Geodaysit 2023

Debris covered glaciers are common in many parts of the world and contribute to the hydrological cycle and freshwater availability in arid regions. In the Italian Alps, some of the largest debris covered glaciers are located in the Mont Blanc group and among them Brenva glacier (5.95 km2 in the latest glacier inventory, Paul et al. 2020) reaches the lowest terminus elevation on the southern side of the Alps at 1415 m a.s.l.. The debris supply originated from several rockfall events throughout the Holocene, with the most recent ones in 1920s and in 1997. In 2004, the ice flow was interrupted from the icefall to the glacier tongue, and this led to enhanced ice stagnation and mass wasting. To investigate the recent evolution of the glacier tongue, we carried out two UAV surveys in 2019 and 2020, using a DJI Mavic and DJI Phantom 4 RTK drones. During the first survey, ground control points were used to increase the accuracy of the final products, while during the second survey we relied on RTK corrections to improve geolocation. The acquired images were processed using a structure from motion pipeline and yielded high resolution orthomosaics and DEMs. By comparing the DEMs from the two photogrammetric surveys, we were able to describe the rapid thinning of the ice tongue, which lost more than 40 m over one year only. Downwasting of the ice was faboured by the formation of epiglacial lakes, which enhance melt. By generating DEMs and orthomosaics from aerial data, we reconstructed the recent history of the glacier, showing an initial phase of mass transfer from the rockfall and the subsequent melt out of the ice tongue.