Storms early this year and the impact on sea level in western France (2-5/01/2014)

The passage of several low pressure systems (2-5 January 2014) over a large part of western France caused a series of storm surges measured by SHOM's RONIM tide gauges installed at the ports of Roscoff, Le Conquet, Brest, Concarneau, Port Tudy, Le Crouesty and Saint-Nazaire. The maximum water level measured for each port exceeded the level of the highest astronomical tide. This is because the series of low pressure systems occurred at the same time as the strong tidal coefficients greater than 100 (calculated for the port of Brest) during this period.

Vue de la presqu'île de Kermovan, Finistère (N. Pouvreau, Crédits SHOM - 3 janvier 2014)

 

Data: tide gauge observations, tide predictions, calculation of positive/negative storm surges

Three types of data, all expressed in Coordinated Universal Time (UTC) from January 2, 2014 at 12:00 to January 5, 2014 at 12:00, are used:

  • sea level observations measured by each tide gauge (heights expressed relative to chart datum). Note that these tide gauges are installed in stilling wells, which filter out some of the signal caused by waves and chop;
  • official tidal predictions produced by SHOM for each site (predicted heights expressed relative to chart datum);
  • positive/negative storm surges, the difference between the sea level observation and the predicted height.

The following graphs present the sea level observations and predictions (top graph) and the calculation of storm surges (bottom graph) for each tide gauge observatory (in order Roscoff, Le Conquet, Brest, Concarneau, Port Tudy, Le Crouesty and Saint-Nazaire).

Roscoff

Observations et prédictions du niveau de la mer à Roscoff (graphique du haut) et calcul des surcotes (graphique en bas) du 2 au 5 janvier 2014 lors du passage de plusieurs dépressions - Cliquer sur la figure pour l'agrandir

Observations and predictions of sea level in Roscoff (top graph - heights expressed relative to chart datum) and calculation of storm surges (bottom graph) for 2 to 5 January 2014 - Click on the figure to enlarge

 

Le Conquet

Observations et prédictions du niveau de la mer au Conquet (graphique du haut) et calcul des surcotes (graphique en bas) du 2 au 5 janvier 2014 lors du passage de plusieurs dépressions - Cliquer sur la figure pour l'agrandir

Observations and predictions of sea level in Le Conquet (top graph - heights expressed relative to chart datum) and calculation of storm surges (bottom graph) for 2 to 5 January 2014 - Click on the figure to enlarge

 

Brest

Observations et prédictions du niveau de la mer à Brest (graphique du haut) et calcul des surcotes (graphique en bas) du 2 au 5 janvier 2014 lors du passage de plusieurs dépressions - Cliquer sur la figure pour l'agrandir

Observations and predictions of sea level in Brest (top graph - heights expressed relative to chart datum) and calculation of storm surges (bottom graph) for 2 to 5 January 2014 - Click on the figure to enlarge

 

Concarneau

Observations et prédictions du niveau de la mer à Concarneau (graphique du haut) et calcul des surcotes (graphique en bas) du 2 au 5 janvier 2014 lors du passage de plusieurs dépressions - Cliquer sur la figure pour l'agrandir

Observations and predictions of sea level in Concarneau (top graph - heights expressed relative to chart datum) and calculation of storm surges (bottom graph) for 2 to 5 January 2014 - Click on the figure to enlarge

 

Port Tudy

Observations et prédictions du niveau de la mer à Port Tudy (graphique du haut) et calcul des surcotes (graphique en bas) du 2 au 5 janvier 2014 lors du passage de plusieurs dépressions - Cliquer sur la figure pour l'agrandir

Observations and predictions of sea level in Port Tudy (top graph - heights expressed relative to chart datum) and calculation of storm surges (bottom graph) for 2 to 5 January 2014 - Click on the figure to enlarge

 

Le Crouesty

Observations et prédictions du niveau de la mer au Crouesty (graphique du haut) et calcul des surcotes (graphique en bas) du 2 au 5 janvier 2014 lors du passage de plusieurs dépressions - Cliquer sur la figure pour l'agrandir

