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 coby Jd and >