Glacier-permafrost interactions and GLOF’s. Insights from 7 decades of kinematics and elevation changes in the Southern French Alps.

The mountain cryosphere is currently undergoing substantial modifications in an unprecedented short period of time. As effects of climate change becomes important, understanding the glacier and periglacial dynamics that lead to complex and delayed responses is timely. The spatial and functional interactions between landforms within these environments may strongly influence their processes (e.g., ablation, accumulation), usually studied in two separate research paths (i.e., glaciology and geomorphology). Very little research has focused on glacial and periglacial systems, where several perennial cryospheric elements (debris-covered glaciers, rock glaciers) are intertwined.

Here, a multidisciplinary approach is proposed combining (i) Structure from Motion on historical, modern aerial images and spaceborne images, (ii) geophysical with Electrical Resistivity Tomograms , and (iii) geomorphological surveys. The purpose is to quantify and describe morphometric changes over seven decades (1940 - 2020) at the Chauvet glacial and periglacial system (Southern French Alps, 44.85°N, 6.84°E). This study site is critical in terms of natural hazards because at least six Glacier Lake Outburst Floods were recorded during the 20^th and 21^th centuries, likely related to the permafrost degradation, the presence of a thermokarstic lake and an englacial conduit within the ice.

Complex spatio-temporal patterns and functional interactions between different landforms were evidenced. In the upper part of the valley, a small debris-free glacier turns downvalley into a debris-covered glacier occupying most of the central part of the valley. Further downslope, a rock glacier developed. The contrasting developments and landform responses are documented with multi-temporal DEMs and ortho-images. Very low thinning rates and surface velocities (< 0.5 m a^-1) were observed on the rock glacier, whereas the adjacent debris-covered glacier presents intermediate thickness losses (> 1 m a^-1) and higher surface velocities. However, the contact zone between the dead debris-covered and the rock glacier shows clear signs of mass down-wasting and complex interplay of phenomena such as thermokarst melting of massive ice and the flow towards the topographic depression.

An important speed-up of the horizontal displacements since the 1990s and an important surface lowering have most probably conditioned the dynamics of the observed outburst-floods. Those seem to be a complex combination of several processes affecting the different cryospheric elements. (i) the well-developed thermokarst lake over debris-covered area, on the topographic depression (i.e., bucket shape with barriers damming the lake) which is mainly controlled by the bedrock morphology and evolution of the surface topography. The current capacity of this depression has been estimated to 180,000 ± 350 m³. (ii) the specific glacio-geomorphological dynamics of debris-covered and the rock glacier units, which dynamics influence the opening/enlargement and closure of the englacial conduit. (iii) the hydro-meteorological conditions (e.g., enhanced snow melt in late spring) probably impacts the hydrology, water filling and the lake outflow.

The findings of this study highlight the relationships between glacial and periglacial features and their long-term evolution. The systemic study of GLOF formation processes would lead to a better identification of sites at risk and to the implementation of more robust prevention procedures in order to face the environmental and societal challenges of climate change.