Observations and predictions of sea level in Le Crouesty (top graph - heights expressed relative to chart datum) and calculation of storm surges (bottom graph) for 2 to 5 January 2014 - Click on the figure to enlarge

 

Saint-Nazaire

Observations et prédictions du niveau de la mer à Saint-Nazaire (graphique du haut) et calcul des surcotes (graphique en bas) du 2 au 5 janvier 2014 lors du passage de plusieurs dépressions - Cliquer sur la figure pour l'agrandir

Observations and predictions of sea level in Saint-Nazaire (top graph - heights expressed relative to chart datum) and calculation of storm surges (bottom graph) for 2 to 5 January 2014 - Click on the figure to enlarge

 

Maximum water levels observed in Roscoff, Le Conquet, Brest, Concarneau, Port Tudy, Le Crouesty and Saint-Nazaire

 

Observatoire marégraphique RONIM du Conquet lors du passage d'une tempête le 3 janvier 2014. Cliquer sur la figure pour l'agrandir et localiser le marégraphe (Crédits SHOM - N. Pouvreau, 2014)

Le Conquet tide gauge observatory on 3 January 2014 Click on photo to enlarge and locate the observatory (Photo credits SHOM - Nicolas Pouvreau, January 3, 2014)

 

The maximum sea levels reached between 2 and 5 January 2014 were recorded in the following table (heights expressed relative to IGN69 and chart datum) and compared to the highest astronomical tide (HAT, referenced to datum) determined for each port in SHOM's Maritime Altimetric Reference Values product, 2013 edition.

Maximum water levels measured by the different tide gauges between 2 and 5 January 2014
Port Maximum water level (expressed relative to IGN69) Maximum water level (expressed relative to chart datum) HAT (expressed relative to chart datum)
Roscoff 5,04 m 9,80 m 9,80 m
Le Conquet 4,30 m 7,80 m 7,69 m
Brest 4,44 m 8,07 m 7,93 m
Concarneau 3,47 m 6,00 m 5,74 m
Port-Tudy 3,28 m 6,15 m 5,86 m
Le Crouesty 3,48 m 6,33 m 6,10 m
Saint-Nazaire 3,64 m 6,80 m 6,59 m

 

The maximum water level measured for each port exceeded the level of the highest astronomical tide (HAT). This is because the series of low pressure systems occurred at the same time as the strong tidal coefficients greater than 100 (calculated for the port of Brest) during this period.

 

Maximum storm surges calculated

The following table shows for each port the value of the maximum storm surge calculated for the period from January 2 to 5, the time when the maximum storm surge was observed and the corresponding phase of the tide cycle.

 

Maximum storm surge observed for the ports between 2 and 5 January 2014
Port Maximum value of the storm surge (m) Event date (UTC) Tide phase
Roscoff 0,55 m 3 janvier à 11h40 UTC mid-ebb
Le Conquet 0,50 m 4 janvier à 11h30 UTC slack low tide*
Brest 0,54 m 2 janvier à 20h30 UTC mid-ebb
Concarneau 0,54 m 4 janvier à 06h50 UTC slack high tide
Port Tudy 0,77 m 4 janvier à 06h40 UTC beginning of ebb tide
Le Crouesty 0,72 m 4 janvier à 06h50 UTC slack high tide
Saint-Nazaire 0,78 m

4 janvier à 07h10 UTC

4 janvier à 12h20 UTC
 

beginning of ebb tide

slack low tide
 

* Note that measurements made by the Le Conquet tide gauge of up to 0.8 m above chart datum are potentially incorrect

 

The peak maximum storm surge was measured at Port-Tudy at the beginning of ebb tide on January 4 at 6:40 UTC: the observed sea level was nearly 0.80 m above the predicted level (5.31 m). Similarly, a double peak storm surge was identified at the port of Saint-Nazaire at 7:10 UTC and 12:20 UTC on 4 January 2013 respectively at the beginning of ebb tide and at slack low tide.

The tide gauges at the ports of Roscoff, Le Conquet, Brest and Concarneau measured storm surges of approximately 0.60 m during the period from January 2 to 5.

 

References

 

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