The estimates, prepared for a massive exercise last year to test how national and local authorities would respond to disastrous floods, calculated that 300,000 homes would be inundated. This compares to 160,000 now under water in New Orleans.
Officials add that there could also be a horrific death toll from a British flood - not least because the Government has no powers to make people leave their threatened homes. Electricity supplies to some parts of the country could be out of action for up to nine months, and coastal nuclear power stations endangered.
The three-day exercise, code-named Triton, was the biggest of its kind ever conducted in the country, and involved more than 1,000 people from 60 organisations including chief police officers, local government strategy managers, senior officials from several government departments and a top team from the Environment Agency, which masterminded it.
It tested what would happen if storm surges on the coasts joined in a very rare but catastrophic combination, with high spring tides and strong winds, causing the sea to "overtop" and breach flood defences from the Humber to Southampton Water and in North and South Wales. Water would sweep "tens of kilometres" inland in many places, covering "millions of hectares of lowland England and Wales, including areas of heavy industry and residential development".
Organisers of the exercise say that the scenario envisaged the flooding of all or part of several dozen towns - including Boston, Skegness, King's Lynn, Great Yarmouth, Thamesmead, Barking, the Medway towns, Hayling Island, Portsmouth, Newport and Rhyl - but said many others could be affected, depending on where flood defences failed. London would escape serious damage, thanks to the Thames barrier.
The official report of the exercise predicts "a mass fatality incident", mainly because of the difficulty of evacuating people. They fear that it may be even harder to persuade people to escape to safety than it was in New Orleans, partly as people in the American South are familiar with the power of hurricanes while Britons are unused to the effects of less threatening-sounding storm surges.
"A lot of people would stay," said one senior official at the Environment Agency. "It is human nature. They would think that it would not happen, that if it did occur it would not be that bad, and that even if it was serious it would not turn out to be bad for them."
If everyone was evacuated, deaths could be kept to around 200, the organisers estimated. But if not, the toll would be "awful".
The report also predicts "widespread loss and disruption to power supplies" - and estimated that in Lincolnshire, to take one example, "it would take six to nine months to fully restore the high-voltage power supply network". And it envisages damage to nuclear plants and factories containing toxic chemicals. The exercise also raised doubts about "the adequacy and availability of dedicated emergency support equipment such as pumps, generators and mobile radios" and "revealed inadequate recovery planning at all levels for such an incident".
More positively - and in sharp contrast to the situation in New Orleans - it found that local and government authorities worked well together.
The report stresses that it is based on an "extreme scenario" that might not occur for centuries, but adds that global warming is making it more likely. And as one very senior figure at the Environment Agency put it: "Sod's law dictates that it could happen much sooner than we expect."
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