Impact of the storm Ulla on the sea level (14/02/2014)

The storm Ulla (14 February 2014) along the Atlantic coast of western French caused a significant sea level rise, particularly in Normandy, Brittany and Pays-de-la-Loire. The maximum storm surge at high tide was measured in Brest at 16:20 UTC and in Cherbourg at 20:10 UTC: the sea level observed at the two ports was 0.91 m above the predicted level. The maximum water level reached respectively 7.48 m and 6.93 m above chart datum. This episode took place while the tidal coefficients (calculated for the port of Brest) were around 80. The effects of this storm on water levels were also measured by other RONIM tide gauges in Cherbourg, Saint-Malo, Roscoff, Le Conquet, Concarneau, Port-Tudy, Le Crouesty and Saint-Nazaire.

View of the shoreline in Plougonvelin (Finistère) during the storm Ulla at high tide (L. Pouvreau, Photo credits SHOM - February 14, 2014) - Click photo to enlarge

View of the shoreline in Plougonvelin (Finistère) during the storm Ulla at high tide (L. Pouvreau, Photo credits SHOM - February 14, 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 February 13, 2014 at 12:00 to February 15, 2014 at 12:00, 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 Cherbourg, Saint-Malo, Roscoff, Le Conquet, Brest, Concarneau, Port Tudy, Le Crouesty and Saint-Nazaire).

 

Cherbourg

Observations and predictions of sea level in Cherbourg (top graph) and calculation of storm surges (bottom graph) from 13 to 15 February 2014 during the storm Ulla - Click on the figure to enlarge

Observations and predictions of sea level in Cherbourg (top graph) and calculation of storm surges (bottom graph) from 13 to 15 February 2014 during the storm Ulla - Click on the figure to enlarge

 

Saint-Malo

Observations and predictions of sea level in Saint-Malo (top graph) and calculation of storm surges (bottom graph) from 13 to 15 February 2014 during the storm Ulla - Click on the figure to enlarge

Observations and predictions of sea level in Saint-Malo (top graph) and calculation of storm surges (bottom graph) from 13 to 15 February 2014 during the storm Ulla - Click on the figure to enlarge

 

Roscoff

Observations and predictions of sea level in Roscoff (top graph) and calculation of storm surges (bottom graph) from 13 to 15 February 2014 during the storm Ulla - Click on the figure to enlarge

Observations and predictions of sea level in Roscoff (top graph) and calculation of storm surges (bottom graph) from 13 to 15 February 2014 during the storm Ulla - Click on the figure to enlarge

 

Le Conquet

Observations and predictions of sea level in Le Conquet (top graph) and calculation of storm surges (bottom graph) from 13 to 15 February 2014 during the storm Ulla - 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 13 to 15 February 2014 during the storm Ulla - 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 13 to 15 February 2014 during the storm Ulla - Click on the figure to enlarge

Observations and predictions of sea level in Brest (top graph) and calculation of storm surges (bottom graph) from 13 to 15 February 2014 during the storm Ulla - 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 13 to 15 February 2014 during the storm Ulla - Click on the figure to enlarge

Observations and predictions of sea level in Concarneau (top graph) and calculation of storm surges (bottom graph) from 13 to 15 February 2014 during the storm Ulla - 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 13 to 15 February 2014 during the storm Ulla - 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 13 to 15 February 2014 during the storm Ulla - 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 13 to 15 February 2014 during the storm Ulla - 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 13 to 15 February 2014 during the storm Ulla - 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 13 to 15 February 2014 during the storm Ulla - 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 13 to 15 February 2014 during the storm Ulla - Click on the figure to enlarge

 


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

 

View of the shoreline in Trégana (Finistère) during the storm Ulla before high tide (L. Pouvreau, Photo credits SHOM - February 14, 2014) - Click photo to enlarge

View of the shoreline in Trégana (Finistère) during the storm Ulla before high tide (L. Pouvreau, Photo credits SHOM - February 14, 2014) - Click photo to enlarge

 

The maximum sea levels and the calculated storm surges reached during the storm Ulla were recorded in the following table (height expressed relative to IGN69 and chart datum at the port).

Maximum sea levels and the calculated storm surges reached during the storm Ulla were recorded on the following table (height expressed relative to IGN69 and chart datum at the port)
Port Maximum water level (expressed relative to IGN69) Maximum water level (expressed relative to chart datum) Time of HW (UTC) Storm surge calculated at the maximum water level reached
Cherbourg 3.64 m 6.93 m 20:10 0.91 m
Saint-Malo 5.79 m 12.08 m 18:20 0.75 m
Roscoff 4.34 m 9.10 m 17:20 0.76 m
Le Conquet 3.71 m 7.21 m 16:20 0.85 m
Brest 3.84 m 7.48 m 16:20 0.91 m
Concarneau 2.88 m 5.41 m 15:40 0.64 m
Port-Tudy 2.70 m 5.44 m 15:50 0.60 m
Le Crouesty 3.01 m 5.86 m 15:50 0.77 m
Saint-Nazaire 3.11 m 6.27 m 15:50 0.74 m

 

For each site, the storm surge calculated for the maximum water height was greater than 0.60 m (surge at high water). However, the graphs show that higher instantaneous surges were observed, but during ebb tide. In Saint-Malo, for example, the storm surge was equal to 1.20 m.

 

Comparaison entre les mesures de hauteurs d'eau et la pression atmosphérique moyenne au niveau de la mer

The following graph for the port of Brest shows the mean sea level pressure curve provided by Météo-France in Guipavas.

 

Observations and predictions of sea level in Brest (top graph) and calculation of storm surges + mean atmospheric pressure at sea level (bottom graph) from 13 to 15 February 2014 during the storm Ulla - Click on the figure to enlarge

Observations and predictions of sea level in Brest (top graph) and calculation of storm surges + mean atmospheric pressure at sea level (bottom graph) from 13 to 15 February 2014 during the storm Ulla - Click on the figure to enlarge

 

The correlation between the calculated storm surges and atmospheric pressure is clearly visible on the chart above. Atmospheric pressure does indeed affect sea level. Tide forecasts are calculated for a mean atmospheric pressure of 1013 hPa. For each 1 hPa drop in pressure, the sea level increases by about 1cm. At 15:00 UTC on February 14, the atmospheric pressure measured by Météo-France in Guipavas (about 10 km from the Brest tide gauge) was 981.7 hPa, or a barometric surge of about 31 cm. The rest of the storm surge observed was caused by other meteorological effects, including wind, waves and swell.

 

References

 

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