Was Bristol Channel Hit By A Tsunami?

"Despite the recent Indian Ocean tsunami of 2004, tsunamis along most coastlines are currently viewed as an underrated hazard," write Edward A. Bryant (University of Wollongong, New South Wales, Australia) and Simon K. Haslett (Bath Spa University, UK). "But an examination of the prehistoric record along many of these coasts gives conclusive evidence of depositional and erosional impact of tsunamis, in some cases of immense proportions."

They continue: "Our purpose here is only to present field evidence showing that Bristol Channel in the United Kingdom has been subject to tsunamis."

In a paper appearing in the May issue of The Journal of Geology, the researchers explore the area affected by the 1607 flood, extending from Barnstaple (Devon) and the Carmarthenshire coast to the head of the Severn Estuary at Gloucester. They use an established catalog of geomorphic features indicating the presence of tsunamis in coastal landscapes, such as sand layers, overlapping boulder deposits, cavitation, bedrock sculpturing, and landscape erosion.

Of these features, bedrock sculpturing and landscape erosion require the greatest water depth and are least subject to the contention that they may be the result of storm surges. Bedrock sculpturing by a tsunami requires a water depth of 10-60 meters (approximately 32-196 feet), while landscape erosion has been found only under the most extreme conditions.

The researchers found evidence in four locales of the sort of bedrock sculpturing linked elsewhere to tsunamis. In Infracombe on the northwest Exmoor coast, they found juxtaposed cliff-types -- cuestas and scarps -- and muschelbruche (scallop-shaped depressions) that reveal substantial unidirectional water flow from the west.

In Ogmore, in southeast Wales, the researchers found both rock and boulder "dumps" and shallow vortex pools with forms that indicate their formation was rapid, occurring in a matter of minutes. The researchers also found evidence of coastal erosion at Sully Island and Ball Rock, where the Bristol Channel decreases in width by 30 percent. According to the researchers, any tsunami traveling up the channel would have been constricted at this point and forced to increase in height dramatically.

"This article adds to recent evidence juxtaposing the geological and historic records," write Bryant and Haslett. "The fact that the signatures of tsunami are found in Bristol Channel up to the more sheltered Severn Estuary is surprising because the northwest European coast in general is not considered tsunami prone."

According to the authors, likely triggers for a tsunami in this region are an earthquake, an underwater landslide, or a combination of both. They point out that an active fault zone lies off the coast of Ireland and, indeed, second-hand reports mention a tremor felt on the morning of January 30, 1607.

Reference: Edward A. Bryant and Simon K. Haslett, "Catastrophic Wave Erosion, Bristol Channel, United Kingson: Impact of Tsunami?" The Journal of Geology: 115, p. 253-269.