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We propose a study on the use of laser techniques to monitor altimetric variations in the tidal flat of the Bay of Mont-Saint-Michel. The Bay of Mont-Saint-Michel has been strongly anthropised. Because of impoldering, wandering rivers were not able to sap salt-meadow and modern tidal flooding of the Mont-Saint-Michel has lowered. Through modern studies and projects aimed at restoring the marine nature of the bay it appears that flushes are useful to discard sediment tending to silt the bay. The major aim of our work consists in the better understanding of the effect of the dam built recently (May 2009) in that purpose. Laser scanning is commonly used for topographic surveys and the generation of Digital Elevation Model (DEM). Repeating surveys, allow to quantify topographic changes and therefore sediment budgets. Our study is based on aerial topographic surveys of the intertidal zone acquired before the operational start up of the dam (in 1997, 2002, 2007 and February 2009). Sediment budgets computations confirm that the bay tends to accrete but at annual rates quite variable in time. The value computed between 2002 and 2007 is 2.3 times and 3.5 times smaller than the deposition rates compute d fo r the 1997/2002 and 2007/2009 periods. Terrestrial laser surveys acquired on a more restricted area after the beginning of the flushes are also used in a more qualitative way to investigate their impacts. The preliminary results show that flushes allow to reduce sediment deposition but also that their efficiency is geographically contrasted. Another remarkable result consists in the identification of coastal dynamics: location of erosion versus deposition zones, of evolutions of vegetation cover and drainage channels activity.

Sediment budget and morphological evolution in the Bay of Mont-Saint-Michel (Normandy, France): aerial (LIDAR) and terrestrial laser monitoring