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Previously completed projects

30.10.2009 | 09:55

Tunnel under Engabreen, 2006. Photograph: Hellgeir Elvehøy.


Glacier bed deformation: Iowa State University
Simulated till was installed in a trough at the bed of Engabreen.  The till contained instruments that recorded shear deformation (tiltmeters), dilation and contraction, total normal stress and pore-water pressure. Shear strain generally increases upward in the bed toward the glacier sole, consistent with previous measurements beneath thinner ice near glacier margins.  Results showed that at low effective normal stresses, ice sometimes decouples from underlying till, presumably resulting in slip over the bed.  Overall, bed deformation accounted for 10-35% of basal motion, although this range excludes shear in the uppermost 0.05 m of till, which was not measured but was probably significant. 

Glacier sliding: Iowa State University
A smooth, instrumented rock bed was installed at the base of the glacier.  Results showed that ice containing 2-11% debris exerted shear traction of 60-200 kPa on a smooth rock bed, comparable to the total shear traction beneath glaciers and contrary to the usual assumption that debris-bed friction is negligible. Imposed pore-water pressure that was 60-100% of the normal stress in a subglacial debris layer reduced shear traction on the debris sufficiently to halt its deformation and cause slip of ice over the debris. Flow resistance was thus less than debris shearing resistance. 

Ice flow over obstacles: Iowa State University
An instrumented obstacle (cone-shaped bump) was installed flush with the bed surface and measured sliding speed, temperatures, normal stresses and the shear stress on the obstacle. Through 3-D finite element modelling of ice flow past the obstacle, it was possible to determine the viscosity parameter in the flow law. As expected, it was smaller than published values for clean ice. Temperature-perturbation experiments aimed at determining the water content of the basal ice were also performed.

Basal ice mapping: NVE and University of Stockholm
The stratigraphy of the lowermost 2 m (1% of total ice thickness) of ice was studied. Sediment content in the layers varies from 0 - 25 %. Cation and isotopic analyses were also performed. These analyses and the presence of water pockets in the basal ice suggest that there is significant water movement therein.

Chemistry of subglacial waters: University of Aberystwyth
Subglacial waters draining into the tunnel system (as well as at other locations) have been sampled as part of an investigation into the spatial variation in meltwater chemistry. This can give information about subglacial drainage system characteristics and how they vary both spatially and temporally. Also, by sampling in the tunnel system it is possible to investigate the meltwater quality characteristics in the more hydrologically isolated parts of the drainage system. Chemical and isotopic (d 18O and d D) analysis of a two metre long basal ice core was performed to investigate the mechanisms by which different ice facies at Engabreen have been formed, and possible relationships with subglacial water quality. Proglacial meltwater was also sampled as part of this study.

Ice fabric studies: University of Minnesota
Fabric studies were performed of the basal ice. Analysis of fabric in debris-laden ice is rare because of difficulties in preparing the thin sections. With increasing proximity to the glacier bed, sediment density increased and the fabric weakened from a distinct three-maxima fabric to a random fabric. It is thought that near the glacier bed where the ice flows past obstacles, the state of stress is constantly changing so new grains nucleate in widely different orientations, hence the lack of a pronounced fabric.

Subglacial hydrology: NVE
Tracing experiments were performed to study the subglacial hydrology. By injecting dyes into the glacier at different points and monitoring the return at appropriate output sites it is possible to delineate the catchment areas. Two of these sampling sites were within the tunnel system. From these experiments it was possible to determine the extent of the catchment areas upstream of the two subglacial intakes functioning beneath Engabreen during that time, and to characterise the connections between the tracer injection points and the intakes.

Subglacial pressure variations: University of Oslo
The correlation between the pressure on load cells at the glacier bed and the drainage system of Engabreen throughout the year was explored. Load cell signals may reflect the pressure contribution of any combination of basal ice, water, or sediment particles. In general, most pressure events occur as sharp, short-lived peaks, with complicated phase relations between the various load cells that depend on the placement of the sensor and on the time of year. In one experiment there was a positive correlation between surface velocities and the time rate of change of proglacial water discharge, and a negative correlation between surface velocities and the load cell pressures. Apparently, the load cells were sensing water pressures in isolated cavities. At other times, however, timing and amplitudes of pressure peaks as well as steady, significant discharge from a nearby tunnel borehole, indicate more direct contact with the subglacial drainage system. 

Subglacial sediment studies: NVE and University of Oslo
The sediment-loaded subglacial waters were automatically sampled. Sediment yield and other measurements have been performed and analysed. The accumulated sediment in the sedimentation reservoir is flushed annually. This event was used as a study of the effect of a controlled discharge pulse on the subglacial drainage system and ice flow.