Gaute Lappegard

Soft-bed experiments beneath Engabreen, Norway: Regelation, infiltration, basal slip and bed deformation

To avoid some of the limitations of studying soft-bed processes through boreholes, a prism of simulated till (1.8 m � 1.6 m � 0.45 m) with extensive instrumentation was constructed in a trough blasted in the rock bed of Engabreen, a temperate glacier in Norway. Tunnels there provide access to the bed beneath 213 m of ice. Pore-water pressure was regulated in the prism by pumping water to it. During experiments lasting 7-12 days, the glacier regelated downward into the prism to depths of 50- 80 mm, accreting ice-infiltrated till at rates predicted by theory. During periods of sustained high pore- water pressure (70-100% of overburden), ice commonly slipped over the prism, due to a water layer at the prism surface. Deformation of the prism was activated when this layer thinned to a sub-millimeter thickness. Shear strain in the till was pervasive and decreased with depth. A model of slip by ploughing of ice-infiltrated till across the prism surface accounts for the slip that occurred when effective pressure was sufficiently low or high. Slip at low effective pressures resulted from water-layer thickening that increased non-linearly with decreasing effective pressure. If sufficiently widespread, such slip over soft glacier beds, which involves no viscous deformation resistance, may instigate abrupt increases in glacier velocity.

Characteristics of subglacial drainage systems deduced from load-cell measurements

Rock tunnels beneath Engabreen, northern Norway, permit access to the ice-bedrock interface beneath a 210 m thick glacier. Eight load cells have been installed in the bedrock of the glacier sole along a 22 m transect. With some interruptions, the load cells have been logged at 15 min intervals since December 1992; here we analyse the records until 2003. Load-cell signals measure stresses acting normal to the bedrock, and usually log the pressure of the thin water film between the basal ice and the bed. Occasionally there are distinct pressure events, characterized by short-lived (hours) local minima, often followed by a maximum before decaying to background load-cell pressure. The amplitudes of these pressure events are of the order of 0.01-1 MPa and depend on the placement of the sensor and the state of the subglacial drainage system. We identify winter and summer pressure regimes. The winter regime is characterized by few pressure events of large pressure amplitude influencing all load cells. A lag of 0-6 days is observed between surface forcing (rain) and pressure events during winter. The summer regime typically has periods of daily pressure events of low amplitude. No delay is seen between surface forcing and pressure events during the summer regime. In summer, the onset of a pressure event is correlated with a local maximum of the derivative of the subglacial discharge record, whereas no such relation is found during the winter regime. The transition from winter to summer (May/June) is easily detectable and is strongly correlated with a rapid increase in subglacial discharge and the transition to a dominating R-channel system. The autumn transition is less clearly defined, but has usually occurred by the beginning of November. Stress bridging, an increase in bed pressure at the edge of low-pressure channels, is recorded during the summer regime. Water pressures at the bed are connected or unconnected to the drainage system. Pressure increases in the connected system, beyond local normal stress values, lead to an uplift of the connected system and a pressure drop in the unconnected system. The occurrence of pressure events is determined by the capacity of the drainage system. Uplift is controlled by local normal stress values and not mean ice-overburden pressure.

Interannual variability of glacier basal pressure from a 20 year record

Abstract Basal pressure has been recorded at the Svartisen Subglacial Laboratory, northern Norway, for 20 years, and is measured by load cells installed at the ice–rock interface under ~200m of glacier ice. Synchronous pressure variations between load cells are investigated as evidence of stress redistribution and hydrological bed connectivity. A running Pearson correlation is used to study the temporal variation in the response of several sensors. By studying the nature of this correlation as well as the correlation between sensor pairs, it is possible to investigate the evolution of the degree of synchronous response, and to some extent basal connectivity, at the glacier bed. Persistent seasonal variations associated with the melt season are observed throughout the measurement period, indicating dependence on surface hydrological forcing. Overlying this pattern, specific years with longer periods of positive and negative correlation of pressure between sensors are presented to show contrasting interannual variability in basal pressure. An anticorrelated connectivity is associated with a local increase in the rate of daily subglacial discharge, and is caused by load transfer or passive cavity opening. Stable weather appears to enhance connectivity of the sensors, which is attributed to the development of a persistent drainage system and stress redistribution.

