2024-01-03 21:24:07
A passerby in a street in Arques, Wednesday January 3, 2024. DENIS CHARLET / AFP
After the record floods in November, Pas-de-Calais experienced a new episode of rain between Sunday and New Year’s Eve. In its bulletin of Wednesday January 3, Vigicruesthe information service on the risk of flooding of the main rivers in France, has classified the Liane – Boulogne-sur-Mer estuary – and the Hem in orange flood vigilance and the Aa in red vigilance.
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Although he points out that heavy rainfall is not rare in the region, geologist Francis Meilliez, professor emeritus at the University of Lille and director of the Northern Geological Society, points out the damage caused by the artificialization of soils. , induced by human development.
Is the region facing an exceptional rain episode?
No, just above average. It rains in the north of France, it is well known. But after a few years of drought, we forget that the rain can be heavy: during the first half of November, 350 mm of water fell, that is to say more than half of what had fallen throughout of the year 2022! Note that such a rainfall amount is low compared to “Cévennes episodes” which affect the south-east of France every autumn.
In December, the Annals of the Northern Geological Society recalled that annual accumulations vary on average from 800 mm per year on the west coast (Boulogne-sur-Mer) to 650 mm in the south of the region. Haut-Boulonnais is an area where the amount of rain is close to one meter per year. In 1999, 2000 and 2001, rainfall was extremely high, exceeding 900 mm each year.
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More recently, the region has already experienced episodes of heavy rain: thus, from October 1 to 10, 2020, accumulations greater than 100 mm over ten days were recorded in the west of the region, in the Somme and the Pas- from Calais. You just need to consult the Météo France website which lists memorable events to be convinced.
In this case, what are the causes of repeated flooding?
This fall, Météo France explains that a very powerful stratospheric current is maintaining the depressions which cross the North Atlantic over the northern half of France. The effect of relative height of Haut-Boulonnais (between 160 and 210 m) compared to the surrounding plains (less than 20 m) means that precipitation is concentrated simultaneously on the sources of the Course, a right bank tributary of the Canche, of the Aa, the Hem, the Liane, and other small rivers of Boulonnais (Slack, Wimereux, etc.). The water descends quickly in narrow valleys and very quickly reaches the lower parts of their course which are very flat; the flow is therefore very difficult.
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