2023-11-02 13:32:38
Is the hardest part over? The red “violent winds” vigilance was lifted on Thursday, November 2, at 10 a.m., in Manche, the last of the three departments of the North-West still on maximum alert during the passage of storm Ciarán, which left two dead , one in Aisne, the other in Normandy. Another consequence: 820,000 homes were still without electricity in the middle of the afternoon. As of 7 a.m. Thursday morning, the network had 1.2 million affected customers. In total, six departments were still on orange alert Thursday evening across the entire Atlantic coast and the English Channel, as well as in the South-East, notably Corsica, indicates Météo-France.
Orange vigilance concerns risks linked to wind, rain, but also “submersion waves”. But what exactly are we talking about? According to Météo-France, these are linked to an extreme rise in sea level, itself due to the combination of several phenomena. First, there is the tide, a normal and well-known process, the strength of which is defined by a coefficient. The higher this coefficient, the higher the sea level at high tide. Added to this is the force of the wind: when there is a storm, this produces a rise in sea level, called a “surge”. Thus the strong swell or waves contribute to increasing the height of the water. There is also the wind, “which exerts friction on the surface of the water, which generates a change in currents and sea level (accumulation of water as it approaches the coast)”, explains Météo- France.
Finally, the storm causes a decrease in atmospheric pressure (force exerted by the weight of the air). On Wednesday evening, Storm Ciarán dropped the pressure to 960 hectopascals (hPa) near its center, while it rose to 980 hPa in the morning, at 9 a.m.
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“The weight of the air then decreases at the sea surface and, mechanically, the sea level rises. A decrease in atmospheric pressure of 1 hPa is approximately equivalent to a rise of 1 centimeter in the water height” , resumes Météo-France. When waves break, a movement of water masses spreads across the foreshore – in other words, the area covered and uncovered by the tide. Consequences: coastal infrastructure can then be weakened or damaged, which is what the authorities fear. Thursday, November 2 in the morning, a wave of 21 meters was measured off the coast of Finistère, swept by storm Ciarán.
A peak between noon and 2 p.m. in the Channel
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When do these risks of marine submersion become maximum? Still according to Météo-France experts, they should have reached their peak on Thursday between noon and 2 p.m. on the Channel coast. And what precautions should be taken? Almost everywhere on the coast, calls for caution, particularly for walkers and boaters, have been relayed in the media or on social networks. Submersion waves are particularly devastating on the coast, where walkers are regularly swept away. Whether on foot or by car, you should avoid driving along the seaside.
Due in particular to the risk of falling trees due to violent winds, the transport sector is operating slowly on Thursday in the west of the country. The Ministry of the Interior therefore recommends that residents of the coasts concerned keep abreast of developments in the situation by regularly following the news. Waves can also carry objects or pebbles with them, “which then become projectiles likely to injure people, damage property or obstruct traffic along the seaside”. warned Météo-France. The Atlantic maritime prefecture has also put itself in working order. It has positioned the powerful tugboat Abeille Bourbon in Ouessant so that it can intervene as quickly as possible in the event of difficulties encountered by ships.
Global warming can amplify the phenomenon
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Extreme weather events (cyclones, heat waves, floods, drought, etc.) are natural phenomena. But global warming caused by greenhouse gas emissions generated by human activity can amplify them. Concerning more precisely the wave-submersion phenomena on the coasts, these risk becoming more dangerous with the rise in sea level linked to the melting of the ice, particularly during storms. The frequency of appearance of these giant waves would have increased by almost 50% on a global scale in the space of twenty years.
Strong waves and marine flooding can be destructive phenomena. Floods due to marine submersion can, however, invade the coastline several kilometers inland and reach a water height of several meters. We remember the passage of storm Xynthia on February 27 and 28, 2010: the sea water had risen in places to more than 2 meters in the houses. That night, atmospheric conditions caused sea levels to rise by 1.53 meters in La Rochelle, while sea levels were at their peak. The sea had then exceeded the level of the highest tides already observed by more than 1 meter.
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