Analyse multiscalaire et multifactorielle de l'évolution et du comportement géomorphologique des systèmes côtiers sud bretons

Soizig Pian 1
1 LETG - Rennes - Littoral, Environnement, Télédétection, Géomatique
LETG - Littoral, Environnement, Télédétection, Géomatique UMR 6554
Abstract : This PhD deals with the geomorphologic behaviour of the South Brittany coast and more precisely, the part located between the pointe du Talut next to Lorient and the mouth of the river Vilaine near Penestin. Consisting essentially of low weathered cliffs and sand-dune/beach systems, the coast is particularly sensitive to erosion processes leading to shoreline retreat. Moreover, since the 1950s, the coast has been subjected to ever increasing man-induced pressures contributing to an ever growing artificial remodelling of the coastal fringe. In such a context, this study aimed at understanding the effect of the intricate interrelations between nature and society on the geomorphologic behaviour of the coast, with particular emphasis given to the occurrence of erosion processes. The analysis is based on a multi-scalar and multifactorial approach backed by three years of field work, as well as on a spatial and statistical treatment of shoreline variations over various time intervals. From a long-term viewpoint with a multi-decennial timescale, the analysis consisted in relating shoreline variations measured from aerial photographs to a GIS database describing the factors controlling coastline evolution. In this way, it was possible to establish the spatial and statistical relations between the shoreline movements and the controlling factors in order to classify them in accordance with importance of their respective role. The analysis was carried out over several time intervals depending on the availability of aerial photographs. It focused on five coastal systems: the Gâvres-Penthièvre dune massif, the Quiberon rocky peninsula, the Quiberon bay, the Gulf of Morbihan and the beaches located south-east of the Rhuys peninsula. From a short-term viewpoint with a timescale based on half-year intervals, the study aimed at establishing the morphodynamic behaviour characteristics of twenty five beaches located within the Gâvres-Penthièvre dune massif, the Quiberon bay and the Rhuys peninsula, using granulometric and topographic data as well as offshore and nearshore modelled wave data. The combination of the results allowed to build conceptual models of the geomorphologic behaviour of each of these coastal systems, and to analysis interrelationships occurring between "natural" and "anthropogenic" constraints. At coastal system scale, coastline variations are driven by morphological and hydrodynamical constraints. Protections established on the sand-dunes could favour fronts dunes advances. Over a shorter space scale, shoreline retreat is controlled either by accelerated erosion or by man-induced erosion processes. From these results, it possible to propose a typology of the vulnerability affecting the South Brittany coast, which takes into account so called natural processes and the relative forcing of man-induced factors in the spatial localisation of erosion processes.
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Submitted on : Tuesday, October 30, 2012 - 2:09:16 PM
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Soizig Pian. Analyse multiscalaire et multifactorielle de l'évolution et du comportement géomorphologique des systèmes côtiers sud bretons. Géomorphologie. Université Rennes 2; Université Européenne de Bretagne, 2010. Français. ⟨NNT : 2010REN20067⟩. ⟨tel-00747056⟩

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