scholarly journals Application of a general sliding law to simulating flow in a glacier cross-section

1992 ◽  
Vol 38 (128) ◽  
pp. 182-190 ◽  
Author(s):  
Jonathan M. Harbor
Keyword(s):  
Shear Stress ◽  
Cross Section ◽  
Empirical Data ◽  
Effective Pressure ◽  
Land Form ◽  
Development Models ◽  
Basal Sliding ◽  
Basal Shear ◽  
Form Development ◽  
Glacier Motion

AbstractObservations at Athabasca Glacier and elsewhere suggest that basal sliding can account for a very significant part of total glacier motion, and that sliding rates vary significantly across a glacier section. The ability to model such spatial variations in basal velocities is important in understanding flow in valley glaciers, as well as in predicting spatial patterns of glacial erosion that drive land-form development models. With a sliding law in which the basal velocity is dependent on the basal shear stress and inversely dependent on the effective pressure at the bed, it is possible to predict an overall flow pattern that is consistent with the empirical data, if it is assumed that friction increases close to the margin of a glacier.

scholarly journals Application of a general sliding law to simulating flow in a glacier cross-section

1992 ◽  
Vol 38 (128) ◽  
pp. 182-190 ◽  
Author(s):  
Jonathan M. Harbor
Keyword(s):  
Shear Stress ◽  
Cross Section ◽  
Empirical Data ◽  
Effective Pressure ◽  
Land Form ◽  
Development Models ◽  
Basal Sliding ◽  
Basal Shear ◽  
Form Development ◽  
Glacier Motion

AbstractObservations at Athabasca Glacier and elsewhere suggest that basal sliding can account for a very significant part of total glacier motion, and that sliding rates vary significantly across a glacier section. The ability to model such spatial variations in basal velocities is important in understanding flow in valley glaciers, as well as in predicting spatial patterns of glacial erosion that drive land-form development models. With a sliding law in which the basal velocity is dependent on the basal shear stress and inversely dependent on the effective pressure at the bed, it is possible to predict an overall flow pattern that is consistent with the empirical data, if it is assumed that friction increases close to the margin of a glacier.

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.

scholarly journals Basal shear stress under alpine glaciers: insights from experiments using the iSOSIA and Elmer/Ice models

2016 ◽  
Vol 4 (1) ◽  
pp. 159-174 ◽  
Author(s):  
C. F. Brædstrup ◽  
D. L. Egholm ◽  
S. V. Ugelvig ◽  
V. K. Pedersen
Keyword(s):  
Shear Stress ◽  
Landscape Evolution ◽  
Empirical Studies ◽  
Three Dimensional ◽  
Temporal Distribution ◽  
Higher Order ◽  
Glacial Erosion ◽  
Significant Control ◽  
Basal Sliding ◽  
Basal Shear

Abstract. Shear stress at the base of glaciers exerts a significant control on basal sliding and hence also glacial erosion in arctic and high-altitude areas. However, the inaccessible nature of glacial beds complicates empirical studies of basal shear stress, and little is therefore known of its spatial and temporal distribution. In this study we seek to improve our understanding of basal shear stress using a higher-order numerical ice model (iSOSIA). In order to test the validity of the higher-order model, we first compare the detailed distribution of basal shear stress in iSOSIA and in a three-dimensional full-Stokes model (Elmer/Ice). We find that iSOSIA and Elmer/Ice predict similar first-order stress and velocity patterns, and that differences are restricted to local variations at length scales of the order of the grid resolution. In addition, we find that subglacial shear stress is relatively uniform and insensitive to subtle changes in local topographic relief. Following the initial comparison studies, we use iSOSIA to investigate changes in basal shear stress as a result of landscape evolution by glacial erosion. The experiments with landscape evolution show that subglacial shear stress decreases as glacial erosion transforms preglacial V-shaped valleys into U-shaped troughs. These findings support the hypothesis that glacial erosion is most efficient in the early stages of glacial landscape development.

