A storm surge is a large scale increase in sea level due to a storm. Low atmospheric pressure allows sea level to rise, and gale force winds combined with the Earth’s rotation force water towards the coastline.

The same physics apply to storm surges caused by European weather systems (extra-tropical cyclones) and tropical cyclones (hurricanes). Storm surges can last from hours to days and span hundreds of square kilometres, affecting coastlines worldwide and causing significant damage and loss of life. In 1970 a devastating storm surge was responsible for hundreds of thousands of deaths in Bangladesh. More recently, in the USA Hurricane Katrina and Superstorm Sandy remind us of the power of these hazards.

Storm surges can raise sea levels by up to 8m in tropical areas and by over 3m in European seas. Coastal flooding occurs when some combination of high tide, storm surge and wave conditions overtops or breaches coastal defences. Storm surges are often the additional factor required to exceed thresholds or to raise the water level so that powerful waves can inflict damage.

Coastal flooding around the UK is a threat to life as well as to economic and environmental assets. The worst natural disaster to affect the country in modern times was the North Sea storm surge of 31 January – 1 February 1953. Flood defences were breached by huge waves and coastal towns in Lincolnshire, Norfolk, Suffolk, Essex and Kent were devastated as seawater rushed into the streets. In the Netherlands, 1800 lives were lost while in England and Scotland 326 people were killed. The estimated cost of the floods was £40–50 million, which is around £1 billion in current prices.

NOC scientists have developed computer models to predict the occurrence of storm surges. These models run on supercomputers at the UK Met Office and are a critical part of today’s coastal flood warning system, the UK Coastal Monitoring and Forecasting (UKCMF, Environment Agency). It is a partnership that combines models of storm surges and waves with real-time monitoring of coastal sea levels, all interpreted by a team of expert forecasters working around the clock. The system also makes use of a technique called ensemble forecasting to quantify the inherent uncertainty in short-term weather prediction. Multiple runs are made, adjusting model conditions and parameters, to provide a range of outcomes that can then be used to judge the reliability of the forecast. Similar models are used to protect lives in more vulnerable parts of the world.