Field measurements

Measurements were carried out in the ablation area of Lauteraargletscher, Switzerland (Fig. 68.1) from 22 to 27 August 2001. A 6.5-m-long aluminium pole was installed on the surface and continuously surveyed with GPS (Global Positioning System) to obtain hourly records of horizontal flow speed and vertical displacement. Three other poles were also surveyed twice a day at 0600 hours and 1800 hours to compute the horizontal surface strain rate. The precise distance (±5 mm) from the base of a ca. 173-m-long borehole to a surface reference was also measured in order to calculate the vertical strain of ice over the depth of the borehole (Gudmundsson, 2002; Sugiyama & Gudmundsson,

Figure 68.1 Map of the study site on Lauteraargletscher, showing the locations of the surface and borehole measurement sites and the strain grid. The two ellipses are strain rate ellipses during the indicated time periods of the days from 22 to 27 August 2001. These ellipses represent the deformation of the circle (broken line) subjected to the computed surface strain rate in units of 10-4 day-1.

656 easting (km)

Figure 68.1 Map of the study site on Lauteraargletscher, showing the locations of the surface and borehole measurement sites and the strain grid. The two ellipses are strain rate ellipses during the indicated time periods of the days from 22 to 27 August 2001. These ellipses represent the deformation of the circle (broken line) subjected to the computed surface strain rate in units of 10-4 day-1.

2003). A water-pressure transducer was also installed in a nearby borehole to measure the water level every 10min.

Results of the measurements are shown together with air temperature recorded at the glacier flank, in Fig. 68.2. During the study period, 40-60mm w.e. of daily ablation took place, and the borehole water level oscillated more than 100 m diurnally. The surface flow-speed showed clear diurnal variations, reaching a maximum in the afternoon and a minimum in the morning. Diurnal variations are also observed in the vertical displacement and the borehole length. Peaks in flow speed coincide with waterlevel maxima, suggesting that basal motion was enhanced by high subglacial water pressure. These peaks also coincide with the maximum rate of upward vertical displacement. Surface uplift during fast-flow events has been considered to result from the formation of water-filled basal cavities (Iken et al., 1983). Nevertheless, the observed uplift cannot be attributed totally to basal cavity formation as the borehole length measurements showed vertical tensile strains at times when the surface moved upward. These vertical strain rates reached 5 x 10-4 day-1. The horizontal surface strain data (Fig. 68.1) indicate compression along the flow direction during the daytime (0600-1800 hours), which is consistent with the vertical extension. These changes in the strain regime indicate consistent systematic diurnal variations in the spatial pattern of flow speed along the glacier.

During the same period, we operated two further GPS receivers located 1.5 km up-glacier and down-glacier from the main study site. Although diurnal velocity variations were recorded at all three sites, those up-glacier of the main study site were greatest. This suggests that the measured vertical extension during the daytime was probably caused by a compressive flow established at the study site. Because the seasonal snow line was about 1 km up-glacier, it is plausible that the highest peak diurnal pressure

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August 2001

Figure 68.2 Records of water level in a borehole, horizontal flow speed, vertical displacement of the surface relative to the mean elevation, borehole length and air temperature. The dash-dot line at the top indicates the water level corresponding to the ice overburden.

23 25 27

August 2001

Figure 68.2 Records of water level in a borehole, horizontal flow speed, vertical displacement of the surface relative to the mean elevation, borehole length and air temperature. The dash-dot line at the top indicates the water level corresponding to the ice overburden.

was greater in the upper reach owing to less developed subglacial drainage conditions (Nienow et al., 1998). The compressive flow field dissipated overnight as subglacial water drained down-glacier and flow speed slowed down.

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