25 years later, a review of the "hurricane" that hit western France and its impact on sea level (15-16 October 1987)

25 years ago, on the night of October 15, 1987, a hurricane struck western France. With gusts recorded on the island of Ouessant approaching 180 km/h, the impact of this violent storm on the sea level was quantified by a maximum storm surge of 1.60 m.

Ile d'Ouessant, vue du phare du Creach (Crédits SHOM - Florence Labous, septembre 2012)

 

Presentation of the storm of October 1987

In October 1987 a very violent storm hit mainly Brittany and Normandy as well as the North and the South of the British Isles. It was so severe that it caused considerable damage, and it came to be known as the "Great Storm of 1987" in the collective memory of the affected regions. Gale force winds reaching 180 to 216 km/h were measured during the storm.

Valeurs maximales de "vent maximal instantané" lors de la tempête des 15-16 octobre 1987 (Source Météo-France)

A storm is a low pressure system that also causes a decrease in atmospheric pressure whose average value is 1013 hPa. At the meteorological observatory of Brest, the minimum pressure was measured at 948 hPa during the storm. This is the lowest pressure ever observed at the station since it opened in 1945.

 

Sea level observations available during the storm

Float tide gauges, mechanical devices that continuously measure the sea level, were in place during the storm in Brest and Le Conquet. They recorded the water levels on sheets of paper, known as tide curves or marigrams.

 

Brest marigram between 5 and 16 October 1987 (Copyright SHOM)

Brest marigram between 5 and 16 October 1987

Conquet marigram between 10 and 20 October 1987 (Copyright SHOM)

Conquet marigram between 10 and 20 October 1987

Click to enlarge the marigrams and view the maximum intensity of the storm

 

Effect of storm on water levels

The passage of a storm produces a rise in sea level (called storm surge) through three main processes:

  • strong swell and waves cause the sea level to rise;
  • the wind exerts friction on the surface of the water, which changes the currents and sea level (accumulation of water near the coast);
  • a decrease in the atmospheric pressure. The weight of the air decreases at the surface of the sea, and as a consequence, the sea level rises.

Given this fact, the decrease in atmospheric pressure to 948 hPa caused the sea level to rise by about 65 cm compared to what it would have been without the passage of the storm.

 

Measured storm surge

The measured storm surge is the difference, at time t, between the observed water level and the predicted water level. In this case, the surge was positive since the water level was higher than the expected tide (it is a negative surge if the level is lower). The maximum surge recorded at Brest was about 1.60 meters. Fortunately, even though the storm surge was very high, it took place while the tidal coefficient was low, around 30, which greatly limited its impact. 
 

Reproduction du marégramme de Brest avec les hauteurs d'eau (en rouge) et en vert, la marée prédite - détail sur la nuit du 16 octobre 1987 (Crédits SHOM)

 

Availability of tidal observations

Dans le In the context of the digitization campaigns conducted by SHOM, the water level values for Brest and Le Conquet were taken from the marigrams and have been made available in digital format hour by hour. They are free and available to anyone who registers on the REFMAR website.

 

To find out more:

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