"We think these [features] were caused by whirlpools pulling up cobbles [round stones], which acted like a drill to erode doughnut-shaped depressions in the rock," Haslett explained.
Such bedrock-sculpting action requires water between 33 and 197 feet (10 and 60 meters) deep, which is unlikely to be generated by a storm surge.
By calculating the energy required to shift the boulders and sculpt the bedrock, Bryant and Haslett estimated that the tsunami reached a height of 20 feet (6 meters) at the channel's narrowest point and gushed between 38.7 and 59.3 feet a second (11.8 and 18.1 meters a second).
This ferocious wave is consistent with historical descriptions of "mighty hilles of water" and a wave that is "affirmed to have runne … with a swiftness so incredible, as that no gray-hounde could have escaped by running before them."
The most likely trigger of the wave was an earthquake, an underwater landslide, or both, the researchers said.
"An active fault zone lies off the coast of Ireland, and second-hand reports mention a tremor felt on the morning of January 30, 1607," Haslett said.
(Learn the basic facts about tsunamis.)
Bryant and Haslett think that locations along the Bristol Channel suffered the most devastation because of local formations that created a magnification effect.
"The Bristol Channel gets very narrow at one point, which funnels the water and would have amplified the wave," Haslett said.
Highest Tide
Not everyone agrees with Bryant and Haslett's interpretation of the 1607 disaster.
Kevin Horsburgh of the Proudman Oceanographic Laboratory in Liverpool is convinced that the event was caused by a storm surge rather than a tsunami.
"It occurred at precisely high water during the highest tide for a century," he said.
"The rudimentary river bank defenses, if any, would have easily been overwhelmed by the known tide plus a routine, modest storm surge."
Horsburgh, who recently published a paper on the flood in the journal Weather, thinks that the bedrock features seen by Haslett and Bryant can be explained by the channel's strong currents.
"Bristol Channel has the second largest tide in the world and tidal currents of up to 8 meters per second [26 feet a second], twice a day, every day," he said.
Horsburgh agreed, however, that tsunamis still pose a risk for the U.K.
"We recently reviewed the risk of a U.K. tsunami for DEFRA [the British Department for Environment, Food and Rural Affairs] and found it to be small but not zero," he said.
The most recent tsunami to hit British shores was probably a wave that arrived in southwestern England after an earthquake struck Lisbon, Portugal in 1755.
"There is good evidence of the Lisbon 1755 tsunami being detected in Cornwall," Horsburgh said.
Geological records also show that a major tsunami hit Scotland's Shetland Islands and eastern coast around 8,000 years ago, when a large underwater landslide occurred off Norway.
A Disaster Waiting to Happen
A group of scientists not involved in the new study says it has spotted a site under the Atlantic that could trigger a potential mega-tsunami.
"Our research has shown that the world's biggest active landslide is occurring on the flanks of Cumbre Vieja, a volcano on the Canary Island of La Palma," said Bill McGuire, director of the Benfield Hazard Research Centre at University College London. (See a map of the Canary Islands and Morocco.)
When this volcano erupts, McGuire and his colleagues predict, half of the island will slide into the ocean, precipitating an Atlantic Ocean tsunami.
The East Coast of the United States and large swaths of Western Europe will be swamped with wave heights of up to 33 feet (10 meters), the experts said.
"One day this eruption will occur. It is a case of when, not if," McGuire said.
About 2 percent of tsunamis occur in the Atlantic Ocean, but this figure could become higher in the future, he added.
"If global warming causes catastrophic melting of the Greenland ice sheet, then we can expect large landslides to occur from the glacial sediment sitting offshore," McGuire said.
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