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.

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

2015 ◽  
Vol 3 (4) ◽  
pp. 1143-1178 ◽  
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 ◽  
Basal Sliding ◽  
Order Stress ◽  
Basal Shear

Abstract. Shear stress at the base of glaciers controls basal sliding and is therefore immensely important for glacial erosion and landscape evolution 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 over length-scales on the order of the grid resolution. In addition, we find that subglacial shear stress is relatively uniform and insensitive to suble changes in local topographic relief. Following these initial stress benchmark experiments, 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.

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 Shear Stress at the Base of a Rigidly Rotating Cirque Glacier

1971 ◽  
Vol 10 (58) ◽  
pp. 31-37 ◽  
Author(s):  
J. Weertman
Keyword(s):  
Shear Stress ◽  
Rigid Body ◽  
Three Dimensional ◽  
Spherical Shape ◽  
Ice Thickness ◽  
Shear Stresses ◽  
Two Dimensional ◽  
Body Rotation ◽  
Recent Compilation ◽  
Basal Shear

AbstractThe value of the basal shear stress is derived for two-dimensional and three-dimensional cirque glaciers. It is assumed that a cirque glacier moves primarily by a rigid-body rotation over a bed of cylindrical or spherical shape. In the region of maximum ice thickness the new value of the basal shear stress is only about one half that derived from equations in common use in the literature. The new expression for the basal shear stress of a cirque glacier is used to correct a data point in Paterson’s recent compilation of measured sliding velocities and basal shear stresses of glaciers.

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 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 Shear Stress at the Base of a Rigidly Rotating Cirque Glacier

1971 ◽  
Vol 10 (58) ◽  
pp. 31-37 ◽  
Author(s):  
J. Weertman
Keyword(s):  
Shear Stress ◽  
Rigid Body ◽  
Three Dimensional ◽  
Spherical Shape ◽  
Ice Thickness ◽  
Shear Stresses ◽  
Two Dimensional ◽  
Body Rotation ◽  
Recent Compilation ◽  
Basal Shear

The value of the basal shear stress is derived for two-dimensional and three-dimensional cirque glaciers. It is assumed that a cirque glacier moves primarily by a rigid-body rotation over a bed of cylindrical or spherical shape. In the region of maximum ice thickness the new value of the basal shear stress is only about one half that derived from equations in common use in the literature. The new expression for the basal shear stress of a cirque glacier is used to correct a data point in Paterson’s recent compilation of measured sliding velocities and basal shear stresses of glaciers.

scholarly journals Comparison of Three Contemporary Flow Laws in a Three-Dimensional, Time-Dependent Glacier Model

1973 ◽  
Vol 12 (66) ◽  
pp. 361-373 ◽  
Author(s):  
L. A. Rasmussen ◽  
W. J. Campbell
Keyword(s):  
Shear Stress ◽  
Steady State ◽  
Large Scale ◽  
Three Dimensional ◽  
Time Dependent ◽  
Flow Law ◽  
Glacier Flow ◽  
Basal Shear ◽  
Basal Flow ◽  
Flow Laws

A numerical model for three-dimensional, time-dependent glacier flow (Campbell and Rasmussen, 1970) treated the ice as a Newtonian viscous fluid and related its dynamics to two large-scale bulk parameters: the viscosity v determining the ice-to-ice friction, and a basal friction parameter A determining the ice-to-rock friction. The equations were solved using the relatively simple flow law of Bodvarsson (1955) in which the basal shear stress is proportional to volume transport. Recent research suggests that a more realistic basal flow law is one in which the basal shear stress to some lower power (1–3) is either proportional to the vertically averaged velocity (Glen, 1958; Nye, 1960, 1963[a], [b], [c], 1965[a], [b], [c]) or to the ratio of the vertically averaged velocity to glacier thickness (Budd and Jenssen, in press).In the present study a generalized flow law incorporating all of the above bulk basal flow laws is applied to the Campbell–Rasmussen momentum equation to form a generalized two-dimensional transport equation, which, when combined with the continuity equation, yields a numerically tractable set of equations for three-dimensional, time-dependent glacier flow. Solutions of the model are shown for steady-state flow and surge advance and recovery for a typical valley glacier bed for powers 1, 2, and 3 for each of the basal flow laws for a steady-state climate input and a given ice-to-ice viscosity parameter.

