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The movement and basal sliding of the Nisqually Glacier, Mount Rainier

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Author(s): Steven M. Hodge

Document Type: Ph.D Thesis
Publisher: University of Washington
Published Year: 1972
Pages: 433
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Accurate and detailed measurements of the positions of stakes along the centerline of the lower Nisqually Glacier were made on the average every twelve days for a period of two years. Changes in surface velocity, surface slope and thickness were calculated, with corrections made for tilting and bending of the stakes. Run-off from the glacier over the same period was measured continuously at a stream gage below the terminus.

Bedrock topography was determined by measurements of the gravity field over the glacier. Accurate terrain corrections and three dimensional modelling were done using a modification of the technique of Talwani and Wing (1960). The regional gravity field was determined by requiring the calculated bedrock to agree with areas of known bedrock and with two depths determined by drilling. The glacier was found to be thinner than expected, with an average centerline depth of only 71 m.

The glacier flow model of Nye (1952) was used to reduce surface velocities to sliding velocities on the bed. Variables were smoothed over distances of several times the ice thickness to minimize the effect of longitudinal stress gradients and shape factors were used to allow for the friction of the valley walls. Existing data on the flow law of ice were examined and an average flow law and a least viscous flow law were chosen. The effect of uncertainties in the flow model was investigated by using these two flow las and by simultaneously perturbing the thickness, surface slope and shape factor by acceptable amounts.

Internal deformation of the glacier is found to account for much less than 50% of the observed surface motion. Sliding of the glacier is negligible only if the ice obeys the least viscous flow law, the depths are everywhere to shallow by at least 10 m and the shape factors are close to unity. Furthermore, internal deformation contributes progressively less to the surface motion with distance up-glacier; in the area near the equilibrium line sliding probably accounts for 80-90% of the total motion.

The surface velocity of the glacier has a pronounced seasonal fluctuation superimposed on along period trend. At the equilibrium line the maximum velocity occurs in late May or June and the minimum in November. The existence of a "seasonal wave" is verified, the maximum and minimum velocity occurring progressively later with distance down-glacier. The wave travels from the equilibrium line to the terminus with a speed of about 20 km a-1. This is one to two orders of magnitude greater than a normal kinematic wave, assuming that ice motion is due to internal deformation and/or the pressure melting/enhance plastic flow mechanism of basal sliding. It is also two orders of magnitude smaller than that of a kinematic wave in a water film 0.5 mm thick.

The maximum deviation of the surface velocity from the long period trend is approximately ±25% and does not vary significantly with distance along the glacier. The maximum velocity occurs 2-3 months after the maximum loading of the glacier and about one month before the peak in run-off. The minimum velocity occurs 1-2 months after the minimum loading and four months before the minimum run-off. The acceleration of the glacier through the winter, while the run-off is still decreasing, is clearly demonstrated.

It is concluded that the seasonal variations in the motion of a glacier are due to variations in the amount of basal sliding and are controlled not by the run-off, the surface melting or the loading but the amount of liquid water stored within the glacier. The peak in the liquid water storage curve of the South Cascade Glacier correlates well, on the average, with the peak in the surface velocity of the Nisqually Glacier. The seasonal wave can be explained with this hypothesis also.

The velocity variations of the Nisqually Glacier are considered to be independent verification that glaciers store water in the fall, winter and spring and then release it in the summer, after the hydrostatic head of water becomes great enough to open the drainage channels. The data suggest that relatively more water is stored higher up the glacier and that possibility most of the storage occurs at the equilibrium line.

The results support the idea that the dominant controlling parameter in the basal sliding mechanism is the hydrostatic pressure of water having access to the bed. Any dependence of sliding on the basal shear stress is probably masked by the varying water pressure.

Finally, it is suggested that the volume of liquid water stored in a glacier may vary annually and that jökulhlaups represent the catastrophic release of any accumulated water, and glacier surges may be caused by a more gradual release. It might be possible to predict jökulhlaups by detecting abnormal accelerations in the surface motion of glaciers.

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In Text Citation:
Hodge (1972) or (Hodge, 1972)

References Citation:
Hodge, S.M., 1972, The movement and basal sliding of the Nisqually Glacier, Mount Rainier: Ph.D Thesis, University of Washington, 433 p..