A repeat of a storm surge that caused severe flooding in south Wales and south west England 400 years ago would be the UK's costliest natural disaster, claims a new report.
The shaded area of the map was hit by the 1607 disaster
A risk management company has calculated that a similar event in the Bristol Channel could cost £13bn.
Experts on freak weather and tsunamis hold a forum in Newport on Saturday.
High tides and storms or a tsunami are theories for causing the deaths of up to 2,000 people on 30 January 1607.
It is estimated 200 square miles (520 sq km) of land in south Wales and south west England were covered by water.
1607 FLOOD FACTS
The flood reached a speed of 30mph and a height of 25ft
It swept up to four miles inland in north Devon, Pembrokeshire, Glamorgan, Monmouthshire, Cardiff and Somerset.
In low-lying Somerset levels, it reached 14 miles inland
In 2002, experts from Bath and Australia, proposed the tsunami explanation over the idea that the flood was caused by a freak high tide
Dr Claire Souch, director of model management at Risk Management Solutions (RMS) and a co-author of the report claims that an "exceptional event" on the scale of the 1607 flood, with a storm surge of nine metres, could overwhelm existing flood defences.
More than 80% of the total losses from the same event today would occur in the cities of Bristol, Cardiff and Gloucester, with the remaining losses along the south-western coast of Wales and around Barnstaple in Devon.
Dr Souch said: "We've run a variety of simulations of a storm surge coming up the Bristol Channel today and in the worst case scenario the flood heights would be so high, they would over-top existing defences and cause flooding over an extremely large area.
'Wall of water'
"This could lead to damage totalling as much as £13bn. which would indeed make it the costliest disaster in the UK."
Dr Souch said it had to be made "very clear" that the chance of a repeat was small, with a probability of it being an event which could take place on average every 500 to 1,000 years.
"It's not something we can expect over the next few years but it could happen - and it last happened 400 years ago".
Timewatch used computer graphics to recreate what the wave could have looked like
One theory was that a tsunami caused the flood, based on accounts from the time and also geological research of mud deposits and boulders found in south Wales.
But Dr Souch believes it was caused by an exceptional set of circumstances.
"That theory has been around for a while, but there's been a lot of other scientific research which has now effectively disproved that theory.
"The work done by the Proudman Oceanographic Laboratory [in Liverpool] has shown it was really a combination of a very high spring tide and almost at exactly the same time a storm system coming in off the Atlantic, driving the waves ahead of it and pushing up this wall of water, which can be funnelled up the estuary."
Eyewitness accounts of the 1607 disaster told of "huge and mighty hills of water" and only receding 10 days later.
Extreme tidal forces are estimated to occur about every 4.5 years and are next expected on 20 March 2007.
The forum is being held at the University of Newport, Caerleon.