scholarly journals Ice-stream surface texture, sticky spots, waves and breathers: the coupled flow of ice, till and water

1998 ◽  
Vol 44 (148) ◽  
pp. 589-614 ◽  
Author(s):  
Richard C. A. Hindmarsh
Keyword(s):  
Water Flow ◽  
Parameter Space ◽  
Rate Constants ◽  
Water Pressure ◽  
Water System ◽  
Effective Pressure ◽  
Coupled Flow ◽  
Ice Surface ◽  
Ice Stream ◽  
Meso Scale

AbstractThis paper addresses the coupling of flows of ice, till and water, and the issue of whether such coupling provides mechanisms for meso-scale (kilometres to tens of kilometres) variability in ice-sheet flow and texture. The question of whether effective pressures at the ice-bed interface are statically or hydraulically controlled is examined in this paper. The answer is scale dependent, and has a significant effect on the relationship between ice surface and basal topography.The consequences of these considerations on till flow, coupled ice–till flow and coupled ice, till and water flow are examined. An analysis of till-flow kinematics and shock formation is carried out. The linear stability of coupled long-wavelength ice-till flow is analysed, and regions in parameter space where this flow is unstable, with rather small rate constants are found. Upstream-moving ice surface waves are predicted. The linear stablity of coupled ice–till–water flow is examined, where water flow is modelled using a basal flow system with effective-pressure-dependent properties. Again, regions in parameter space where the system is linearly unstable are found, this time with relatively rapid rate constants. The water pressure exhibits “breather” modes.These analyses assume that there is a substantial basal traction. A problem with models of ice streams wholly restrained at the side is identified: they seem to predict erosion rates which are unfeasibly large.There appears to be sufficient variability in the ice–till– water system to potentially explain texture in ice-stream surfaces, variations in ice-stream thickness of tens of metres not directly relatable to topography, and waves moving upstream or downstream. Most importantly, the ice-stream–bed system is shown to exhibit meso-scale variability simply by coupling ice flow according to the shallow-ice approximation, till flow according to the hydrostatic thin-till approximation and water flow according to an effective-pressure-dependent hydraulics.

scholarly journals Ice-stream surface texture, sticky spots, waves and breathers: the coupled flow of ice, till and water

1998 ◽  
Vol 44 (148) ◽  
pp. 589-614 ◽  
Author(s):  
Richard C. A. Hindmarsh
Keyword(s):  
Water Flow ◽  
Parameter Space ◽  
Rate Constants ◽  
Water Pressure ◽  
Water System ◽  
Effective Pressure ◽  
Coupled Flow ◽  
Ice Surface ◽  
Ice Stream ◽  
Meso Scale

AbstractThis paper addresses the coupling of flows of ice, till and water, and the issue of whether such coupling provides mechanisms for meso-scale (kilometres to tens of kilometres) variability in ice-sheet flow and texture. The question of whether effective pressures at the ice-bed interface are statically or hydraulically controlled is examined in this paper. The answer is scale dependent, and has a significant effect on the relationship between ice surface and basal topography.The consequences of these considerations on till flow, coupled ice–till flow and coupled ice, till and water flow are examined. An analysis of till-flow kinematics and shock formation is carried out. The linear stability of coupled long-wavelength ice-till flow is analysed, and regions in parameter space where this flow is unstable, with rather small rate constants are found. Upstream-moving ice surface waves are predicted. The linear stablity of coupled ice–till–water flow is examined, where water flow is modelled using a basal flow system with effective-pressure-dependent properties. Again, regions in parameter space where the system is linearly unstable are found, this time with relatively rapid rate constants. The water pressure exhibits “breather” modes.These analyses assume that there is a substantial basal traction. A problem with models of ice streams wholly restrained at the side is identified: they seem to predict erosion rates which are unfeasibly large.There appears to be sufficient variability in the ice–till– water system to potentially explain texture in ice-stream surfaces, variations in ice-stream thickness of tens of metres not directly relatable to topography, and waves moving upstream or downstream. Most importantly, the ice-stream–bed system is shown to exhibit meso-scale variability simply by coupling ice flow according to the shallow-ice approximation, till flow according to the hydrostatic thin-till approximation and water flow according to an effective-pressure-dependent hydraulics.

