Thesis defended on December 20th 2017
Understanding the morpho-sedimentary dynamics of cohesive and non- cohesive beaches in the South-Brittany region (France) at different spatiotemporal scales
At the world scale, coastal areas can be divided in several categories: rocky, sandy muddy and mixed coasts. However, these sedimentary coasts are regularly found mixed. Temporary or permanent mud inputs are observed on the sandy beaches. These atypical mixed environments, coupling erosional, transports and deposits processes associated to cohesive and non-cohesive sediments have been poorly studied. So, understanding theirs morphological and sedimentary dynamics and the identification of regional and local forcings driving them are essential. Four sandy-muddy or fully sandy Low-Tide-Terrace beaches from the bay and estuary of Vilaine (South-Brittany) have been monthly monitored over two years to characterize their morphodynamics beahaviors. Specific monitoring of coupled hydrodynamics and morpho-sedimentary surveys and photogrammetry experiments have been conducted on mixed sandy-muddy or full sandy beaches. At a regional scale, this work highlights the role of the initial morphology and regional geology inheritance on the morphological response of the littoral zones to extreme energy conditions. Despite some erosion patterns in local parts of beaches, particularly close to shore protection structures, the mid-term recovery capacity of the south-Brittany coastal area after extreme wave energy conditions have been proved. In a more specific approach, the works regarding the sandymuddy beach dynamic show a morphodynamics behaviour close to that of a fully sandy beach during high energy event. The mudflat elevation changes and the dynamic of the ridges and runnels sedimentary patterns are mainly controlled by the incoming waves but also by the own physical properties of the sediment, particularly those induced by the interaction between the sand from the reflective section and the mudflat. A sand deposit between mud layers potentially decreases the wave erosion capacity on the mudflat.