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Recent evolution of the Mont Saint-Michel Bay as seen by ALOS-AVNIR-2 data

Jean-Paul DEROIN(1), Clélia BILODEAU(1) and Benoît DEFFONTAINES(1)

(1) Université Paris-Est, 5 boulevard Descartes, 77454 Marne la Vallée, France

Abstract

The Mont Saint-Michel Bay at the base of the Gulf of Saint-Malo dividing Normandy and Brittany in western France, is characterized by an exceptional tidal amplitude : 14.5 m during spring tides. Associated with a very light northwest-trending slope, the width of the foreshore reaches 15 km along a NW-SE axis. The emerged foreshore can then be as much as 240 km², that is, half the bay. More than 400,000 m3 per year of soft sediments stay uneroded in the internal part of the bay (leading to about 1 mm of elevation per year). The finer part contributes to the development of the halophytic vegetation (schorre), whereas the coarser part helps the formation of large sand bodies and dunes.

Due to the link between local mean time and the higher tidal amplitudes, the sun-synchronous satellites allow to cover only 80% of the emerged foreshore in the better conditions (about 2.3 m of sea elevation). These particular conditions have been obtained for the first time on October 24, 2007 by the AVNIR-2 sensor onboard the Japanese Space Agence (JAXA) ALOS satellite. ALOS AVNIR-2 data were used to make a new geological map of the macrotidal sedimentary prism including the drawing of the channels and the identification of the main type of sediments (coarse sand, fine sand, mud, etc.). They allow also to update the mapping of the schorre in the upper part of the tidal flat.

The results have been compared with those previously get using archive satellite data such as the Landsat, Spot, JERS-1 or ISS spacecraft systems, covering the 1972-2007 period, and aerial photographs available from the end 1940’s onward. Whether the growing of halophytic plants (and then the fixation of sediment) in the upper part of the foreshore is now well established, the potential migration of meanders and channels on more than 1 km/year renders unexpected any change in the morphology of the bay.

 

 

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