The Group's aim is to identify, survey, protect and promote geological and geomorphological sites in the former County of Avon - the modern unitary authorities of Bath and North East Somerset, Bristol, North Somerset and South Gloucestershire. RIGS are selected for their educational, research, historical and aesthetic value.

Wednesday, 10 July 2013

The Bristol 'tsunami': Flood or fallacy?

This post was originally featured on http://betweenarock.co.uk/

30th January 1607*.
The day dawns sunny and bright. You are ploughing a field in your smallholding deep in the Somerset Levels. As the sweat drips down your back, you hear a distant rumbling sound but think nothing of it; the wind has been blowing a gale all night. Suddenly, a shout from a neighbour makes you look up in alarm. At the end of the far field you see a great cloud hugging the ground, light dazzling off the whiteness. At first you are confused: is it fog, or smoke from a fire? But then you realise, it's water. Within ten seconds, the tumbling, roaring mass has advanced the length of the paddock. You try to run but it's too late. Knocked off your feet by the force of the wave, your head dips below the surface and you inhale a lungful of salty water...
*The exact date depends on whether you have a preference for the Julian or Gregorian calendar

From eyewitness reports, this is what it felt like to be caught up in the most catastrophic flood ever to hit western Britain. Striking in January 1607*, its effects were felt all over the south-west of England, extending over 570 km of coastline from Barnstaple to south Wales and as far inland as Glastonbury (approximately 22km). Contemporary sources put the death toll at over 2,000, though modern estimates have revised this to 500 - 10001. The water flow is said to have been so fast "... that no gray-hounde could have escaped by running before them." But what was the cause?

Contemporary woodcut depicting the scene in Monmouthshire on 30th January 1607.

Prior to a modern-day brush with fame, the Bristol Channel Floods were variously attributed an extreme spring tide (the maximum extent of a tidal range that occurs when the Earth, Moon and Sun are in alignment, roughly every fortnight), a storm surge (high water levels associated with a low pressure weather system) or a combination of both. This type of coastal flooding is relatively common in the UK; a particularly deadly occurrence in 1953 killed 307 people in East Anglia.

The tsunami hypothesis was first proposed in 2002 by two academics (Haslett & Bryant - see references 2,3 and 4), and followed up in a series of subsequent papers by the same authors. Their re-interpretation of the events unintentionally coincided with the devastating Boxing Day tsunami of 2004, and so was perfectly poised to percolate the national consciousness. Numerous media articles publicised the theory, and the floods were featured in two BBC2 TV programmes (Timewatch - "The Killer Wave of 1607" and "Britain's Forgotten Floods").
Flood plaque in Goldcliff parish church, Newport. Reads "1606. On the XX day of January even as it cames to pass it pleased God the flud did flow to the edge of this same bras [brass], and in this parish theare was lost 5000 and od pownds besides xxii [22] people was in this parrish drown.". Photo credit: Robin Drayton.

Of course, publicity is not the mark of whether a theory is right or wrong, but proving this particular watery dispute one way or the other has been hindered by a couple of confounding conundrums: the subjectivity of historical sources and the ambiguous nature of tsunami deposits.

At the turn of the 17th century, literacy levels in the UK were still relatively low. There were no newspapers (or Twitter!), thus first-hand accounts are mostly limited to privately printed pamphlets which tend to offer contrasting reports. For example, the weather on the day in question is conflictingly described as being "most fayrely and brightly spred", "tempestuously moved by the windes" and in the grip of "a mightie storm". The most supportive evidence for a tsunami comes from "Gods [sic] warning to the people of England" , a publication funded by the Church. Its coverage of the event is predictably zealous, describing the flood as a "universal, punishment by Water."
As geologists, the obvious solution would be to look to the rock record; however, tsunami deposits are notoriously tricky to identify because their physical markers are incredibly hard to distinguish from other sources of coastal flooding. Pro-tsunami authors Haslett & Bryantt cite sand "storm" layers in sediments, erosion of salt marshes, vortex pools, and imbricated boulder dumps as supporting evidence for a 'killer wave'; all features imply rapid deposition from a forceful flow of water. Their proposed mechanism for the tsunami is either a submarine landslide or earthquake in the sea between Ireland and Cornwall.

Imbricated boulders
Prof. Simon Haslett atop imbricated boulders in the Severn Estuary. Photo was taken during filming of the BBC2 programme “The Killer Wave”. Source: http://profsimonhaslett.blogspot.co.uk

Perhaps the most compelling evidence against the tsunami hypothesis is that severe flooding in Norfolk is documented on the same day. Most tsunami models agree that it is geometrically impossible for the effects of a tsunami to wrap around the entire coast of England. It seems like the most plausible cause of the floods is a storm surge imposed on an unusually high spring tide. Indeed, the Severn Estuary has the second highest tidal range in the world. The contemporary reports of windstorms driving up the seas is reminiscent of storm surges in New Orléans during Hurricane Katrina in 2005.

Regardless of the cause, it is important to consider the impact that a repeat of the 1607 floods would have today, in order to mitigate against future disasters. The Severn estuary is home to the (active) Hinkley Point and (closed) Oldbury nuclear power stations, and is the proposed site of the controversial Severn Tidal Barrage. Other notable infrastructure includes two motorway bridges, a working port (Avonmouth) and half a million people living in Bristol alone! One risk assessment puts the cost of such an event at £7 - 13 billion1.

In the wake of the 2004 Boxing Day tsunami, the UK government recognised they did not have a quantitative assessment of threat to the UK. This was despite another infamous tsunami study5 (the results of which are now viewed with scepticism) which predicted that a landslide off La Palma would generate waves "higher than Nelson's column" and smash into the west coast of Britain - mass media loved it. Happily for us, the government reports conclude "tsunami-type events [affecting the UK] are unlikely to exceed those anticipated for major storm surges", and "all major centres of development on coasts and estuaries have defences that have been designed to withstand such surge waves."

Should we have these in Bristol City Centre?

Despite their assurances, a small part of me feels pretty smug about a living and working a good 50 metres above sea level!

Charly Stamper

[1] "1607 Bristol Channel Floods: A 400-Year Retrospective" - Online publication by Risk Management Solutions.
[2] Bryant EA & Haslett SK (2007) Catastrophic Wave Erosion, Bristol Channel, United Kingson: Impact of Tsunami? The Journal of Geology: 115, p. 253-269.
[3] Bryant EA & Haslett SK (2002) Was the AD 1607 coastal flooding event in the Severn Estuary and Bristol Channel (UK) due to a tsunami? Archaeology in the Severn Estuary. 13: 163 - 167.
[4] Haslett & Bryant (2004) The AD 1607 coastal flood in the Bristol Channel and Severn Estuary: historical records from Devon and Cornwall (UK). Archaeology in the Severn Estuary. 13: 81 - 89.
[5] Ward, SN & Day, SJ (2001) Cumbre Vieja Volcano; potential collapse and tsunami at La Palma, Canary Islands. Geophys. Res. Lett. 28-17, 3397-3400.