The Uplift of Unteraargletscher at the Beginning of the Melt Season—A Consequence of Water Storage at the Bed?

Iken, Almut; Röthlisberger, Hans; Flotron, A.; Haeberli, Wilfried

doi:10.1017/s0022143000005128

“…Similar to the concepts expressed by Iken and others (1983), we believe the mini-surges occur at the beginning of the melt season because water input from surface melt is faster than can be transmitted by the constricted hydraulic system which has contracted over the previous winter. This leads to back-up of water in the system, water storage, and increased water pressure at the bed.…”

Section: Descriptive Model Of Mini-surge Mechanismsupporting

confidence: 63%

“…Closely spaced measurements of velocity versus time have provided some detailed information about temporal variation at a fixed location (e.g. Flotron, 1973;Iken, 1978;Iken and others, 1983). There is little information about how such velocity variations evolve in space.…”

Section: Introductionmentioning

confidence: 99%

Propagating Strain Anomalies during Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Raymond

¹

,

Malone

²

1986

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ABSTRACT. Wire strain meters and seismometers spaced longitudinally along the upper part of Variegated Glacier, Alaska, showed quasi-periodic episodes of increased velocity (mini-surges), which lasted about I day and recurred at intervals of a few days to 2 weeks during the early part of the melt seasons of 1979, 1980, and 1981. The zone affected by these mini-surges corresponds to the zone of highest velocity and basal stress increase over the previous decade, and the initiation of the most recent surge in 1982. Mini-surges initiate locally; as a single melt season progresses, the later mini-surges start at higher locations and show a distinct down-glacier propagation of a characteristic strain pattern and associated zone of acoustic emissions at speeds of 0

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“…It may be interpreted as due to cavitation associated with the increasing sliding velocities of the "normal" summer-time regime, and is very similar in this respect to (but smaller than) the "extended uplift" of Unteraargletscher that has been similarly interpreted by Iken and others (1983). The contrast in slope between a steeper "winter-time" trend (dash-dot line in Figure 11) and a shallower "summer" trend (dashed line) is a feature also ob served in Unteraargletscher (Iken and others, 1983, fig. 2).…”

mentioning

confidence: 78%

Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Kamb

¹

,

Engelhardt

²

1987

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ABSTRACT. Periods of dramatically accelerated motion, in which the flow velocity increases suddenly from about 55 cm/d to a peak of 100-300 cm/d and then decreases gradually over the course of a day, occurred repeatedly during June and July 1978-8 1 in Variegated Glacier (Alaska), a surging-type glacier that surged in 1982-83. These "mini-surges" appear to be related mechanistically to the main surge. The flow-velocity peak propagates down glacier as a wave at a speed of about 0.3 km/h, over a reach of about 6 km in length. It is accompanied by a propagating pressure wave in the basal water system of the glacier, in which, after a preliminary drop, the pressure rises rapidly to a level greater than the ice-overburden pressure at the glacier bed, and then drops gradually over a period of 1-2 d, usually reaching a new low for the summer. The peak velocity is accompanied by a peak of high seismic activity due to widespread fresh crevassing. It is also accompanied by a rapid uplift of the glacier surface, amounting to 6-1 1 cm, which then relaxes over a period of 1-2 d. Maximum uplift rate coincides with the peak in flow velocity; the peak in accumulated uplift lags behind the velocity peak by 2 h. The uplift is mainly due to basal cavitation driven by the high basal water pressure, although the strain wave associated with the mini-surge motion can also contribute. Basal cavitation is probably responsible for the pulse of high turbidity that appears in the terminal out flow stream in association with each mini-surge. In the down-glacier reach, where the mini-surge waves are attenuating, the observed strain wave corresponds to what is expected for the propagating pulse in flow velocity, but in the reach of maximum mini-surge motion the strain wave has a form quite different, possibly related to special features in the mini-surge initiation process from that point up-stream. The flow acceleration in the mini-surges is due to enhanced basal sliding caused by the high basal water pressure and the consequent reduction of bed friction. A preliminary velocity increase shortly before the pressure wave arrives is caused by the forward shove that the main accelerated mass exerts on the ice ahead of it, and the resulting preliminary basal cavitation causes the drop in water pressure shortly before the pressure wave arrives. The mini-surge wave propagation is controlled by the propaga tion of the water-pressure wave in the basal water-conduit system. The propagation characteristics result from a longi tudinal gradient (up-glacier increase) in hydraulic conductivity of the basal water system in response to the up-glacier increase of the basal water pressure in the mini surge wave. The mini-surge waves are initiated in a succession of areas situated generally progressively up-glacier during the course of the summer season. In these areas, presumably, melt water that has accumulated in subglacial *Contribution No.3734,