scholarly journals Ice flow models and glacial erosion over multiple glacial–interglacial cycles

2015 ◽  
Vol 3 (1) ◽  
pp. 153-170 ◽  
Author(s):  
R. M. Headley ◽  
T. A. Ehlers
Keyword(s):  
Landscape Evolution ◽  
Numerical Models ◽  
Higher Order ◽  
Glacial Erosion ◽  
Distinctive Features ◽  
Flow Models ◽  
Erosion Rates ◽  
Covered Area ◽  
Order Of Magnitude ◽  
Long Timescales

Abstract. Mountain topography is constructed through a variety of interacting processes. Over glaciological timescales, even simple representations of glacial-flow physics can reproduce many of the distinctive features formed through glacial erosion. However, detailed comparisons at orogen time and length scales hold potential for quantifying the influence of glacial physics in landscape evolution models. We present a comparison using two different numerical models for glacial flow over single and multiple glaciations, within a modified version of the ICE-Cascade landscape evolution model. This model calculates not only glaciological processes but also hillslope and fluvial erosion and sediment transport, isostasy, and temporally and spatially variable orographic precipitation. We compare the predicted erosion patterns using a modified SIA as well as a nested, 3-D Stokes flow model calculated using COMSOL Multiphysics. Both glacial-flow models predict different patterns in time-averaged erosion rates. However, these results are sensitive to the climate and the ice temperature. For warmer climates with more sliding, the higher-order model yields erosion rates that vary spatially and by almost an order of magnitude from those of the SIA model. As the erosion influences the basal topography and the ice deformation affects the ice thickness and extent, the higher-order glacial model can lead to variations in total ice-covered area that are greater than 30% those of the SIA model, again with larger differences for temperate ice. Over multiple glaciations and long timescales, these results suggest that higher-order glacial physics should be considered, particularly in temperate, mountainous settings.

scholarly journals Surging Glaciers—The Dilemma Continues

1979 ◽  
Vol 24 (90) ◽  
pp. 502-503 ◽  
Author(s):  
M. F. Meier
Keyword(s):  
Shear Stress ◽  
Phenomenological Approach ◽  
Sliding Velocity ◽  
Recent Interest ◽  
Normal Behavior ◽  
Rapid Flow ◽  
Basal Sliding ◽  
Ice Velocity ◽  
Glacier Flow ◽  
Basal Shear

AbstractA glacier surge, according to most definitions, is a short-lived phase of unusually rapid glacier flow, after which the glacier returns to more normal behavior, with the surge–non-surge phases recurring on a regular or periodic basis. Recent interest is largely directed toward analyzing the effect of water at the bed on the periodic change in flow regime and on the rapid flow during a surge phase. For instance, study of a local depression of basal shear stress that dependson a “friction lubrication factor” which becomes important as the ice velocity increases, is one promising phenomenological approach. An important physical approach focuses on a water “collection zone” that occurs where and when the longitudinal pressure gradient in the subglacial wtaer film approaches zero. The data necessary for properly verifying these and other similar theories do not yet exist. Computer modeling of rapidly-surging glaciers based on a “friction lubrication factor” has been quite successful in duplicating their major features. Once rapid movement (102–103m a–1) has begun, sufficient water is generated at the bed, from ice melted by heat dissipated in sliding, to produce some decoupling of the glacier from its bed and to maintain the surge, but only if this water is not lost by rapid drainage. Some glaciers exhibit periodic pulses in which the basal sliding velocity during the fastest part of the pulses appears to be in the range for “normal” glaciers (<102m a–1). Some evidence suggests a continuum of behavior from steady (normal) glaciers through these “mini-surges” to classic surges. This continuum and the “mini-surges” seem to be difficult to explain quantitatively by existing theories. A few glaciers flow continuously at surging speeds (>103m a–1) in certain reaches. The up-glacier transition reaches show speeds decreasing to “nonrmal” with no indication of intermediate surging regime, but the down-glacier transition reaches may be areas where surges are triggered.

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