Storm surge flood warning system

Storm surge flooding information is produced by Météo-France in collaboration with SHOM. SHOM provides real time sea level observations, tide predictions, expertise in coastal hydrodynamics and information on extreme levels and bathymetry (ocean depth).

 

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The storm surge flood warning system was implemented under the Plan to Prevent Coastal Flooding and Flash Floods, presented by Jean-Louis Borloo, Minister of Ecology, Energy, Sustainable Development and the Sea, at the Council of Ministers on 13 July 2010.

 

What is storm surge flooding?

 

Storm surges can cause severe flash flooding along the coast, in harbours and the mouths of rivers.

They are caused by extreme high sea levels due to the combination of several events:

  • the intensity of the tide (sea level mainly due to astronomical phenomena and geographical configuration). The higher the tidal coefficient, the higher the high tide level.
  • the passage of a storm, producing a rise in sea level (called storm surge) through three main processes:
    • strong swell and waves cause the sea level to rise;
    • the wind exerts friction on the surface of the water, which changes the currents and sea level (accumulation of water near the coast);
    • a decrease in the atmospheric pressure. The weight of the air decreases at the surface of the sea, and naturally, the sea level rises. A decrease in atmospheric pressure of one hectopascal (hPa) is equivalent to an increase of approximately one centimetre of water level. Example: A low pressure system of 980 hPa (a difference of 35 hPa relative to the average atmospheric pressure of 1015 hPa) generates a rise of about 35 cm.

 

Scéma explicatif cliquez sur l'image pour l'agrandir

 

 

Breaking waves result in a movement of water masses propagating along the foreshore (zone covered and uncovered at each tide). Jetties, seawalls and other coastal structures can be overrun, damaged or weakened.

 

Aggravating factors

 

If the events described above occur simultaneously, this aggravates the flooding and allows the sea to breach sea walls and flow into areas that are usually sheltered. The severity of the flooding depends on the water level reached, incoming water volume and how fast the water flows back out. The intensity of these events depends strongly on the configuration of the seabed, the foreshore and the geographical features of the coast, such as:

  • shallower sea (upon arriving on the coast, the wave energy is transformed into rising water levels);
  • the nature of the seabed, which slows or accelerates the propagation of the wave towards the coast (sand, gravel, mud)
  • the orientation of the coast with respect to the direction of wave propagation.

 

 

Two recent examples

During the passage of the storm Xynthia (27-28 February 2010), the sea level rose in places to over 2 meters in homes. That night, the weather conditions caused the sea level to rise (storm surge) by 1.53 m in La Rochelle, at a time when the sea level was at its highest (high tide with a coefficient of 102 and strong swell). The sea rose more than one meter above the level of the highest tides ever observed.

Hauteur d'eau à La Rochelle

 


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