Résumé
The western part of Mont-Saint-Michel Bay (northwestern France), reputed for its high tidal range, is characterized by shelly ridges on the uppermost part of a macrotidal flat. Morphological monitoring and hydrodynamic measurements are undertaken since February 2015 with the aim of characterizing the role of the shelly ridges on wave dissipation and coastal protection. Each month, 11 topographic profiles are surveyed using a DGPS, as well as water levels and wave characteristics are measured using thirty miniature pressure sensors deployed in the tidal zone. Analysis of the collected data shows that wave characteristics were strongly influenced by water depth and ridge geometry. The incident waves measured on the lower part of tidal flats during experiments were predominantly wind-wave and swell, depending on meteorological conditions. At the back of the well-formed shelly, ridges, residual waves were extremely low throughout with a dominance of infragravity frequencies. Calculations of energy dissipation indicated an energy reduction of 92% to 98 % as waves traveled across shelly ridge systems. In contrast, behind degraded or breached shelly ridges, waves were not so attenuated (reduction of 70% to 90%) and the spectral decomposition showed a mix between gravity and infragravity energy. These preliminary results show the role of the shelly ridges on wave attenuation and shoreline protection. Further measurements and stronger wave conditions are needed to strengthen the first observations.