Etude des dynamiques du trait de côte de la région Bretagne à différentes échelles spatio-temporelles

Emmanuel Blaise 1
1 LETG - Brest - Littoral, Environnement, Télédétection, Géomatique
LETG - Littoral, Environnement, Télédétection, Géomatique UMR 6554
Abstract : This research is part of current context of increasing of human beings and human activities vulnerability face to coastal risks (erosion hazards and marine submersion), linked to global environmental changes (global relative sea level rise and growth of storms’ frequency and intensity) and the current concentration of the population on the coastline. Following the storm Xynthia (February 2010), France has adopted a national strategy dealing with coastal erosion and shoreline retreat, in order to define a new integrated national strategy of coastline management, promoting the relocation of stakes and properties. This strategy focuses on the monitoring of shoreline changes and the identification of coastal risks due to erosion.This work fall within this issue. The main objective was to study sedimentary coasts dynamics (sand dunes and gravel barriers) of Brittany, following different spatial and temporal scales. In the long term, the analysis consisted, first of all, in establishing a typology of sedimentary coasts according to several morpho-sedimentary criteria; then, coastline kinematic has been analyzed over the last sixty years (1950-2010) through a diachronic study using photogrammetry. In the medium term, analysis was carried out as part of topographic surveys performed at multi-annual to multi-decadal scale, on several Observatoire du Domaine Côtier (IUEM) and/or Service National d’Observation - Dynalitt survey sites. The morpho-sedimentary study was combined with a wave, tide, wind and atmospheric pressure analysis, in order to explain the role of these forcing in the erosion and recovery successive phases. Finally, in the short term, analysis was based on high frequency of topomorphological surveys carried out during the 2013-2014 stormy winter. Additionally, an experimental study to quantify runup process, in order to analyze extreme water levels impact on shoreline erosion.Overall, the quantification of coastline variations over the long term revealed an erosion tendency on 27% of the studied linear, a non-negligible progression of the shoreline on 20.5% of the studied linear, and a significant shoreline anthropogenic impact (34.5% of the studied linear). The similarity of the morpho-sedimentary responses of the littoral strands studied to the meteorological conditions allowed to identify five distinct phases. Three morphogenetic phases (1998 to 2003; winter 2006/07 to spring 2008; winter 2012/13 to the present), characterized by severe shoreline erosion combined with a high frequency of storm episodes, and two phases (2003 to winter 2006/07 ; spring 2008 to winter 2012/13) characterized by milder weather conditions, allowing shoreline recovery. During 2013-2014 winter, three storms were particularly morphogenetic for sedimentary coasts, because they were combined with high spring tides: January 4-5, February 1-3, and March 2-3. During this winter, the average retreat for all studied sites reached -6,3 m, with a maximum of -30,1 m and a minimum of -0,2 m.
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Emmanuel Blaise. Etude des dynamiques du trait de côte de la région Bretagne à différentes échelles spatio-temporelles. Géographie. Université de Bretagne occidentale - Brest, 2017. Français. ⟨NNT : 2017BRES0010⟩. ⟨tel-01563652⟩

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