Evidence for historic coastal high-energy wave impact (tsunami?) in North Wales, United Kingdom

Abstract

Understanding the contribution of high-energy events (e.g. storms, tsunami) to coastal evolution is currently much debated. Field investigations in North Wales on Anglesey and the Lleyn Peninsula have identified four sites where imbricated boulder trains occur that are discriminators of wave characteristics. Clast analysis indicates that storm wave heights (ca. 20 m), in excess of known extremes (5 to < 9 m), are required to transport them. A plausible explanation is the historic impact of tsunami (≥ 5 m high) that may have been caused either by a) a submarine slide situated offshore on the edge of the continental shelf or Rockall Trough, b) through seismic activity, as the region is one of the most seismically active regions of the British Isles, or c) impact of comet debris in the North Atlantic, with a candidate event around AD 1014. Field evidence suggests that such a high wave-energy event has not recurred along this coast within the last 400 years. RÉSUMÉL'interprétation de la contribution des phénomènes de haute énergie (p. ex. tempêtes, tsunamis) à l'évolution des côtes fait présentement l'objet de beaucoup de débats. Des études sur le terrain dans le Nord du pays de Galles sur l'île d'Anglesey et sur la presqu'île de Lleyn ont défini quatre emplacements qui présentent des traînées de blocs rocheux constituant des discriminateurs des caractéristiques des vagues. Une analyse des clastes révèle qu'il faut des hauteurs de vagues de tempête (environ 20 m), supérieures aux extrêmes connus (5 à moins de 9 m), pour les transporter. Une explication plausible réside dans l'incidence historique des tsunamis (≥ 5 m de hauteur) ayant pu être causés a) par un glissement sous-marin au large de la bordure de la plate-forme continentale ou de la dépression Rockall, b) par l'activité sismique, car la région constitue l'une des régions les plus actives des îles Britanniques sur le plan sismique ou c) par l'incidence de débris de comète dans l'Atlantique nord, un événement candidat étant survenu vers 1014 de notre ère. Des indices sur le terrain laissent supposer qu'aucun phénomène de haute énergie de vagues de ce genre ne s'est reproduit le long de cette côte au cours des 400 dernières années. [Traduit par la redaction]

A wave-competence approach to distinguish between boulder and megaclast deposits due to storm waves versus tsunamis

“…Scicchitano et al (2007) used measures of distance from the shoreline, size and weight together with statistical analysis of storm regime in the area to distinguish between tsunami and storm deposits. Several investigators have used shoreward distance and clast size together as metrics to conclude that the landward extent of the deposits they studied everywhere exceeded that of the storm wave deposits (Scheffers and Kelletat, 2005;Bryant and Haslett, 2007;Haslett and Bryant, 2007;Kennedy et al, 2007;Scicchitano et al, 2007;Goto et al, 2010a,b). Kennedy et al (2007) using the Nott equations Nott (2003) established hydrodynamic relationships between the energy required to move 2-m long boulders and the historic record of marine inundations of that coast to conclude that the boulders were deposited by a tsunami between 2 and 3 m high.…”

“…However, other disturbances can displace a large water mass from its equilibrium position and cause tsunamis such as, submarine landslides, a violent marine volcanic eruption, subaerial landslides, glacial caving and asteroid impacts. Indeed, Haslett and Bryant (2007) correlate a flood event along the British Coast in 1014 AD to comet debris impact and discuss other UK tsunamis including a 20 m high wave that hit Scotland 7000 years ago, following a massive landslip in Norway. Bryant and Nott (2001) report that paleo-tsunamis have impacted the West Australian coast with wave heights exceeding 30 m produced most likely by large submarine slides on the continental slope or the impact of meteorites with the adjacent ocean.…”

The ‘lost’ islands of Cardigan Bay, Wales, UK: insights into the post-glacial evolution of some Celtic coasts of northwest Europe

“…Whether such an inundation(s) was due to storm surge or tsunami is not possible to discern but an interesting account (the legend of Aber Llyn Lliwan) included in the 9 th -century Historia Brittonum, and repeated by Geoffrey of Monmouth (Reeve and Wright 2007), recounts a marine phenomenon linked to the 5 th or 6 th century that describes unusual tidal conditions and a mountainous wave in the Severn Estuary along the south Wales coast (Rhŷs 1901). Evans et al (2008) have suggested that this refers to a specific site where whirlpools occur near the coast; however, it may also be describing a tsunami event and, moreover, Bryant and Haslett (2007) and Haslett and Bryant (2007) present historic and physical evidence indicative of historic tsunami events in south and north Wales respectively. It is clear that this question requires further investigation in the future.…”

Historic tsunami in Britain since AD 1000: a review

“…This could include coastal flooding in the extensive coastal lowlands of the Solway Firth and Morecombe Bay of Cumbria where flooding may result in the loss of livestock and people. In North Wales, it has been suggested that recently described field evidence for tsunami impact may be related to this event (Haslett and Bryant, 2007b). The flood is also mentioned in the Chronicle of Quedlinburg Abbey (Saxony), where it states many people died as a result of the flood in The Netherlands, and it is remembered in a North American account by Johnson (1889).…”