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The Cryosphere An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/tc-2019-243
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2019-243
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 08 Nov 2019

Submitted as: research article | 08 Nov 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal The Cryosphere (TC).

Seasonal and Diurnal Dynamics of Subglacial Channels: Observations Beneath an Alpine Glacier

Ugo Nanni1, Florent Gimbert1, Christian Vincent1, Dominik Gräff2, Fabian Walter2, Luc Piard1, and Luc Moreau3 Ugo Nanni et al.
  • 1University of Grenoble Alpes, CNRS, IRD, IGE, Grenoble, France
  • 2Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zürich, Zürich, Switzerland
  • 3Edytem, CNRS, Université de Savoie, Chambéry, France

Abstract. Water flowing below glaciers exerts a major control on glacier basal sliding speeds. However, our knowledge on the physics of subglacial hydrology and its link with sliding is limited by lacking observations. Here we use a two-year long dataset made of on-ice measured seismic and in-situ measured glacier basal sliding speed records on the Glacier d’Argentière (French Alps) to investigate the physics of subglacial channels and its potential link with glacier basal sliding. Using dedicated theory and concomitant measurements of water discharge, we quantify temporal changes in channels hydraulic radius and hydraulic pressure gradient. At seasonal timescales we observe, for the first time, that hydraulic radius and hydraulic pressure gradient present a four-fold increase from spring to summer, followed by a comparable decrease towards autumn. At low discharge during the early and late melt season channels respond to changes in discharge mainly through changes in hydraulic radius, a regime that is consistent with predictions of channels behaving at equilibrium. In contrast, at high discharge and high short-term water-supply variability (summertime), channels undergo strong changes in hydraulic pressure gradient, a behavior that is consistent with channels being out-of-equilibrium. This out-of-equilibrium regime is further supported by observations at the diurnal scale, which demonstrate that channels pressurize in the morning and depressurize in the afternoon. During summer we also observe high and sustained basal sliding speeds, supporting that the widespread inefficient drainage system (cavities) is likely pressurized concomitantly with the channel-system. We propose that pressurized channels help sustain high pressure in cavities (and therefore high glacier sliding speeds) through an efficient hydraulic connection between the two systems. Using the two regimes herein observed in channels seasonal-dynamics as constraints for subglacial hydrology/ice dynamics models may allow to strengthen our knowledge on the physics of subglacial processes.

Ugo Nanni et al.
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Short summary
Our study tackles the current lack of observations of the subglacial hydrology components. Here we provide unique two-year long continuous measurements of seismic power induced by subglacial water-flow together with in-situ measured glacier basal sliding speeds. Using these observations, we directly invert for key subglacial channels properties and investigate the links between channels, cavities and basal slip from seasonal to diurnal timescales.
Our study tackles the current lack of observations of the subglacial hydrology components. Here...
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