Impacts des décisions et actions de protection sur l’évolution morphologique du littoral lacustre de Thonon-les-Bains (France, Haute-Savoie)

Abstract

The landscape heritage of Thonon-les-Bains’ Lake Geneva shoreline is a key component of the local population's living environment and tourist activity. Since the mid-1990s however, coastal erosion has been observed. With this worrying development, experts were called in to monitor the shoreline’s morphology. The first study (2011-2014) showed that the majority of the shoreline was eroding (60%) at an average rate of 0.40 m.yr and a maximum of 1.5 m. The primary cause is sediment deficit linked to hydroelectric developments in the Dranse catchment area, and sediment extraction at the mouth of the river Dranse on Lake Geneva (Maleval et al., 2016). In light of the study’s findings, the Prefecture refused the extraction company's (Sagradranse) application to continue its activity, and Thonon-les-Bains municipality undertook renaturation work on its shoreline using soft protection methods (plant engineering and removal of hard structures). The latest study (2019), using the same methodology as the first (mainly manual topography and acoustic bathymetry), revealed an encouraging change: between 2014 and 2019, the portion of eroded shoreline decreased to 42%, and the percentage of severely-eroded shorelines decreased from 39% to 5%. On the basis of these latest data, the average annual erosion rate of the Thonon-les-Bains shoreline can be considered as minor (0.13 m), thereby supporting the public authorities’ decisions.