Is Wales at risk from a tsunami?

10.09.2018

In his latest interview featured on WalesOnline, one of the leading academics on this theory is professor of physical geography at the University of Wales Trinity Saint David, Professor Simon Haslett.

Simon Haslett and Dr Ted Bryant

According to Professor Haslett all coasts are at some risk.

Professor Simon Haslett, also a Pro Vice-Chancellor at UWTSD and Dr Ted Bryant of the University of Wollongong (Australia) researched the possibility that a devastating coastal flood that struck southwest Britain in 1607 was caused by a tsunami. They published their tsunami theory in a paper in 2003 in the journal Archaeology in the Severn Estuary that hit the news and were subsequently involved in the filming of a BBC Timewatch programme Killer Wave of 1607 about it during summer 2004. The programme was first broadcast in April 2005 and has continued to attract a lot of public interest.

Professor Haslett said: "All coasts are at risk of tsunami and the 1755 tsunami, generated from an earthquake offshore Lisbon, hit Cornwall, Devon and southern Ireland with waves up to 3m (9ft) high, and is very likely to have hit the western south Wales coasts of the outer Bristol Channel, although there are no records I can recall.

"The 2004 Boxing Day tsunami in the Indian Ocean travelled around the world and seems to have been recorded on the Milford Haven tide gauge, albeit only a few centimetres high by the time it reached south Wales, but it indicates the global reach of some of these events.

"In my review paper I also identified small tsunami waves created in Milford Haven by an earthquake in 1892 but they caused no damage. However Dr Ted Bryant and myself proposed a theory that the 1607 flood in the Bristol Channel was caused by a tsunami and caused around 2,000 deaths."

Where in Wales would be most at risk?

When it comes to a what the impact of wave would be it depends on three things – the wave size, the tide, and the coast.

According to Professor Haslett, living near cliffs is safer.

"The extent of the inland penetration of a tsunami is dependent not only on the size of the wave but on the topography of the coast," he said.

"For example on cliffed coastlines even large tsunami cannot get much further than the cliffs themselves, although tsunami can sometimes mount the clifftops and deposits boulders there."

Some parts of Wales are more at risk than others with communities several kilometres inland at risk.

Professor Haslett said: "For coastal lowlands, like the Gwent Levels for example, once a tsunami gets over the sea defences it may travel inland for several kilometres until it runs out of steam.

"This was seen tragically in both the 2004 Indian Ocean tsunami and the 2011 Sendai tsunami in Japan where there were locations where the tsunami mounted and inundated low-lying agricultural land as a high bore that was very destructive and penetrated up to 5km inland in the case of Japan.

"In south Wales and Somerset there is the added factor that the coastal lowlands like the Gwent Levels and Somerset Levels slope inland, so even when a tsunami that is travelling across the lowland runs out of steam the water would continue to run downhill until the ground rises.

"For the 1607 event, Ted and I suggested that was how the flood waters seemed to have reached the foot of Glastonbury Tor some 22km (14 miles) inland from the sea."

The geography of south Wales could also make a potential tsunami more damaging still. This is because the Bristol Channel could increase the size of the wave.

"Our modelling linked to the 1607 event suggests that a moderate-sized tsunami entering the Bristol Channel could be amplified by the funnel-shape of the channel and Severn Estuary," said Professor Haslett.

"Our data suggested that a possible 1607 tsunami wave may have been around 4m (12ft) high at the mouth of the Bristol Channel but amplified to around 6m (18ft) high by the time it reached the Severn Estuary.

"This would have been on top of the high tide at the time and easily flooded the Gwent Levels causing great damage and fatalities."

How big does a wave need to be to pose a threat?

The Bristol Channel has huge tidal range and this will be crucial in determining the impact a wave will have.

"Regarding wave height, the key point for the Bristol Channel and the Severn Estuary is the state of the tide at the time of the event," said Professor Haslett.

"The Severn Estuary has around a 15m (45ft) maximum tidal range, so if a tsunami hit the coast at low tide it would need to be in excess of the tidal range to cause any flooding at all.

"However if the tsunami arrived at high tide, when the water level is close to the top of the sea defences, then even a small wave on top of the tide would cause flooding and pose a risk to life."

https://www.walesonline.co.uk/news/wales-news/asked-leading-experts-risk-wales-15126333

Note to Editor

The 400th anniversary of the 1607 flood was commemorated by a public scientific forum organised by Professor Haslett on the cause and impact of the 1607 coastal flooding event on Sat 27th January 2007 at the University of Wales, Newport. In addition, BBC2 repeated Timewatch Killer Wave of 1607 in January 2007 to coincide with the commemorative events, BBC Somerset Soundproduced a special 2 hour programme, and BBC News featured it on the national and regional news.

The local communities of the Gwent Levels in South Wales also combined to commemorate Flood 400. The commemoration began Tuesday 30th January 2007 at Redwick Church. A weekend of events was held on 25-28th May 2007 at which Professor Haslett and Dr Bryant presented a poster.

The 400 Years On! public forum included a distinguished panel of speakers: Professor Simon Haslett (then at Bath Spa University), Dr Ted Bryant (University of Wollongong, Australia), Dr Kevin Horsburgh (Proudman Oceanographic Laboratory), Dr Philippe Blondel (University of Bath), Richard Brunning (Somerset County Council), Dr Andrew Skellern (Bath Spa University), and Chaired by Dr Paul Davies (Bath Spa University). The forum was kindly sponsored by Aquatility.

Initial field and laboratory research undertaken by Professor Haslett and Dr Bryant to test their tsunami theory for the 1607 flood is published in 2007 in academic journals.

In the Journal of Geology (vol. 115, pp. 253-269), they presented evidence from erosional features and boulders that a large tsunami has almost certainly struck the coasts of the Bristol Channel and Severn Estuary in the past. Evidence derived from rates of cliff retreat at Dunraven Bay, Glamorgan suggests that the 1607 event is a candidate for this tsunami.

In Marine Geology (vol. 242, pp. 207-220), evidence from coarse sediment layers throughout the region is examined and dated using the radiocarbon method. Whilst some coarse layers could have been deposited during the 1607 flood, layers featured on the BBC2 Timewatch programme at Rumney Great Wharf and in North Devon return radiocarbon dates that are too young to have been laid down by the 1607 flood, and are more likely to be the result of the Great Storm of 1703. However, this dating evidence from Rumney confirms a previous view that for some reason all the salt marshes fringing the Severn Estuary had been completely eroded away prior to the mid 17th century, possibly by the 1607 event.

Their current thinking is that a tsunami has indeed hit the shores of the Bristol Channel and Severn Estuary sometime in the past, but although considerable evidence supports 1607 as the most likely candidate this cannot be confirmed, and perhaps there has been more than one tsunami. Indeed, after Professor Haslett and Dr Bryant published a major review they made a follow-up BBC2 Timewatch programme entitled Britain's Forgotten Floods, which was broadcasted in October 2008 that investigated a number of possible tsunami events that have affected other parts of the British Isles.

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