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 Basal hydraulic system of a West Antarctic ice stream: constraints from borehole observations

1997 ◽  
Vol 43 (144) ◽  
pp. 207-230 ◽  
Author(s):  
Hermann Engelhardt ◽  
Barclay Kamb
Keyword(s):  
Water Level ◽  
Water Pressure ◽  
Water System ◽  
Water Levels ◽  
Effective Pressure ◽  
Large Hole ◽  
Basal Pressure ◽  
Ice Stream ◽  
Gap Opening ◽  
Conduit System

AbstractPressure and tracer measurements in boreholes drilled to the bottom of Ice Stream B, West Antarctica, are used to obtain information about the basal water conduit system in which high water pressures are developed.These high pressures presumably make possible the rapid movement of the ice stream. Pressure in the system is indicated by the borehole water level once connection to the conduit system is made. On initial connection, here also called “breakthrough” to the basal water system, the water level drops in a few minutes to an initial depth in the range 96–117 m below the surface. These water levels are near but mostly somewhat deeper than the floation level of about 100 m depth (water level at which basal water pressure and ice overburden pressure are equal), which is calculated from depth-density profiles and is measured in one borehole. The conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed; the thickness of the gap δ has to be about 2 mm to account for the water-level drop on breakthrough, and about 4 mm to fit the results of a salt-tracer experiment indicating downstream transport at a speed of 7.5 mm s−1. The above gap-conduit model is, however, ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole, and also by the large hole-to-hole variation in measured basal pressure, which if present in a gap-conduit system with δ = 2 or 4 mm would result in unacceptably large local water fluxes. An alternative model that avoids these objections, called the “gap opening” model, involves opening a gap as injection proceeds: starting with a thin film, the injection of water under pressure lifts the ice mass around the borehole, creating a gap 3 or 4mm wide at the ice/bed interface. Evaluated quantitatively, the gap-opening model accounts for the volume of water that the basal water system accepts on breakthrough, which obviates the gap-conduit model. In order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits, of which the most promising type is the “canal” conduit proposed theoretically by Walder and Fowler (1994): flat, low conduits incised into the till, ∼0.1 m deep and perhaps ∼1 m wide, with a flat ice roof. The basal water-pressure data suggest that the canals are spaced ∼50–300 m apart, much closer than R-tunnels would be. The deepest observed water level, 117 m, is the most likely to reflect the actual water pressure in the canals, corresponding to a basal effective pressure of 1.6 bar. In this interpretation, the shallower water levels are affected by loss of hydraulic head in the narrow passageway (s) that connect along the bed from borehole to canal(s). Once a borehole has frozen up and any passageways connecting with canals have become closed, a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till, given enough time for pressure equilibration. This pressure varies considerably with time, over the equivalent water-level range from 100 to 113 m. Basal pressure sensors 500 m apart report uncorrelated variations, whereas sensors in boreholes 25 m араrt report mostly (but not entirely) well-correlated variations, of unknown origin. In part of the record, remarkable anticorrelated variations are interspersed with positively correlated ones, and there are rare, abrupt excursions to extreme water levels as deep as 125 m and as shallow as 74 m. A diurnal pressure fluctuation, intermittently observed, may possibly be caused by the ocean tide in the Ross Sea. The lack of any observed variation in ice-stream motion, when large percentagewise variations in basal effective pressure were occurring according to our data, suggests that the observed pressure variations are sufficiently local, and so randomly variable from place to place, that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed.

scholarly journals Basal hydraulic system of a West Antarctic ice stream: constraints from borehole observations

1997 ◽  
Vol 43 (144) ◽  
pp. 207-230 ◽  
Author(s):  
Hermann Engelhardt ◽  
Barclay Kamb
Keyword(s):  
Water Level ◽  
Water Pressure ◽  
Water System ◽  
Water Levels ◽  
Effective Pressure ◽  
Large Hole ◽  
Basal Pressure ◽  
Ice Stream ◽  
Gap Opening ◽  
Conduit System

