The Geometry and Hydraulics of Englacial Conduits, Storglaciären, northern Sweden
Master's Thesis work of Sara E. Frödin
Department of Geology - Portland State University - Portland, Oregon
Englacial conduits are the primary structure responsible for transporting surface water to the base of a glacier, where it supplies the subglacial hydraulic system and, in turn affects glacier movement. Despite the well-known theoretical descriptions of englacial conduits, little direct evidence exists about their geometry and hydraulics. In July 2001, a field effort was initiated at Storglaciären, northern Sweden, to intersect englacial conduits by drilling into the glacier using a hot water drill. A submersible camera was used to image the conduits and measure the physical properties of the features. By attaching a compass and a ruler to the camera housing a sense of direction and size of the opening was attained. A companion project led by Prof. Robert W. Jacobel, St. Olaf College, Northfield, MN, attempted to detect the englacial conduits using ground-penetrating radar.
The study site for this field investigation is Storglaciären (67° 54`N, 18° 35`E) in northern Sweden, located in Tarfala Valley (fig.1). Storglaciären (fig. 2), situated in Tarfala valley, provide a perfect ground for research study because of the extensive background of hydrological work on the glacier, and great infrastructure and facilities at Tarfala research station. Storglaciären was selected for a long-term study on climatic impact on glaciers in 1945 and an annual mass balance programme was initiated.
Figure 1. Tarfala valley where Tarfala Research Station is situated (1130 m.a.s.l.).
Figure 2. Photo of Storglaciären with marked study area, July 2001.
Hot Water Drilling
By using a hot water drill (fig. 3-6) the ice is penetrated at a rate of ~ 1 m/min. During drilling water level and drill depth is monitored. A drop in water level at any time prior to reaching the bed suggests a hydraulic connection, and if the water level remains low the connection is presumably an englacial conduit.
Fig. 3-6 Drilling scenes from Storglacären, July 2002.
Figure 3. Robert and Mart drilling.
Figure 4. Robert drilling. The South summit of Kenekaise's (~2 117 m.s.a.s.l.),
the highest peak in Sweden, in the background.
Figure 5. Andrew drilling?
Figure 6. Drilling set up.
The boreholes were drilled in 3 ×3 grids with square grid cells of 10 ×10 meters at different locations at the glacier (fig. 7). By having a 10 meters distance between the boreholes the drilling equipment can be centrally located for drilling numerous holes before moving to a new spot and the risk of tapping into the same local crevasse is minimized.
Figure 7. Location map of Storglaciären with drilling areas.
Once an englacial connection was discovered a small video camera with attachments (fig. 8) was lowered in the borehole to examine the the englacial feauture for opening size, geometry, orientation, and water flow speed. A plastic ruler and a small compass provided the necessary scales and orientation. Naturally-occuring particles in the water was being used to estimate the flow speed. Pressure transducers were installed in some of the boreholes to obtain a continuous record of basal- and englacial water levels.
Figure 8. Borehole camera (dye experiment with a balloon..).
On a total 22 borehole were drilled at three separate locations (fig. 7), 17 (77 %) connected englacially, 2 drained at the bed, and the remaining 3 reached the bed without draining at all. Channels appeared to be creavasse-like features (fig.9 ), with planar walls separated by a few centimeter. The vertical extent was difficult to ascertain. The depth of connection varied between 11m and 45m. Most channels were oriented down slope, parallell to the orientation of the surface crevasses.
Figure 9. Photo of a typical englaical drainage feature spotted.
In contrast to previous studies we have observed no channels of circular cross-section during this first field season. Our channels are highly elliptical and most likely occupy relict crevasses.
The slow water flow speed (~1 cm s-1) in the channels and the highly non-circular cross-sectional geometry of the channels indicates a sluggish flow system. This implies that the ratio of discharge to cross-sectional area is small, suggesting a small drainage area for the channel. Hence, many channels exist and the hydraulic gradient in the system is low. The observed cross-sectional geometry suggests that melt enlargement is not a significant process. This observation is consistent with the notion of sluggish flow system.
Water level data suggests that the hydraulics of the englacial flow system is quite distinct from the subglacial system. The englacial system exhibits rather large diurnal variations in pressure (~ meters) whereas the subglacial pressures vary over centimeters. The one hole which connected both englacial and subglacial hydraulic systems exhibited large diurnal variations in water pressure indicating substantial interaction beween the two systems. This suggests that such boreholes are not useful for indicating hydraulic conditions in either system.
Acknowledgements: This work is funded by NSF/OPP 0097137 within Arctic Natural Sciences. We are grateful to the faculty and staff of the Tarfala Research Station and Glaciology Program within the Department of Physical Geography at the University of Stockholm, Sweden for their assistance.
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