Sliding with Cavity Formation

1987 ◽  
Vol 33 (115) ◽  
pp. 255-267 ◽  
Author(s):  
A. C. Fowler
Keyword(s):  
Exact Solution ◽  
Shear Stress ◽  
Power Law ◽  
Drainage System ◽  
Cavity Formation ◽  
Effective Pressure ◽  
Dynamic Phenomenon ◽  
Basal Shear ◽  
Tunnel System

AbstractWe present a model for the determination of a sliding law in the presence of subglacial cavitation. This law determines the basal stress at a clean ice‒bedrock interface in terms of the velocity and effective pressure. The method is based on an exact solution of the Nye—Kamb (linearly viscous) sliding problem with cavities, and uses ideas of Lliboutry (1979) to construct, via renormalization methods, an approximate law for general bedrock form. We show that, for a bedrock whose spectrum has a power‒law behaviour, one obtains a sliding law which gives the basal shear stress proportional to a power of the velocity, and to a power of the effective pressure.The effect of subglacial cavitation on the drainage system is examined, using recent ideas of Kamb. For sufficiently high velocities, drainage through a Röthlisberger tunnel system is unstable, and drainage takes place through the linked system of cavities. This leads to a reduction of the effective pressure, and by taking account of this, one can rewrite the sliding law in terms of stress and velocity only.This sliding law can be multi‒valued, and it is suggested that this underlies the dynamic phenomenon of surges.

scholarly journals Evolution of Variegated Glacier, Alaska, U.S.A., Prior to its Surge

1988 ◽  
Vol 34 (117) ◽  
pp. 154-169 ◽  
Author(s):  
C. F. Raymond ◽  
W. D. Harrison
Keyword(s):  
Shear Stress ◽  
Seasonal Variation ◽  
Normal Stress ◽  
Deformation Rate ◽  
Surface Slope ◽  
Thickness Increase ◽  
Effective Normal Stress ◽  
Inverse Effect ◽  
Basal Sliding ◽  
Basal Shear

AbstractDuring the decade prior to its 1982–83 surge, Variegated Glacier experienced progressive changes in geometry and velocity. It thickened in the upper 60% and thinned in the lower 40% of its 20 km length. Thickness changes were up to 20%. Annual velocity increased by up to 500%, reaching a maximum of 0.7 m d−1 in the year before surge onset. Amplitude of seasonal variation in velocity increased up to 0.3 m d−1 by 1978, but did not increase markedly after that. The changes in velocity were larger than predicted from changes in deformation rate caused by changes in shear stress and depth. This anomalous velocity was especially large after 1978 in the zone of thickening on the upper glacier. If it is assumed to arise from basal sliding, the inferred pattern of sliding shows qualitative features consistent with a direct effect from basal shear stress and an inverse effect from effective normal stress. A drop in effective normal stress in a zone of decreasing surface slope up-glacier from the largest thickness increase may have been significant in the initiation of surge motion in 1982.

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 Evolution of Variegated Glacier, Alaska, U.S.A., Prior to its Surge

1988 ◽  
Vol 34 (117) ◽  
pp. 154-169 ◽  
Author(s):  
C. F. Raymond ◽  
W. D. Harrison
Keyword(s):  
Shear Stress ◽  
Seasonal Variation ◽  
Normal Stress ◽  
Deformation Rate ◽  
Surface Slope ◽  
Thickness Increase ◽  
Effective Normal Stress ◽  
Inverse Effect ◽  
Basal Sliding ◽  
Basal Shear

