Sliding of ice past an obstacle at Engabreen, Norway

Denis Cohen,Jack Kohler,Neal R Iverson,Roger Leb Hooke

doi:10.3189/172756500781832747

Denis Cohen, Jack Kohler + Show 2 more

Open Access

https://doi.org/10.3189/172756500781832747

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Abstract

AbstractAt Engabreen, Norway, an instrumented panel containing a decimetric obstacle was mounted flush with the bed surface beneath 210 m of ice. Simultaneous measurements of normal and shear stresses, ice velocity and temperature were obtained as dirty basal ice flowed past the obstacle. Our measurements were broadly consistent with ice thickness, flow conditions and bedrock topography near the site of the experiment. Ice speed 0.45 m above the bed was about 130 mm d−1, much less than the surface velocity of 800 mm d−1. Average normal stress on the panel was 1.0–1.6 MPa, smaller than the expected ice overburden pressure. Normal stress was larger and temperature was lower on the stoss side than on the lee side, in accord with flow dynamics and equilibrium thermodynamics. Annual differences in normal stresses were correlated with changes in sliding speed and ice-flow direction. These temporal variations may have been caused by changes in ice rheology associated with changes in sediment concentration, water content or both. Temperature and normal stress were generally correlated, except when clasts presumably collided with the panel. Temperature gradients in the obstacle indicated that regelation was negligible, consistent with the obstacle size. Melt rate was about 10% of the sliding speed. Despite high sliding speed, no significant ice/bed separation was observed in the lee of the obstacle. Frictional forces between sediment particles in the ice and the panel, estimated from Hallet’s (1981) model, indicated that friction accounted for about 5% of the measured bed-parallel force. This value is uncertain, as friction theories are largely untested. Some of these findings agree with sliding theories, others do not.

Highlights

Conditions at the glacier bed are of fundamental concern to glaciologists
Simultaneous measurements of sliding speed, normal and shear stresses and temperature were obtained as dirty basal ice flowed past an instrumented obstacle installed flush with the bed beneath 210 m of ice at Engabreen
The ice speed at the ice/bed interface measured with video cameras was 70 mm d^1.This is smaller than the sliding speed recorded higher in the ice, because of the presence of the conical obstacle, which effectively slowed the ice approaching it

Summary

INTRODUCTION

Conditions at the glacier bed are of fundamental concern to glaciologists. The motion of temperate glaciers is strongly influenced by basal sliding and by deformation of ice near the bed. An instrumented panel containing a conical obstacle was mounted flush with the bed surface beneath 210 m of ice. Sliding speed, normal and shear stresses, and temperature were measured simultaneously over a period of several days in April 1996, and again in November 1997, in order to estimate the rheology of the basal ice layer. Experiments done in a laboratory cold room indicated that, at the expected load and temperature, the coefficient of friction between the pads and the panel was 0.04 Æ 0.02 for uncontaminated lubricant, and 0.10 Æ 0.05 for lubricant that was contaminated with a small amount of grit The latter value is used in subsequent calculations because, during the field experiments, water containing fine sediment melted from the basal ice above the shaft and dripped onto the polished plate. The speed measured at the ice/bed interface in experiment 1 was even smaller (70 mm d^1), owing to the presence of the obstacle

DISCUSSION

Findings

P trD2 is the effective strain rate and

CONCLUSIONS