Periglacial cascading process in an alpine environment: an example of an ice avalanche-induced debris flow in Ferret Valley (Courmayeur, Italy)

Abstract

On 23 June, 2022, a debris flow occurred in the Montitaz Stream (Mont Blanc area), which flows off the Planpincieux Glacier snout. It destroyed the bridge that links the hamlets of Planpincieux and Rochefort. Using a multi-source dataset from UAV, satellite and terrestrial sensors belonging to the Planpincieux Glacier monitoring network, we reconstructed the series of events that led to the debris flow. We found evidence that this resulted from a cascading process which started with an ice avalanche of 4200 m³ falling over a reformed glacieret that lies 500 m downstream from the glacier front. Subsequently, the deposited ice avalanche formed an unstable ice dam along the Montitaz Stream riverbed, causing a mixed accumulation of water and ice debris. Finally, the dam collapsed, originating a debris flow consisting of ice, water and debris from the glacial fan. DEM differencing showed that approximately 14,000 m³ of material were mobilised overall.