The story of the Thames Barrier is one of impressive numbers. The 520-metre-long band of 10 steel gleaming gates spanning the River Thames near Woolwich has protected 125 square kilometres of central London from flooding since 1982.

But can the structure carry on protecting the capital against increased risk of flooding into the next century? We examine the calculations the government must make to decide.

Building the Thames barrier

This winter, the barrier protected the capital from the worst potential flood event in 60 years. And you don’t have to go far back in history to understand the damage the flood could have caused had it not been built.

Tides have not been as high since 1953, when the greatest surge on record, combined with a high spring tide, drowned more than 2,000 people. In London, 100 metres of sea wall collapsed as 640,000 cubic metres of water engulfed 1,000 houses.

Credit: Andy Roberts

In the aftermath the 1953 flood, the government-appointed Waverley committee recommended the building of a barrier that could be left open to allow shipping up the river when tides are at safe levels. After over 20 years of consultation, construction began in 1974, and in 1982 the Thames Barrier was complete.

Tidal surges, fluvial floods

The barrier was designed to stop tidal surges flooding central London. The Thames is particularly vulnerable to these surges of water. When they are generated in the Atlantic, they can funnel down the North Sea, into the English Channel, and up the Thames Estuary toward London. When such surges coincide with high tides, they can raise the sea level in eastern England by more than two metres.

The barrier stops the water in its tracks –  its circular gates are closed when high tides are expected. Once water levels start to fall, the gates can be partially opened to allow water levels to slowly equalise.

The risk from tidal surges is growing. The high water mark at London bridge has gone up by over 1.5 metres since 1780, while the south of England is slowly sinking. Scientists expect parts of southern England, Wales and southern Ireland to sink by up to five centimetres this century as a result of changes from the last ice age. All of that adds up to greater flood risk.

It’s not just water from the sea that the barrier protects against. It’s increasingly being used to prevent fluvial floods, as heavy rainfall pours off the land into the Thames.

The barrier can actually prevent fluvial flooding by closing at low tide, keeping the sea out and creating a ‘gap’ where additional river water can flow during the few hours of high tide.

About two thirds of closures last year were to prevent fluvial floods, with the other third preventing tidal floods.

Data from the Environment Agency, graph by Carbon Brief. The agency’s data given online only runs to January this year – the barrier continued to be raised throughout February. Each bar is for a year range (e.g. 1982-1983). The final bar (2013-2014) is incomplete.

The floods the UK has experienced over the past few months have seen the barrier raised more frequently than any other year, after southern England experienced more than 200 per cent of its average rainfall in January alone. By 18 February, the gates were raised 43 times in the year 2013/14: five in response to tidal flooding and 38 to counter fluvial flooding.

But even before this year’s floods, the barrier has had to be closed more frequently. The Environment Agency uses the number of Thames Barrier closures as an indicator for assessing London’s flood risk – and its records show a steady increase over the decades: just four closures in the 1980s, 35 in the 90s, and over 100 since 2000.

Climate change and the Thames estuary

Climate change will increase the risks of flooding to the Thames estuary this century, but quantifying exactly how much is difficult.

Modelling from the Met Office in a 2012 report for the Environment Agency suggests that climate change could raise sea levels in the area between 20 and 90 centimetres by the end of the century, and warns that eventual sea level rise could be much higher if greenhouse gas emissions are not cut. The same modelling suggests there could be 40 per cent more water flowing from tributaries into the Thames by 2080, due to higher winter rainfall.

Similarly, the Intergovernmental Panel on Climate Change (IPCC) predicts a warming world will lead to more extreme rainfall: The UK is expected to receive about 10 per cent more rainfall on average per year by 2100, compared to 1986-2005.

The UK is set to see about a 10 per cent rise in annual average rainfall by 2100 (right) compared to the period 1985-2005 (left). Source: IPCC 5th Assessment Report Summary for Policymakers (p20).

The IPCC’s latest report predicts Europe and the UK are “very likely” to see more heavy rainfall events by the end of the century.

A lot of rain falling in a short space of time raises flood risk, and there’s already evidence heavy rainfall events are getting more frequent in the UK due to climate change, as a Met Office report explains.

Heavier rainfall plus sea level rise – which make storm surges bigger and more likely to breach coastal defences – has scientists warning of a greater flood risk in the UK as the climate warms.

Professor Nigel Arnell, director of the Walker Institute for Climate System Research at the University of Reading tells Carbon Brief:

“The IPCC’s Working Group I report demonstrated that sea level will continue to rise, and extreme precipitation events are likely to become more frequent across mid-latitude regions such as the UK. These will both increase the risk of flooding to London, and the [Environment Agency] plan is designed to address these extra risks.”

Arnell says the consequences of these predictions for the Thames Barrier are likely to be more complicated, however:

“Change in the risk of river flooding from the Thames and other large rivers in London is less clear, because this depends on changes in the amounts of rainfall accumulated over days or indeed weeks.”

On a national scale, scientists are increasingly certain flooding will become more of a problem. But as Arnell’s comments illustrate, zeroing in on the effects of climate change in specific areas like the Thames is more challenging.

Future flood defence

The Environment Agency has outlined a range of options for the future of the Thames Estuary flood defences. None are particularly cheap.

As a minimum, the EA estimates that the cost of maintaining the defences until 2035 will be around £1.5 billion, with an additional £1.8 billion needed to repair and upgrade the defences until 2050. Particular bits of marshland could also be set aside to store tide waters.

More ambitiously, the government could fund a new barrier in either Tilbury in Essex or Long Reach in Kent. Such a barrier would be designed to resist the highest surge tides identified by the Met Office’s analysis of how conditions will change this century. The EA estimates a new barrier could cost as much as £7 billion, though that figure could go up if conditions change significantly as the climate changes.

Finally, the existing barrier could be converted to include locks, which could open and close more flexibly and extend the life of the defences. The EA report says a decision on a new barrier will have to be made by 2050.

It may turn out that there’s a more pressing reason to spend. The EA says 50 is the maximum number of times the barrier should close each year, and beyond that, the barrier could start to fail.

The Environment Agency says:

“[R]eaching this figure will mean we will need to intervene and improve the tidal defence system”.

2013/14 has seen more than 40 closures.