Formation et évolution de cheniers en contexte macrotidal. Approches expérimentales et in-situ.

摘要

The objective of this thesis is to characterize the genesis, growth and stabilization of cheniers in a macrotidal setting. These transgressive beach ridges, anchored in a silty to muddy prograding sequence, are the result of a balance between fine sediment inputs from fluvial or tidal dynamics, and reworking of coarse sediment by waves. Through the example of the bay of Mont-Saint-Michel, the problem is tackled through an integrated and multi-disciplinary approach, from hydrodynamic processes (scale of a few seconds) to the evolution of a coastline and the construction of a sedimentary body (several decades). Laboratory studies (sedimentation velocity, threshold of motion) and wave flume modelling have highlighted the importance of the flat shape and high permeability of bioclastic particles in their suspension, transportation, and deposition in sedimentary beds. GPR prospecting in the field and core analysis (X-ray and permeability) have revealed the internal structure and the geotechnical characteristics of cheniers, with a detailed description of lithological facies. Finally, field observations and wave flume modelling demonstrates the importance of flooding levels in the dynamics of cheniers, and allow to propose a depositional model controlled by 4.5 and 18.6 years low frequency tidal cycles, modulated by wave intensity.