Ground-water intrusions in a mine beneath HÖ ganesbreen, Svalbard: assessing the possibility of evacuating water subglacially

Abstract Evacuation of the ground-water intruding into a coal mine beneath Høganesbreen, Svalbard, is difficult and expensive. To solve this problem, it was proposed that the mine be connected to the ice–bedrock interface. Pumping hot water from the mine should establish a flow path along the glacier bed where the ground-water would drain gravitationally. In this paper, we assess the requirements for maintaining such a drainage system in open-channel conditions. To obtain the bedrock topography, we determined the ice thickness by ground-penetrating radar and subtracted it from the surface elevation measured by global positioning system. A measured temperature profile at the site where the mine should connect to the glacier bed (140m depth) revealed that the basal ice is below the pressure-melting point. The locations of major subglacial conduits were estimated using a hydraulic-potential approach. We adopted a model oftime-dependent discharge through a Röthlisberger channel to calculate a set of scenarios using different flow-law parameters. Results of the simulations suggest that for the given conditions, water flow would be pressurized, thereby inhibiting the gravitational drainage of the mine.

Determination of basal hydraulic systems based on subglacial high-pressure pump experiments

Abstract We have conducted short-term pump experiments with pump pressures exceeding ice overburden to study the seasonality of the subglacial hydraulic system of Engabreen, Norway. Data were collected from load cells installed flush with the ice–bedrock interface and pressure transducers installed in boreholes leading from bedrock tunnels underneath the glacier to the ice–bedrock interface. The water-pressure recordings, seen in relation with the load-cell record, show the existence of hydraulically connected vs unconnected bed areas. Monitored boreholes have been used to inject water at high pressures. Each experiment led to the growth of a high-pressure water cavity whose spatial extent could be inferred from load-cell and pressure transducer records. Post-pump pressures were low after summer pump tests and close to ice-overburden level after winter pump experiments. We conclude that drainage takes place in a fast-flow, low-pressure, channel-based drainage system during summer, and a low-flow, high-pressure, linked-cavity drainage system during winter.

Stress Redistribution Explains Anti-correlated Subglacial Pressure Variations

We used a finite element model to interpret anti-correlated pressure variations at the base of a glacier to show the importance of stress redistribution in the basal ice. Two pairs of load cells are installed 20 m apart at the base of the 210 m thick Engabreen glacier in Northern Norway. Pressurisation of a subglacial channel located over one pair led to anti-correlation in pressure between them. A full Stokes 3D model of a 210 m thick and 25-200 m wide glacier with a pressurised subglacial channel represented as a pressure boundary condition was used to investigate the anti-correlated response at the bed. The model reproduced the anti-correlated pressure response at the glacier bed and variations in pressure of the same order of magnitude as the load cell observations. The anti-correlation pattern was shown to depend on the bed/surface slope such that the anti-correlated pressure variations were reproduced at a distance greater than 10-20 m from the channel when the bed slope was zero, whereas anti-correlation occurred within 10 m of the channel when the bed was inclined by 5 degrees. Pressurisation of the channel led to lateral or vertical ice flow away from the channel, support of the overlying ice and reduction of the normal stress on the bed. If the modelled cross-section was laterally constrained and the bed flat, the resulting bridging effect diverted some of the normal forces acting on the bed to the sides. In contrast, if the bed was inclined, then channel support was vertical only. The model showed that the effect of stress redistribution depends on the slope as well as the geometry of the subglacial channel and glacier, and can lead to an opposite response in pressure at the same distance from the channel.