Monitoring of recent morphological changes of the dune of Vougot beach (Brittany, France) using differential GPS

The dune of Vougot beach is a massive drifting sand body approximately 250 to 400 m wide and 2 km long. It is located in the municipality of Guissény, on the north coast of Finistère (Brittany). This dune, with a southwest to northeast position, protects a vast polder area which was disconnected from the sea by a dike construction in 1834. For several decades the eastern part of this dune experienced erosion mainly due to the construction of an artificial jetty in 1974 (Curnic jetty), which entirely modified the hydrodynamics and sedimentation processes. In order to determine the actual trend of evolution, the advance rate, and the resultant sand drift that is occurring, a survey of the dune was achieved between 2004 and 2009. Shoreline changes were monitored on a yearly basis; in addition, two detailed topography surveys (2005 and 2009) were carried out. Topography surveys were made using a Trimble DGPS respectively with 5 cm and 1,5 cm of horizontal and vertical accuracy. ArcView GIS was used to process the data and display the results. Shoreline change rates were determined using Digital Shoreline Analysis (DSA) ArcView tools to measure erosion and accretion transects. 3D surface analysis was based on Digital Elevation Model (DEM) calculations using Surfer software. Foredune changes showed that dune evolution since 2004 is in accordance with the trend observed during the last decades. However, the speed of dune retreat has increased from 0,6 m/yr to 1,5 m/yr. The comparison between DEMs obtained from the 2005 and 2009 surveys confirmed this evolution. A volume of eroded sand from the dune amounting to -10,677 ± 110 m3 with an erosion of the foreshore beach of -10,933 ± 1,396 m3 was determined. These results confirm the fact that the Curnic jetty is constantly interrupting the sand drift inducing an increase in sediment loss from the Vougot beach/dune system.