BRITAIN鈥檚 sea walls are as much as 2 metres too low to protect coastal
areas from serious risk of flooding, according to a report being studied by
government scientists. The shortfall is due to an error discovered in the way
storm surges are forecast.
The report reveals a major flaw in calculations underpinning the design of
Britain鈥檚 800 kilometres of sea defences, many of which were built to prevent a
repeat of the devastation caused by the storm surge in the winter of 1953, which
killed more than 300 people on the east coast.
Sophisticated forecasting techniques based on extreme value theory (EVT) are
currently used to work out the height of sea walls needed to reduce the risk of
flooding to below 1 in 100 per century. Based on the mathematical properties of
extreme events (see 鈥淔ar out forecasting鈥�, p 36), the theory converts past
records from tide gauges into predictions of the maximum likely storm surge over
the next century. These forecasts are then used to set the standards for sea
walls around Britain鈥檚 coast.
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But according to the new report, compiled for the Ministry of Agriculture by
mathematicians Mark Dixon and Jonathan Tawn of Lancaster University, EVT assumes
that any nonrandom effects, such as tides, have first been subtracted out of the
raw data. Yet the records from tide gauges reflect the effects of both the
regular tides and freak weather conditions.
Until now, experts have believed the effect of using the uncorrected gauge
measurements would be minimal. But the report warns that the effect is far from
negligible, and can lead to substantial underestimates of the maximum possible
storm surge. Some parts of Britain, especially around the Bristol Channel near
Ilfracombe, may need sea defences as much as 2 metres higher to ensure that the
chances of flooding are cut to reasonable levels. The underestimates 鈥渕ay have
severe repercussions for current designs鈥�, concludes the report.
A spokesman for the government鈥檚 Environment Agency, which is responsible for
the nation鈥檚 sea defences, says the agency has yet to study the report鈥檚
implications, but added: 鈥淲e take very seriously any research concerning the
safety of Britain鈥檚 coastal defences.鈥