COASTAL EROSION

CASE OF THE AQUITAINE COAST

For several decades, a particularly marked decline in the coastline has been observed in the Médoc. In Soulac, this retreat is of the order of 200m in 50 years, i.e., an average of 4m per year. In addition, more and more experts are predicting a sea level rise of one meter by the end of the century with an accelerating trend. To estimate the extent of the phenomenon in the years to come, there are numerous studies and not always concordant. Yet a simple extrapolation of what has happened in past millennia should give us an indication of this evolution. What can we notice when we look at the map below, (based on Cemagref/Adour-Garonne, Actes des journées techniques sur les lacs et étangs aquitains, 1992)?

In 18000 years, the sea level rose by 120 m and the coastline retreated by about 50 km at the level of Biscarrosse and Arcachon. A simple calculation gives us a recoil of 400 m for the year 2100.
Is there an error in this calculation?

Obviously, there are at least two uncertainties: Will the sea level actually rise by one meter and the reaction time? But what is already happening in the Médoc is not reassuring. As for the Dune du Pilat, it is receding by 3 to 5 meters per year according to observers.

Is it reasonable to let the back of the first dune strip be built?
Shouldn’t we create no-build zones, such as in the mountains for avalanche paths or in plains in flood-prone areas?

PROTECTIONS

To combat this retreat of the coastline, several solutions have been devised. Most are listed in J. W. Kamphuis, Introduction to coastal engineering and management, 2000.
Toutefois, page 270 de cet ouvrage, on peut lire :
“When the hotels replace the dunes, the beautiful beaches quickly erode to the point that no beach is left. The hotels and other structures are then endangered and are often protected by seawalls and groins (Ch. 15). Fewer tourists will now come to sunbathe (on the seawalls). The hotels need both protection and recreational beach and this can be provided by artificial beach nourishment.”

EXAMPLES AT SAINTES MARIES DE LA MER

The case of the works carried out in Saintes Maries de la Mer and nearby is interesting because several techniques have been used with varying fortunes despite a fairly similar environment.

Simple protections east of Saintes Maries:

More complex protections for the “plage des épis”.

EXAMPLE AT PORTUGAL – PRAIA DE CORTEGACA

From a technical point of view, this is an illustration of the text of “coastal engineering and management” mentioned earlier.

From the point of view of the general interest, the work carried out seems very questionable:

1. significant reduction in the bathing area (cf. “Bains publics” on the picture) ;
2. site denatured by dikes and groynes ;
3. the cost of work and maintenance is most likely to be borne by taxpayers.

Obviously, from the point of view of the few owners of bars, restaurants and other waterfront constructions, it is different.