Mapping and study of morphological monitoring of the beaches of Lorient Agglomeration

Lorient Agglomeration has 132 km of coastline, one of the major attractions of its territory. In response to the vulnerability of this space and the anthropogenic and natural pressures of this coastline, Lorient Agglomeration has launched a monitoring programme whose first phase (1999-2013) was to bring together the information on the different parameters that affect the evolution of the coastline, to identify signs of erosion* and to estimate the degree of sensitivity of the coastal fringe. As a result of this operation, a photographic monitoring tool was put in place. Several further studies have thus been carried out, in particular on Kerguelen, for example. However, these qualitative data do not allow quantitative monitoring. Thus, the objective of this study is to launch multi-year monitoring on the beaches of Lorient Agglomeration showing signs of erosion, based on a bi-annual topo-bathymetric survey and particle size analyses*. These constant and regular data will help to better understand the hydrosedimentation* of its beaches over the long term and to define the best management and/or development tools. **Collection context** Bathymetric surveys take place offshore towards the foreshore and at high tide, and surveys topographics at low seas, in order to obtain a cover allowing the setting data. High tidal coefficients are preferred for interventions. The interpretation of topo-bathymetric data should be an aid to managers to determine the most appropriate accommodations to combat against coastal erosion and the risk of flooding. Monitoring of topo-bathymetric data will allow as campaigns to know the characteristics of each site and to understand the sediment movements in the profile at different time scales. **Collection method** The topo-bathymetric readings were made by the firm Ingeo. The method used is as follows: The bathymetric reading is carried out using a multibeam probe* high definition, coupled for the exhaustive topography of the foreshore* with a LiDAR survey*, rigged on the roof of a 4x4. Topographical and bathymetric coasts are reported to zero of the lowest seas. Reminder: I.G.N 69 = CM + 2.63. Taking into account the extent of topo-bathymetric surveys of longitudinal profiles have been realised. These have been set to the particle size sampling profiles, the positioning of which is stipulated in the IGO report. For this first campaign, the analysis of topobathymetric surveys was based on the range profiles that were also used for particle size sampling and on the analysis of isobath 0 CM, considered here (taking into account SHOM and Candhis data on the sector)as a depth close to the limit of action of the houles. For more information, see [the metadata on the Isogeo catalog](https://open.isogeo.com/s/0bd17a2a330842c68ec4a412d1df6824/AiCyPK6W8fg012l-vHV0s5tPry5b0/r/32e90f95741e4d0ba494120a85f57cb5).