Investigating the slope failures at the Lou rock glacier front, French Alps

Abstract

On August 14 th 2015 a large debris flow initiated by the occurrence of two slope failures at the front of the Lou rock glacier flooded part of the town of Lanslevillard, France. The present study aims to understand the meteorological and geomorphological context that led to these failures. Investigations were conducted by combining meteorological data, surface movements and geophysical transects. The analysis indicates that the Lou rock glacier is directly connected to an active torrential channel and has a natural predisposition to frontal failure due to the steepness of its front. The slope failures were triggered after a heat wave followed by a three-week period of almost continuous rainfall. Water flowing on top of the permafrost table was observed in the two head scarps, suggesting that regressive erosion consecutive to this concentrated subsurface water flow triggered the failures.For one of the slides, traces of previous failures were observable on historical aerial imagery dating back to the 1950's, while the second slide corresponded to a novel event and developed on the frontal slope of a fast moving and destabilized rock glacier lobe. We also discuss the increase in local predisposition to failure related to the remarkable morphological modifications such as frontal advance and development of surface cracks associated with the lobe destabilization.

A general theory of rock glacier creep based on in‐situ and remote sensing observations

“…The deformation of the active layer is mostly linked to tilting or sliding of the boulders on top of the permafrost table. Rare events of active layer detachments have been observed at rock glacier sites, characterised by extreme rates of displacement due to sliding 36,37 . In some cases, boulders can move very rapidly for a short time also in relation to tilting.…”

Debris flows originating in the mountain cryosphere under a changing climate: A review

“…Moreover, a positive feedback process may exist for mountain permafrost slope deformation in response to atmospheric and ground warming; that is, the potential infiltration of water promotes slope deformation, which in turn enhances infiltration of water and further increases frozen debris deformation (Ikeda et al, 2008), though the increased frozen debris deformation does not always mean more instabilities and more hazards, which in general depends mainly on specific topographical and geomorphological predispositions. High concentrations of water above the permafrost table, which originate mainly from precipitation and/or ice/snowmelt, may even result in active-layer failures as well as subsequent debris flows or other mass flows (Kääb et al, 2005a, 2005b; Kummert et al, 2018; Lugon and Stoffel, 2010; Marcer et al, 2020; Stoffel et al, 2011).…”

“…rapid topographic changes and high creeping rates), which often form a natural predisposition toward a possible rock glacier destabilization or collapse, and act as preparatory factors (storage of morainic debris, forming of steep front) for the development of DFMC under extrinsic triggering disturbances (e.g. heatwave and/or heavy rainfall) (e.g, Kummert et al, 2018; Marcer et al, 2019, 2020; Vivero and Lambiel, 2019).…”

“…In general, two potential consequences may be induced from accelerated slope processes (or slope failure) in a changing climate: (a) it promotes the formation, accumulation, and export of loose slope debris (e.g. Frank et al, 2019; Keiler et al, 2010; Lane et al, 2017; PERMOS, 2013, 2016; Tie, 2012), or develops an instability-prone slope with a steep front (e.g., Kummert et al, 2018; Marcer et al, 2020), which acts as a preparatory factor for the initiation of debris flows under suitable conditions (intense rainfall or high temperature); or (b) in the case of large-scale slope failure (e.g. rock avalanche, landslide) and necessary conditions (sufficient soil water content, pore fluid pressure, or entrainment of ice/snow and sediment), the slope failure is prone to transform directly into debris flows (c.f.…”

Consensus-Based Rock Glacier Inventorying in the Torngat Mountains, Northern Labrador

“…Rock glaciers are key indicators of contemporary and past permafrost conditions (Humlum 1998) and are amongst the most recognizable permafrost landforms in mountain environments (Haeberli et al 2006). Recent studies show that rock glaciers exhibit complicated kinematic responses to climate change (Eriksen et al 2018;Scotti et al 2017;Sorg et al 2015) but can be associated with geocryological hazards (Marcer et al 2020) and can distribute large quantities of freshwater into local ecosystems (Jones et al 2019b). Further, rock glaciers are of significant ecological importance as they provide long-term cold habitat for local biodiversity (Brighenti et al 2020).…”