Short internal velocity variations in 1953: Supplement 5 from "Mode of flow of Saskatchewan Glacier, Alberta, Canada" (Thesis)

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Research in 1952-54 on Saskatchewan Glacier was directed toward the measurement of velocity on the surface and at depth, the surface and bedrock topography, ablation, and structures produced by flow. These field data are used to test current theories of flow and to derive new conclusions about the flow of a valley glacier. Positions in space of 51 velocity stations fixed in the ice were computed from triangulation surveys. Summer velocities are generally greater than yearly velocities. Short interval (1/2-1 day) observations recorded great velocity fluctuations and occasional backward movements. Some of these fluctuations represent domains not over 100 feet in extent. Dispersion values indicate that jerkiness is probably due to irregular shearing and is not predominantly perpendicular to crevasses. Dispersion of velocity decreases with increasing time intervals of measurement. Maximum surface velocity of 383 fpy occurs at the firn limit; velocity decreases unevenly along the midglacier line to 12 fpy at the terminus. Velocity vectors plunge below the surface along the centerline from above the firn limit to 1.3 miles below. Further downglacier the vectors rise out from the surface and the angular divergence increases both downglacier and toward the margins. The flow of ice toward the surface is constant at 10 fpy in the lower 3 miles. Rates of surface lowering computed from these data and ablation data agree roughly with independently measured thinning. Velocity gradients in an area of detailed study are analyzed to determine the surface strain rate field. Deformation is largely caused by the transverse gradient of the longitudinal velocity. Longitudinal and transverse extensions and compressions were measured. One principal strain rate trajectory lies along the flow centerline; a trajectory of maximum shearing strain rate parallels the valley wall at the margin. Velocity to a depth of 140 feet decreases exponentially. The flow law of ice is determined by an analysis of this short vertical profile and a transverse velocity profile on the surface. The two sets of data give consistent results which agree with results from other glaciers, and suggest that the flow law is unaffected by either hydrostatic pressure or extending or compressing flow. The strain rate cannot be expressed as a simple power function of the stress. A viscous-like flow appears to predominate at low stresses. Above a shear stress of 0.7 bar the flow velocity changes much more rapidly with slight changes in stress. The derived flow law is used to compute velocity as a function of depth and the mass-budget. These results show that the ice currently being supplied to the surface is not as great as the surface ablation but is just sufficient to keep the glacier thinning at an unchanged rate in time. Computed streamlines parallel the bedrock channel closely. Three main classes of features in the ice are distinguished: (1) primary sedimentary layering, (2) secondary flow foliation and (3) secondary cracks and crevasses. Primary stratification is flat-lying in general but wrinkled longitudinally in detail. Foliation generally dips steeply, strikes longitudinally, and shears other structures. However, some foliation attitudes do not relate to measured directions of maximum shearing strain rate at the point of observation or at any conceivable point of origin. The orientation of the most prominent set of cracks agrees approximately with measured trajectories of principal compressing strain rate. Other minor sets of cracks are related to trajectories of maximum shearing strain rate.

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