Coastal erosion by wind and rain is a phenomenon which occurred even before humans were around to spot it. Yet with the urbanisation of the coastlines and the increased frequency of storms and cold events these last years, it feels like the problem is getting serious. Focusing mainly on my hometown Dieppe and its chalk cliff, I’ll talk about the causes, consequences and evolution of coastal erosion.
The Alabaster Coast in the north of Normandy (France) owes its name to its white cliffs, formed from the remnants of Cretaceous coccolithophorids. The most famous part of the cliffs, the needle in Étretat, attracts thousands of tourists every year1. But most of them ignore the dangers of such a beautiful display: each year, about 1 million m3 of cliff collapses and drives the coastline 30 cm inland on average2.
video: 30 000 tonnes of cliff collapsing last summer by Le Havre
There are two ways chalk cliffs can be eroded: from the bottom by the sea, and from the top by the weather. The whole phenomenon is quite complex, but in both cases the water digs its way through the chalk, and the more acid the water the faster it goes (for chalk is made of calcium carbonate). With every wave, every tide, every rainfall, the chalk contains more and more water. When the whole structure has been dissolved too much, or in winter when the water freezes in the chalk, whole parts of the cliff collapse (see an example of a brutal collapse on the video).
The collapse of parts of the cliff can create beautiful natural structures like the needle in Étretat. However, most times they destroy buildings and roads which are too close to its changing edge. A year ago in my city, several houses had to be evacuated urgently after a storm caused the cliff to become unstable. Now one of them is in the sea, and three others are just on its edge. Blockhauses from WWII which were built on the cliff by the Germans to see the British arrive can now be observed only at low tides, 50 m away from the cliff edge. After several emergency strengthenings, the Council finally admitted that the road which is now too close to the cliff edge has to be diverted. All of this may sound anecdotical, but with 10% of the French population living by the coast3, the problem can not easily be ignored. Plus it is not only limited to Normandy: over 15% of the European Union coastlines are being eroded4.
Especially when you live there, you know cliffs are dangerous and can collapse at any time, but people have the feeling that the phenomenon is accelerating. Is climate change causing more cliff to collapse? This is hard to say until we have longer time series. But we know that climate change is already causing sea level rise, ocean acidification and more extreme weather5, all of them contributing to the destruction of chalk cliffs.
1 Office du Tourisme d’Etretat
2France 3 Haute Normandie, “L’inexorable recul des falaises”
3Memento du maire, “RN4 – Risques littoraux”
4European Environment Agency, “Coastline Dynamics in Europe”
5IPCC working group I, “Climate Change 2013: The Physical Science basis”