scholarly journals Rheology of till beneath Storglaciären, Sweden

1997 ◽  
Vol 43 (143) ◽  
pp. 172-179 ◽  
Author(s):  
Roger LeB. Hooke ◽  
Brian Hanson ◽  
Neal R. Iverson ◽  
Peter Jansson ◽  
Urs H. Fischer
Keyword(s):  
Strain Rate ◽  
Water Pressure ◽  
Force Balance ◽  
Surface Velocity ◽  
Shear Strain Rate ◽  
Effective Pressure ◽  
Frictional Material ◽  
Image Position ◽  
Gain Access

AbstractIn order to study, in situ, the rheology of a deforming subglacial till, various instruments were emplaced in till beneath Storglaciären, Sweden. Boreholes were used to gain access to the till beneath about 100 m of ice. Tiltmeters provided an estimate of the shear strain rate in the till. Two other instruments yielded measures of till strength. In addition, water pressures were recorded in boreholes and in the till, a computer-controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag, based on a force-balance calculation.Tilt rates varied directly with effective pressure, so decreases in water pressure apparently increased the coupling between the glacier and the bed. Surface speed was either out of phase with tilt or varied independently of tilt. Thus, increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed; they were not a result of till deformation! Till strength varied directly with effective pressure, which is consistent with it being a Mohr – Coulomb, or frictional material. The devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed. This implies that the residual strength of the till is insensitive to strain rate. Thus, the appropriate constitutive equation for till rheology may be of the form:where k is a constant. This is consistent with experimental data reported in the geotechnical literature.

scholarly journals Rheology of till beneath Storglaciären, Sweden

1997 ◽  
Vol 43 (143) ◽  
pp. 172-179 ◽  
Author(s):  
Roger LeB. Hooke ◽  
Brian Hanson ◽  
Neal R. Iverson ◽  
Peter Jansson ◽  
Urs H. Fischer
Keyword(s):  
Strain Rate ◽  
Water Pressure ◽  
Force Balance ◽  
Surface Velocity ◽  
Shear Strain Rate ◽  
Effective Pressure ◽  
Frictional Material ◽  
Image Position ◽  
Gain Access

AbstractIn order to study, in situ, the rheology of a deforming subglacial till, various instruments were emplaced in till beneath Storglaciären, Sweden. Boreholes were used to gain access to the till beneath about 100 m of ice. Tiltmeters provided an estimate of the shear strain rate in the till. Two other instruments yielded measures of till strength. In addition, water pressures were recorded in boreholes and in the till, a computer-controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag, based on a force-balance calculation.Tilt rates varied directly with effective pressure, so decreases in water pressure apparently increased the coupling between the glacier and the bed. Surface speed was either out of phase with tilt or varied independently of tilt. Thus, increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed;they were not a result of till deformation!Till strength varied directly with effective pressure, which is consistent with it being a Mohr – Coulomb, or frictional material. The devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed. This implies that the residual strength of the till is insensitive to strain rate. Thus, the appropriate constitutive equation for till rheology may be of the form:wherekis a constant. This is consistent with experimental data reported in the geotechnical literature.

scholarly journals A 3 Year Record of Seasonal Variations in Surface Velocity, StorglaciÄren, Sweden

1989 ◽  
Vol 35 (120) ◽  
pp. 235-247 ◽  
Author(s):  
Roger LeB. Hooke ◽  
Peter Calla ◽  
Per Holmlund ◽  
Mats Nilsson ◽  
Arjen Stroeven
Keyword(s):  
Strain Rate ◽  
Shear Strain ◽  
Seasonal Variations ◽  
High Velocity ◽  
Water Pressure ◽  
Force Balance ◽  
Surface Velocity ◽  
Time Interval ◽  
Strain Rates ◽  
Lateral Shear

