Innovative new flood defence projects

“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’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.

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.