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“…It is thus very likely that the Unteraargletscher events are caused by peaks of high basal water pressure, as argued by Iken (1977), Rothlisberger (1980), and Iken and others (1983), although direct evidence for this is lacking. Certain detailed characteristics of the Unteraargletscher events, particularly the relatively long duration and the lack of asymmetry of the movement peaks, make these events appear more nearly akin to the mini surges in their source regions (section 10.7) than in the reach where well-defined down-glacier propagation is observed.…”

Section: Comparison With Movement Events In Other Glaciersmentioning

confidence: 99%

Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Kamb

¹

,

Engelhardt

²

1987

View full text Add to dashboard Cite

ABSTRACT. Periods of dramatically accelerated motion, in which the flow velocity increases suddenly from about 55 cm/d to a peak of 100-300 cm/d and then decreases gradually over the course of a day, occurred repeatedly during June and July 1978-8 1 in Variegated Glacier (Alaska), a surging-type glacier that surged in 1982-83. These "mini-surges" appear to be related mechanistically to the main surge. The flow-velocity peak propagates down glacier as a wave at a speed of about 0.3 km/h, over a reach of about 6 km in length. It is accompanied by a propagating pressure wave in the basal water system of the glacier, in which, after a preliminary drop, the pressure rises rapidly to a level greater than the ice-overburden pressure at the glacier bed, and then drops gradually over a period of 1-2 d, usually reaching a new low for the summer. The peak velocity is accompanied by a peak of high seismic activity due to widespread fresh crevassing. It is also accompanied by a rapid uplift of the glacier surface, amounting to 6-1 1 cm, which then relaxes over a period of 1-2 d. Maximum uplift rate coincides with the peak in flow velocity; the peak in accumulated uplift lags behind the velocity peak by 2 h. The uplift is mainly due to basal cavitation driven by the high basal water pressure, although the strain wave associated with the mini-surge motion can also contribute. Basal cavitation is probably responsible for the pulse of high turbidity that appears in the terminal out flow stream in association with each mini-surge. In the down-glacier reach, where the mini-surge waves are attenuating, the observed strain wave corresponds to what is expected for the propagating pulse in flow velocity, but in the reach of maximum mini-surge motion the strain wave has a form quite different, possibly related to special features in the mini-surge initiation process from that point up-stream. The flow acceleration in the mini-surges is due to enhanced basal sliding caused by the high basal water pressure and the consequent reduction of bed friction. A preliminary velocity increase shortly before the pressure wave arrives is caused by the forward shove that the main accelerated mass exerts on the ice ahead of it, and the resulting preliminary basal cavitation causes the drop in water pressure shortly before the pressure wave arrives. The mini-surge wave propagation is controlled by the propaga tion of the water-pressure wave in the basal water-conduit system. The propagation characteristics result from a longi tudinal gradient (up-glacier increase) in hydraulic conductivity of the basal water system in response to the up-glacier increase of the basal water pressure in the mini surge wave. The mini-surge waves are initiated in a succession of areas situated generally progressively up-glacier during the course of the summer season. In these areas, presumably, melt water that has accumulated in subglacial *Contribution No.3734,

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The Uplift of Unteraargletscher at the Beginning of the Melt Season—A Consequence of Water Storage at the Bed?

Cited by 248 publications

References 7 publications

Propagating Strain Anomalies during Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Propagating Strain Anomalies during Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

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