Abstract
Glacial and paraglacial processes have a major influence on rock slope stability in alpine environments. Slope deglaciation causes debuttressing, stress and hydro-mechanical perturbations that promote progressive slope failure and the development of slow rock slope deformation possibly evolving until catastrophic failure. Paraglacial rock slope failures can develop soon after or thousands of years after deglaciation, and can creep slowly accelerating until catastrophic failure or nucleate sudden rockslides. The roles of topography, rock properties and deglaciation processes in promoting the different styles of paraglacial rock slope failure are still elusive. Nevertheless, their comprehensive understanding is crucial to manage future geohazards in modern paraglacial settings affected by ongoing climate change. We simulate the different modes and timing of paraglacial slope failures in an integrated numerical modelling approach that couples realistic deglaciation histories derived by modelling of ice dynamics to 2D time-dependent simulations of progressive failure processes. We performed a parametric study to assess the effects of initial ice thickness, deglaciation rate, rock-slope strength and valley shape on the mechanisms and timing of slope response to deglaciation. Our results allow constraining the range of conditions in which rapid failures or delayed slow deformations occur, which we compare to natural Alpine case studies. The melting of thicker glaciers is linked to shallower rockslides daylighting at higher elevation, with a shorter response time. More pronounced glacial morphologies influences slope lifecycle and favour the development of shallower, suspended rockslides. Weaker slopes and faster deglaciations produce to faster slope responses. In a risk-reduction perspective, we expect rockslide differentiation in valleys showing a strong glacial imprint, buried below thick ice sheets during glaciation.
Highlights
Different types of rock slope instabilities, including both slow rock slope deformations and catastrophic failures, are commonly observed in formerly glaciated, alpine environments
Field evidence, monitoring and modelling studies suggest that the stability of large rock slopes in alpine environments is strongly controlled by their glacial history, including the extent and timing of glaciation, deglaciation and paraglacial landscape adjustment (Church and Ryder 1972; Ballantyne 2002; Ambrosi and Crosta 2006; Ballantyne et al 2014)
We systematically investigate the relationships among valley morphology, rock properties and deglaciation processes and their impact on the mode and timing of paraglacial large rock slope instability
Summary
Different types of rock slope instabilities, including both slow rock slope deformations and catastrophic failures, are commonly observed in formerly glaciated, alpine environments. Glacial erosion leads to valley steepening, deepening and undercutting (Penck 1905; Herman et al 2011; Sternai et al 2013) These can have critical effects on rock slope stability by causing stress-redistribution within the slope (Augustinus 1995) and exposing persistent, potentially weak stratigraphic boundaries (Agliardi et al 2019) or inherited persistent fractures, shear zones or pre-damaged slope sectors, which may promote slope failure by structural controls. Rock damage accumulation during and after deglaciation results in progressive slope failure, mirrored by time-dependent displacements (slope creep; Emery 1978) and leading to the differentiation of rockslides increasingly sensitive to external factors These include ice mass distribution and varying hydrogeological conditions, which greatly change across glacial, paraglacial and postglacial conditions, as well as rainfall and permafrost degradation (Church and Ryder 1972; Ballantyne 2002; Grämiger et al 2017; Riva et al 2018)
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