Model output

Although the computed strain rate is still smaller than that measured in the borehole, the basal sliding zone established in the upper reach and its temporal evolution was able to explain the diurnal flow variations in Lauteraargletscher.

Horizontal flow speed and vertical strain rate were computed for a region between 3 km up-glacier and down-glacier of the study site (Fig. 68.3). Ice thickness at the study site was modelled to be 400 m. No sliding c(-3000 < x < 3000) = 0 was assumed at 0800 hours when the surface speed was at a minimum, and then c(x) was determined for each time of day so that the computed surface-flow speeds at x = 0 agree to within 10% of the measurements on 25 August (Fig 68.3a). This 10% tolerance was within GPS measurement errors. Based on the hypothesis that the basal motion was higher in the upper reach, a sliding zone with greater value of c was allocated up-glacier of the study site at 1000 hours and its magnitude increased during the daytime. The position of the sliding zone was then gradually moved down-glacier at night, reducing the magnitude of c toward next morning.

Output from the model recreates the principal features of the field measurements:

1 vertical strain rate is tensile during the day from 1000 to 1800 hours;

2 magnitude of the strain rate reaches 10-4 day-1 at 1500 hours at x = 0;

3 diurnal velocity variation is greater up-glacier.

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