What would be the UK's costliest disaster?

Risk Management Solutions (RMS), which analyses the risks of catastrophes, presented their findings this week at a scientific meeting about the Bristol Channel floods of 1607. They showed that insured losses from a modern day repeat of the conditions which led to the floods, could be up to £13 billion in the worst case scenario.

More than 80% of the total losses from the same event today would occur in the inner Bristol Channel, including the cities of Bristol, Cardiff and Gloucester, with the remaining losses along the south-western coast of Wales and around Barnstaple in Devon.

The study estimated that an event of the magnitude of the 1607 floods would, on average, occur about once every 500 to 1000 years.

Major surge

Between 500 and 2000 people were killed by the flooding on January 30 1607 in villages and farms along the low-lying coastlines around the Bristol Channel and Severn Estuary.

Despite recent suggestions that the floods were the result of a tsunami, recent research confirms that the cause was a major storm surge created by persistent gale force winds and low air pressures, coupled with an exceptionally high tide.

A similar combination of factors today could raise water levels up to 9.5 metres above sea level in the inner Bristol Channel, overtopping current flood defences and submerging hundreds of square kilometres of coastal floodplain around the Bristol Channel and Severn Estuary of south-west England and South Wales.

High tide levels in the region have risen by more than a metre in the past 400 years, mainly due to a rise in relative sea level, so flood waters from the same event today would significantly exceed the high water marks recorded in 1607.

At up to £13 billion, the potential cost of a repeat of the 1607 Bristol Channel floods would be even greater than the £7 billion in damage that it is estimated would result from a repeat today of the 1953 floods along the east coast of the UK.

'Catastrophic risk'

Dr Robert Muir-Wood chief research officer of RMS and a co-author of the report said: "It is well-known that the east coast of Britain is vulnerable to a storm surge flood and flood defences have been significantly upgraded since the catastrophic flooding of 1953.

"This study shows that storm surges also pose a potentially catastrophic risk to the west coast of Britain."

He added: "Current flood defences might protect areas from a storm surge of up to 8.5 metres, but would not be able to contain a truly exceptional event on the scale of the 1607 floods. Although the risk of a repeat of the 1607 floods is comparatively low, it is not negligible."

The high tide on January 30 1607 was exceptional because the semi-diurnal tidal forces were at their most extreme, with the sun and moon both overhead at the equator and the moon at its closest to the Earth. These extreme tidal forces are estimated to occur about every 4.5 years and are next expected on March 20 2007.

A copy of the report '1607 Bristol Channel Floods: 400-Year Retrospective' can be downloaded from the RMS website - see link above/right.