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Flash flooding risk

PUBLISHED: 07:31 25 March 2009 | UPDATED: 13:27 03 July 2010

Flooding wake-up calls: Tewkesbury cut off by flash flooding in the summer of 2007.

Flooding wake-up calls: Tewkesbury cut off by flash flooding in the summer of 2007.

MORE than 15,000 homes and 30 schools and colleges in Norfolk are at risk from flash-flooding of the kind that devastated areas of Hull, Sheffield and other towns and cities in 2007, according to new estimates.

MORE than 15,000 homes and 30 schools and colleges in Norfolk are at risk from flash-flooding of the kind that devastated areas of Hull, Sheffield and other towns and cities in 2007, according to new estimates.

In addition 153 electricity sub-stations, seven water storage and treatment plants, 334 shops and 41 offices in the county could be deluged, research by BBC Look East has found.

The programme has obtained maps which, for the first time, identify the risk across the entire East region for what is termed surface water flooding. Analysis of that data shows that an estimated 127,948 homes in the region could be affected.

The maps identify those areas where water naturally collects and are based solely on the shape of the land, not taking into account buildings, sewer capacity or flood defences.

They show which areas are likely to be flooded to a depth of 30cm (1ft) in the case of a one-in-200-year rainfall event, which for Norfolk would equate to 75mm (3ins) of rain over a six-hour period.

It is understood that even the most robust sewer systems in the UK have a capacity to cope with one-in-30-year rainfall events at most.

The maps have already been given to emergency responders, including fire, police and ambulance personnel, as well as councils and utility companies across the UK.

They will be used to help them plan their response to any extreme rainfall alert issues by the Environment Agency and Met Office's new Flood Forecasting Centre, which is due to begin operations next month.

John Ellis, head of emergency planning at Norfolk County Council, said: “We have received the maps and will now analyse the data to see if there is anything we need to incorporate in any of our emergency plans.

“Whenever any such new information arrives we always try and see if there is anything we can learn from it and whether we need to make any amendments to our emergency plans to make sure they are as robust as possible."

The maps, along with the forecasting centre, arose from the recommendations of the Pitt Review, published last year, into the 2007 floods which caused widespread damage in towns and cities including Hull, Sheffield, Gloucester, Worcester and Barnsley.

The report's author Sir Michael Pitt described those floods as the country's largest peacetime emergency since World War II, adding: “The impact of climate change means that the probability of events on a similar scale happening in future is increasing.”

The maps were produced by a company called JBA Consulting, based on data collected from the air. Urban areas were surveyed by an aircraft fitted with a Lidar unit that fires a laser beam at the ground 100,000 times a second.

Look East asked the company to calculate how many buildings lay within the risk areas identified by the maps, which it did by cross-referencing this information with other date pinpointing the precise location of every building in the region.

Jill Boulton, technical director for JBA Consulting, said: “What seems to be happening is that you're not getting more rain in a month but you're getting it much more intensely, so where you might have had six inches in a whole month, you're now getting it in two hours.

“The sewers, basically, cannot cope and therefore you're getting a lot more surface water incidents. The kind of incidents we saw in Hull, Worcester, Yarmouth - we speak of them as being one-in-175, one-in-200-year events - they're actually going to become a lot more frequent as this century progresses.”

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