What is the 1000-year flood?

The term is statistical, a probability. It means that there is a 1 in 1000 chance the Barrier will be overwhelmed in any particular year. The flood could occur tomorrow, or the next day, or any time in one thousand years.

The odds of 1000 to 1 are true only for a specific period, because of the parameters used in the calculations. After 2030 the odds decline. And, if global warming accelerates the rise in sea level, they will decline sooner and more sharply than the Barrier designers intended.

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How does the risk compare?

In Holland the surge tide of 1953 killed 3000 people. The Dutch have since built their sea defences to a risk level of 1 in 10,000.

In the UK the recommended risk level to the public from an accident at a nuclear power station is 1 in 1,000,000 and from an industrial plant, 1 in 10,000.

The Thames Barrier was built to a risk level of 1 in 1000. Slightly worse odds than contracting fatal cancer (1 in 3000) and much worse odds than of dying in a traffic accident (1 in 10,000).

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What is the trigger for Barrier closure?

The trigger for Barrier closure is any combination of tide and surge that will generate a tide height at London Bridge of 4.85 metres ODN.

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How high are the banks in Central London?

The banks in Central London are at a range of different levels:

Teddington 6.10 metres
Chiswick 5.54 metres
Putney to London Bridge 5.41 metres
Greenwich Peninsula 5.18 - 5.23 metres
St Catherine's Dock 5.28 metres
Barrier 7.20 metres

(all measurements above ODN)

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What is ODN?

In the UK heights above sea level are defined in terms of Ordnance Datum Newlyn. ODN is the mean level of the sea at Newlyn, Cornwall between 1915 and 1921.

All countries have their own standards. In France, heights are relative to mean sea level at Marseilles at a particular epoch.

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Has London been flooded in the past?

"In the year 1236 the River Thames, overflowing its banks, caused the marshes all about Woolwich to be all a sea wherein boats and other vessels were carried by the stream, so that besides cattle a great number of inhabitants there were drowned, and in the great Palace of Westminster men did row with wherries in the midst of the Hall."
(John Stow, The Chronicles of England)

"There was last night the greatest tide that ever was remembered in England to have been in this river, all Whitehall having been drowned."
(Samuel Pepys's Diary, 7 December 1663)

In the twentieth century the most damaging storm surges to enter the Thames Estuary were in 1928 and 1953.

On January 6, 1928 the flood tide in central London was 1.8m above predicted level.

Flood reports started coming in from Battersea, Poplar and Greenwich; the embankment at Temple Station was awash and the whole of Old Palace Yard, Westminster. The training ship President rode the waves at parapet level. The embankment opposite the Tate Gallery was the first to give way and the gallery was flooded almost to the tops of the doors on ground level. A collection of Turners was damaged. Lots Power Station was partly flooded as was Wandsworth Gas Works and the Blackwall Tunnel. Failure of a twenty five metre stretch of embankment near Lambeth Bridge left fourteen people drowned in their basements. Four thousand were made homeless.

On January 31/February 1, 1953 the main focus of the storm was the East Coast and lower Estuary with a death toll of 175. By the time the storm surge reached central London winds were abating and only Docklands was seriously flooded. The area was largely industrial at this time and there were no fatalities.

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Is there a season for flooding?

There is for storm surges: the danger period runs from October through to April. Fluvial flooding - rivers breaking their banks due to high rainfall - can occur at any time.

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Is the Barrier closing more frequently?

It would appear so. Since 1984 the Barrier has been closed 64 times in response to tidal surges. 15 of these closures took place in 2001.

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Can the height of the Barrier be increased?

In theory it is possible to rotate the gates round slightly further, adding perhaps half a metre to the overall height. Barrier staff admit that this would be impractical unless the banks either side were also raised, a massive undertaking. In addition there are doubts about the extra strain that would be put on the gates.

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Which government agency is in charge of flooding?

The Environment Agency (EA) oversees flood prevention. The responsible ministry is the Department of the Environment, Food and Rural Affairs (DEFRA).

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What is the Emergency Powers Act?

The Emergency Powers Act gives power to the Crown to proclaim a state of emergency in order to make regulations for securing the essentials of life to the community.

In short it gives Ministers executive powers. In the event of tidal flooding they would be able to commandeer private property such as boats, order evacuations, etc. There are also provisions for summary trials of individuals for non-cooperation.

Such powers are invoked by proclamation signed by the Queen in Council.

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What is Triple-C?

Triple-C stands for Cabinet Contingency Committee. It is a committee drawn from government ministers, senior civil servants, military and emergency services and such experts as may be needed to coordinate the response to a particular crisis. It is normally chaired by the Home Secretary and meets in Cabinet Office Briefing Room A (COBRA). The secretary is normally a military officer such as a Commander RN on secondment from the MOD.

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