scholarly journals Investigating changes in basal conditions of Variegated Glacier prior to and during its 1982–1983 surge

2011 ◽  
Vol 5 (3) ◽  
pp. 659-672 ◽  
Author(s):  
M. Jay-Allemand ◽  
F. Gillet-Chaulet ◽  
O. Gagliardini ◽  
M. Nodet
Keyword(s):  
Flow Line ◽  
Stokes Problem ◽  
Water Pressure ◽  
Drainage System ◽  
Surface Velocity ◽  
Parameter Distribution ◽  
Basal Friction ◽  
Friction Parameter ◽  
Basal Sliding ◽  
Regular Increase

Abstract. Variegated Glacier (Alaska) is known to surge periodically after a sufficient amount of cumulative mass balance is reached, but this observation is difficult to link with changes in the basal conditions. Here, using a 10-yr dataset, consisting of surface topography and surface velocity observations along a flow line for 25 dates, we have reconstructed the evolution of the basal conditions prior to and during the 1982–1983 surge. The model solves the full-Stokes problem along the central flow line using the finite element method. For the 25 dates of the dataset, the basal friction parameter distribution is inferred using the inverse method proposed by Arthern and Gudmundsson (2010). This method is here slightly modified by incorporating a regularisation term in the cost function to avoid short wavelength changes in the friction parameter. Our results indicate that dramatic changes in the basal conditions occurred between 1973 to 1983. Prior to the surge, periodic changes can be observed between winter and summer, with a regular increase of the sliding from 1973 to 1982. During the surge, the basal friction decreased dramatically and an area of very low friction moved from the upper part of the glacier to its terminus. Using a more complex friction law, these changes in basal sliding are then interpreted in terms of basal water pressure. Our results support that dramatic changes took place in the subglacial drainage system of Variegated Glacier, moving from a relatively efficient drainage system prior to the surge to an inefficient one during the surge. By reconstructing the water pressure evolution at the base of the glacier it is possible to propose a scenario for the hydrological history leading to the occurrence of a surge.

scholarly journals Investigating changes in basal conditions of Variegated Glacier prior and during its 1982–1983 surge

2011 ◽  
Vol 5 (3) ◽  
pp. 1461-1494
Author(s):  
M. Jay-Allemand ◽  
F. Gillet-Chaulet ◽  
O. Gagliardini ◽  
M. Nodet
Keyword(s):  
Flow Line ◽  
Stokes Problem ◽  
Water Pressure ◽  
Wave Length ◽  
Drainage System ◽  
Surface Velocity ◽  
Parameter Distribution ◽  
Basal Friction ◽  
Friction Parameter ◽  
Length Changes

Abstract. The Variegated Glacier (Alaska) is known to surge periodically after a sufficient amount of cumulative mass balance is reached, but this observation is difficult to link with changes in the basal conditions. Here, using a 10-year dataset, consisting in surface topography and surface velocity observations along a flow line for 25 dates, we have reconstructed the evolution of the basal conditions prior and during the 1982–1983 surge. The model solves the full-Stokes problem along the central flow line using the finite element method. For the 25 dates of the dataset, the basal friction parameter distribution is inferred using the inverse method proposed by Arthern and Gudmundson (2010). This method is here slightly modified by incorporating a regularisation term in the cost function to avoid short wave length changes in the friction parameter. Our results indicate that dramatic changes in the basal conditions occurred between 1973 to 1983. Prior to the surge, periodical changes can be observed between winter and summer, with a regular increase of the sliding from 1973 to 1982. During the surge, the basal friction decreased dramatically and an area of very low friction moved from the upper part of the glacier to its terminus. Using a more complex friction law, these changes in basal sliding are then interpreted in terms of basal water pressure. It confirms that dramatic changes took place in the subglacial drainage system of Variegated Glacier, moving from a relatively efficient drainage system prior to the surge to an inefficient one during the surge. By reconstructing the water pressure evolution at the base of the glacier it is possible to infer realistic scenarios for the hydrological history leading to the occurrence of a surge.