AbstractPressure and tracer measurements in boreholes drilled to the bottom of Ice Stream B, West Antarctica, are used to obtain information about the basal water conduit system in which high water pressures are developed.These high pressures presumably make possible the rapid movement of the ice stream. Pressure in the system is indicated by the borehole water level once connection to the conduit system is made. On initial connection, here also called “breakthrough” to the basal water system, the water level drops in a few minutes to an initial depth in the range 96–117 m below the surface. These water levels are near but mostly somewhat deeper than the floation level of about 100 m depth (water level at which basal water pressure and ice overburden pressure are equal), which is calculated from depth-density profiles and is measured in one borehole. The conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed; the thickness of the gap δ has to be about 2 mm to account for the water-level drop on breakthrough, and about 4 mm to fit the results of a salt-tracer experiment indicating downstream transport at a speed of 7.5 mm s−1. The above gap-conduit model is, however, ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole, and also by the large hole-to-hole variation in measured basal pressure, which if present in a gap-conduit system with δ = 2 or 4 mm would result in unacceptably large local water fluxes. An alternative model that avoids these objections, called the “gap opening” model, involves opening a gap as injection proceeds: starting with a thin film, the injection of water under pressure lifts the ice mass around the borehole, creating a gap 3 or 4mm wide at the ice/bed interface. Evaluated quantitatively, the gap-opening model accounts for the volume of water that the basal water system accepts on breakthrough, which obviates the gap-conduit model. In order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits, of which the most promising type is the “canal” conduit proposed theoretically by Walder and Fowler (1994): flat, low conduits incised into the till, ∼0.1 m deep and perhaps ∼1 m wide, with a flat ice roof. The basal water-pressure data suggest that the canals are spaced ∼50–300 m apart, much closer than R-tunnels would be. The deepest observed water level, 117 m, is the most likely to reflect the actual water pressure in the canals, corresponding to a basal effective pressure of 1.6 bar. In this interpretation, the shallower water levels are affected by loss of hydraulic head in the narrow passageway (s) that connect along the bed from borehole to canal(s). Once a borehole has frozen up and any passageways connecting with canals have become closed, a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till, given enough time for pressure equilibration. This pressure varies considerably with time, over the equivalent water-level range from 100 to 113 m. Basal pressure sensors 500 m apart report uncorrelated variations, whereas sensors in boreholes 25 m араrt report mostly (but not entirely) well-correlated variations, of unknown origin. In part of the record, remarkable anticorrelated variations are interspersed with positively correlated ones, and there are rare, abrupt excursions to extreme water levels as deep as 125 m and as shallow as 74 m. A diurnal pressure fluctuation, intermittently observed, may possibly be caused by the ocean tide in the Ross Sea. The lack of any observed variation in ice-stream motion, when large percentagewise variations in basal effective pressure were occurring according to our data, suggests that the observed pressure variations are sufficiently local, and so randomly variable from place to place, that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed.

scholarly journals In search of ice-stream sticky spots

1993 ◽  
Vol 39 (133) ◽  
pp. 447-454 ◽  
Author(s):  
Richard B. Alley
Keyword(s):  
Shear Stress ◽  
Large Scale ◽  
Water Pressure ◽  
Pressure Measurements ◽  
West Antarctica ◽  
Ice Surface ◽  
Ice Stream ◽  
Bed Roughness ◽  
Basal Shear

AbstractThe basal shear stress of an ice stream may be supported disproportionately on localized regions or “sticky spots”. The drag induced by large bedrock bumps sticking into the base of an ice stream is the most likely cause of sticky spots. Discontinuity of lubricating till can cause sticky spots, but they will collect lubricating water and therefore are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps. Raised regions on the ice-air surface can also cause moderate increases in the shear stress supported on the bed beneath. Surveys of large-scale bed roughness would identify sticky spots caused by bedrock bumps, water-pressure measurements in regions of thin or zero till might reveal whether they were sticky spots, and strain grids across the margins of ice-surface highs would show whether the highs were causing sticky spots. Sticky spots probably are not dominant in controlling Ice Stream Β near the Upstream Β camp, West Antarctica.

