scholarly journals Cyclic Surging of Glaciers

1973 ◽  
Vol 12 (64) ◽  
pp. 3-18 ◽  
Author(s):  
G. de Q. Robin ◽  
J. Weertman
Keyword(s):  
Shear Stress ◽  
Pressure Gradient ◽  
Water Pressure ◽  
Stress Gradient ◽  
Longitudinal Direction ◽  
Phenomenological Theory ◽  
Trigger Zone ◽  
Basal Shear ◽  
Velocity Zone ◽  
Theory And Model

AbstractA partly phenomenological theory and model are constructed of cyclically surging glaciers. During the after-surge portion of a surge cycle the lower portion of a glacier becomes increasingly stagnant. The upper part of the glacier gradually becomes more active as both its thickness and the magnitude of its basal shear stress increase. In the region between these two parts, called by us the trigger zone, the value of the derivative of the basal shear stress in the longitudinal direction of the glacier gradually increases with time. The pressure gradient in the water at the base of a glacier is related to the derivative of the basal shear stress. The pressure gradient decreases as the basal shear-stress gradient increases. The pressure gradient actually can take on negative values, a condition which produces “up-hill” water flow at the base of a glacier. A surge is started in the trigger zone when water is dammed there by a zero water-pressure gradient. The zone of fast-sliding velocities propagates up the glacier from the trigger zone with a velocity of the order of a surge velocity. The fast-sliding velocity zone also propagates down the glacier because of increased melt-water production.

scholarly journals Cyclic Surging of Glaciers

1973 ◽  
Vol 12 (64) ◽  
pp. 3-18 ◽  
Author(s):  
G. de Q. Robin ◽  
J. Weertman
Keyword(s):  
Shear Stress ◽  
Pressure Gradient ◽  
Water Pressure ◽  
Stress Gradient ◽  
Longitudinal Direction ◽  
Phenomenological Theory ◽  
Trigger Zone ◽  
Basal Shear ◽  
Velocity Zone ◽  
Theory And Model

AbstractA partly phenomenological theory and model are constructed of cyclically surging glaciers. During the after-surge portion of a surge cycle the lower portion of a glacier becomes increasingly stagnant. The upper part of the glacier gradually becomes more active as both its thickness and the magnitude of its basal shear stress increase. In the region between these two parts, called by us the trigger zone, the value of the derivative of the basal shear stress in the longitudinal direction of the glacier gradually increases with time. The pressure gradient in the water at the base of a glacier is related to the derivative of the basal shear stress. The pressure gradient decreases as the basal shear-stress gradient increases. The pressure gradient actually can take on negative values, a condition which produces “up-hill” water flow at the base of a glacier. A surge is started in the trigger zone when water is dammed there by a zero water-pressure gradient. The zone of fast-sliding velocities propagates up the glacier from the trigger zone with a velocity of the order of a surge velocity. The fast-sliding velocity zone also propagates down the glacier because of increased melt-water production.

scholarly journals The role of bed separation and friction in sliding over an undeformable bed

1992 ◽  
Vol 38 (128) ◽  
pp. 77-92 ◽  
Author(s):  
Jürg Schweizer ◽  
Almut Iken
Keyword(s):  
Shear Stress ◽  
Functional Relationship ◽  
Water Pressure ◽  
Important Variable ◽  
Sliding Velocity ◽  
Sliding Motion ◽  
Basal Ice ◽  
Bed Separation ◽  
Basal Shear

AbstractThe classic sliding theories usually assume that the sliding motion occurs frictionlessly. However, basal ice is debris-laden and friction exists between the substratum and rock particles embedded in the basal ice. The influence of debris concentration on the sliding process is investigated. The actual conditions where certain types of friction apply are defined, the effect for the case of bed separation due to a subglacial water pressure is studied and consequences for the sliding law are formulated. The numerical modelling of the sliding of an ice mass over an undulating bed, including the effect of both the subglacial water pressure and the friction, is done by using the finite-clement method. Friction, seen as a reduction of the driving shear stress due to gravity, can be included in existing sliding laws which should contain the critical pressure as an important variable. An approximate functional relationship between the sliding velocity, the effective basal shear stress and the subglacial water pressure is given.

