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Characterization of the basal hydraulic system of a surge-type glacier : Trapridge Glacier, 1989-92 Stone, Daniel B.


Subglacial hydrology is a critical issue in understanding glacier and ice-sheet dynamics. This is especially true for surge-type glaciers, which are distinguished by their regular, quasi-periodic alternation between slow and fast flow regimes (Meier and Post, 1969); the fast flow during a surge is thought to be caused by rapid sliding brought about by sustained high basal water pressure. How sustained high pressures develop and how the subglacial distribution of water influences glacier sliding are fundamental unsolved questions. We describe an investigation of the water drainage system beneath Trapridge Glacier, a surge-type glacier in Yukon Territory, Canada. We take two different approaches to characterizing the basal hydraulic system of the glacier. The first approach is borehole response testing, which involves changing the basal water pressure in the vicinity of a borehole and simultaneously observing the drainage system's response. We develop a theoretical model that describes the movement of water, induced by response tests, in a coupled borehole—subglacial flow layer. The model encompasses a broad range of flow regimes, from laminar Darcian flow in a thick permeable unit to turbulent sheet flow in a very thin layer. Important terms in the model are highlighted by a dimensional analysis. We show how the model can be used in numerical simulations to generate predicted data, which can be compared with field observations. We use our model as the basis for a formal inversion scheme that is aimed at objectively quantifying model parameters associated with subglacial water flow. Response test data from Trapridge Glacier are inverted to obtain estimates of hydraulic properties of the basal drainage system. The second approach that we have taken involves direct measurement of the properties of subglacial water. Year-round measurements of subglacial water pressure, turbidity, and electrical conductivity were made at intervals ranging from 2-20 minutes for three consecutive years. Turbidity and conductivity were measured using new sub-glacial sensors that we designed and constructed. The data in this study are unique and important because they were obtained directly at the glacier bed and because they span multiple summer and winter seasons. Spontaneous changes in the subglacial drainage system have been recorded and are sometimes accompanied by a release of stored basal water. In general, a single, stable drainage configuration cannot be identified. Instead, we infer a seasonal progression of drainage structures from the data. The complex behavior of the drainage system points to a dynamic subglacial environment, one in which the basal hydrology is governed by the combined influences of the glacier thermal regime, meltwater input, sediment movement, and mechanical interactions with the overlying ice.

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