La baie d'Arcachon (document en anglais)

The Bay of Arcachon

Introduction

This bay is a priceless environment between earth and sea. Locates long of the Atlantic coast at 40 miles in the west of Bordeaux it's corresponds to the only one indentation of this coast.

According to his formation, origins and topography it is considers like a bass coastal area where the inner coast is made of sandy-muddy estran.

It's as well a lagoon of triangular shape semi-closed by 18 232 hectares which when the wter withdraws, we can see a landscape of channels.

Moreover, it is a space to high population density, attractive more and more tourists each years and it's a maritime home thanks to oyster farming.

The problem of this Bay is that it's undergone to a strong marine influence involving a important movement and transport of sediment which alters the Bay morphology.

I) The dynamics which are exerted on the Bay.

The coastlines are fragile spaces, which suffers a very specific hazards and accommodate a population more and more concentrated on these territories so increasing the vulnerability of the Bay.

A) The natural dynamics

The Bay of Arcachon possesses important coastlines phenomena bound to the hydro-sedimentary dynamics coastal.

The majority coastline risks met on the sector of Arcachon concern the erosion of the line of coast and the movement of dune.

The line of coast tends to move back in front of the important marine erosion.

This erosion is translated by a movement inward land. Indeed, the coastline which is formed by sand and by dune knows phases of recession alternated about phases of respite.

The factors erosive responsible here for these phenomena are the oceanic swell, the current of tides and storms. Moreover, the climatic modifications are going to stress in the future the erosive trends.

Indeed , today the recession of the line of coast in on average of 0.5 to 1.5meters a years.

The unstable character of this Bay corresponds to a combination of two natural phenomena.

The first take into account the presence of important sea currents which follow the Atlantic coast. This current carries every years a lot of sand which accumulate on the banks blocking the entrance of the Bay. So this action modifies at the same time the morphology of channels which become unstable and evolve in the time.

(croquis)

Thanks to this sketch, we can see the evolution of channels and the line of coast.

We can observe in this way that nowadays dunes doesn't take up the same position and the sandbanks have been considerably moved changing by the same opportunity the trajectory of channels.

Moreover, this coastline drift has for consequence the formation of the arrow of the Cape Ferret. This is the environment which underwent most erosion in this Bay.

As we can see on the pictures, the Cape Ferret has got a important erosion about 17 meters a years.

Next the second phenomenon is the game of tide of whose powers erode and weaken sandbanks. Of this fact, the advanced of dunes are made inward lands because of the push of winds and sea. For example, this process has been making decreased the Dune of Pilat from 3 to 4 meters

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