivers to provide homes at the heart of city centres,” he says. “It’s good sustainable development in terms of transport and locating people where the jobs are, but it’s also another way of resisting urban sprawl into the green belt.”

  • Robyn Wilson is an award-winning journalist and the former news editor of Construction News.

This article originally appeared in the Coastal Issue of Modus (May 2019).