Objectives

Glacier dynamics governs the advection of ice towards warmer areas with faster melt, and thus exerts a key control on glacier mass loss and associated sea level rise under a warming climate. Glacier dynamics is set partly by internal ice deformation, a relatively well-known and constrained component, and partly by sliding at the ice-bedrock interface, which comparatively remains much more poorly known, mostly because of missing observations. Our lack of knowledge on the physics of glacier basal sliding is such that we are unable to discriminate which physical law to use in large scale models, which causes large uncertainties in predictions of glacier retreat and sea level rise. Although several theoretical frameworks have been proposed over the past 60 years to describe basal sliding and its potential links with hydrology, so far very few studies have allowed measuring all the relevant physical parameters at the natural scale, and, to our knowledge, none of them was able to validate existing or propose new physical laws based on their field observations. In the SAUSSURE project, our objective is to evaluate, improve and validate various friction laws in a natural, geophysical scale configuration.

In order to reach this goal, we will proceed in two steps. First, we will elaborate an advanced field measurement methodology that will enable us to overcome traditional observational difficulties and measure, simultaneously and at relevant spatio-temporal scales, the key physical variables of interest. This dataset newly acquired at high temporal and spatial resolution is ideally completed by a unique multi-decade-long dataset at lower resolution but covering a time period of large changes in basal conditions (WP1). Second, we will elaborate a thorough theoretical and modelling strategy to quantitatively test existing or propose new basal friction laws based on our findings. The control of each physical process and parameters on basal sliding will be evaluated at multiple temporal scales (seconds to decades). First we will investigate the meso spatial scale (meters to tens of meters spatially, WP2) at which friction laws are defined. Then, the friction laws will be implemented at a larger glacier scale (hundreds of meters to few kilometres spatially) using large scale observational constraints on subglacial hydrology and long term observational constraints on glacier dynamics and mass balance (WP3).

The aforementioned strategy will be conducted on the Argentière Glacier (Mont Blanc, France, see the interactive map), which offers two essential advantages that are crucial to our objective. First, key components of glacier characteristics (surface mass balance, basal sliding velocities, subglacial discharge, glacier thickness, glacier surface velocities and glacier extent) have been measured for most of them at both the meso-scale and the glacier scales and continuously over decades. Over this time-period, the Argentière Glacier has experienced significant thinning, and thus significant changes in basal stress conditions that will be used to fully test basal friction laws at multiple spatio-temporal scales. Second, the Argentière Glacier offers unique instrumentation possibilities thanks to being easily accessible and thanks to subglacial tunnels (drilled in bedrock) offering direct access to the glacier base. Sliding velocities can thus be measured in-situ and at the very high temporal resolution of the meso-scale and other measurements such as seismic and water pressure can be conducted from both above and below the glacier, thus ensuring the observation of processes of interest with unique spatio-temporal coverage allowing to link the meso-scale with the glacier scale.

Although our scientific findings obtained for a hard bedrock of an Alpine glacier will by construction not be generic, and thus not necessarily directly applicable to all glaciers around the world, they will still demonstrate, we believe for the first time, whether theoretical descriptions generally applied to a wide range of glaciers worldwide hold or not in a real setting. We will provide evidences as to why and in which context certain types of glacier basal friction theories and formulations hold or not, giving the scientific community new clues for application of friction laws on glaciers where less observational constraints are available. Finally, our expertise gained in this project on the Argentière Glacier will allow us, in future projects, to export our innovative field measurement methods to other glaciers.

Work Package WP1

In this work package, we will acquire during two field campaigns a new dataset with high temporal and spatial resolution that will ideally complement the already existing decades-long dataset covering large changes in basal shear stress conditions.

We will combine classical measurement techniques such as DGPS positioning, ground penetrating RADAR and pressure measurements in boreholes with more innovative ones that offer continuous (often multi-year), monitoring at particularly high temporal resolutions (down to the minute to the hour) and over a wide range of spatial scales (from metric up to glacier scale). All these measurements are presented here.

Work Package WP2

The objective of this work package is to incorporate relevant physical processes not yet accounted for in the existing friction laws by a conjugate use of meso-scale finite-element modelling and physical analysis of the Argentière Glacier datasets. Simple to moderately complex theoretical and/or numerical models will be designed to investigate separately the role of each relevant physical process of interest, such as mainly the effects of non-steady water pressure, of real bed roughness configurations and of solid-like friction at the ice-to-bed interface. In each of these experiments the prescribed parameters such as water pressure changes, bed roughness characteristics and solid-like friction patches will be scaled based on those observed on the Argentière Glacier. Ultimately, model predictions will be compared with sliding measurements made at a similar scale in the subglacial observatory. Predictions will be compared to measurements at both short time scales, during periods where a highly fluctuating discharge, pressure and/or particularly high stick slip activity are observed, and long timescales at which large changes in basal shear stress conditions have occurred over the past decades.

Work Package WP3

The objective of WP3 is to construct a modelling framework coupling ice flow and basal hydrology in order to test and quantify the effect already existing and newly inferred (see WP2) friction laws have on glacier scale dynamics. To avoid imposing complex and unphysical boundary conditions the entire Argentière Glacier (in average about 5 km long by 2 km wide) will be modelled, although only the instrumented area (about 1 by 1 km long) will have a very refined mesh allowing proper representation of the involved processes. All acquired and analysed data in WP1 will be used to construct the model initial state, to force the transient simulation and evaluate results. In complement of this new, high spatial and temporal resolutions dataset acquired during the two field campaigns of the SAUSSURE project, we will benefit from a fully complementary, already existing, multi-decadal long dataset, describing high changes in basal shear stress conditions due to the large glacier evolution over that period. All this modelling will be conducted using the finite-element model Elmer/Ice.