-Channel coast lowlands may be washed away by rising tide
Rising tides threaten the shape of France, says the country’s national geological research agency BRGM, with coastal erosion meaning estuaries flooding more regularly and possibly even the disappearance of some smaller islands off the coast.
Greenhouse gases, global warming, melting ice caps and rising sea levels are not new, but what has been less recognised is that levels are rising faster year on year, leading to changes in the way tides operate, particularly in the Channel and on France’s western and northern coasts.
One effect is that as sea level rises, so do low and high tide levels and the height gap (marnage) between the two is increasing in some areas. These include the coast between Cotentin and Calais although further west, such as Saint-Michel bay, the gap reduces.
This, along with tidal and coast defences built by humans, is changing the way currents flow along the coast, which also affects the way the coastline is eroded, as well as the way underwater deposits are made.
A BRGM team headed by Déborah Idier has published results of its study in Continental Shelf Research and said that globally, since 1870, sea-level has risen by 20cm. But while the rise in the 20th century was 1.7mm a year, this has increased now to 3.2cm and, although difficult to predict, if this continued it could mean “a rise of 50cm to 1metre and possibly more”.
It added: “Sea level rise will have repercussions on coastal erosion and the risks of flooding of shore areas.
“On this last point, the sea level rise may be an aggravating factor in two scenarios: on the one hand with the permanent flooding of coastal marshlands and low islands, on the other, seasonal coastal flooding would be more intense and more frequent.”
So coastal lowlands such as the pré-salé sheep grazing marshes round Mont Saint-Michel could vanish and more areas could face seasonal floods.
Higher seas and higher tides will cause increased erosion along the eastern Channel coast, at places like Calais, but here there will also be a higher risk of seasonal flooding in an area where more than 70 communes were declared catastrophes naturelles after floods in May and June last year.
Although the study looked at sea level rising by up to 10m – possible by 2300 – it could not simulate where eroded sediments would end up and this would further change the coast.