scholarly journals Combined measurements of Subglacial Water Pressure and Surface Velocity of Findelengletscher, Switzerland: Conclusions about Drainage System and Sliding Mechanism

1986 ◽  
Vol 32 (110) ◽  
pp. 101-119 ◽  
Author(s):  
Almut Iken ◽  
Robert A. Bindschadler
Keyword(s):  
Functional Relationship ◽  
Water Pressure ◽  
Drainage System ◽  
High Water ◽  
Water Levels ◽  
Surface Velocity ◽  
Glacier Surface ◽  
Measured Velocity ◽  
Sliding Mechanism ◽  
Velocity Variations

AbstractDuring the snow-melt season of 1982, basal water pressure was recorded in 11 bore holes communicating with the subglacial drainage system. In most of these holes the water levels were at approximately the same depth (around 70 m below surface). The large variations of water pressure, such as diurnal variations, were usually similar at different locations and in phase. In two instances of exceptionally high water pressure, however, systematic phase shifts were observed; a wave of high pressure travelled down-glacier with a velocity of approximately 100 m/h.The glacier-surface velocity was measured at four lines of stakes several times daily. The velocity variations correlated with variations in subglacial water pressure. The functional relationship of water pressure and velocity suggests that fluctuating bed separation was responsible for the velocity variations. The empirical functional relationship is compared to that of sliding over a perfectly lubricated sinusoidal bed. On the basis of the measured velocity-pressure relationship, this model predicts a reasonable value of bed roughness but too high a sliding velocity and unstable sliding at too low a water pressure. The main reason for this disagreement is probably the neglect of friction from debris in the sliding model.The measured water pressure was considerably higher than that predicted by the theory of steady flow through straight cylindrical channels near the glacier bed. Possible reasons are considered. The very large disagreement between measured and predicted pressure suggests that no straight cylindrical channels may have existed.

scholarly journals Basal Sliding and Conditions at the Glacier Bed as Revealed by Bore-hole Photography

1978 ◽  
Vol 20 (84) ◽  
pp. 469-508 ◽  
Author(s):  
H. F. Engelhardt ◽  
W. D. Harrison ◽  
Barclay Kamb
Keyword(s):  
Flow Direction ◽  
Water Pressure ◽  
Surface Flow ◽  
Water Levels ◽  
Surface Velocity ◽  
Internal Motions ◽  
Basal Sliding ◽  
Bed Roughness ◽  
Atmospheric Factors ◽  
Basal Shear

AbstractBore-hole photography demonstrates that the glacier bed was reached by cable-tool drilling in five bore holes in Blue Glacier, Washington. Basal sliding velocities measured by bore-hole photography, and confirmed by inclinometry, range from 0.3 to 3.0 cm/d and average 1.0 cm/d, much less than half the surface velocity of 15 cm/d. Sliding directions deviate up to 30° from the surface flow direction. Marked lateral and time variations in sliding velocity occur. The glacier bed consists of bedrock overlain by a ≈ 10 cm layer ofactive subsole drift, which intervenes between bedrock and ice sole and is actively involved in the sliding process. It forms a mechanically and visibly distinct layer, partially to completely ice-free, beneath the zone of debris-laden ice at the base of the glacier. Internal motions in the subsole drift include rolling of clasts caught between bedrock and moving ice. The largest sliding velocities occur in places where a basal gap, of width up to a few centimeters, intervenes between ice sole and subsole drift. The gap may result from ice—bed separation due to pressurization of the bed by bore-hole water. Water levels in bore holes reaching the bed drop to the bottom when good hydraulic connection is established with sub-glacial conduits; the water pressure in the conduits is essentially atmospheric. Factors responsible for the generally low sliding velocities are high bed roughness due to subsole drift, partial support of basal shear stress by rock friction, and minimal basal cavitation because of low water pressure in subglacial conduits. The observed basal conditions do not closely correspond to those assumed in existing theories of sliding.

scholarly journals The impact of jökulhlaups on basal sliding observed by SAR interferometry on Vatnajökull, Iceland

