Modelling risk from a potential Cornish tsunami

By Rory Walshe

If you live in England, tsunamis are probably not a major worry. When people think of tsunamis, they think of distant, tropical locations in seismically active regions such as Japan and South East Asia. Few would think of England as being at risk from tsunamis, but some scientists believe a Cornish tsunami could be just around the corner.

There have actually been several deadly tsunamis to strike England in the past, and there may be an even deadlier one brewing in the Atlantic. There is some debate about a few of England’s historical tsunamis (with several chalked up to ‘meteotsunamis’), but two key historical tsunamis are the Bristol Channel tsunami in 1607 and Lisbon earthquake tsunami of 1755.

The Bristol Channel tsunami of 1607

In 1607, a tsunami struck the Bristol Channel with a 20 meter high wave which inundated large areas of the south of England and reached as far as 14 miles inland. This originated from a disputed seismic event but regardless of the source, the devastation was catastrophic, with approximately 2000 fatalities recorded¹.

A tapestry depicting the 1607 tsunami
Public domain image

A commemorative plaque
Photography by Robin Drayton, CC-image from Geograph

Lisbon earthquake and tsunami of 1755

148 years later, an earthquake in Lisbon in 1755 created a tsunami that struck the Cornish coast. The wave was around 3 meters high, and although there is no official death toll, a literary record states that a “great loss of life and property occurred upon the coasts of Cornwall”.

Copper plate record of the Lisbon tsunami
Public domain image

Artists rendering of the Lisbon tsunami
Public domain image

So England is far from invulnerable to tsunamis, but what about a future tsunami?

The Cumbre Vieja Tsunami of 2016?

In 2001, Steven Ward and Simon Day from University College London’s Hazard Research Centre, published geological evidence and computer modelling of a potential tsunami, caused by the collapse of a volcano in the Canary Islands². Ward and Day warn that an eruption of the volcano, called Cumbre Vieja, could trigger a catastrophic landslide event on its western flank, dropping 150 to 500 km³ of rock into the sea.

Although the exact probability and timeframe of this event is a controversial topic among geologists (a 2006 paper argued the island is more stable than first thought³) few dispute that it is indeed a risk. So what would happen to England if a piece of rock the size of the Isle of Man fell into the Atlantic?

The resulting tsunami would strike across most of the Atlantic basin with wave heights between 10 and 25m (the length of three London buses). Within 6 hours of the collapse, the UK would be hit by 4-7m waves (just under a single bus length – phew!), with Cornwall, on the South-Western tip, being the first part of England to be hit by the wave.

If you’re reading this in Cornwall, this will probably come as rather alarming news. While there has been some research into tsunami hazard for the UK⁴ this has focused on wave height. An estimation of water elevation above still water level is certainly valuable, but it does not account for the topography of the land the wave is striking, and therefore is of little use for examining impact. No one has ever modelled the tsunami risk zones once the wave makes landwall in Cornwall.

Since the wave propagation and evolution has already been calculated, it was fairly simple to construct a basic inundation map of Cornwall. I began by overlaying the height and direction of the predicted wave on topographic data, and added information about human settlements and population sizes from the Ordinance Survey 2014 maps.

This is a basic map, admittedly; it lacks several key factors, such as sub-oceanic topography and human coastal defences such as seawalls. It also doesn’t consider population exposure and vulnerability, which should be considered in any disaster risk reduction strategy. That said, the map has some obvious value. If you live in the only city in Cornwall – Truro – with a population of approximately 19,000 people, this map should at least pique your interest. What with the potential for you and everyone you love to be suddenly and perhaps permanently underwater…

So the map shows that several large urban areas are at risk, but it also suggests which areas are perhaps safer. This is important, because there is a quite understandable and well-documented tendency for people faced with a tsunami – especially those from developed countries – to either freeze, or run inland (directly away from the wave) rather than simply heading uphill, regardless of direction.

The map illustrates that a Cornwall tsunami could entirely inundate and wash out the low-lying narrow area around St Erth, separating two green ‘dry zones’. Even if advance warning could be provided during the waves’ 6-hour journey across the Atlantic, without maps like this, communities’ further inland might incorrectly assume that they are safe.

References

1. 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, 2002, 163-167.

2. Ward, S. N., & Day, S. (2001). Cumbre Vieja volcano—potential collapse and tsunami at La Palma, Canary Islands. Geophysical Research Letters, 28(17), 3397-3400

3. Nieuwenhuis, J. & Janneke van Berlo, I.R.(2006) New research puts ‘killer La Palma tsunami’ at distant future, Delft Integraal

4. Richardson, S., Musson, R., Horsburgh, K. (2007) Tsunami – assessing the hazard for the UK and Irish coast. Proceedings of the 41st Defra Flood and Coastal Management conference 2006, 4th July to 6th July 2006, University of York, London. London, Defra, 10.1.1-10.1.11.