By David Shukman

BBC environment correspondent

Please turn on JavaScript. Media requires JavaScript to play. Advertisement In a cloudless sky, above the market towns and green and pleasant fields of Warwickshire, it is hard to believe that it is worth the effort of the Environment Agency to map in painstaking detail every bump and dip of the terrain down below. But the project - to plot the topography of England and Wales to within six inches of accuracy - is seen as a vital first step in preparing the country to face the emerging threat of flash flooding from tropical-style downpours. Fitted to the floor of a leased aircraft is a Lidar unit which fires a laser beam downwards 100,000 times every second, scanning a swath of land up to 600m wide. So far, 56% of England and Wales have been surveyed, with the focus now on the urban areas most at risk from torrential surface water during heavy storms. Please turn on JavaScript. Media requires JavaScript to play. Advertisement On board, scientist Chris O'Dwyer activates the laser for a series of passes one mile long and watches the data stream into a bank of consoles. During our flight, he told me: "The difference between an area flooding and not flooding is 6ins and the data we are gathering here has an accuracy in elevation of about 15cm. That could be the difference between the water flowing over the threshold of somebody's doorstep and not." The result is a highly detailed series of three-dimensional maps. I was shown one of Hull, one of the most vulnerable cities and badly hit last summer. Unexpected deluge The aim is to create a basic database to which local authorities, water companies and private businesses can then add their own information about drains and water courses - to try to provide the best possible estimate of the locations most at risk from flash flooding. It is a huge task. Although Hull is used to the threat of flooding from the sea and from the River Humber, the flooding last June was caused by an intense and unexpected deluge. A total of 8,647 homes was affected along with 91 out of 99 schools in the area. And even now, nearly one year on, 547 families are still living in caravans. The head of flood risk at the Environment Agency, Phil Rothwell, told the BBC that an urgent and nationwide effort to prepare for downpours was now needed. "I think this is a very big issue. We saw last week monsoon-style rainfall in Oxfordshire. This is the sort of thing we can expect more of in the future - very local, very heavy rain. "We saw last year the floods demonstrated that over half the property was flooded by water on its way to the rivers, not coming from the rivers. It's flash flooding, urban flooding." And Mr Rothwell appealed for better co-ordination to tackle surface water flooding - to have a single organisation in charge of overseeing the running of all the systems for handling flash floods. "If we don't get the management of our cities' drainage structure right, this will be a continuing problem, even an increasing one," he says.



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