2007 ◽  
Vol 53 (181) ◽  
pp. 232-240 ◽  
Author(s):  
Eyjólfur Magnússon ◽  
Helmut Rott ◽  
Helgi Björnsson ◽  
Finnur Pálsson
Keyword(s):  
Initial Phase ◽  
Water Pressure ◽  
Drainage System ◽  
Sar Interferometry ◽  
Ice Dynamics ◽  
Velocity Increase ◽  
Basal Sliding ◽  
Tunnel System ◽  
The Impact ◽  
Local Uplift

AbstractWe have analyzed InSAR data from the ERS-1/ERS-2 tandem mission, to study the ice dynamics of Vatnajökull, Iceland, during jökulhlaups from the Skaftá cauldrons and the Grímsvötn geothermal area, which drained under the Tungnaárjökull and Skeiðarárjökull outlets, respectively. During the initial phase of a Grímsvötn jökulhlaup in March 1996, the velocity of Skeiðarárjökull increased up to three-fold (relative to observed velocities in December 1995) over an area up to 8 km wide around the subglacial flood path. Accumulation of water was observed at one location in the flood path. During a small jökulhlaup from the Skaftá cauldrons in October 1995 the velocity on Tungnaárjökull increased up to four-fold over a 9 km wide area. The velocity increase was observed 1.5 days before the floodwater was detected in the river Skaftá. A reduced glacier speed as the flood peaked in Skaftá indicates evolution of the subglacial drainage system from sheet to tunnel flow. The glacier acceleration and local uplift, observed in the early phase of both jökulhlaups, supports the concept that increased water inflow in a narrow tunnel system causes water pressure to rise and forces water into areas outside the channels, thus reducing the coupling of ice with the glacier bed.

scholarly journals Short-term velocity and water-pressure variations down-glacier from a riegel, Storglaciären, Sweden

1998 ◽  
Vol 44 (147) ◽  
pp. 359-367 ◽  
Author(s):  
Brian Hanson ◽  
Roger LeB. Hooke ◽  
Edmund M. Grace
Keyword(s):  
Water Pressure ◽  
Spatial Coherence ◽  
Drainage System ◽  
Pressure Rise ◽  
Surface Velocity ◽  
Storm Events ◽  
Pressure Cycle ◽  
Large Water ◽  
Local Water ◽  
Subglacial Drainage

AbstractDuring the 1991 94 summer held seasons, time-correlated measure-merits of water pressure and surface speed were made over and down-glacier from a major riegel on Storglaciären, Sweden. Measurements were made at sub-hourly time-scales in order to discern details in the diurnal cycle. Large water-input events, typically associated with rain storms, produced coherent, lagged surface-velocity responses that could be understood in terms of till deformation or decoupling, and these have been discussed elsewhere. The consequences of smaller diurnal water-pressure events w ere more enigmatic, in that acceleration of ice flow generally preceded the onset of the local water-pressure rise. From consideration of these data and other work done on the hydrology of Storglaciären, we infer that the ice in this area is generally pushed from behind via a relaxation in extensional strain across the riegel. Hence, accelerations occur in response to increases in water pressure that occur up-glacier and that precede local water-pressure rises. In addition, following a period of large storm events, surface speeds became more spatially coherent and were in phase with the diurnal water-pressure cycle. This suggests that the large water-pressure events lead to a spatially more homogeneous subglacial drainage system. Sliding laws need to take into account such temporal changes in spatial coherence of the subglacial drainage system.

scholarly journals Water pressure and basal sliding on Storglaciären, northern Sweden

1995 ◽  
Vol 41 (138) ◽  
pp. 232-240 ◽  
Author(s):  
Peter Jansson
Keyword(s):  
Water Pressure ◽  
Surface Velocity ◽  
Northern Sweden ◽  
Overburden Pressure ◽  
Weakly Coupled ◽  
Basal Sliding ◽  
Order Of Magnitude ◽  
Image Position ◽  
Hydraulic Jacking ◽  
Small Valley

