Impact of the storm Xaver in the North Sea on the sea level in the Nord-Pas de Calais region (5-6/12/2013)

 

The storm Xaver (5-6 December 2013) north of the North Sea caused a significant rise in sea level in several European countries. As it passed over the North Sea, the storm Xaver generated a major storm surge that spread to the south and caused a significant rise in the sea level on the French coast and especially in the Nord-Pas Calais. The maximum storm surge was measured in Dunkirk during low tide around 20h40 UTC: the observed sea level was 2.40 m above the predicted level. The maximum water level reached 7.44 m above chart datum. Finally, note that this episode took place while the tidal coefficients (calculated for the port of Brest) were above 100.

(N. Pouvreau, Crédits SHOM - 2012)

Tide gauge observations in Dunkirk

Observatoire marégraphique RONIM de Dunkerque (Crédits SHOM - 2007)

Dunkirk tide gauge observatory (photo credits SHOM, 2007)

Until 9:00 UTC on December 5, the water level data coincided with tidal predictions. After that time and until 17:00, a difference between the observations and the predictions appeared, showing a negative storm surge with water levels up to 50 cm lower than predicted. Then the positive storm surge occurred, with a maximum of approximately 2.40 m at LW at 20:40 UTC and a storm surge of approximately 1.20 m at the next HW around 2:00 UTC on December 6. The maximum water height reached 7.44 m above chart datum or 4.75 m relative to IGN69. This is greater than the numerical value of the extreme high water levels calculated for a return period of 100 years, which is 4.70 m (based on the statistics of extreme sea levels on the coasts of France (Channel and Atlantic) 2012 - SHOM - CETMEF).

Observations et prédictions du niveau de la mer à Dunkerque (graphique du haut) et calcul des surcotes (graphique en bas) du 5 au 6 décembre 2013 lors de la tempête Xaver - Cliquer sur la figure pour l'agrandir

Observations and predictions of sea level in Dunkirk (top graph - heights expressed relative to chart datum) and calculation of storm surges (bottom graph) for 5 to 6 December 2013 during the storm Xaver - Click on the figure to enlarge

 

Observations of water levels in Calais

Marégraphe RONIM de Calais (Crédits SHOM - 2010)

Calais tide gauge observatory (photo credits SHOM, 2010)

Significant noise appears on this series of measurements. This is related to the design of the observatory itself. Unlike the Dunkirk tide gauge that measures the sea level inside a stilling well, the radar sensor installed in Calais measures the sea level directly, along with the entire high-frequency signal (wave, chop, etc.) with no mechanical filtration system. The maximum storm surge of around 1.70 m was observed at low tide around 20:50 UTC on 5 December. The storm surge at the next high tide was about 1 m. The maximum water height reached approximately 8.60 m above chart datum or 5.15 m relative to IGN69. This is greater than the numerical value of the extreme high water levels calculated for a return period of 100 years, which is 5.04 m (based on the statistics of extreme sea levels on the coasts of France (Channel and Atlantic) 2012 - SHOM - CETMEF).

Observations et prédictions du niveau de la mer à Calais (graphique du haut) et calcul des surcotes (graphique en bas) du 5 au 6 décembre 2013 lors de la tempête Xaver - Cliquer sur la figure pour l'agrandir

Observations and predictions of sea level in Calais (top graph - heights expressed relative to chart datum) and calculation of storm surges (bottom graph) for 5 to 6 December 2013 during the storm Xaver - Click on the figure to enlarge

 

Sea level observations in Boulogne-sur-Mer

Marégraphe RONIM de Boulogne-sur-Mer (Crédits SHOM - 2010)

Boulogne-sur-Mer tide gauge observatory (photo credits SHOM, 2010)

The Boulogne-sur-Mer tide gauge observatory is identical to the one in Calais, namely a radar tide gauge on a beam. However, this observatory is installed in a harbor environment that is more protected than in Calais, and there is less high-frequency noise. The maximum storm surge (about 1 m) was observed at falling tide around 6:00 UTC on December 6. The maximum water height reached approximately 10.00 m above chart datum or 5.62 m relative to IGN69. This is within the numerical values of the extreme high water levels calculated for a return period of between 20 and 50 years, which are respectively 5.57 and 5.62 m (based on the statistics of extreme sea levels on the coasts of France (Channel and Atlantic) 2012 - SHOM - CETMEF).

Observations et prédictions du niveau de la mer à Boulogne-sur-Mer (graphique du haut) et calcul des surcotes (graphique en bas) du 5 au 6 décembre 2013 lors de la tempête Xaver - Cliquer sur la figure pour l'agrandir

Observations and predictions of sea level in Boulogne-sur-Mer (top graph - heights expressed relative to chart datum) and calculation of storm surges (bottom graph) for 5 to 6 December 2013 during the storm Xaver - Click on the figure to enlarge

 

References

 

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