COULD a huge wave swamp Britain? Almost certainly. Historical evidence suggests that it has happened in the past.

At 9am on January 20, 1607, there occurred along the shores of the Bristol Channel the greatest environmental disaster in British history.

According to the few witnesses who survived, “one or more mighty hills of water” swept over the sea defences and “running with a swiftness so incredible as that no greyhound could have escaped by running before them” swept over the flat hinterlands of more than 20 parishes. Thousands of people were drowned, houses and villages swept away, farmland inundated, flocks overwhelmed and the development of Cardiff and the surrounding region set back for more than a century.

No one can be absolutely certain that it was a tsunami, but the conditions suggest so. The sky was blue, the tide was high, there is a secondhand report of an earth tremor felt earlier that morning. It all fits together.

Much of our archival evidence comes from God’s warning to the people of England by the great overflowing of the waters or floods, a 12-page pamphlet printed in London in 1607. The innocent folk of Cardiff, the writer states, on a bright day in January 1607 “might see and perceive a far off, as it were in the element, huge and mighty hilles of water, tumbling one ouer another, in such sort, as of the greatest mountains in the world had ouer-whelmed the lowe valeyes or marshy grounds.”

On the Welsh side, the devastation stretched from Laugharne in Carmarthenshire to above Chepstow on the English border. There remain plaques up to 8ft above sea level to show how high the waters rose on the sides of surviving churches. At St Mary’s Church in the centre of Cardiff, some walls fell and the corpses were washed out of the graveyard.

Survivors clung to the steeples of churches and to the roofs of the few buildings that remained standing. One woman saved her young children, it is said, by putting them into an empty trough and floating with them to safety.

The Atlantic is not surrounded by the subduction zones that regularly generate tsunamis in the Pacific and, less often, the Indian Ocean but devastating tsunamis do occur here too. Earthquakes can set off monstrous submarine landslides in which tens of cubic kilometres of sediment perched unstably at the edges of the continental shelf plunge down into the abyss at 500km an hour.

We have studied the problem of saving the population of Cardiff, probably one of the cities on Earth most at risk from tsunamis. The majority can be saved simply by walking to the nearest high ground. For those who cannot, there should be designated tsunami shelters, which need be no more than overhead car parks. The rushing waters can pass underneath, and with no sidewalls to feel the pressure they should withstand almost any conceivable event.

There has to be a proper evacuation plan, the citizens need to know what it is, and there has to be a rehearsal from time to time. Buildings can be protected by a stout sea wall, not necessarily very high.

Whether it is worth taking the trouble depends where you live, and how often dangerous tsunamis occur. In low-lying cities such as Cardiff, London, Lisbon and Amsterdam the risk is not negligible. If the 1607 event was a tsunami, then a resident of Cardiff, for instance, is ten times as likely to be killed by a tsunami as by a road accident.

• Michael Disney is professor of astronomy at Cardiff University and consultant to Nasa and the European Space Agency. He and his students and colleagues have studied tsunamis for years.

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