Mechanical conditions beneath a surge-type glacier
Creators: name: Fischer, Urs Heinrich
Publisher: name: University of British Columbia
Year: 2009

description: The interaction of basal processes with the subglacial drainage system is a critical issue in understanding glacier dynamics. This is especially true for glaciers that exhibit a flow instability known as “surging”, characterized by cyclical alternation between slow and fast flow regimes. It is accepted that sustained high subglacial water pressure causes glacier surging by decoupling the glacier from its bed, but how basal hydrological conditions control coupling at the ice—bed interface remains the subject of debate. I have applied new investigative techniques for measuring basal sliding and ex ploring mechanical conditions of the subglacial material at Trapridge Glacier, a small surge-type glacier in the St. Elias MOuntains, Yukon Territory, Canada. The data pre sented in this thesis are unique and important because they were obtained directly at the glacier bed using new subglacial sensors, designed and constructed at the University of British Columbia. Basal sliding is measured with a “drag spool” which consists of a multi-turn potentiometer connected to a spooled string. The drag spool is suspended within the borehole close to the glacier bed, and measures continuously the length of string payed out to an anchor in the bed. Mechanical interactions between the glacier and its bed are sensed with a device, dubbed a “ploughmeter”, which is essentially a steel rod instrumented with strain gauges. The ploughmeter is installed at the bottom of a borehole with its tip protruding into subglacial sediment, and measures the bend ing moment acting near the rod tip which is dragged through the sediment as a result of glacier sliding. Field data from the drag spool instruments show that, during the melt season, basal sliding can account for up to 45—65% of the total flow observed at the glacier sur face. The contribution from ice creep is known to be small, so most of the remaining surface motion must be attributed to subglacial sediment deformation. Ploughmeter measurements reveal a spatial variability in subglacial processes or sediment texture. Quantitative analysis of the interaction of the ploughmeter with the basal layer yields estimates of rheological parameters. If the sediment is assumed to behave as a Newto nian viscous fluid, the estimated effective viscosity is 3.0 x 10⁹—3.1 x 10¹⁰ Pas; if it is assumed to behave as an ideal plastic solid, the estimated yield strength is 48—57 kPa. For clast-rich sediments, the rate of collision between clasts and the ploughmeter pro vides us with an estimated basal sliding rate of 30—50 mmd⁻¹, in good agreement with the drag spool results. Diurnal signals recorded with both types of instruments appear to be correlated to fluctuations in subglacial water pressure. These diurnal variations in the response of both instruments can be interpreted in terms of changes in sliding velocity and basal resistance as the mechanical conditions at the glacier bed vary in response to changes in the subglacial water system. I have deveioped theoretical models that describe the sliding motion of ice over a surface with variable basal drag and have demonstrated how the models can be used in numerical simulations to generate data, which can be compared with my field observations. The results from my model calculations provide strong evidence for time-varying sticky spots and for stick—slip sliding motion, both of which are linked to changes in basal lubrication in response to fluctuations in subglacial water pressure.