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

• Main Author: Fahnestock, Mark Allen
• Format: Thesis
• Language: English
• Published: 1991
• Subjects: glacier; glaciers; Alaska
• Online Access: https://thesis.library.caltech.edu/2920/; https://thesis.library.caltech.edu/2920/1/Fahnestock_ma_1991.pdf; https://resolver.caltech.edu/CaltechETD:etd-07182006-134101

Short term variations in the velocities of glaciers reflect changes in the processes which determine sliding velocity. The role of water in these processes is considered for certain types of variable behavior observed on two glaciers in Alaska. Pulses of increased velocity on Variegated Glacier (mini-surges) prior to its 1982-83 surge have been attributed to pulses of water at the glacier bed. Our field program in 1986 demonstrated that mini-surges still occurred following the surge; the propagation of two such disturbances over part of the upper reach of the glacier was documented. The mini-surges of 1986 had substantially lower peak velocities but only slightly lower propagation velocities than the pre-surge mini-surges. The first of the mini-surges observed in 1986 originated in the tributary, the second originated in the upper reach of the main glacier. A model of the basal water system with pressure dependent conductivity and storage is developed to investigate the conditions necessary for propagation of a pulse of water. The response of this system to the introduction of a localized increase in input is followed with a finite difference formulation. The extra input of water produces a downglacier propagating front, which, for reasonable values of porosity and pre-event conditions, moves at speeds similar to those observed for mini-surges. The relation between the non-linearity of the pressure dependence in this model and the shape and history of the propagating disturbances is investigated using conductivity relations which have linear, quadratic, and cubic dependence on pressure, and one with an inverse dependence on the effective pressure. The modeling indicates that a system with a non-linear change of conductivity in response to a change in water pressure (suggestive of a cavity system) is required to match the field observations. The shapes of waves which propagate with unchanging form in this system can be found theoretically; the numerical model generates these waves when the input rate is held ...