scholarly journals In search of ice-stream sticky spots

1993 ◽  
Vol 39 (133) ◽  
pp. 447-454 ◽  
Author(s):  
Richard B. Alley
Keyword(s):  
Shear Stress ◽  
Large Scale ◽  
Water Pressure ◽  
Pressure Measurements ◽  
West Antarctica ◽  
Ice Surface ◽  
Ice Stream ◽  
Bed Roughness ◽  
Basal Shear

Abstract The basal shear stress of an ice stream may be supported disproportionately on localized regions or “sticky spots”. The drag induced by large bedrock bumps sticking into the base of an ice stream is the most likely cause of sticky spots. Discontinuity of lubricating till can cause sticky spots, but they will collect lubricating water and therefore are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps. Raised regions on the ice-air surface can also cause moderate increases in the shear stress supported on the bed beneath. Surveys of large-scale bed roughness would identify sticky spots caused by bedrock bumps, water-pressure measurements in regions of thin or zero till might reveal whether they were sticky spots, and strain grids across the margins of ice-surface highs would show whether the highs were causing sticky spots. Sticky spots probably are not dominant in controlling Ice Stream Β near the Upstream Β camp, West Antarctica.

scholarly journals Water-Pressure Coupling of Sliding and Bed Deformation: I. Water System

1989 ◽  
Vol 35 (119) ◽  
pp. 108-118 ◽  
Author(s):  
R.B. Alley
Keyword(s):  
Shear Stress ◽  
Water Supply ◽  
Water Pressure ◽  
Water System ◽  
Effective Pressure ◽  
Bed Deformation ◽  
Varying Thickness ◽  
Melt Water ◽  
Basal Shear

AbstractAnalysis of the likely behavior of a water system developed between ice and an unconsolidated glacier bed suggests that, in the absence of channelized sources of melt water, the system will approximate a film of varying thickness. The effective pressure in such a film will be proportional to the basal shear stress but inversely proportional to the fraction of the bed occupied by the film. These hypotheses allow calculation of the sliding and bed-deformation velocities of a glacier from the water supply and basal shear stress, as discussed in the second and third papers in this series.

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 (4) ◽  
pp. 1649-1661 ◽  
Author(s):  
S. H. R. Rosier ◽  
G. H. Gudmundsson ◽  
J. A. M. Green
Keyword(s):  
Water Pressure ◽  
Drainage System ◽  
Water System ◽  
Coupled Model ◽  
Temporal Variations ◽  
Decay Length ◽  
Stream Velocity ◽  
Tidal Period ◽  
Ice Stream ◽  
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 modelling 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 tidally induced subglacial water pressure variations, transmitted through a highly conductive drainage system at low effective pressure. Furthermore, the basal sliding law requires a water pressure exponent that is strongly nonlinear with q = 10 and a nonlinear basal shear exponent of m = 3. Coupled model results show that sub-ice shelf tides result in a ∼12 % increase in mean horizontal velocity of the adjoining ice stream. 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 Water-pressure Coupling of Sliding and Bed Deformation: III. Application to Ice Stream B, Antarctica

1989 ◽  
Vol 35 (119) ◽  
pp. 130-139 ◽  
Author(s):  
R.B. Alley ◽  
D.D. Blankenship ◽  
S.T. Rooney ◽  
C.R. Bentley
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Water Pressure ◽  
Effective Pressure ◽  
West Antarctica ◽  
Bed Deformation ◽  
Ice Stream ◽  
Basal Sliding ◽  
Ice Stream B ◽  
Basal Shear

AbstractGeophysical studies and glaciological analyses suggest strongly that Ice Stream B, West Antarctica, moves primarily by pervasive deformation of a meters thick subglacial till. Analysis of the longitudinal profile of the ice stream up-stream of the ice plain suggests that basal sliding is slow everywhere, that effective pressure decreases slowly down-stream, and that the strain-rate of pervasive shear is proportional to the basal shear stress and inversely proportional to the square or cube of the effective pressure. Discrete shearing may occur beneath the pervasively deforming zone. These and other hypotheses, which build on the analyses of the first two papers in this series, can be tested in the field.

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