AbstractThe subglacial hydrology of the ablation area of Storglaciären, a small valley glacier in northern Sweden, is dramatically affected by a subglacial ridge, or riegel. Water pressures above this riegel are relatively constant, while down-glacier from it they vary significantly. The lower part of the glacier accelerates in response to peaks in basal water pressure. The upper part may be weakly coupled to the lower part during these peaks.A power-law fit of observed basal water pressures and measured surface velocities yieldswhereusis the surface velocity andPEis the effective water pressure (ice overburden pressure minus subglacial water pressure). Data from Findelengletscher, reported by Iken and Bindschadler (1986), yield an identical exponent and a coefficient one order of magnitude larger. The similar exponent implies that the process producing the velocity variations on both glaciers is similar. The variations in velocity are inferred to be due to hydraulic jacking on both glaciers.

scholarly journals The relationship between subglacial water pressure and velocity of Findelengletscher, Switzerland, during its advance and retreat

1997 ◽  
Vol 43 (144) ◽  
pp. 328-338 ◽  
Author(s):  
Almut Iken ◽  
Martin Truffe
Keyword(s):  
Water Pressure ◽  
Pressure Fluctuations ◽  
Drainage System ◽  
Surface Strain ◽  
Aerial Photographs ◽  
Surface Velocity ◽  
Surface Geometry ◽  
Sliding Speed ◽  
Pressure Surface ◽  
The Relationship

AbstractFindelengletscher, Switzerland, advanced about 250 m between 1979 and 1985, and retreated thereafter. Subglacial water pressure, surface velocity and surface strain rate were determined at several sites. The measurements were made early in the melt seasons of 1980, 1982, 1985 and 1994 and in the autumn of 1983 and the winter of 1984. Changes of surface geometry were assessed from aerial photographs.The estimated basal shear stress changed little between 1982 and 1994. Nevertheless, large changes in the relationship of subglacial water pressure and surface velocity were observed, which cannot be reconciled with the most commonly used sliding law unless it is modified substantially. Consideration of possible reasons indícales that a change in the subglacial drainage system occurred, probably involving a change in the degree of cavity interconnection. Isolated cavities damp the variations in sliding velocity that normally result from changes in water pressure, because the pressure in isolated cavities decreases as the sliding speed increases. In contrast, by transmitting water-pressure fluctuations to a larger area of the bed, interconnected cavities amplify the effect of water-pressure fluctuations on sliding speed. Thus, we suggest that an observed decrease in velocity (for a given water pressure) between 1982 and 1994 was a consequence of a decrease in the interconnectedness of the subglacial cavity system.

scholarly journals Seasonal patterns of velocity and strain across the tongue of the polythermal glacier midre Lovénbreen, Svalbard

2005 ◽  
Vol 42 ◽  
pp. 445-453 ◽  
Author(s):  
David Rippin ◽  
Ian Willis ◽  
Neil Arnold
Keyword(s):  
Water Pressure ◽  
Drainage System ◽  
Surface Strain ◽  
Force Balance ◽  
Surface Velocity ◽  
Spring Period ◽  
Vertical Extension ◽  
Polythermal Glacier ◽  
Longitudinal Extension ◽  
Annual Period

AbstractTerrestrial surveys to 17 markers distributed across the tongue of the polythermal glacier midre Lovénbreen, Svalbard, are used to calculate annual and seasonal (summer 1998, autumn/winter/ spring 1998/99, summer 1999) patterns of surface velocity and strain. The annual period and the three seasonal periods have similar velocity azimuths and patterns, with fastest velocities along the centre line and in the upper tongue. Velocities in both summers are of similar magnitude, and greater than those in the autumn/winter/spring period. In all periods, longitudinal compression (increasing towards the snout) and transverse and vertical extension dominate the surface strain field. However, an area of longitudinal extension develops in the middle tongue during the 1998 summer. Surface strain patterns are used to estimate the components of the force balance. Basal drag is the dominant force resisting flow, but patterns are rather different between the three seasons. In summer 1998, a slippery spot in the upper-middle tongue is confined to the central part of the glacier. In autumn/winter/spring, this slippery spot has expanded towards the western glacier margin. In summer 1999, it has disappeared, and a slight sticky spot now covers virtually the entire upper and middle tongue. The location and extent of the slippery spot are explained in terms of the distribution of warm and cold ice, and the location and morphology of the subglacial drainage system, which control the patterns of water pressure beneath the glacier tongue.

