MI5 and flood risk experts are at odds over whether to publish inundation maps highlighting areas under threat if any of the country's dams were to collapse. The Security Service says that the information could show terrorists where an attack on a dam might have the most impact.
Experts in the Cabinet Office and the Environment Agency feel the time has come to make the information public, as the risk of major flooding rises with climate change. Just how grave the threat is was illustrated yesterday when it was revealed that record rainfall nearly caused a Yorkshire dam to fail, with catastrophic consequences.
The scale of an alert at Ulley reservoir, near Rotherham, emerged for the first time at the publication of the independent review by Sir Michael Pitt into the lessons from the 2007 floods. Sir Michael, an engineer and former local authority chief executive, added that new research showed that the likelihood of more "extreme event" floods in Brit-ain in the future was greater than previously thought because of heavier rainfall.
He made 92 recommendations about future action, including setting up a joint Environment Agency-Met Office national flood warning centre within two years.
Asked if Britain's flood defences had improved since last year, he said: "I have no doubt that there is now a much greater awareness of the risk of flooding." Some preparation work had been done, he said, and critical sites such as electricity substations and water pumping stations had been reviewed for flood risk. "But there is a great deal more to be done," he said.
A major threat to infrastructure made the situation at the Ulley reservoir dam so critical. The dam came close to failure when water flowing down a spillway undermined its base. It was saved by emergency repairs after a midnight evacuation of 1,000 people and the closure of the M1.
The consequences of a collapse would have extended far beyond the immediate area. Waters from a breach would have knocked out the main electricity switching station in the Sheffield area for an indefinite period, perhaps leading to a breakdown in social cohesion. A dam flood would also have destroyed the main 42-inch gas main serving Sheffield, which would have created a hazard to aircraft from the gas escaping into the atmosphere. A five-mile aerial exclusion zone was imposed around the point where the main might have fractured. All this would have been in addition to hundreds of deaths caused by a wall of water driving down the valley below the dam if it burst.
Yet emergency services facing this crisis, Sir Michael said, had no maps of potential inundation to work from and had to work it out themselves. He added that because of climate change, flooding in Britain was itself likely to become a risk as great as terrorism or an influenza pandemic.
According to Cabinet Office and Environment Agency sources, the Water Act 2003 had specified that the maps should be made public, but this had not happened after MI5 objected.