Coastal erosion, a natural phenomenon manageable for a while, but a losing battle in the long term

Question: What is coastal erosion, which threatens 50.000 homes on the French coast by 2100?

Answer: "It's a natural phenomenon, where the ocean eats away at the beaches and dunes. The sand was formed during an ice age, with rivers carrying a lot of sediment.

With the natural warming of the planet, observed between 18.000 and 6.000 years ago, sea levels rose, coastlines receded by tens of kilometers and rivers brought in much less material.

Today we live on a stock built up several millennia ago and which is gradually being eaten away. In winter, the waves carry the sand deposited the previous summer by the wind offshore. Depending on the tides and sea level, they can strike right up to the base of the dune.

Classic storms can then cause the beaches to recede by 10 to 20 meters in places, this was the case on the island of Oléron this fall. They then re-fatten during the summer but without necessarily compensating for the losses. For example, in Gironde, the coastline recedes on average by 2,5 meters each year."

Q: What effects will climate change have on erosion?

A: “The ongoing change will influence several climatic and oceanographic variables.

It can affect the patterns of waves, winds, storms and bring more intense rains which will act on the erosion of the cliffs. But the most important variable is sea level rise.

In 2050, whatever the level of greenhouse gas emissions, we will have a rise of 20 cm in sea level. Depending on the calculation methods, this will cause an additional decline of 5 to 20 cm on the beaches. meters from the coastline.

By 2100, between the most optimistic scenario of emissions reductions and the continuation of the current trend, the rise in sea level will be between 42 and 73 cm: the base of the dune will be attacked more frequently by waves ."

Q: How can rib recession be managed?

A: "It depends on the local context and the dynamics of retreat. First, there is everything that concerns ecological engineering. We can garden the dunes: put windbreaks, branches or nets there to keep the sand.

Then, flexible active control consists of taking sand where there is excess and depositing it where it is lacking. It is virtuous if it is done well but it requires being fair in the evaluation of stocks.

Hard protections, often longitudinal riprap preventing waves from attacking the dune, or perpendicular groynes which keep the sand drifting towards the south, help maintain the seafront. But these structures require heavy maintenance and accelerate erosion. on either side of the hard points.

Scientific observation, to know where, how much and how we are moving backwards, and sustainable development are essential. Today, land use planning takes into account all hazards and this can go as far as the relocation of buildings.

All of these methods are effective but whatever we do, the battle is lost in advance. The question is when? Are we able to adapt? How ? And these management strategies belong to the territories."