scholarly journals Inversion for basal friction coefficients with a two-dimensional flow line model using Tikhonov regularization

2012 ◽  
Vol 2 (2) ◽  
pp. 11 ◽  
Author(s):  
Yuri V. Konovalov
Keyword(s):  
Inverse Problem ◽  
Friction Coefficient ◽  
Sea Level ◽  
Flow Line ◽  
Regularization Parameter ◽  
Surface Velocity ◽  
Line Model ◽  
Basal Friction ◽  
Ice Surface ◽  
Level Shifts

We present results of basal friction coefficient inversion. The inversion was performed by a 2D flow line model for one of the four fast flowing ice streams on the southern side of the Academy of Sciences Ice Cap in the Komsomolets Island, Severnaya Zemlya archipelago. The input data for the performance of both the forward and the inverse problems included synthetic aperture radar interferometry ice surface velocities, ice surface elevations and ice thicknesses obtained by airborne measurements (all were taken from Dowdeswell <em>et al.</em>, 2002). Numerical experiments with: i) different sea level shifts; and ii) randomly perturbed friction coefficient have been carried out in the forward problem. The impact of sea level changes on vertical distribution of horizontal velocity and on shear stress distribution near the ice front has been investigated in experiments with different sea level shifts. The experiments with randomly perturbed friction coefficient have revealed that the modeled surface velocity is weakly sensitive to the perturbations and, therefore, the inverse problem should be considered ill posed. To mitigate ill posedness of the inverse problem, Tikhonov’s regularization was applied. The regularization parameter was determined from the relation of the discrepancy between observed and modeled velocities to the regularization parameter. The inversion was performed for both linear and non-linear sliding laws. The inverted spatial distributions of the basal friction coefficient are similar for both sliding laws. The similarity between these inverted distributions suggests that the changes in the friction coefficient are accompanied by appropriate water content variations at the glacier base.

scholarly journals Combined measurements of Subglacial Water Pressure and Surface Velocity of Findelengletscher, Switzerland: Conclusions about Drainage System and Sliding Mechanism

1986 ◽  
Vol 32 (110) ◽  
pp. 101-119 ◽  
Author(s):  
Almut Iken ◽  
Robert A. Bindschadler
Keyword(s):  
Functional Relationship ◽  
Water Pressure ◽  
Drainage System ◽  
High Water ◽  
Water Levels ◽  
Surface Velocity ◽  
Glacier Surface ◽  
Measured Velocity ◽  
Sliding Mechanism ◽  
Velocity Variations

AbstractDuring the snow-melt season of 1982, basal water pressure was recorded in 11 bore holes communicating with the subglacial drainage system. In most of these holes the water levels were at approximately the same depth (around 70 m below surface). The large variations of water pressure, such as diurnal variations, were usually similar at different locations and in phase. In two instances of exceptionally high water pressure, however, systematic phase shifts were observed; a wave of high pressure travelled down-glacier with a velocity of approximately 100 m/h.The glacier-surface velocity was measured at four lines of stakes several times daily. The velocity variations correlated with variations in subglacial water pressure. The functional relationship of water pressure and velocity suggests that fluctuating bed separation was responsible for the velocity variations. The empirical functional relationship is compared to that of sliding over a perfectly lubricated sinusoidal bed. On the basis of the measured velocity-pressure relationship, this model predicts a reasonable value of bed roughness but too high a sliding velocity and unstable sliding at too low a water pressure. The main reason for this disagreement is probably the neglect of friction from debris in the sliding model.The measured water pressure was considerably higher than that predicted by the theory of steady flow through straight cylindrical channels near the glacier bed. Possible reasons are considered. The very large disagreement between measured and predicted pressure suggests that no straight cylindrical channels may have existed.

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