Dynamique de comblement d'un bassin sédimentaire soumis à un régime mégatidal : Exemple de la Baie du Mont-Saint-Michel

This work has been performed in a DRET/CNRS convention and related to the problems of sedimentary dynamics in a bay dominated by tidal currents. It combines the descriptive study of the morphosedimentary units with the modeling of bedload processes. At the South channel Isles, the Mont-Saint-Michel bay is well known for his exceptional tidal range. It's open to the offshore with two broad master channels on both sides from the Chausey's archipelago. On the coarse sedimentary cover of pebbles and gravels, sediments dynamic series of some plurikilometres sandy ribbons. These sands are temporarily stocked in lonely bedforms and in coastal maritime banks, sandwaves field of Granville and tide delta. The coastal banks are connected in the littoral prism. They are controlled by tide currents chenalized between hollows of the Grouin headland. This rocky promontory induces a large-scale residual rotary currents that refract transport directions to Cancale. The oriental limit of this clockwise vorticity divide the maritime bay and the estuarine bay. The shallow conditions and the water level agitation change the morphology of the banks and very large sandwaves near Granville according to the meteorologic conditions, their migration could be opposited to their tidal polarity. The tidal delta is composed of mobile sand bars with a strong sedimentary balance. Great spring tides and storms rework near subtidal sandy floor. The rate of bedload sand transport calculated by the numerical modeling have been estimated to 500 000 m3/y and the suspended deposit volume to 900 000 m3/y on the sand beach near the Mont-Saint-Michel. These computed values have been obtained for a nil agitation so they are underestimated. The proliferation of crepidule for 30 years is a new significant element in the sedimentary facies distribution. 214 000 tons of alive and dead biomass entail the growing of faecal pellets mud flats (about 332 000 t/y) where seabed was made up sands and gravelly sands. Most of marine technics has been used (side scan, grab samplings, corings, self-recording current measurements, ...) to analyse sedimentary environments and draw up a residual hydrodynamic map of bedload directions. Net sedimentary transports are defined with sonographs studying, relationships of grain-size distribution (Gao and Collins's method), analysis of sediment color and with a 2D hydrodynamic numerical model combined with Bagnold bedload transport rate equation. Tidal currents remain the main source for the holocene sedimentary prism construction and are temporarily opposed or strengthened by swell currents. Bedforms are numerous and various in the bay scale. This "sand sheet facies" is distributed to the shoreline in accordance with currents falling off.