open access

https://doi.org/10.14943/doctoral.k6481

https://hdl.handle.net/2115/28088

温暖山岳氷河における底面状態に起因した短期流動

英語

2003-03-25

Hokkaido University

vii, 89p.

This thesis studies how a temperate valley glacier flows under the control of basal conditions. To investigate the influence of basal conditions on the glacier flow field, field measurements were carried out with high temporal resolutions at Lauteraargletscher in the Swiss Alps during the ablation season. The measurements and subsequent data analyses focus on the relationship between surface velocity and subglacial water pressure, surface vertical movement (uplift), and temporal variations in vertical strain. A glacier flow model is developed to study general effects of basal perturbation on the flow field, and is used to interpret the observed short-term flow variations. Clear diurnal variations in surface velocity were observed in the mid ablation season, and the velocity was correlated with subglacial water pressure during this period. The glacier was accelerated significantly when the water pressure raised up close to the overburden pressure. Detailed examination of the relationship showed that the velocity is larger when the pressure increases than it decreases under the identical water pressure. This hysteresis is understood as the result of pushing force conveyed through ice from a few kilometers upper reach of the glacier. Diurnal variations were also found in vertical strain, which was tensile during the daytime and compressive during the nighttime with an order of 10-4. This observation implies that the glacier flow regime switched from extending to compressive flow in a diurnal manner. Nonuniformly distributed basal lubrication and its diurnal evolution can cause such change in the flow regime. The short-term variations in vertical strain and the hysteresis in surface velocity indicate that the glacier flow is not a local phenomenon, but influenced by neighbouring flow conditions.Uplifts were frequently observed with surface velocity variations. Surface vertical displacement was compared with vertical strain to determine the mechanism of the uplifts, and it was concluded that vertical straining was the major cause in most cases. However, a sudden upward movement measured at a motion event cannot be ascribed to vertical strain, and volume increase in subglacial water cavities is a plausible reason of the uplift. General effects of spatially limited basal perturbation on the flow field was investigated with a finite element flow model. Transmission of basal motion to the surface through non-linear viscous material (ice) was computed by numerical experiments. It was shown that the rate of the transmission is dependent on the spatial scale of the basal perturbation. The experiments also showed that the influence of local basal motion can reach a horizontal distance of 10 times ice thickness. The relationship between surface velocity and water pressure was simulated including the effect of spatially distributed basal lubrication. The observed velocity hysteresis was reproduced by allocating more lubricated bed condition in the upper reach of glacier. The diurnal vertical strain was also reproduced qualitatively by assuming diurnal evolution of a basal slippery zone. The field measurements show that subglacial water pressure controls short-term flow variations in temperature valley glaciers. Furthermore, detailed examinations of the data insist that the flow of a glacier is not determined by local water pressure alone, because the influence of neighbouring basal condition cannot be neglected. Numerical investigations also confirm the importance of basal lubrication in up- and downglacier areas to compute a flow field. Therefore, the mechanical interaction with surrounding ice mass should be taken into account, when glacier flow variations under the control of basal conditions are discussed.

2003-03-25

甲第6481号

博士(地球環境科学)

博士論文

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