Walham electricity switching station had a close escape from last summer's floods. David Shukman reports
Hundreds of UK power substations and water treatment plants are potentially at risk from flooding, a confidential government study suggests.
BBC News has seen the conclusions of research commissioned after the devastating floods of 2007.
Yorkshire and Humberside, the Midlands and Gloucestershire were among the worst affected areas after heavy rain.
And a separate study suggests that the UK is entering a "flood-rich" period where more flooding is likely.
The government report calls on companies, regulators and ministers to act.
The confidential report warns that "there are likely to be hundreds of sites at the highest levels of criticality" and says that "the risks posed by natural hazards are already rising and are predicted to rise further".
It concludes that it would "be imprudent to rest on the basis that events on the lines of those which happened last summer were so infrequent as to reply on a reactive response alone".
Early estimates of the cost of strengthening the flood resistance of key sites run into the region of £1bn.
The catalyst for this investigation was the near-loss of a major power switching station at Walham, near Gloucester, in July last year.
It provides electricity for 500,000 homes and businesses in Gloucestershire and acts as a key relay for supplies to south Wales.
Only with emergency work supported by the military was the floodwater kept inches away from overwhelming the plant.
I think many of us were surprised by the degree to which critical infrastructure was affected
At one stage during a meeting of Cobra, the government's emergency planning committee, ministers ordered officials to prepare plans for a mass evacuation.
The site is now defended by a massive flood barrier.
The flooding of a treatment works, at Mythe, also in Gloucestershire, at the same time led to 350,000 people losing water supplies for up to three weeks.
The summer floods saw about 13 people killed and 44,600 homes and 7,100 businesses flooded, with the damage caused costing £3bn.
What has alarmed officials is the potential impact on the normal functioning of society - and the speed with which last summer's rainstorms led to flash flooding.
This internal government study come as Sir Michael Pitt puts the finishing touches to his official review of last summer's floods.
His inquiry - due to publish its final report next month - has already found that more than 1,000 electricity and water works were affected, along with 12 sections of railway line and eight stretches of motorway.
Sir Michael told the BBC: "There is no doubt that the network was vulnerable, that the loss of Walham would have been a major issue and many hundreds of thousands of people would have had their power affected.
"I think many of us were surprised by the degree to which critical infrastructure was affected.
"Tens of thousands of people were out of their homes, thousands of businesses were directly affected, but I honestly believe that we could have been a great deal worse."
Spokesmen for the industry associations representing the electricity network and water companies said the summer floods had served as a wake-up call.
But they added that urgent research into the risks was under way - and in many cases further flood defence work had been carried out.
Last June, CE Electric's control centre flooded (CE Electric)
Meanwhile, Professor Stuart Lane, of Durham University's new Institute of Hazard and Risk, has published a report in the academic journal Geography, which suggests the UK will experience more floods in future.
He said: "We have also not been good at recognising just how flood-prone we can be.
"More than three-quarters of our flood records start in the flood-poor period that begins in the 1960s.
"This matters because we set our flood protection in terms of return periods - the average number of years between floods of a given size.
"We have probably under-estimated the frequency of flooding, which is now happening, as it did before the 1960s, much more often than we are used to."
And the Commons Environment Select Committee has said the infrastructure to deal with the same scale of flooding seen last year is in "an unclear and chaotic state".
The nation's current flood defences are focused almost entirely on river and coastal flooding, but about two thirds of last summer's floods were caused by surface water, often following heavy rainfall.
MPs said there was no organisation with overall responsibility for surface water flooding at a national or local level, no-one was responsible for issuing flood warnings and it was unclear who had responsibility for overflowing drains.
How the floods still affect life in Hull
Ministers, the committee added, had repeatedly suggested that the £800m a year for flood management by 2010/2011 would allow the government to deal effectively with future cases of flooding.
But a report has warned the settlement for flood defences made under the Comprehensive Spending Review was "far less impressive under close analysis".
Environment Secretary Hilary Benn told the BBC that the government needed to "learn the lessons from the summer" and that the issue of green belt land being concreted over needed to be dealt with.
He said: "That's a product of development in our society.
"One of the things that the terrible floods of last summer has taught all of us is we need to think about the consequences of that.
"That's why we're changing the rule on front gardens, that's why we're reviewing the ability of new developments to automatically connect to the public sewerage system, because we need to think about where the water is going to go."
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