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Fahnestock, M.A., 1991

Hydrologic control of sliding velocity in two Alaskan glaciers: Observation and theory

Bibliographic Reference

Fahnestock, M.A., 1991, Hydrologic control of sliding velocity in two Alaskan glaciers: Observation and theory: Pasadena, California, California Institute of Technology, Ph.D. dissertation, 111 p., illust., maps.


Hydrologic control of sliding velocity is considered for two glaciers in Alaska. Mini-surges on Variegated Glacier prior to its 1982-83 surge were attributed to pulses of water at the bed. 1986 fieldwork documented the propagation of two of these disturbances. A model of the basal water system with pressure-dependent conductivity and storage is used to investigate the propagation of a water pulse. A localized input produces a downglacier propagating front with speeds similar to mini-surges. A system with a non-linear conductivity response to water pressure (suggestive of a cavity system) is required to match field observations. A model tunnel system with a storage term will propagate a disturbance, but will not match the abrupt rise times seen in boreholes. Columbia Glacier, a tidewater glacier, experiences velocity fluctuations in response to storms and increased ablation. 1987 fieldwork by the University of Colorado, the USGS and Caltech collected records of velocity, water input and discharge, and basal water pressure. These data show a complex pattern of behavior correlated with variations in hydrologic parameters. An estimate of the change in volume of stored water is made by relating input (estimated from the filling rate of a lake and also from meteorological records) to discharge. Variations in velocity on time scales longer than one day are explained by changes in the volume of stored water. A slowdown was coincident with a drop in stored water volume of 0.1 m3/m2. A cavity/tunnel model is proposed to explain the melt season behavior of Columbia Glacier. The cavities are responsible for the correlation between stored water volume and velocity, while tunnels are responsible for seasonal variations in velocity. A model based on a sliding law and a pressure distribution at the bed determined by discharge through a tunnel system can explain the difference between winter and summer behavior. This model produces the late-melt-season pulse in velocity and calving at the terminus. The model may explain the previously recognized correlation between calving rate and water discharge. The complicated behavior of these glaciers can be understood at a simple level from variations in the hydrologic systems.

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