Abstract Between 3 June 1982 and 8 July 1985, a stake net consisting of up to 32 stakes covering the greater part of Storglaciären was surveyed 70 times, yielding roughly 2000 separate determinations of vertical and horizontal velocity. The time interval between surveys averaged about 1 week during the summer and 2 months during the winter. Horizontal velocities were normally highest during periods of high daily temperature or heavy rain early in the melt season. Comparable or sometimes higher temperatures or rainfalls later in the season usually had less effect, though minor velocity peaks were often present in August and early September. During periods for which bore-hole water-level measurements are available, velocity peaks generally coincided with periods of high basal water pressure, but not all periods of high water pressure resulted in velocity peaks. Despite increasing basal water pressures, velocity decreased gradually during the winter. Vertical velocities also vary seasonally. Beneath the upper part of the ablation area the glacier bed is overdeepened. Vertical velocities here are ˜3 mm/d higher during the summer. Down-glacier from the overdeepening, vertical velocities are ˜1 mm/d lower during the summer. These and other characteristics of the vertical velocity pattern are best explained by appealing to: (1) a decrease in strain-rate with depth, and (2) seasonal variations in this depth-dependence. Five periods of high velocity lasting from 3 to 11d were studied in detail. In an area where the bed is overdeepened, force-balance calculations suggest that basal drag decreased between 16 and 40% during these high-velocity events. This resulted in a decrease in compressive strain-rate at the up-glacier end of the overdeepening, an increase at the down-glacier end, and a slight increase in lateral shear strain-rates. Down-glacier from the overdeepening, basal drag increased during two events owing to an increased push from up-glacier and pull from down-glacier. Lateral shear strain-rates increased sharply here.

scholarly journals Erebus Glacier Tongue, Mcmurdo Sound, Antarctica

1974 ◽  
Vol 13 (67) ◽  
pp. 27-35 ◽  
Author(s):  
G. Holdsworth
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Shear Strain ◽  
Longitudinal Strain ◽  
Strain Rates ◽  
Shear Strain Rate ◽  
Mcmurdo Sound ◽  
Glacier Tongue ◽  
The Past ◽  
Image Position

Examination of the past and present behaviour of the Erebus Glacier tongue over the last 60 years indicates that a major calving from the tongue appears to be imminent. Calculations of the regime of the tongue indicate that bottom melt rates may exceed 1 m a−1. By successive mapping of the ice tongue between the years 1947 and 1970, longitudinal strain-rates were determined using the change in distance between a set of 15 teeth, which are a prominent marginal feature of the tongue. Assuming a flow law for ice of the form where τ is the effective shear stress and is the effective shear strain-rate, values of the exponent n = 3 and B = 1 × 108 N m−2 are determined. These are in fair agreement with published values.

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 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 Temporal variations in the flow of a large Antarctic ice-stream controlled by tidally induced changes in the subglacial water system

2015 ◽  
Vol 9 (2) ◽  
pp. 2397-2429 ◽  
Author(s):  
S. H. R. Rosier ◽  
G. H. Gudmundsson ◽  
J. A. M. Green
Keyword(s):  
Water Pressure ◽  
Water System ◽  
Surface Velocity ◽  
Decay Length ◽  
Model Parameters ◽  
Effective Pressure ◽  
Tidal Period ◽  
Ice Stream ◽  
Highly Nonlinear ◽  
Basal Sliding

Abstract. Observations show that the flow of Rutford Ice Stream (RIS) is strongly modulated by the ocean tides, with the strongest tidal response at the 14.77 day tidal period (Msf). This is striking because this period is absent in the tidal forcing. A number of mechanisms have been proposed to account for this effect, yet previous modeling studies have struggled to match the observed large amplitude and decay length scale. We use a nonlinear 3-D viscoelastic full-Stokes model of ice-stream flow to investigate this open issue. We find that the long period Msf modulation of ice-stream velocity observed in data cannot be reproduced quantitatively without including a coupling between basal sliding and tidal subglacial water pressure variations. Furthermore, the subglacial water system must be highly conductive and at low effective pressure, and the relationship between sliding velocity and effective pressure highly nonlinear in order for the model results to match GPS measurements. Hydrological and basal sliding model parameters that produced a best fit to observations were a mean effective pressure N of 105 kPa, subglacial drainage system conductivity K of 7 × 109 m2d-1, with sliding law exponents m = 3 and q =10. Coupled model results show the presence of tides result in a ~ 12% increase in mean surface velocity. Observations of tidally-induced variations in flow of ice-streams provide stronger constraints on basal sliding processes than provided by any other set of measurements.

scholarly journals Spatial and temporal variations in surface velocity and basal drag across the tongue of the polythermal glacier midre Lovénbreen, Svalbard

