Abstract
AbstractTheory and experiments indicate that ice–bed separation during glacier slip over 2-D hard beds causes basal shear stress to reach a maximum at a particular slip velocity and decrease at higher velocities. We use the sliding theory of Lliboutry (1968) to explore how friction between debris particles in sliding ice and a rock bed affects this relationship between shear stress and slip velocity. Particle–bed contact forces and associated debris friction increase with increasing slip velocity, owing to increased rates of ice convergence with up-glacier facing surfaces. However, debris friction on diminished areas of the bed counteracts this effect as cavities grow. Thus, friction from debris alone increases only slightly with slip velocity, and for sediment particles larger than ~60 mm in diameter, debris friction peaks and decreases with increasing slip velocity. The effect on the sliding relationship is to steepen its rising limb and shift its shear stress peak to a slightly higher velocity. These results, which exclude the effect of debris friction on cavity size and debris concentrations above ~15%, indicate that the effect of debris in ice is to increase basal shear stress but not significantly change the form of the sliding relationship.
Highlights
The relationship between glacier sliding speed and the shear stress at glacier beds has been a major uncertainty in efforts to model glacier flow for over 60 years (Weertman, 1957)
Of many proposed sliding rules, none has greater potential for contributing to fast glacier flow than the one first advocated by Lliboutry (1965, 1968, 1979) in which ice can separate from the lee surfaces of sinusoidal bed undulations
In this case, neglecting regelation, the shear stress increases with steady sliding speed, reaches a maximum, and decreases over a commonly wide range of sliding speed. This decrease in stress with increasing speed, which we call rate-weakening drag, results from lee side cavities that increase their size with increasing speed; with increasing speed, diminished zones of ice–bed contact on convex bumps are inclined up-glacier at smaller angles, decreasing drag
Summary
The relationship between glacier sliding speed and the shear stress at glacier beds has been a major uncertainty in efforts to model glacier flow for over 60 years (Weertman, 1957). Of many proposed sliding rules, none has greater potential for contributing to fast glacier flow than the one first advocated by Lliboutry (1965, 1968, 1979) in which ice can separate from the lee surfaces of sinusoidal bed undulations In this case, neglecting regelation, the shear stress increases with steady sliding speed, reaches a maximum, and decreases over a commonly wide range of sliding speed. Enhanced rates of basal melting and bed-parallel ice extension, caused by increasing normal stresses over diminished zones of ice–bed contact This effect motivates the hypothesis we wish to test: that higher particle bed-contact forces at higher slip velocities may lessen or eliminate the rate-weakening drag of sliding models with ice– bed separation (e.g. Lliboutry, 1968; Fowler, 1986). We find that debris-bed friction does not eliminate or significantly reduce rate-weakening drag
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