Abstract
Abstract. The processes leading to a glacier instability depend on the thermal properties of the contact between the glacier and its bedrock. Assessing the stability of temperate glacier (i.e. the glacier can slide on its bedrock) remains problematic. In order to scrutinize in more detail the processes governing such "sliding" instabilities, a numerical model designed to investigate gravitational instabilities in heterogeneous media was further developed to account for the presence of water at the interface between the bedrock and the glacier for Allalingletscher. This model made it possible to account for various geometric configurations, interaction between sliding and tension cracking and water flow at the bedrock. We could show that both a critical geometrical configuration of the glacier tongue and the existence of a distributed subglacial drainage network were the main causes of the Allalingletscher catastrophic break-off. Moreover, the analysis of the modelling results diagnosed the phenomenon of recoupling of the glacier to its bed followed by a pulse of subglacial water flow as a potential new precursory sign of the final break-off in 1965. This model casts a gleam of hope for a better understanding of the ultimate rupture process resulting from such glacier sliding instabilities.
Highlights
Gravity-driven instabilities include landslides, mountain collapse, rockfalls, ice mass break-off and snow avalanches
At each time step of the run, the friction coefficient under each block is modified according to the local subglacial discharge. μ0 is evaluated with Eq (1) based on the runoff at the outlet given by the model of Farinotti et al (2011) and the matrix Zev calculated from the Digital elevation models (DEM) of the bedrock topography
Instabilities occurring on temperate glacier tongues are strongly affected by the subglacial hydrology: infiltrated meltwater may cause (i) a lubrication of the bed and (ii) a decrease in the effective pressure at the glacier bed and a decrease in basal friction
Summary
Gravity-driven instabilities include landslides, mountain collapse, rockfalls, ice mass break-off and snow avalanches. The accurate prediction of the occurrence of such phenomena remains a somewhat daunting task In this context, studying glacier break-off is of particular interest because a glacier consists of a unique natural material (ice) where the interface between ice and bedrock is well defined. The present paper is devoted to the study of such instabilities, taking the Allalingletscher as an example This glacier is of particular interest because it experienced 2 catastrophic break-offs (in 1965 and 2000) and because a unique set of data was collected since the first one. To address the open questions on the initiation of the instability, we apply a general numerical model developed to investigate gravity-driven instabilities in heterogeneous media (Faillettaz et al, 2010) This model was already applied successfully to a polythermal glacier becoming partly temperate at its bedrock, i.e. the Altelsgletscher (Faillettaz et al, 2011b).
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