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

The Thames Barrier in action in 2014. Image: Getty.

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.


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.

 
 
 
 

Here’s how we plant 2 billion more trees in the UK

A tree in Northallerton, North Yorkshire. Image: Getty.

The UK’s official climate advisor, the Committee on Climate Change (CCC), recently published a report outlining how to reduce the 12 per cent of greenhouse gas emissions that come from land use by two thirds by 2050. Alongside recommending cutting meat and dairy consumption by 20 per cent, the report calls for the annual creation of up to 50,000 hectares of broadleaf and conifer woodland for the next three decades. This would increase forest cover from 13 per cent to at least 17 per cent – a level not seen in Britain since before the Norman invasion.

Reforestation at that rate would mean creating roughly the area of the city of Leeds every year for the next three decades. At typical stocking densities of 1,500 stems per hectare, the ambition is to establish some 2.25 billion additional trees. Given that the UK, as with most of Europe, is in the grip of ash dieback, a disease likely to prove fatal for many millions of native ash trees, the scale of the challenge is massive.

On a crowded and intensively farmed island like Britain, unlocking a million and a half hectares of land will be no mean feat. But it’s not impossible – and is an unprecedented opportunity not only to tackle the climate crisis but also the biodiversity crisis that is every bit as detrimental to our wellbeing.

Trees and farms

One million and a half hectares is just 6 per cent of the mainland UK’s land area. To give some sense of perspective on this, 696,000 hectares of “temporary grassland” were registered in 2019. So if land supply is not the problem, what is? Often it’s cultural inertia. Farmers are firmly rooted to the land and perhaps understandably reluctant to stop producing food and instead become foresters. But the choice need not be so binary.

The intensification of agriculture has caused catastrophic declines in many species throughout the UK by reducing vast wooded areas and thousands of miles of hedgerows to small pockets of vegetation, isolating populations and making them more vulnerable to extinction.

Integrating trees with the farmed landscape delivers multiple benefits for farms and the environment. Reforestation doesn’t have to mean a return to the ecologically and culturally inappropriate single-species blocks of non-native conifers, which were planted en masse in the 1970s and 1980s. Incentivised under tax breaks to secure a domestic timber supply, many of the resulting plantations were located in places difficult or in some cases impossible to actually harvest.

Productive farmland needn’t be converted to woodland. Instead, that 4 per cent of land could be found by scattering trees more widely. After all, more trees on farmland is good for business. They prevent soil erosion and the run-off of pollutants, provide shade and shelter for livestock, a useful source of renewable fuel and year-round forage for pollinating insects.

The first tranche of tree planting could involve new hedgerows full of large trees, preferably with wide headlands of permanently untilled soils, providing further wildlife refuge.


Natural regeneration

Where appropriate, new woody habitats can be created simply by stopping how the land is currently used, such as by removing livestock. This process can be helped by scattering seeds in areas where seed sources are low. But patience is a virtue. If people can learn to tolerate less clipped and manicured landscapes, nature can run its own course.

A focus on deliberate tree planting also raises uncomfortable truths. Most trees are planted with an accompanying stake to keep them upright and a plastic shelter that protects the sapling from grazing damage. All too often, these shelters aren’t retrieved. Left to the elements, they break down into ever smaller pieces, and can be swept into rivers and eventually the ocean, where they threaten marine wildlife. Two billion tree shelters is a lot of plastic.

The main reason for using tree shelters at all is because the deer population in the UK is so high that in many places, it is all but impossible to establish new trees. This also has serious implications for existing woodland, which is prevented from naturally regenerating. In time, these trees will age and die, threatening the loss of the woodland itself. Climate change, pests and pathogens and the lack of a coordinated, centrally supported approach to deer management means the outlook for the UK’s existing treescape is uncertain at best.

An ecologically joined-up solution would be to reintroduce the natural predators of deer, such as lynx, wolves, and bears. Whether rewilding should get that far in the UK is still the subject of debate. Before that, perhaps the focus should be on providing the necessary habitat, rich in native trees.

A positive response would be to implement the balanced recommendations, made almost a decade ago in a government review, of creating more new habitat, improving what’s already there, and finding ways to link it together. Bigger, better, and more connected habitats.

But the UK is losing trees at increasing rates and not just through diseases. The recent removal of Victorian-era street trees in Sheffield and many other towns and cities is another issue to contend with. As the climate warms, increasing urban temperatures will mean cities need shade from street trees more than ever.

Trees aren’t the environmental panacea that the politicians might have people believe – even if they do make for great photo opportunities – but we do need more of them. Efforts to expand tree cover are underway across the world and the UK will benefit from contributing its share. Hitting the right balance – some commercial forestry, lots of new native woodland and millions of scattered trees – will be key to maximising the benefits they bring.

Nick Atkinson, Senior Lecturer in Ecology & Conservation, Nottingham Trent University.

This article is republished from The Conversation under a Creative Commons license. Read the original article.