Soizig Pian (PhD)

Multiscalar and multifactorial analysis of the evolution and geomorphological behaviour of the coastal systems of Southern Brittany

Abstract

The work carried out through this PhD thesis focused on the geomorphological behavior of the South Brittany coasts, located between the Talut tip (Lorient) and the Vilaine estuary (Penestin). Mainly made up of low coastlines, composed of altered cliffs or dune-beach systems, these coasts are sensitive to erosion processes favouring the retreat of the coastline. In addition, anthropic pressures exerted on the coastal fringe have increased sharply since the 1950s and favour an increasing artificialisation of coastal spaces. In this context, this work has sought to understand the impact of the complexity of "nature" - "society" relations in the geomorphological functioning of these coasts, with a particular interest in the occurrence of erosive processes. The analysis is based on a multiscalar and multifactorial approach, based on three years of fieldwork, as well as on a spatial and statistical analysis of coastline variations over different time steps. Over the long term (multi-decadal time scale), the analysis consisted in relating the coastline variations measured from aerial photographs to a GIS database describing the factors controlling the evolution of the coastline. In this way, spatial and statistical dependency relationships between coastline movements and these factors could be established in order to prioritize their respective roles. The analysis was carried out over several time steps according to the availability of aerial photographs. It focused on five coastal systems : the Gâvres-Penthièvre dune system, the rocky peninsula of Quiberon, the Bay of Quiberon, the Gulf of Morbihan and the south-eastern beaches of the Rhuys peninsula. In the short term (biannual time scale), the analysis sought to characterize the morphodynamic behavior of 25 beaches located within the Gâvres-Penthièvre dune system, the Bay of Quiberon and the Rhuys peninsula, based on granulometric data, topographic data and modeled swell data from the offshore and the coast. The results obtained were compared in order to establish a typology of morphodynamic and geomorphological behaviours. The combination of the results obtained made it possible to propose conceptual models of the geomorphological functioning of these coastal systems and to analyse the complexity of the relationships between "natural forcing" and "anthropogenic forcing". At the scale of coastal systems, the coastline variations, on the long and medium term, are mainly explained by morphological and hydrodynamic forcing. On certain time steps, the protection measures of the coastal dunes favour the programming of the dune front. On a finer scale, the retreat of the coastline is controlled by accelerated erosion processes or by anthropic erosion processes. The analysis of these interactions has enabled us to propose a typology of the forms of vulnerability affecting the coasts of Southern Brittany, integrating the relative importance of the control of the anthropic element in the occurrence and location of erosive processes.

Supervisors
Hervé Regnauld
David Menier

Keywords
Scouring (geomorphology)
Photo-interpretation
Beach
Erosion
Littorals
Modifications
France
Morbihan
XXIst century