Abstract During the decade prior to its 1982–83 surge, Variegated Glacier experienced progressive changes in geometry and velocity. It thickened in the upper 60% and thinned in the lower 40% of its 20 km length. Thickness changes were up to 20%. Annual velocity increased by up to 500%, reaching a maximum of 0.7 m d−1 in the year before surge onset. Amplitude of seasonal variation in velocity increased up to 0.3 m d−1 by 1978, but did not increase markedly after that. The changes in velocity were larger than predicted from changes in deformation rate caused by changes in shear stress and depth. This anomalous velocity was especially large after 1978 in the zone of thickening on the upper glacier. If it is assumed to arise from basal sliding, the inferred pattern of sliding shows qualitative features consistent with a direct effect from basal shear stress and an inverse effect from effective normal stress. A drop in effective normal stress in a zone of decreasing surface slope up-glacier from the largest thickness increase may have been significant in the initiation of surge motion in 1982.

scholarly journals Flow in a Transverse Section of Athabasca Glacier, Alberta, Canada

1971 ◽  
Vol 10 (58) ◽  
pp. 55-84 ◽  
Author(s):  
C.F. Raymond
Keyword(s):  
Strain Rate ◽  
Shear Strain ◽  
Cross Section ◽  
Shear Strain Rate ◽  
Sliding Velocity ◽  
Longitudinal Flow ◽  
Glacier Surface ◽  
Basal Sliding ◽  
Basal Shear ◽  
Longitudinal Strain Rate

Measurements of ice deformation at the surface and at depth in the Athabasca Glacier, Canada, reveal for the first time the pattern of flow in a nearly complete cross-section of a valley glacier, and make it possible to test the applicability of experimental and theoretical concepts in the analysis of glacier flow. Tilting in nine bore holes (eight holes essentially to the bottom at depth about 300 m) was measured with a newly developed electrical inclinometer. The new instrument permitted bore-hole configurations to be determined with greater speed and accuracy than possible with earlier methods. The measurements define the distribution of the velocity vector and the strain-rate tensor over about 70% of the area of the glacier cross-section.The main longitudinal component of flow has the following general features: (1) basal sliding velocity which exceeds 70% of the surface velocity over half of the width of the glacier, (2) marginal sliding velocity (not more than a few meters per year) much less than basal sliding velocity at the center-line (about 40 m a-1), (3) marginal shear strain-rate near the valley walls two to three times larger than the basal shear strain-rate near the center-line (0.1 a-1).The observed longitudinal flow is significantly different from that expected from theoretical analysis of flow in cylindrical channels (Nye, 1965). The relative strength of marginal and basal shear strain-rate is opposite to that expected from theory. In addition, the longitudinal flow velocity averaged over the glacier cross-section (which determines the flux of ice transported) is larger by 12% than the average flow velocity seen at the glacier surface, whereas it would be essentially the same if the theoretical prediction were correct. These differences are caused to a large extent by the contrast between the actual distribution of sliding velocity and the constant sliding velocity for which the theoretical analysis holds. The observed relation between marginal and basal sliding velocity is probably a general flow feature in valley glaciers, and may be caused by lateral variation of water pressure at the ice-rock contact. The observed pattern of longitudinal velocity over the section also shows in detail certain additional features incompatible with the theoretical treatment, even after the difference in boundary conditions (distribution of sliding velocity) is taken into account.Longitudinal strain-rate (a compression of about 0.02 a-1at the surface) decreases with depth, becoming nearly o at the bed in the center of the glacier, which confirms a prediction by Savage and Paterson (1963). The depth variation cannot be explained completely by overall bending of the ice mass as a result of a longitudinal gradient in the curvature of the bed, and is at variance with existing theories, which require the longitudinal strain-rate to be constant with depth.Motion transverse to the longitudinal flow occurs in a roughly symmetric pattern of diverging margin-ward flow, with most of the lateral transport occurring at depth in a fashion reminiscent of extrusion flow. The observed lateral velocities averaged over depth (up to 1.9 m a-1) are compatible with the lateral flux required to maintain equilibrium of the marginal portions of the glacier surface under ablation (about 3.7 m of ice per year) and are driven by the convex lateral profile of the ice surface.

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

AbstractAnalysis 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 Neff 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 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.

Sign in / Sign up

Export Citation Format

Share Document