The endless interaction between the Earth and the stars

The tide refers to the upward and downward movement of the oceans and seas. It is the result of the attraction of the moon and the sun on the sea, but also of the rotation of the earth which generates a centrifugal force.

This so-called gravitational attraction varies according to the position of the moon and the sun in relation to the earth: if these two stars are on the same side or diametrically opposed, there will be a high tide. If they are 90° apart, it will be low tide.

When the sun and moon form a right angle to the earth, the effect of their attractions is subtracted. The tides will then be of very low amplitude, this is what is called dead water. It was Isaac Newton who discovered, for the first time in 1687, this phenomenon of gravitational attraction of the stars on the earth.

What are tidal coefficients ?

They make it possible to know if the tide of the day has a large or a small tidal range, the tidal range being the difference in level between the high tide and the low tide.

The coefficients are between 20 and 120 and are the result of a calculation which notably takes into account the height of the high tide and the mean sea level. The high tides observed on the coasts are those which exceed 95.

High tides

Spring tides, also called spring tides, occur when the sun and the moon are aligned on the same axis and their effects combine. The tides are then of strong amplitudes.

During these high tides, several ports in the Basin are submerged for a few hours.

The particularities of the tides in the Bassin d'Arcachon

We can say that the Bassin d'Arcachon fully experiences the tides and that this phenomenon has a strong impact on its geography and its ecosystem. First of all, the Bassin d'Arcachon is a semi-enclosed lagoon with very diverse landscapes.

On the side of the La Leyre estuary on the Teich and Biganos side, the landscape is more “Pré-salés style”.

Then many esteys and channels in the lagoon as well as sandbanks, beaches and passes at the level of the opening towards the ocean change according to the tides, the wind and the swell.

Indeed, the tides move the sandbanks, modify the passes, and carry millions of M3 of water with each tide.

The delta of the mouth is different depending on the rising tide or the falling tide.

At low tide, we discover the sandbanks of the Bassin d'Arcachon as well as the omnipresent mud, i.e. more than 100 km2 of foreshore.

Conclusion, we prefer high tide for swimming and low tide for walks and shore fishing!