MODELLING AND CHARACTERIZING GLACIER-BED OVERDEEPENINGS AS POTENTIAL SITES FOR FUTURES LAKES IN THE DEGLACIATING FRENCH ALPS
Résumé
Since the end of the Little Ice Age, the French Alps areaffected bya general glacierretreat. This results in major landscape changessuchas the formation of new lakes in Glacier-Bed Overdeepenings (GBOs). These lakes are sometimes located below potentially unstable slopes subject to debuttressing and/or permafrost degradation,and can be sources or amplifiers of natural hazards (e.g.Glacial Lake Outburst Floods)whichcanreachvalley floors. Nevertheless,they alsocanbe sourcesof new economic opportunities(e.g.tourismattractivity,hydropower, etc.).This work aims to modeland characterise potential GBOsin which lakes could form,in order to provide a relevant database for anticipating future risks and opportunities in the French Alps.To do so, we use GlabTop (Glacier-bed Topography) and GlabTop 2, two GIS schemes which calculate the glacier icethicknessand map potential GBOs. The level of confidence of the GlabTop outputs is estimated by analysing the glacier morphology at the GBOs location (i.e. slope angle, crevasse field and lateral narrowing at bedrock thresholds)and analysing recent lake formation.139 GBOs(> 0.01 km²) aredetected. Among them, 59 have a medium to very high level of confidence. In the Vanoise massif, many predicted GBOs have low confidence because of the presenceof ice-cap like glaciers for which the constant shear stress approximationusedby GlabTop becomes questionable. 20lakes are already forming at some glaciermargins, including 4in predicted GBOs< 0.01km². 43% of the GBOsdetected are in the Vanoise, but the Mont Blanc massif gathers larger, deeper and more voluminous ones. GBOsare mostly below 3500 m a.s.l., as related to the glacier elevation. Furthermore, 58 potential GBOs were detected by analysing visuallyglacier morphologies. The study highlights the relevance of combining various methods to detectGBOs and is a first step towards anticipating the risks and opportunities related to the formation of new lakes in the French Alps