The Mont Saint Michel estuary is a megatidal setting (tidal range up to 14 m). It is characterized by a strong tidal asymmetry during spring tides, with the flood stage much shorter and quicker than the ebb, reaching commonly a velocity of 2m/s into the estuarine channels. In estuaries with tidal ranges greater than 6 m, the difference of elevation between the rising tide and the river creates a discontinuity of velocity and pressure, called tidal bore (or "mascaret" in French). Visually, a tidal bore can be described as a wave or series of waves propagating upstream. This study takes place into a national project "ANR Mascaret". Part of the field work we performed recently on the tidal bores that propagate into the Mt St Michel estuary, aims in studying the impact of the bore fluid dynamics on sediment transport. This is an important issue for a better understanding of the complex fluid-sediment interactions and for the operation of restoration of the Mont-Saint-Michel's maritime character