Tsunami or meteotsunami? | South West England | June 2011 |

On 28 and 29 June 2011 it was reported in the press that on 27 June a tsunami struck South West England between Penzance and Portsmouth; approximately 200 miles of coastline were affected.

Personal anecdotes on the event were published from locations at St Michael’s Mount, Cornwall, and the Yealm Estuary.

Apparently the tsunami struck the Yealm at about 10.30. Wave heights were between 0.5 and 0.8 metres.

The video evidence (SDFitch/YouTube) indicated a bore in the Yealm Estuary.  The sea withdrew from the coast before the wave came in.

At St Michael’s Mount there were reports of ‘hair standing on end’ before the wave struck.

Interpretations of tide data (collected by EMU Limited) were used to identify an east to west progression of the wave.

An interpretation seemingly confirmed by accounts from St Michael's Mount that water had built up on the east side of the causeway (0.2 m higher than on the west) before overtopping this.

According to the Tidal Gauge Anomaly measure, which records the difference between the forecast tide and actual tide, the wave was higher by 20 cm in Newlyn, 30 cm in Plymouth and 40 cm in Portsmouth.

Newspaper reports, like the one published in the Guardian, mainly concluded that the tsunami was created by a submarine landslide on the continental slope off of the South West; 200 miles south-west of Newlyn.

Preliminary BGS interpretations

Based on the available reporting BGS scientists concluded that a submarine landslide was an unlikely source because it did not fit the observations:

  • The tsunami decreased in height from east to west, indicating a source in the eastern English Channel: this rules out a source to the west.

    Thus a submarine landslide on the South West continental slope is discounted; a source here would have resulted in a tsunami decreasing in height the opposite direction, from west to east.

    In addition, if a submarine landslide was located here there would have been a tsunami wave at other locations, such as in southern Ireland and France — none has been reported.

  • Most likely geological sources in the eastern channel are considered to be either an earthquake or a submarine landslide.

    No earthquakes were reported at this time.

    It is also unlikely that a submarine landslide took place in the eastern channel. Submarine landslides need unstable sediment to form, additionally they need a trigger, most commonly an earthquake.

    In addition, a submarine landslide would have created a tsunami focused on the nearest coastline, and the area of inundation would have been limited — less than 200 miles.

    None of these conditions are known to be satisified; there was no earthquake, no locations of unstable sediment are known that could be mobilised and there seems to be no area of focused tsunami waves.

Meteotsunami or meteorological tsunami?

Meteotsunami (© iStock/leonid_titleonid_tit)

One aspect that is novel, and may bear on the answer, is the ‘hair standing on end’ before the tsunami that was reported from St Michael’s Mount, that suggests an atmospheric¬†source, as it is known that this phenomena can take place in association with lightning.

Preliminary BGS conclusions as of 5 July

BGS scientists concluded, therefore, that the tsunami was not geological in origin.

It was probably caused by a meteorological effect, such as a squall over the ocean developed during summer storms; these events are called meteotsunami.

These types of waves are common all over the world and are known by their local names, such as: rissaga (Spain), milghuba (Malta), marrobbio (Italy) and abiki (Japan).

We know that there were storms in the south east of England on the afternoon of Monday 27 June; at Wimbledon play was restricted to the covered Centre Court and there was thunder and lightning.

BGS new interpretations as of 7 July 2011

Google map shows the key locations where the Tsunami was observed

Since 5 July, we now have reviewed tidal data from the English Channel and also (in response to our first posting) received reports from France.

We now know that the tsunami was not a localised event. It was recorded at UK tide stations in the Channel and in South Wales at Mumbles and Milford Haven.

It also struck the French coast (see map), but rather than just being confined to the English Channel, or La Manche, it can be identified as far south as St Jean de Luz, that lies in the Bay of Biscay on the border with Spain.

In the English Channel it was traced as far east as Calais. The wave travelled from west to east and not vice versa.

In addition the maximum height of the wave recorded at numerous tide stations shows no overall variation in an east–west or north–south direction.

French coast records

Along the French coast the wave was recorded at numerous tide stations (identified on the map). The first indications of the wave are at Le Conquet on the west coast of Brittany, at about 7 am British Summer Time (BST) on 27 June. From Le Conquet it took about five hours to reach St Jean de Luz a distance of 350 miles (~600 km).

In the English Channel, the wave was also identified on north coast of France as far eastward as Calais. From le Conquet it took about seven hours to reach Calais a similar distance of 350 miles (600 kms).

The evidence still suggests the most likely source to be meteorological.

More than one event

What is becoming evident, from the timing of the wave impact and its height, is that the tsunami is probably not due to one event. It looks as if a weather front moved east along the Channel and associated squalls created individual tsunamis.

Once we have completed our analysis we will publish the supporting data.

In conclusion it seems as if the most likely source is meteorological, but in the west, not the east, and it moved from west to east.

Are meteostunamis common in Britain?

Meteotsunamis are not unusual in the UK (Haslett et al, 2009). With the following historical events recognised along the south coast of England:

Yealm (Devon) and Fowey estuaries (Cornwall), 18 August 1892 — thunderstorms reported, The Times (1892a) also reports this event in the River Yealm as well as stating that '‘there was a rapid rise in the River Fowey as a great tidal wave, but this immediately subsided'. Davison (1924) considers that they generated the large tsunami-like waves.

Folkestone (Kent) and Brighton (Sussex), 20 July 1929 — large tsunami-like wave struck the Kent and Sussex coasts, busy with tourists, and drowned two people, at Brighton and Worthing the wave was accompanied by sudden downpours of rain and high winds, but at Folkestone and Hastings, where one person drowned at each, the weather was clear and the unexpected wave was estimated to be c. 3.5 and 6 m high, respectively. Douglas (1929) suggests the wave was caused by a squall-line travelling up the English Channel, coincident with thunderstorms, and so may be referred to as a meteotsunami.

Sources:

Haslett, S K, Mellor, H E and Bryant, E A. 2009. Meteo-tsunami hazard associated with summer thunderstorms in the United Kingdom. Physics and Chemistry of the Earth, Parts A/B/C, 34(17–18): 1016–1022.

Davison, C. 1924. A History of British Earthquakes. Cambridge University Press, Cambridge.

Douglas, C K M. 1929. The line-squall and channel wave of July 20th, 1929. Meteorological Magazine 64, 187–189.

Contact

Contact Dave Tappin or David Long for further information