The relationship between subglacial water pressure and velocity of Findelengletscher, Switzerland, during its advance and retreat

Abstract

AbstractFindelengletscher, Switzerland, advanced about 250 m between 1979 and 1985, and retreated thereafter. Subglacial water pressure, surface velocity and surface strain rate were determined at several sites. The measurements were made early in the melt seasons of 1980, 1982, 1985 and 1994 and in the autumn of 1983 and the winter of 1984. Changes of surface geometry were assessed from aerial photographs.The estimated basal shear stress changed little between 1982 and 1994. Nevertheless, large changes in the relationship of subglacial water pressure and surface velocity were observed, which cannot be reconciled with the most commonly used sliding law unless it is modified substantially. Consideration of possible reasons indícales that a change in the subglacial drainage system occurred, probably involving a change in the degree of cavity interconnection. Isolated cavities damp the variations in sliding velocity that normally result from changes in water pressure, because the pressure in isolated cavities decreases as the sliding speed increases. In contrast, by transmitting water-pressure fluctuations to a larger area of the bed, interconnected cavities amplify the effect of water-pressure fluctuations on sliding speed. Thus, we suggest that an observed decrease in velocity (for a given water pressure) between 1982 and 1994 was a consequence of a decrease in the interconnectedness of the subglacial cavity system.