3-D Pattern of Stress and Velocity in Glaciers: Haut Glacier d'Arolla, Switzerland

An integrated field-work and modelling strategy was used to study how basal conditions affect the 3-D distribution of stress and velocity within the ablation area of Haut Glacier d'Arolla, Switzerland. Fieldwork took place between May 1998 and August 1999 when we monitored spatial and temporal variations in basal water pressure (pressure transducers), sediment thickness, texture and strength (penetrometer and ploughmeters), surface motion (terrestrial surveying), internal ice deformation (tilt cells and inclinometer), basal sliding (drag spools) and subglacial sediment deformation (tilt cells). There were distinct patterns of surface, internal and basal motion that varied between spring, summer, and fall/winter, which reflect patterns of basal water pressure and sediment characteristics. These, in turn, are influenced by the proximity to subglacial drainage axes. For example, during spring, the glacier surface speeds up from ~2 cm/d to >10 cm/d over short periods of a few days. The zone of maximum surface velocity shifts from the centre of the glacier towards the major drainage axes where water-pressure fluctuations are greatest and sediments are relatively thin and coarse grained. The relative importance of basal motion to surface motion increases during these "spring events", particularly towards the drainage axes and less so far away from them. The field data have been used to drive and test a 3-D glacier-flow model. The model is able to reproduce the spring, summer, fall/winter, and annual patterns and magnitudes of movement very accurately. Discrepencies between model calculations and field measurements suggest that glacier ice, particularly at depth, may become softer during spring and summer, especially around subglacial drainage axes.