Height of water observed by SHOM during the passage of storm Xynthia

 

During passage of the storm Xynthia, in the night of 27 to 28 February 2010, the height of water was measured by the tide gauges of the RONIM network (Sea level observation network).

The RONIM network contains 19 tide gauges on the Channel and Atlanticcoasts (Figure 1).

Fig.1 : RONIM in mainland France

The storm generated significant positive surges (difference between the observed height of water and the predicted tidal height). The coinciding of the storm, the moment of high tide, and the equinoctial spring tides (large tidal range of 102) explains why the levels reached were also high.

The observed heights of high tide (with reference to the hydrographic datum) and the values of the positive surges over high tide are summarised in the Table below.

 

Heights of water observed and positive surges during the passage of storm Xynthia
TIDE GAUGE Heigth observed at high tide Positive surge over high tide
Dunkerque 6,90m 0,80 m
Calais 8,05 m 0,70 m
Boulogne-sur-mer No measurement, undergoing maintenance  
Dieppe 10,47 m 1,05 m
Le Havre 8,84 m 0,87 m
Cherbourg 7,14 m 0,57 m
Saint-Malo 13,18 m 0,88 m
Roscoff 9,63 m 0,53 m
Le Conquet 7,63 m 0,51 m
Brest 7,89 m 0,53 m
Concarneau 5,90 m 0,63 m
Le Crouesty 6,58 m 0,89 m
Saint-Nazaire 7,34 m 1,16 m
Sable d'Olonne > 6,89 m (problem with data acquisition) Not measured
La Rochelle 8,01 m 1,53 m
Pointe de Grave 6,57 m 0,98 m
Arcachon 5,46 m 0,90 m
Bayonne 4,92 m 0,38 m
Saint Jean de Luz 4,96 m 0,38 m

 

      The observations, predictions and positive surges at La Rochelle are shown in Figure 2 (below). The positive surge at high water (1.53 m) is particularly large. This degree of positive surge has not been observed since the tide gauge was installed in 1997. Incomparison, it is larger than the largest positive surge ever recorded at Brest, where there are more than 150 years of measurements (the maximum positive surge at high tide at Brest is 1.42 m, and this was observed during the storm on 15 October 1987 (extract from tide gauge record).).

Heights of water at La Rochelle

Fig 2 : Heights of water at La Rochelle

 

The level reached at La Rochellewas 8.01 m, a level never before observed since the tide gauge was installed in 1997. Incomparison, the level reached at La Rochelleduring the 1999 storm was 6.76 m.

Analysis of the observations enables us to estimate the return periods associated with the extreme levels reached. At La Rochelle, at Les Sables d'Olonne and at St-Nazaire these return periods are estimated to be more than 100 years.

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