Climate change. Coastal erosion, when are homes threatened?

Violent floods marked Hauts-de-France in 2023 and 2024. But heavy rains also have longer-term consequences on the region’s landscape. Collapse of ramparts, erosion of the coastline and retreat of the coastline which makes residents close to the sea shudder… What is the impact of precipitation on the tangible and intangible heritage of the region? Elements of answers.

Showers, again and again. 2023 and 2024 were record years: never before had so much rain fallen on Hauts-de-France since 1959. Pas-de-Calais faced unprecedented floods. But in the long term, these extreme weather phenomena, amplified by global warming, are profoundly changing the face of our territory, threatening coastal homes.

Month after month, year after year, these heavy rains attack our natural, but also historical, heritage. HAS Montreuil-sur-Mer, on November 10, 2023, a section of the city ramparts collapsed, under the stunned gaze of passers-by, leaving a gaping wound of 40 meters which still disfigures the heritage jewel of the city today.

If they have survived multiple wars since their construction in the 13th century, these fortifications, classified as a Historic Monument, did not withstand the heavy rains which hit Pas-de-Calais in the fall of 2023. For residents, “it’s sad to see this heritage destroyed because of bad weather“.

“Water has accumulateddetails the mayor (without label) of Montreuil-sur-Mer Pierre Ducrocq. Since it could not evacuate downwards, it created water pressure and this push caused the section of the ramparts to fall. A direct consequence of the accumulation of water, it doesn’t necessarily come to mind immediately… It was really a surprise to receive this call announcing the collapse.”

Between now and spring, work to secure and reconstruct the site will be undertaken. Their cost ? 1 350 000 euros. But to avoid other landslides in the future, larger projects are to be planned, as explained Philippe Cousin, vice-president of the Montreuillois urban community in charge of heritage. “A study was carried out to quantify all the work that would need to be done. The amount needed is 38 million euros. This is the image of Montreuil. It is important to preserve and conserve our territory for future generations.”

Historical heritage, victim of global warming ? The example of the Montreuil ramparts is not an isolated case. HAS Boulogne-sur-Mer, a 5-meter section of the retaining wall of the ramparts dating from the 16th century collapsed last October.

“The pressure of the currently waterlogged lands has led to a partial collapseexplains Angélique Demon, head of the city’s archeology department. In any case, we have part of the passageway which has simply failed. In recent history, this is the first time this has happened. We really have an immediate impact of climate change and in any case the heavy rainfall that we have had recently on the fortified heritage of Boulogne..”

Heavy rainfall which also lastingly transforms the region’s natural sites. In recent months on the coast, cliff landslides have increased. Whether in Mers-les-Bains, on the customs officers’ path between Le Portel and Equihen Plage, or even to the south of Cap Blanc-Nez. The cause each time is the runoff of rainwater which weakens the rock.

“As soon as we have a degree of global warming, we have 7% more water in the atmosphere, alerts Pierre Bracq, professor of hydrogeology at the University of Littoral Côte d’Opale. And precipitation will have an impact on the retreat of the coastline. We see steps on the cliffs. It is not the sea that attacks, it is the collapse that occurs behind it, linked to the quantities of water that fall and infiltrate..”

The coastline can recede by 3 or 4 meters in certain years. On a geological scale, we will be in Lille very quickly!

Pierre Bracq

Professor of hydrogeology at the University of Littoral Côte d’Opale

“Regarding the extent of this shrinkage, we sometimes average 0.5 to 1 meter per year, but it can reach 3 or 4 meters in certain years. This may not seem like much on a human scale, but over thousands of years on a geological scale, we will very quickly be in Lille!“

The proliferation of these black cracks visible in the cliffs shows that the phenomenon will increase in the years to come, with an impact on coastal homes. According to experts, if nothing is done to combat climate change, more than 110 000 homes are threatened by the retreat of the coastline in the region by 2100.