During the snow-melt season of 1982, basal water pressure was recorded in 11 bore holes communicating with the subglacial drainage system. In most of these holes the water levels were at approximately the same depth (around 70 m below surface). The large variations of water pressure, such as diurnal variations, were usually similar at different locations and in phase. In two instances of exceptionally high water pressure, however, systematic phase shifts were observed; a wave of high pressure travelled down-glacier with a velocity of approximately 100 m/h.The glacier-surface velocity was measured at four lines of stakes several times daily. The velocity variations correlated with variations in subglacial water pressure. The functional relationship of water pressure and velocity suggests that fluctuating bed separation was responsible for the velocity variations. The empirical functional relationship is compared to that of sliding over a perfectly lubricated sinusoidal bed. On the basis of the measured velocity-pressure relationship, this model predicts a reasonable value of bed roughness but too high a sliding velocity and unstable sliding at too low a water pressure. The main reason for this disagreement is probably the neglect of friction from debris in the sliding model.The measured water pressure was considerably higher than that predicted by the theory of steady flow through straight cylindrical channels near the glacier bed. Possible reasons are considered. The very large disagreement between measured and predicted pressure suggests that no straight cylindrical channels may have existed.

ABSTRACT. In order to interpret observed short-term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model. In particular the transient stages of grow ing or shrinking water-filled cavities at the icebedrock interface were analysed. It was found that the sl iding veloci ty was larger when cavities were growing than when they had reached the steady-state size for a given water pressure. The smallest sliding velocities occurred while cavi ties were shrinking. When cavitation is substantial a small drop of water pressure below the steady-state value (e.g. by 0.5 bar) can temporarily cause backward sliding. A limiting water pressure at which sliding becomes unstable is derived. The consequences of more realistic assumptions than those of the model are discussed. RESUME. Injluence de la pression hydrauligue sous-glaciaire sur la vitesse de glissement d'un glacier dans un modete numerigue idealise. Dans le but d'interpreter les variations de vitesse a court terme qui ont ete observees, on a etudie a l'aide d'un modele numerique idealise l'effet exerce par une pression hydraulique sous-glaciaire variable sur la vitesse de glissement d'un glacier. On a etudie en particulier les etats transitoires de la croissance et de la decroissance des poches d'eau situees a l'interface glace-bedrock. II s'est revele que pendant la phase de croissance des cavites, la vitesse de glissement est plus grande qu'a l'etat stationnaire pour une pression hydraulique donnee. Les vitesses de g lissement les plus faibl es se produisent lors du retrecissement des cavites. Lorsque le volume des cavites est suffisamment grand, une faible diminution de la pression (p.ex. de 0,5 bar) peut temporairement provoquer un glissement vers l'amont. On a determine une valeur limite de la pression hydraulique pour laquelIe de glissement devient instable. On discute les consequences d'hypotheses plus realistes que celIes sur lesquelles le modele repose. ZUSAMMENFASSUNG. Der Einjluss des suhglazialen Wasserdruckes au] die Gleitgeschwindigkeit eines Gletsehers, untersueht mit einem idealisierten numerischen Modell.Mit dem Ziel, beobachtete kurzzeitige Geschwindigkeitsschwankungen zu erklaren, wurde der EinAuss eines veranderli chen subglazialen Wasserdruckes auf die Gleitgeschwindigkeit mit einem idealisierten numerischen Modell untersucht. Insbesondere wurden die Ubergangsphasen von wachsenden od er sch rumpfenden, wassergefUllten Hohlraumen an der GrenzAache von Eis und Felsbett analysiert. Es zeigte sich, dass die Gleitgeschwindigkeit grosser war, wenn Hohlrliume zu wachsen begannen, als wenn sie die fUr einen bestimmten Wasserdruck endgUltige Grosse erreicht hatten. Die kleinsten Gleitgeschwindigkeiten kamen vor, wlihrend die Hohlraume schrumpften. Bei genugendem Ausmass der H ohlraumbi ldu ng kann ein geringfUgiges Absinken des Wasserdruckes unter den Gleichgewichtswert (z.B. urn 0,5 bar) vorUbergehend Ruckwlirtsgleiten verursachen. Ein Gr...

