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Humphrey, N.F., 1987

Basal hydrology of a surge-type glacier; observations and theory relating to Variegated Glacier

Bibliographic Reference

Humphrey, N.F., 1987, Basal hydrology of a surge-type glacier; observations and theory relating to Variegated Glacier: University of Washington, Seattle, Ph.D. dissertation, 206 p.


Discharges of water, suspended sediment, and solutes were measured in the outlet streams of Variegated Glacier. The data interval spans 1980 to 1984 and includes the major surge of 1982-1983. The data have been used to infer basal-water velocity distributions. During the early melt seasons, before the surge, the upper half of the glacier was underlain by a zone of low velocity (<0.3 m/s) and water flowed through the lower half at high velocity (>1 m/s). During surge motion, water flowed beneath the surging zone at low velocity (<0.1 m/s) and at high water velocity beneath parts unaffected by the surge. The high water velocities are consistent with flow through tunnels, while the low velocities are compatible with flow through a linked-cavity system. Estimates of basal erosion, based on suspended sediment discharge, indicate a relationship between basal sliding velocity and erosion rate corresponding to slightly less than 10-4 meters of erosion per meter of basal sliding. Total erosion during a complete surge cycle is on the order of 0.25 meters of bedrock. A fully coupled hydraulic and mechanical theory of linked-cavity systems is developed for a simple stepped-bed geometry, and used to investigate the propagation of coupled waves in basal water pressure and in glacier sliding. In steady-state conditions with longitudinally uniform ice flow, the model assumptions lead to a constant water pressure that depends only on bed geometry. With longitudinal variations in ice flow, coupling between water pressure and longitudinal stress in the ice, allows the existence of stationary waves of water pressure and other variables, in which local water pressure, cavity size, water flux and sliding velocity are not related in any simple way. The linked-cavity system supports the propagation of two kinds of down-glacier moving waves in both the basal water flow and coupled ice flow; a kinematic water wave and a generally slower longitudinal stress coupling wave caused by interference of the pressure gradients in the basal water and induced pressure gradients due to longitudinal stress gradients in the ice flow. The longitudinal coupling associated with the waves complicates the interpretation of local sliding velocity and water pressure measurements in boreholes.

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