Shelly cheniers on a modern macrotidal flat (Mont-Saint-Michel bay, France) — Internal architecture revealed by ground-penetrating radar

The relationship between short-term morphodynamics and internal structure of shelly chenier ridges is investigated using ground-penetrating radar and core X-ray analysis, complemented with a comparison of aerial photographs. These cheniers are located on the uppermost part of a macrotidal flat in Mont-Saint-Michel bay (North-Western France). They actively migrate landward over the salt-marshes by washover processes when they are submerged during the coincidence of spring tides and enhanced wave activity. The architecture of the cheniers is imaged using high-frequency radar antennas (400MHz to 2.6GHz). Three types of cheniers are recognized, featuring differences in morphology and internal structure. The altitude of the banks on the tidal flat relative to the level of tidal flooding, as well as local sediment supply, seem to be important forcing parameters in chenier development and stages of evolution. On a multi-annual time scale, evolution of this chenier system is influenced by a combination of the 18-year and the 4-year tidal cycles, superimposed on wave activity. During minima of the 18-year cycle, spring tide level is globally lower on the tidal flat, allowing salt marsh progradation and chenier ridge development. During peak periods of the tidal cycle, former barriers are reworked by wave action and experience onshore migration associated with salt marsh erosion. The 4-year tidal cycle coupled with enhanced wave activity is probably responsible for the deposition of large washover units, observed in GPR reflection profiles.