During the snow-melt season of 1982, basal water pressure was recorded in 11 bore holes communicating with the subglacial drainage system. In most of these holes the water levels were at approximately the same depth (around 70 m below surface). The large variations of water pressure, such as diurnal variations, were usually similar at different locations and in phase. In two instances of exceptionally high water pressure, however, systematic phase shifts were observed; a wave of high pressure travelled down-glacier with a velocity of approximately 100 m/h.The glacier-surface velocity was measured at four lines of stakes several times daily. The velocity variations correlated with variations in subglacial water pressure. The functional relationship of water pressure and velocity suggests that fluctuating bed separation was responsible for the velocity variations. The empirical functional relationship is compared to that of sliding over a perfectly lubricated sinusoidal bed. On the basis of the measured velocity-pressure relationship, this model predicts a reasonable value of bed roughness but too high a sliding velocity and unstable sliding at too low a water pressure. The main reason for this disagreement is probably the neglect of friction from debris in the sliding model.The measured water pressure was considerably higher than that predicted by the theory of steady flow through straight cylindrical channels near the glacier bed. Possible reasons are considered. The very large disagreement between measured and predicted pressure suggests that no straight cylindrical channels may have existed.

At a site on the ice sheet adjacent to the Jakobshavn ice stream in West Greenland, ice deformation rates and temperatures have been measured in boreholes to the bedrock at 830 m depth. Enhanced deformation rates were recorded just below the Holocene^Wisconsin transition at 680 m depth. A 31m layer of temperate ice and the temperature minimum of^22³C at 520 m depth were detected. The good agreement of these data with results of a two-dimensional thermomechanically coupled flow model implies that the model input is adequate. Discrepancies between modelled and measured temperature profiles on a flowline at the ice-stream centre have been attributed to effects not accounted for by the model. We have suggested that the convergent three-dimensional flow leads to a vertical extension of the basal ice entering the stream. A thick basal layer of temperate and Wisconsin ice would explain the fast flow of this ice stream. As a test of this hypothesis, the new core-borehole conductivity (CBC) method has been used to compare conductivity sequences from the ice stream to those of the adjacent ice sheet. The correlation thus inferred suggests that the lowest 270 m of the ice sheet correspond to the lowermost 1700 m of the stream, and, consequently, that the lower part of the ice stream has experienced a very large vertical extension.Englacial temperatures have been recorded in boreholes on Jakobshavn Isbr×, 50 km upstream from the calving front (Iken and others, 1993). The holes, drilled with a hot-water system, reached the bed near the ice-stream margins at approximately 1600 m, but stopped far above the bed in the ice-stream centre where the ice thickness is 2500 m. At the centre line, the shape of the measured temperature profile differed substantially from that of a modelling study (Funk and others, 1994). This discrepancy was attributed to convergent three-dimensional flow into the bedrock channel, not accounted for by the two-dimensional model. It was suggested that the convergent flow causes enhanced vertical extension of the basal ice at the centre line and thereby a thickening of the temperate (``soft'') basal layer.This could explain the high flow velocity of this ice stream. GOALS OF THE STUDYA better understanding of the specific dynamics of the ice stream is obtained by investigating the conditions in the adjacent ice sheet where the ice is of the same origin, but where flow conditions are less complex. In this study we