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 The role of bed separation and friction in sliding over an undeformable bed

1992 ◽  
Vol 38 (128) ◽  
pp. 77-92 ◽  
Author(s):  
Jürg Schweizer ◽  
Almut Iken
Keyword(s):  
Shear Stress ◽  
Functional Relationship ◽  
Water Pressure ◽  
Important Variable ◽  
Sliding Velocity ◽  
Sliding Motion ◽  
Basal Ice ◽  
Bed Separation ◽  
Basal Shear

AbstractThe classic sliding theories usually assume that the sliding motion occurs frictionlessly. However, basal ice is debris-laden and friction exists between the substratum and rock particles embedded in the basal ice. The influence of debris concentration on the sliding process is investigated. The actual conditions where certain types of friction apply are defined, the effect for the case of bed separation due to a subglacial water pressure is studied and consequences for the sliding law are formulated. The numerical modelling of the sliding of an ice mass over an undulating bed, including the effect of both the subglacial water pressure and the friction, is done by using the finite-clement method. Friction, seen as a reduction of the driving shear stress due to gravity, can be included in existing sliding laws which should contain the critical pressure as an important variable. An approximate functional relationship between the sliding velocity, the effective basal shear stress and the subglacial water pressure is given.

An Approximate Method for the Calculation of Turbulent Boundary Layers in Diffusers

1957 ◽  
Vol 8 (1) ◽  
pp. 58-77 ◽  
Author(s):  
J. F. Norbury
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Pressure Gradient ◽  
Approximate Method ◽  
Boundary Layers ◽  
Stress Gradient ◽  
Adverse Pressure Gradient ◽  
Turbulent Boundary Layers ◽  
Form Parameter ◽  
Parameter Equation

SummaryAn approximate method is described for the calculation of turbulent boundary layers in which the turbulence is developed before the commencement of the adverse pressure gradient, as in most diffuser layers. It is based on a method due to Spence which has been modified and also extended to the calculation of three-dimensional diverging layers such as occur in ducts whose breadth is increasing. The velocity profiles occurring in a diverging layer are examined and it is shown that the inner part obeys the universal logarithmic law, as in two-dimensional layers. This result is used to obtain an equation for the form parameter in diverging layers, by substitution in the equation of motion and incorporation of the equations of momentum and continuity for diverging flow. The form parameter equation contains a term involving the gradient of shear stress at y = θ and values of this term are obtained by the analysis of experimental data and the substitution of known values for all the other terms in the form parameter equation. Values of the term involving shear stress gradient are then correlated in terms of known boundary layer quantities, and the resulting correlation allows the formulation of a step-by-step method for the solution of the form parameter equation. This may be used in conjunction with the momentum equation to predict the boundary layer growth. It was not found possible to effect a satisfactory correlation for boundary layers on lifting aerofoils, in which the turbulence develops within the adverse pressure gradient, and the method cannot be used for the prediction of such layers. The results of a number of calculations are given.

scholarly journals Glacier hydromechanics: early insights and the lasting legacy of three works by Iken and colleagues

2010 ◽  
Vol 56 (200) ◽  
pp. 1069-1078 ◽  
Author(s):  
Gwenn E. Flowers
Keyword(s):  
Shear Stress ◽  
Vertical Velocity ◽  
Water Pressure ◽  
Ice Sheets ◽  
Strong Correlations ◽  
Velocity Measurements ◽  
Short Term ◽  
Ice Flow ◽  
Alpine Glaciers ◽  
Basal Shear

AbstractThe association between basal hydrology and glacier sliding has become nearly synonymous with the early work of Almut Iken and colleagues. Their research published in theJournal of Glaciologyfrom 1981 to 1986 made an indelible impact on the study of glacier hydromechanics by documenting strong correlations between basal water pressure and short-term ice-flow variations. With a passion for elucidating the physics of glacier-bed processes, Iken herself made fundamental contributions to our theoretical and empirical understanding of the sliding process. From the theoretical bound on basal shear stress, to the inferences drawn from detailed horizontal and vertical velocity measurements, the work of Iken and colleagues continues to inform the interpretation of data from alpine glaciers and has found increasing relevance to observations from the ice sheets.

scholarly journals Mechanisms of surface-tension-induced epithelial cell damage in a model of pulmonary airway reopening

