Master keys of the basin of Arcachon

The master keys of the basin of Arcachon are the channels of draining of the Bassin of Arcachon, which allow the ships coming from the Atlantic Ocean to penetrate in the basin. This arm of the sea almost closed is located at the limit of the the Gironde and the Landes in France.

The entry of the basin of Arcachon is broad several kilometers but it is encumbered by sand banks prolonging the point of the Cape Ferret like by a large bench emerging permanently in the medium: the bench of Arguin. The master keys themselves are thus relatively narrow and especially very moving; historically two master keys open alternatively according to a cycle which lasts 70 to 80 years: one in the south skirts the coast and the Dune of Pilat, the other passes to the north of the bench of Arguin.

These master keys constitute a zone of particularly dangerous navigation, which regularly make victims among the professionals of fishing and the yachtmen. These master keys are also likely to be closed again in the future, because of a stranding which worsened these last years: the basin of Arcachon would then become a lake with very serious incidences undoubtedly on the local economy.

The crossing of these master keys by a ship is dangerous as soon as the wind forcit or that a swell of west settles. Under these conditions, a bar is formed at the beginning of the master keys with the place or the funds is raised (dimension -4 measures with the lower part of the zero of the charts). The high sandy bottom at the origin of this bar is created by a North-South general current which drives out sand throughout the French coast of the Atlantic until the gouf of Capbreton (located at the level of the Spanish border).

This current is the consequence of a relatively recent modification (in geological term) of the mode of wind and swell on north-western the Atlantique close relation now directed.

The current of Eyre - river leading to the basin of Arcachon - and the downward tide are today just sufficient to maintain the entry of the open master keys.

Since ten year (since the southern channel was closed again), the problem arising from the master keys worsened and there the trawlers of the wearing of Arcachon (located in the basin) today are frequently obliged to unload their fishing in the ports located at the north or the south of the basin fault of being able to return. The spectrum of a complete closing of the basin is frequently evoked like its dramatic consequences for the area in the field of employment (activity Ostréicole, tourist).

The Big lakes landais which are staged along the Landes (Hourtin, Lacanau, Cazaux, Parentis…) are all old basins similar to that of Arcachon which were closed again following a phenomenon of stranding similar to that described.

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