Report on the high tides and the effects of the low pressure system on 1 and 2 February and impact on sea level along the French Atlantic coast

The effects on water heights of the strong tidal coefficients (over 110) in conjunction with the passage of a low pressure system north of the British Isles on February 1 and 2, 2014 were measured by SHOM's RONIM tide gauges. The maximum water level measured for each port exceeded the level of the highest astronomical tide. This result is explained by the storm surges generated by the wind and the atmospheric pressure this weekend of around 40 to 50 cm on average (see graphs below).  These storm surge levels are common for this type of event and explain why, except at the port of Le Conquet, the maximum sea level reached during this episode did not exceed the heights reached during the previous episode in early January, despite higher tidal coefficients.

View of the Le Conquet tide gauge (foreground) and the seawall protecting the port in the background at high tide - coefficient 114, Finistère (N. Pouvreau, Photo credits SHOM - 1 February 2014) - Click photo to enlarge

View of the Le Conquet tide gauge (foreground) and the seawall protecting the port in the background at high tide - coefficient 114, Finistère (N. Pouvreau, Photo credits SHOM - 1 February 2014) - Click photo to enlarge

 

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 31, 2014 at 00:00 to February 3, 2014 at 00:00, are used:

  • sea level observations are measured by each tide gauge (heights expressed relative to chart datum at the port). 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 at the port);
  • 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 Le Conquet, Brest, Concarneau, Port Tudy, Le Crouesty Saint-Nazaire, Les Sables-d'Olonne, La Pallice-La Rochelle, Aix Island, Port-Bloc, and Arcachon).

Le Conquet

Observations and predictions of sea level in Le Conquet (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

Observations and predictions of sea level in Le Conquet (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

 

Brest

Observations and predictions of sea level in Brest (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

Observations and predictions of sea level in Brest (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

 

Concarneau

Observations and predictions of sea level in Concarneau (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

Observations and predictions of sea level in Concarneau (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

 

Port Tudy

Observations and predictions of sea level in Port-Tudy (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

Observations and predictions of sea level in Port-Tudy (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

 

Le Crouesty

Observations and predictions of sea level in Le Crouesty (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

Observations and predictions of sea level in Le Crouesty (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

 

Saint-Nazaire

Observations and predictions of sea level in Saint-Nazaire (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

Observations and predictions of sea level in Saint-Nazaire (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

 

Les Sables-d'Olonne

Observations and predictions of sea level in Les Sables-d'Olonne (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

Observations and predictions of sea level in Les Sables-d'Olonne (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

 

La Pallice - La Rochelle

Observations and predictions of sea level in La Pallice - La Rochelle (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

Observations and predictions of sea level in La Pallice - La Rochelle (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

 

Ile d'Aix

Observations and predictions of sea level in Aix Island (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

Observations and predictions of sea level in Aix Island (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

 

Port-Bloc (Pointe de Grave)

Observations and predictions of sea level in Port-Bloc (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

Observations and predictions of sea level in Port-Bloc (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

 

Arcachon - Eyrac

Observations and predictions of sea level in Arcachon-Eyrac (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

Observations and predictions of sea level in Arcachon-Eyrac (top graph) and calculation of storm surges (bottom graph) from 31 January to 2 February 2014 during the high tidal coefficients and the low pressure system - Click on the figure to enlarge

 

Maximum water levels observed in Le Conquet, Brest, Concarneau, Port Tudy, Le Crouesty, Saint-Nazaire, Les Sables d'Olonne, La Rochelle, Aix Island, Port-Bloc, and Arcachon

 

Combined effects of the low pressure system and the high tidal coefficients on 1 February 2014 at the port of Le Conquet near the tide gauge observatory. Click on the image to enlarge (Photo credits SHOM - N. Pouvreau, 01/02.2014)

Combined effects of the low pressure system and the high tidal coefficients on 1 February 2014 at the port of Le Conquet near the tide gauge observatory. Click on the image to enlarge (Photo credits SHOM - N. Pouvreau, 01/02.2014)

 

The maximum sea levels reached between 31 January and 2 February 2014 were recorded in the following table (heights expressed relative to IGN69 and chart datum at the port) 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 31 January and 02 February 2014
Port Maximum water level (expressed relative to IGN69) Maximum water level (expressed relative to chart datum) HAT (expressed relative to chart datum)
Le Conquet 4,40 m 7,90 m 7,69 m
Brest 4,44 m 8,07 m 7,93 m
Concarneau 3,35 m 5,88 m 5,74 m
Port-Tudy 3,13 m 6,00 m 5,86 m
Le Crouesty 3,41 m 6,26 m 6,10 m
Saint-Nazaire 3,60 m 6,76 m 6,59 m
Les Sables d'Olonne 3.16 m 5,99 m 5,93 m
La Pallice - La Rochelle 3,48 m 6,98 m 6,86 m
Île d'Aix 3,38 m 6,95 m 6,82 m
Port-Bloc 3,37 m 6,20 m 5,96 m
Arcachon - Eyrac 3,18 m 5,16 m 4,91 m

 

The maximum water level measured for each port exceeded the level of the highest astronomical tide (HAT). This is because this low pressure system occurred at the same time as the strong tidal coefficients greater than 110 (calculated for the port of Brest) during this period.

 

Comparison between this low pressure system and the one that struck western France in early January 2014

The following table compares the maximum water levels reached (relative to chart datum) during the low pressure system in early January with the low pressure system in early February 2014.

 

 
Port Maximum height during the low pressure system in January 2014 (m) Maximum height during the low pressure system in February 2014 (m) Comparison between the January and February low pressure systems
Le Conquet 7,80 m 7,90 m +
Brest 8,07 m 8,07 m =
Concarneau 6,00 m 5,88 m -
Port Tudy 6,15 m 6,00 m -
Le Crouesty 6,33 m 6,26 m -
Saint-Nazaire 6,80 m 6,76 m -

 

Only in Le Conquet did the water level reached last weekend exceed the maximum measured in early January. The maximum sea level reached in Brest was the same for both events. For the other ports, the maximum level reached in early January was higher than in early February, despite lower tidal coefficients.

 

References

 

To find out more

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