2005 ◽  
Vol 51 (175) ◽  
pp. 588-600 ◽  
Author(s):  
D.M. Rippin ◽  
I.C. Willis ◽  
N.S. Arnold ◽  
A.J. Hodson ◽  
M. Brinkhaus
Keyword(s):  
Water Pressure ◽  
Temporal Variations ◽  
Force Balance ◽  
Surface Velocity ◽  
Rainfall Events ◽  
Pressure Surface ◽  
Balance Analysis ◽  
Polythermal Glacier ◽  
Hydraulic Jacking ◽  
Enhanced Surface

AbstractWe present results of a detailed investigation of surface motion across the tongue of a polythermal glacier, midre Lovenbreen, Svalbard, during the 1999 summer. Surface velocities in the warm-based upper tongue increased during periods of enhanced surface melting and rainfall events, and force-balance analysis indicates that these velocity variations were locally forced, probably by fluctuations in subglacial water pressure. Surface speed-ups were also observed on the cold-based lower tongue (which acted as a sticky spot, through which there was minimal subglacial drainage for most of the summer), but these were largely non-locally forced by longitudinal coupling to the faster-moving ice up-glacier. On one occasion, however, a large, rapid input of surface water to the glacier reduced the basal drag beneath the cold-based lower tongue, presumably due to hydraulic jacking. This resulted in locally forced enhanced surface velocities across the entire tongue, accompanied by a breaching of the lower tongue and an outburst of subglacially stored water.

scholarly journals Erebus Glacier Tongue, Mcmurdo Sound, Antarctica

1974 ◽  
Vol 13 (67) ◽  
pp. 27-35 ◽  
Author(s):  
G. Holdsworth
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Shear Strain ◽  
Longitudinal Strain ◽  
Strain Rates ◽  
Shear Strain Rate ◽  
Mcmurdo Sound ◽  
Glacier Tongue ◽  
The Past ◽  
Image Position

Examination of the past and present behaviour of the Erebus Glacier tongue over the last 60 years indicates that a major calving from the tongue appears to be imminent. Calculations of the regime of the tongue indicate that bottom melt rates may exceed 1 m a−1. By successive mapping of the ice tongue between the years 1947 and 1970, longitudinal strain-rates were determined using the change in distance between a set of 15 teeth, which are a prominent marginal feature of the tongue. Assuming a flow law for ice of the form where τ is the effective shear stress and is the effective shear strain-rate, values of the exponent n = 3 and B = 1 × 108 N m−2 are determined. These are in fair agreement with published values.

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 yieldswhere us is the surface velocity and PE is 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 Effects of ice deformation on Röthlisberger channels and implications for transitions in subglacial hydrology

2016 ◽  
Vol 62 (234) ◽  
pp. 750-762 ◽  
Author(s):  
COLIN R. MEYER ◽  
MATHEUS C. FERNANDES ◽  
TIMOTHY T. CREYTS ◽  
JAMES R. RICE
Keyword(s):  
Water Pressure ◽  
Circular Cross Section ◽  
Antiplane Shear ◽  
Shear Strain Rate ◽  
Effective Pressure ◽  
Water Drainage ◽  
Ice Streams ◽  
Turbulent Dissipation ◽  
Creep Flow ◽  
Fixed Radius

ABSTRACTAlong the base of glaciers and ice sheets, the sliding of ice over till depends critically on water drainage. In locations where drainage occurs through Röthlisberger channels, the effective pressure along the base of the ice increases and can lead to a strengthening of the bed, which reduces glacier sliding. The formation of Röthlisberger channels depends on two competing effects: (1) melting from turbulent dissipation opens the channel walls and (2) creep flow driven by the weight of the overlying ice closes the channels radially inward. Variation in downstream ice velocity along the channel axis, referred to as an antiplane shear strain rate, decreases the effective viscosity. The softening of the ice increases creep closure velocities. In this way, even a modest addition of antiplane shear can double the size of the Röthlisberger channels for a fixed water pressure or allow channels of a fixed radius to operate at lower effective pressure, potentially decreasing the strength of the surrounding bed. Furthermore, we show that Röthlisberger channels can be deformed away from a circular cross section under applied antiplane shear. These results can have broad impacts on sliding velocities and potentially affect the total ice flux out of glaciers and ice streams.

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