Articles | Volume 4, issue 1
Research article
02 Feb 2016
Research article |  | 02 Feb 2016

Basal shear stress under alpine glaciers: insights from experiments using the iSOSIA and Elmer/Ice models

C. F. Brædstrup, D. L. Egholm, S. V. Ugelvig, and V. K. Pedersen

Abstract. Shear stress at the base of glaciers exerts a significant control on basal sliding and hence also glacial erosion in arctic and high-altitude areas. However, the inaccessible nature of glacial beds complicates empirical studies of basal shear stress, and little is therefore known of its spatial and temporal distribution.

In this study we seek to improve our understanding of basal shear stress using a higher-order numerical ice model (iSOSIA). In order to test the validity of the higher-order model, we first compare the detailed distribution of basal shear stress in iSOSIA and in a three-dimensional full-Stokes model (Elmer/Ice). We find that iSOSIA and Elmer/Ice predict similar first-order stress and velocity patterns, and that differences are restricted to local variations at length scales of the order of the grid resolution. In addition, we find that subglacial shear stress is relatively uniform and insensitive to subtle changes in local topographic relief.

Following the initial comparison studies, we use iSOSIA to investigate changes in basal shear stress as a result of landscape evolution by glacial erosion. The experiments with landscape evolution show that subglacial shear stress decreases as glacial erosion transforms preglacial V-shaped valleys into U-shaped troughs. These findings support the hypothesis that glacial erosion is most efficient in the early stages of glacial landscape development.

Short summary
When studying long-term glacial landscape evolution one must make simplifying assumptions about the nature of glacial flow. In this study we show that for two different numerical models such simplifications are mostly unimportant in the setting of glacial landscape evolution. Following this we find that glacial erosion is most intense in the early stages of glaciation and its effects are reduced with time due to flow patterns in the ice removing areas of highest resistance to flow.