Thermally controlled glacier surging

Bakaninbreen in Svalbard and Trapridge Glacier in Yukon Territory, Canada, are two prominent examples of surging glaciers which are thought to be controlled by their thermal regime. Both glaciers have developed large bulges which have propagated forward as travelling wave fronts, and which are thought to divide relatively stagnant downstream cold-based ice from faster-moving warm-based upstream ice. Additionally, both glaciers are underlain by a wet, metres thick layer of deforming till. We develop a simple model for the cyclic surging behaviour of these glaciers, which interrelates the motion of the ice and till through a description of the subglacial hydrology. We find that oscillations (surges) can occur if the subglacial hydrological transmissivity is sufficiently low and the till layer is sufficiently thin, and we suggest that these oscillations are associated with the development and propagation of a travelling wave front down the glacier. We therefore interpret the travelling wave fronts on both Trapridge Glacier and Bakaninbreen as manifestations of surges. In addition, we find that the violence of the surge in the model is associated with the resistance to ice flow offered by undulations in the bed, and the efficiency with which occasional hydrological events can release water accumulated at the glacier sole.

Surface and bed topography of Trapridge Glacier, Yukon Territory, Canada: digital elevation models and derived hydraulic geometry

Measurements of ice thickness and surface elevation are prerequisite to many glaciological investigations. A variety of techniques has been developed for interpretation of these data, including means of constructing regularly gridded digital elevation models (DEMs) for use in numerical studies. Here we present a simple yet statistically sound method for processing ice-penetrating radar data and describe a technique for interpolating these data onto a regular grid. DEMs generated for Trapridge Glacier, Yukon Territory, Canada, are used to derive geometric quantities that give preliminary insights into the underlying basin-scale hydrological system. This simple geometric analysis suggests that at low water pressures a dendritic drainage network exists that evolves into a uniaxial morphology as water pressure approaches flotation. These predictions are compared to hydraulic connection probabilities based on borehole drilling.

Evidence for temporally varying “sticky spots” at the base of Trapridge Glacier, Yukon Territory, Canada

During the 1992 summer field season we installed arrays of “plough-meters” and water-pressure transducers beneath Trapridge Glacier. Yukon Territory, Canada, to study hydromechanical coupling at the ice–bed interface. Diurnal signals recorded with two of these ploughmeters appear to correlate with fluctuations in sub-glacial water pressure. These diurnal variations can be explained by changes in basal resistance to sliding as mechanical conditions at the bed vary temporally in response to changes in the subglacial hydrological system. We propose that a lubricating water film, associated with high water pressures, promotes glacier sliding, whereas low pressures cause increased basal drag resulting in “sticky” areas. Using a theoretical model, we analyze the sliding motion of glacier ice over a flat surface having variable basal drag and show that a consistent explanation can be developed. Results from our model calculations provide strong support for the existence of time-varying sticky spots which are associated with fluctuations in subglacial water pressure.

Evidence for temporally varying “sticky spots” at the base of Trapridge Glacier, Yukon Territory, Canada

During the 1992 summer field season we installed arrays of “plough-meters” and water-pressure transducers beneath Trapridge Glacier. Yukon Territory, Canada, to study hydromechanical coupling at the ice–bed interface. Diurnal signals recorded with two of these ploughmeters appear to correlate with fluctuations in sub-glacial water pressure. These diurnal variations can be explained by changes in basal resistance to sliding as mechanical conditions at the bed vary temporally in response to changes in the subglacial hydrological system. We propose that a lubricating water film, associated with high water pressures, promotes glacier sliding, whereas low pressures cause increased basal drag resulting in “sticky” areas. Using a theoretical model, we analyze the sliding motion of glacier ice over a flat surface having variable basal drag and show that a consistent explanation can be developed. Results from our model calculations provide strong support for the existence of time-varying sticky spots which are associated with fluctuations in subglacial water pressure.

Surface and bed topography of Trapridge Glacier, Yukon Territory, Canada: digital elevation models and derived hydraulic geometry

Measurements of ice thickness and surface elevation are prerequisite to many glaciological investigations. A variety of techniques has been developed for interpretation of these data, including means of constructing regularly gridded digital elevation models (DEMs) for use in numerical studies. Here we present a simple yet statistically sound method for processing ice-penetrating radar data and describe a technique for interpolating these data onto a regular grid. DEMs generated for Trapridge Glacier, Yukon Territory, Canada, are used to derive geometric quantities that give preliminary insights into the underlying basin-scale hydrological system. This simple geometric analysis suggests that at low water pressures a dendritic drainage network exists that evolves into a uniaxial morphology as water pressure approaches flotation. These predictions are compared to hydraulic connection probabilities based on borehole drilling.