2023-11-10 20:48:17
Hundreds of houses invaded by water, two hundred schools and nurseries closed… For the fifth day in a row, Pas-de-Calais was placed on red alert for floods or rain-floods on Friday, November 10. The Boulonnais, Montreuillois and Audomarois sectors are threatened by the overflow of the Canche, the Liane or the Aa. By what phenomena can we explain these floods described as“historical” and D’“exceptional” ?
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These floods are above all the product of the record intensity of precipitation. According to Météo-France, the cumulative rainfall across the department over the last three weeks has almost reached 250 millimeters, a figure well above the “climatological reference” for the months of October (89 millimeters) and November (99 millimeters). For Bruno Janet, head of the “Modelling and operational hydrology” pole of the Central Hydrometeorology and Flood Forecasting Support Service (Schapi), these precipitations are ” unprecedented “. In places, more than 400 millimeters of water fell in one month, four times what usually falls in November, already the wettest period of the year. Since October 18, Hauts-de-France has not known a day without rain. Beyond the intensity of the precipitation, it is also this duration which is exceptional.
Two storms, Ciaran and Domingos, followed one another in a few days in the region. As a result, the soils are saturated. “The soils are already full of water, all the excess runs off and this worsens the consequences”, explains Stéphane Bonelli, researcher at the National Institute of Agronomic Research (INRAE). For him, “this is an event comparable to a 100-year flood”, that is to say that it has a probability in a hundred of occurring during the twelve months of the year. Stéphane Bonelli summarizes: “This succession of rare hydrological events is the main cause of this exceptional situation. »
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“Low infiltration power”
Affected after the storms by a depression from the British Isles named Elisa, the rivers in the affected area, notably the Aa, the Liane, and the Canche, are overflowing. “We are not really dealing with flooding in the Oise or the Aisne, which are very slow. Here, we are on waterways that are very sensitive to precipitation”explains Bruno Janet.
Other, more minor factors explain why the region may be conducive to this type of phenomenon. “For us in Nord-Pas-de-Calais, with the exception of Artois, it is very flat. And when it overflows, it’s more difficult to evacuate, it stagnates”explains Jamal El Khattabi, hydrogeologist at Polytech Lille. “In the region, there are in places what we call silts, these are coarse clays, which have low infiltration power, they are even described as impermeable. » Combined with marshes and clayey soils, as in Saint-Omer, the result can only be aggravated.
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