Comparison of a three-dimensional model for glacier flow with field data from Haut Glacier d'Arolla, Switzerland

A three-dimensional, Finite-difference model based on a first-order solutio n of the ice-flow equations is applied to Haut Glacier d'Arolla, Switzerlan d. The numerical model successfully converges at horizontal resolutions dow n to 70 m, so a number of detailed comparisons with field data can be made. Modelled surface velocities with no basal sliding component are compared w ith surface velocities observed on the glacier over four different time per iods. The best fit is achieved with over-winter surface velocities (R-2 = 0 .75) using a rate factor; A, in Glen's flow law of 0.003 a(-1) bar(-3). Sur face zones of maximum computed effective stress display a high level of coi ncidence with observed crevassing, the orientation of which is successfully predicted by the direction of the tensile component of the computed princi pal surface stress. Comparison of the relative magnitude and direction of c omputed principal stresses with principal strains measured at the ice surfa ce also correspond closely. In an attempt to simulate the observed annual v elocity distribution within a cross-section of the glacier tongue, Mle inco rporate two basal-motion patterns into the model. By treating net annual ic e motion as a time-weighted composite of three separate flow situations: no rmal sliding, enhanced sliding and no sliding, we are able to reproduce the key features of the observed cross-sectional ice and basal slip velocity d istributions. These experiments indicate there may be substantial decouplin g taking place along an elongated narrow zone at the bed of Haut Glacier d Arolla and that this decoupling interacts ill a complex manner with the eng lacial stress and strain field.