Modelling ice-bed coupling during a glacier speed-up event: Haut Glacier d'Arolla, Switzerland

Short-term increases in surface velocity have long been observed on alpine glaciers and more recently across the Greenland Ice Sheet. Alpine speed-up events typically occur when the subglacial drainage system is unable to accommodate a sudden and large influx of water. The ensuing backup of water in the drainage system commonly leads to a rise in subglacial water pressure, local ice-bed decoupling and enhanced sliding. Through horizontal stress coupling, this locally induced high pressure forcing also increases glacier motion in regions adjacent to the decoupled regions by enhanced, non-locally forced basal motion. These speed-up events demonstrate the importance of subglacial hydromechanical adjustments for patterns of glacier flow. This article applies Kavanaugh and Clarke's (2006) numerical model, which simultaneously solves equations that describe the time-evolution of subglacial sediment pore-water pressure, bed deformation, glacier sliding and ice-dynamical shear stress transfer, to Haut Glacier d'Arolla, Switzerland. Model input parameters are determined from field observations and interpretations of the basal hydrological system beneath the main glacier tongue and in particular during an approximate fivefold increase in surface velocity observed in June 1998 (Mair et al., 2003). Model output, in the form of synthetic subglacial instrument responses, is compared with signals recorded by instruments emplaced at the glacier bed. Sensitivity analyses demonstrate that while the model outputs are sensitive to input parameters, modelled sediment strength and shear strain rate responses show good agreement, in both magnitude and form, with values recorded in the field under specific realistic parameterizations. The qualitative agreement between measurements and modelled output suggests that the key assumptions regarding hydraulically induced changes in sediment strength, sediment deformation and basal shear stress distribution are sound. Copyright (C) 2010 John Wiley & Sons, Ltd.