Seiche in the port of Royan during the night of 15 to 16 February 2011 Seiche in the port of Royan during the night of 15 to 16 February 2011

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Seiche in the port of Royan during the night of 15 to 16 February 2011

 

The magazines Sud-Ouest and Le Marin report a phenomenon having the characteristics of a large amplitude seiche in the port of Royan, which caused damage during the night of 15 to 16 February 2011.

The phenomenon was characterised by a strong current in the entrance to the pleasure port of Royan, causing damage to some pontoons and boats. An earlier event causing damage occurred during the night of 27 to 28 November 2006.

Ponton G of the port of Royan (photograph on Internet site of the journal Sud-Ouest, 16/02/2011)
Ponton G of the port of Royan
(photograph on Internet site of the
journal Sud-Ouest, 16/02/2011)

Analysis was undertaken of the heights of water observed by the tide gauge at Royan and produced by the Atlantic Coast Flood Forecasting Service (SPC) (the digital tide gauge at Royan had not yet entered service in 2006):

Analysis was undertaken of the heights of water observed by the tide gauge at Royan and produced by the Atlantic Coast Flood Forecasting Service (SPC)

The period of data analysed was 5 minutes.

The principal characteristics derived from the tide curves are:

  • Maximum peak-to-peak heights of 1 metre;
  • Attenuation to half at the end of 4 hours;
  • End of the episode at the end of 8 hours;
  • Oscillation periods range between 11-13 min.

Representation of the spectrum:

Representation of the spectrum

Time-frequency representation:

Time-frequency representation

Predominant frequencies (↔ Predominant periods) of the oscillations during the night of 15 to 16 February:

(1) 1.276 mHz ↔ 13 min (2) 1.354 mHz ↔ 12.3 min (3) 1.543 mHz ↔ 10.8 min (4) 0.950 mHz ↔ 17.5 min

The seiche phenomenon at Royan therefore has a large amplitude and a scale approaching that of the seiche at Port-Tudy (Ile de Groix), already documented elsewhere (Fleitour, 2004).

Example of 1 metreseiche at Port-Tudy on 27 November 2006 and 15 February 2011:

Tide curves form Port-Tudy in November 2006

Tide curves form Port-Tudy in November 2006

TIde curves from Port-Tudy from 14 to 17 February 2011

TIde curves from Port-Tudy from 14 to 17 February 2011

These seiche phenomena may be linked to the wind, to atmospheric pressure or, more probably in the case of Port-Tudy, to long waveforms generated by waves (Devaux, 2009 and Ardhuin et al, 2010).

We also note that a heavy swell was observed during the night of 15 to 16 February as is shown by the plot from the Previmer site of significant wave heights measured by the bay of Gascogn (operated by UK Met Office and Météo-France) and 170 nautical miles away from the port of Royan:

Significant wave heights (Bay of Gascogn)

Significant wave heights (Bay of Gascogn)

Modelling the period and direction of dominant waves

Modelling the period and direction of dominant waves

 

 

References :

  • -Sud Ouest, 17 February 2011,
  • -Le Marin, 25 February 2011,
  • -Sud Ouest, 29 & 30 November 2006,
  • -Devaux E. (2009). Analysis of seiches through the examination of tide gauge data. Graphic examination, frequency analysis and search for initiating elements. VA management of water and coastal flows, Ecole Nationale des Travaux Publics de l'Etat, promotion 54. 68pp.,
  • -Fleitour J. B. (2004). Study relating to the seiche phenomenon at Port-Tudy (Groix). Project report at end of ENSIETA study. Host organisation: Morbihan Department Council, 105pp.,
  • -Pons F., Ph. Sergent, A. Lambert. (2008). Calculation of seiches using Refonde software, 10th National Seminar on Coastal Engineering – Civil Engineering, 14-16 October 2008, Sophia Antipolis,
  • -Ardhuin F., E. Devaux, L. Pineau-Guillou (2010). Observation and forecasting of seiches on the French Atlantic Coast , 11th National Seminar on Coastal Engineering – Civil Engineering 22-24 June 2010, Sables d'Olonne, pp. 1-8, DOI: 10.5150/jngcgc.2010.001-A.