Abrupt glacier motion and reorganization of basal shear stress following the establishment of a connected drainage system

Abstract

Three episodes of strong basal motion occurred at Trapridge Glacier, Yukon Territory, Canada, on 11 June 1995 following the establishment of a connected subglacial drainage system. Responses to these``spring events'' are noted in the records for 42 instruments and were recorded throughout the 60 000 m 2 study area. Strong basal motion during the events is indicated by ploughmeter, load-bolt and vertical-strain records, and abrupt pressure changes in several transducer records denote damage caused by extreme pressure pulses. These pressure pulses, generated by the abrupt basal motion, also resulted in the failure of seven pressure sensors. Records for pressure, turbidity and conductivity sensors indicate that basal drainage patterns did not change significantly during the events. Geophone records suggest that the episodes of basal motion were precipitated by the gradual failure of a``sticky spot'' following hydraulic connection of part of the study area. This failure resulted in the transfer of basal stress to the unconnected region of the bed during the course of the events. No evidence for strong basal motion is seen in the instrument records for several weeks following the events, suggesting that the mechanical adjustments resulted in a stable configuration of basal stresses. This event illustrates how unstable situations can be quickly accommodated by mechanical adjustments at the glacier bed.

Subglacial till behaviour derived from in situ wireless multi-sensor subglacial probes: Rheology, hydro-mechanical interactions and till formation

“…Conversely, above a certain critical threshold a positive relationship occurs, and each time this occurs a greater critical pressure is required to trigger the same response. This may reflect initial excess pore-water pressure (generated by sediment compression), followed by strengthening (increase in case stress) once deformation by grain shear, dilation and rolling has occurred (dilatancy hardening e Kavanaugh and Clarke, 2001). This results in an increase in case stress with water pressure, as sediment strength increases, despite a reduction in the applied shear stress of the ice above.…”

Oscillatory subglacial drainage in the absence of surface melt

“…3. One possible instrumental cause of such abrupt jumps is damage to the transducer diaphragm due to transient pressure spikes as discussed in Kavanaugh and Clarke (2001), but we cannot reconstruct with certainty whether this occurred in hole A1.…”

“…We have no inclinometry data for the boreholes, and although we were able to measure borehole lengths and surface elevations, we are unable to locate the bottom of the boreholes exactly; the observed offsets between the pressure records are certainly consistent with the possibility that there are insignificant hydraulic gradients between the holes. In addition, there may be calibration errors due to damage to the transducers caused by pressure spikes (Kavanaugh and Clarke, 2001). This generally affects only the offset in the calibration but not the multiplier, so the measured pressure is the actual borehole pressure plus an offset that generally cannot be determined after the fact.…”

Hydrological controls on patterns of surface, internal and basal motion during three “spring events”: Haut Glacier d’Arolla, Switzerland

“…This input may result in hydraulic and mechanical instabilities (Kavanaugh and Clarke, 2001). Such instabilities typically last a few hours or days and are often called ''spring events'' (R˛thlisberger and Lang, 1987).…”