Modification de la marée par le creusement du chenal d'un estuaire

Résumé

Des mesures des niveaux de basses mers réalisées par les autorités portuaires dans l'estuaire de la Loire montrent qu'entre 1969 et 1993 ce niveau est descendu d'une façon significative. Ceci s'explique par les dragages récents dans le chenal externe de l'estuaire qui ont modifié la propagation de la marée dans ce site, ce qui a conduit à une correction officielle de –0,40 m pour le zéro des cotes marines.

Afin d'expliquer ce phénomène, les niveaux extrêmes de la marée ont été analysés en quatre sites de la côte bretonne occidentale entre 1965 et 1990. L'analyse statistique des données montre une nette amplification des surcotes dans la partie nord-est de l'embouchure de l'estuaire qui contribue à une érosion de ce site.

L'étude phénoménologique des surcotes et des vents de 1985 à 1990 sur les deux rives de la Loire montre que le niveau moyen de la mer y reste stable.

Abstract

 The present study compares the tide measurements on the left and right banks of the mouth of the river Loire (France) which flows into the Atlantic Ocean and two others points on the French Brittany coast (Fig. 1, 2), for the last twentyfive years. In order to simplify the location of the sites, the points have been numbered from the north to the south as follows:

• Point 1 : Le Conquet

• Point 2: Port Tudy, île de Groix

• Point 3: St.-Nazaire

• Point 4: Pointe de St.-Gildas

The observed tide levels in the estuary of the river Loire show important changes since 1903 (Fig. 3). These modifications can be linked to the dredging works in the navigation channel between Paimboeuf and Nantes (55 km upstream) where the bottom level has been excavated from –2.50 m to –3.00 m in 1929 and to –5.50 m in 1969.

Whereas the chart datum (hydrographic zero reading), which in France corresponds to the lowest possible seas for safe navigation, had been set at the end of the last century at 3.03 m under the terrestrial chart datum, on January 1st 1996, the SHOM, Naval Hydrographic and Oceanographic Service, indicated that on all the nautical documents it publishes this chart datum was now being set at 3.43 m under the terrestrial chart datum for the right bank of the mouth of the estuary (corresponding to the northern side of the estuary) while the one for the left bank, although only 15 km away, remained unchanged.

The statistical study of surges for a period of twenty five years (1965–90) on both sides of the external estuary shows that the surges are significantly greater on the north-eastern side. It also highlights the incoherence of certain results. The study of level readings on 3 500 high and low tides effected by 4 tide gauges spread out along 300 km of coastline highlighted that a single badly zeroed tide gauge could lead to false results with regards to the average tide level at point 3.

The work shows that this discrepancy is neither due to the blocking of fresh river water nor to a set of compartment faults but to the displacement and dredging of the channel along the right bank of the mouth of the estuary.

The storm surges have also been studied between 1985 and 1990 using a dynamic model where surge levels depend on the duration of the meteorological disturbances. In this model the surge associated with a stationary meteorology S_∞ depends on the wind (velocity and direction) and the atmospheric pressure.

The theoretical law for the surge due to atmospheric pressure is valid in the 4 points studied; for each hectoPascal of lowering of barometric pressure, the sea level increase is 1 cm.

For a wind of constant velocity and direction, the model shows that the surges are greater at points 3 and 4 than at points 1 and 2. This fact can be explained by the local morphology of the external part of the estuary; it also gives an explanation for the coastal erosion of this area.

This model confirms that the reduction of the extreme low water level observed at point 3 is a local disturbance. In fact, the increase of the mean sea level is approximately 0.10 m per century at points 1 and 2 as in Brest harbour where level measurements have been undertaken since 1808. In contrast, this level appears to be stable at the two points situated on the estuary while the amplitude of the tide increases. This phenomenon is better marked at point 3 in the north of the estuary.

Finally, this study shows that the installation of a mobile dam in the Loire estuary (similar to the one existing in the Thames estuary) would permit regulation of the upper water levels above the dam although the amplitude of the tide could be enhanced downstream of the dam.