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April 4, 2016updated 20 Jul 2021 3:04pm

“We can’t keep building higher walls”: so how can cities deal with flood risk?

The Thames is one of the defining features of London. It’s easy to take for granted, for residents, tourists and commuters alike, all of whom share an assumption that the surrounding land is safe and dry.

But that’s an assumption that’s regularly challenged.  A quick glance at the Environment Agency’s flood maps uncovers swathes of blue floodplain across the city, a reminder of the latent power the river is able to unleash.

London’s extensive flood defences include barriers on both banks, as well as the iconic Thames Barrier. Completed in 1982, the barrier protects the city from devastating tidal surges such as those experienced in 1928 and 1953.  A recent study by the Environment Agency concluded that the barrier would continue to be effective until the 2080s, despite its original design life of 50 years – albeit only in conjunction with upgrading the defences along the river banks.

Not only is the capital at risk of rising sea levels and tidal surges; increasing rainfall intensities, combined with ever reducing permeable areas, creates a conundrum. That is – what happens when the barrier is closed during exceptionally heavy rainfall?  Well, the opportunity to witness such an event occurred in February of this year: parts of the Embankment were submerged as run-off could not discharge into the swollen river.

And it’s not only London that’s at risk.  Across the world, major cities are facing the reality of rising sea levels and increased rainfall.  

In New York, a 10 year programme of coastal protection is being undertaken in the wake of Hurricane Sandy, including a 10 mile long waterfront park.  Although a barrier across New York Harbour was proposed, the consensus was that hard engineering wasn’t the solution – hence the living waterfront approach.  

In Asia, Ho Chi Minh City is at major flood risk from the Saigon River. It’s looking at adopting best practice from the Netherlands, including increasing water storage capacity and building more resilient waterfront infrastructure.

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And events in Cumbria have shown it’s not just cities where our flood defences are at full capacity.  Brand new defences have been overtopped – a reminder that no matter how we define the risk, there’s always potentially a bigger storm around the corner.

In the face of bigger storms and rising seas, perhaps it’s time to redefine our relationship with water. We can’t keep building higher walls to keep out the floods. Instead, we need to see water as an asset which can add to the amenity, ecology and liveability of our cities. We need to allow waters right back into the heart of our communities, bringing it to the surface rather than burying it underground.

This is something being done the Danish city of Copenhagen in response to a “cloudburst” event. On 2 July 2011, 150mm of rainfall fell in just two hours, leaving swathes of the city under up to a metre of water. Insurance claims from this flood exceeded over €800m, and the total socio-economic loss has been estimated to be double this figure.

The responding Cloudburst Mitigation Plan, and the subsequent catchment level plans (prepared by our own global environmental and engineering consultancy  Ramboll, among others), identifies the parts of the city most at risk from future cloudburst events, and proposes a toolkit of solutions to increase the city’s resilience to flooding.

The overall principles of the strategy are to retain rainwater in the higher elevated areas; to provide robust and flexible drainage of lower lying areas; and a focus on green and blue solutions to be implemented in existing projects. 

This network of blue-green infrastructure aims to replicate the natural water cycle that has been disrupted through modern urban development.  As well as the flood relief and water management functions, the solutions also contribute to the amenity and liveability of the city.

Back in the UK, when flood defences are breached, the impact is dramatic. Perhaps we need to be as bravely dramatic in our decision making about how our cities look, feel and function.

As Copenhagen discovered, the economic cost of flooding is staggering. Perhaps we would be wiser to invest up front in our future – and take a leaf out of their book before other major cities are inundated. 

Stuart Divall and Luke Strickland are engineers at global environmental and engineering consultancy Ramboll.

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