2003 ◽  
Vol 94 (2) ◽  
pp. 770-783 ◽  
Author(s):  
Anastacia M. Bilek ◽  
Kay C. Dee ◽  
Donald P. Gaver
Keyword(s):  
Shear Stress ◽  
Epithelial Cells ◽  
Pressure Gradient ◽  
Pulmonary Surfactant ◽  
Cell Damage ◽  
Stress Gradient ◽  
Cellular Damage ◽  
Airway Reopening ◽  
Pulmonary Epithelial Cells ◽  
Observed Damage

Airway collapse and reopening due to mechanical ventilation exerts mechanical stress on airway walls and injures surfactant-compromised lungs. The reopening of a collapsed airway was modeled experimentally and computationally by the progression of a semi-infinite bubble in a narrow fluid-occluded channel. The extent of injury caused by bubble progression to pulmonary epithelial cells lining the channel was evaluated. Counterintuitively, cell damage increased with decreasing opening velocity. The presence of pulmonary surfactant, Infasurf, completely abated the injury. These results support the hypotheses that mechanical stresses associated with airway reopening injure pulmonary epithelial cells and that pulmonary surfactant protects the epithelium from this injury. Computational simulations identified the magnitudes of components of the stress cycle associated with airway reopening (shear stress, pressure, shear stress gradient, or pressure gradient) that may be injurious to the epithelial cells. By comparing these magnitudes to the observed damage, we conclude that the steep pressure gradient near the bubble front was the most likely cause of the observed cellular damage.

scholarly journals On the Analysis of Longitudinal Stress in Glaciers

1985 ◽  
Vol 31 (109) ◽  
pp. 293-302 ◽  
Author(s):  
R.M. McMeeking ◽  
R.E. Johnson
Keyword(s):  
Shear Stress ◽  
Stress Gradient ◽  
Deviatoric Stress ◽  
Surface Strain ◽  
Shear Stresses ◽  
Longitudinal Stress ◽  
Bed Topography ◽  
Standard Solution ◽  
Basal Shear ◽  
Deviatoric Stresses

Abstract In the standard solution for the stresses in a glacier or ice sheet obeying Glen’s law, the down-slope component of the weight is supported by the basal shear stress, and the longitudinal deviatoric stress is second order. However, it has been found necessary to account for the longitudinal stress gradient when relating surface to bed topography with empirical data. In addition, during rapid stretching of the glacier, perhaps during a surge, the longitudinal stress gradient becomes comparable to or larger than the shear stress, and the standard solution is not entirely valid. In this paper, we consider the analysis of the stresses and strain-rates in a glacier when the longitudinal deviatoric stresses are comparable to the basal shear stresses. In some circumstances the down-slope component of weight is not borne completely by basal shear stress to leading order and some of the weight is shifted to the longitudinal deviatoric stress gradient. This case has also been examined. The results are used to obtain expressions for basal shear stress in terms of glacier thickness, slope, surface strain-rate gradient, and ice properties.

scholarly journals Basal Sliding and Bed Separation: Is There a Connection?

1979 ◽  
Vol 23 (89) ◽  
pp. 407-408 ◽  
Author(s):  
Robert Bindschadler
Keyword(s):  
Shear Stress ◽  
Normal Stress ◽  
Water Pressure ◽  
Simple Relationship ◽  
Sliding Velocity ◽  
Separation Parameter ◽  
Basal Sliding ◽  
Bed Separation ◽  
Image Position ◽  
Basal Shear

Abstract Analysis of field data from Variegated Glacier supports the conclusion of Meier (1968) that no simple relationship between basal shear stress and sliding velocity can be found. On the other hand, an index of bed separation is defined and evaluated that correlates very well with the longitudinal variation of summer sliding velocity inferred for Variegated Glacier. This bed separation parameter is defined as where τ is the basal shear stress and is proportional to the drop in normal stress on the down-glacier side of bedrock bumps and N eff is the effective normal stress equal to the overburden stress minus the subglacial water pressure. The water-pressure distribution is calculated assuming water flow to be confined in subglacial Röthlisberger conduits. The excellent agreement between the longitudinal profiles of I and sliding velocity suggests that calculations of the variation of bed separation can be used to deduce the variation of sliding velocity in both space and time. Further, it is possible that a functional relationship can be developed that adequately represents the geometric controls on basal sliding to permit accurate predictions of sliding velocities.

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.

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