The results of systematic movement studies carried out by means of an automatic camera on the Unteraargletscher since 1969 (Flotron, 1973) are discussed together with more recent findings from theodolite measurements made at shorter intervals and over a longer section of the glacier.In addition to the typical spring/early-summer maximum of velocity known from other glaciers, an upward movement of up to 0.6 m has been recorded at the beginning of the melt season. It was followed, after various fluctuations of the vertical velocity, by a similar but slower downward movement which continued at an almost constant rate for about three months. The uplift was not confined to the section covered by the camera but occurred nearly simultaneously in profiles located 1 km below and 2 km above. The times of maximum upward velocity (increases of up to 140 mm/d) coincided approximately with periods of large horizontal velocity and occurred after increases of melt-rate.The following explanations for the variations of vertical velocity are considered: (1) Changes of longitudinal strain-rate. (2) Changes of the sliding velocity in a channel of variable width and with a bed slope deviating from horizontal. (3) Changes of volume due to opening or closing of crevasses. (4) Swelling or contraction of veins at the grain edges. (5) Growth (and closure) of cavities in the interior of the glacier. (6) Changes of large-scale water storage at the bed.Although all of the mechanisms (1)–(5) have some effect on the vertical ice movement, they cannot account for the observed variations of vertical velocity. We therefore conclude that large-scale water storage at the bed is the main cause of the uplift. Apparently the storage system is efficiently connected with the main subglacial drainage channels only during times of very high water pressure in the channels.The findings are of some interest to the concepts of glacier sliding: As mentioned above the maxima of horizontal velocity—and thus of the sliding velocity—have not been measured at the time when the storage had attained a maximum, but at the time of maximum vertical velocity, which we assume to be the time of most rapid growth of cavities at the bed. This behaviour of the sliding velocity agrees with that predicted by a simple finite-element model of the basal ice on a wavy bed with water-filled cavities. In particular, the model shows that the sliding velocity is larger during the process of cavity growth than at the final stage when the cavities have grown to the size which is stable for the applied water pressure.

Results of systematic movement studies carried out by means of an automatic camera on Unteraargletscher since 1969 are discussed together with supplementary theodolite measurements made at shorter intervals and over a longer section of the glacier. In addition to the typical spring/early summer maximum of velocity known from other glaciers, an upward movement of up to 0.6 m has been recorded at the beginning of the melt season. It was followed, after a few fluctuations of the vertical velocity, by an equal but slower downward movement which continued at an almost constant rate for about three months. Possible explanations of the uplift are discussed, the most satisfactory explanation being water storage at the bed. The observations then suggest that this storage system is efficiently connected with the main subglacial drainage channels only during times of very high water pressure in the channels. Detailed measurements showed that the times of maximum horizontal velocity coincided with the times of maximum upward velocity rather than with the times when the elevation of the surveyed poles had reached a maximum. On the basis of the hypothesis of water storage at the bed this finding means that the sliding velocity is influenced mainly by the subglacial water pressure and the actual, transient stage of cavity development, while the amount of stored water is of lesser influence.

Several holes were drilled to depths of 1500–1630 m along a profile across Jakobshavns Isbræ, 50 km upstream from the calving front. Drilling was by hot water and required approximately 20 h. The holes were rapidly closed by refreezing, but it was possible to instrument them with thermistors and tilt sensors before this occurred.Near the margins of the ice stream the holes reached the bed and connected with the subglacial drainage system. Water-level changes recorded in these holes are discussed in terms of the basal hydraulic system. The temperature measurements show that the glacier is temperate-based. Moreover, extrapolation of a measured temperature profile and its curvature suggests that a temperate layer of substantial thickness may exist at the bed near the center of the ice stream. There is a striking difference in the shapes of temperature profiles measured at different locations: beneath the center line the temperature minimum is at a considerably smaller relative depth than near the margins, but it is nearly the same in magnitude (−22.1°C). This may indicate a disproportionately large vertical stretching of the basal ice in the center of the ice stream. Since the basal ice is warmer and much less viscous than the ice above, a thickening of that layer would cause a corresponding increase of surface velocity. We presume that this mechanism contributes to the fast flow of Jakobshavns Isbræ.

ABSTRACT. Findeleng letsc he r, Switze rl a nd , a d\'a nced abo ul 250 m betwee n 1979