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

Abstract

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,

Evolution of Variegated Glacier, Alaska, U.S.A., Prior to its Surge

“…In addition to the seasonal variation in geometry and velocity reported and discussed in this paper, intensified measurements started in 1978 led to the discovery of short time-scale variations in velocity and associated phenomena in the terminal stream discharging water from the glacier. These results have been reported elsewhere (Harrison and others, 1986;Humphrey and others, 1986;Raymond and Malone, 1986;Kamb and Engelhardt, 1987). The dramatic surge motions of 1982 and 1983 have been described by Kamb and others (J 985).…”

“…The holes made in different years were not at exactly the same locations and therefore may have sampled different local variations at the bed. The measurement intervals were short and included the occurrence of mini-surges (Kamb and Engelhardt, 1987) during which there could be substantial perturbation of the internal deformation pattern (Balise and Raymond, 1985). Therefore, there is considerable uncertainty about how ud and the corresponding ub measured in these bore holes relate to the average over large areas of the bed and the long time intervals, which Equations (2)- (9) attempt to predict.…”

“…Measurements of basal water pressure P ware available only from a few locations and short time intervals during summers (Kamb and others, 1985;Kamb and Engelhardt, 1987). In order to extend this information in space and time, it is necessary to calculate the pressure.…”

Basal conditions and glacier motion during the winter/spring transition, Worthington Glacier, Alaska, U.S.A.

“…Numerous studies have compared direct measurements of glacier motion with observations of the subglacial hydrological system (e.g. Iken and others,1983;Kamb and others,1985;Iken and Bindschadler, 1986;Kamb and Engelhardt, 1987;Hooke and others, 1989;Blake and others, 1994;Jansson, 1995;Raymond and others, 1995;Harbor and others, 1997;Iken and Truffer, 1997;Hanson and others, 1998). Yet, a predictable linkage between sliding velocity and measurable parameters of the hydrological system has been elusive.…”

Short-term Variations in Strain and Surface Tilt on Storglaciären, Kebnekaise, Northern Sweden

“…Recognition (Holmlund and Hooke, 1983) of phenomena on Storglaciaren that are similar to the mini-surges of Variegated Glacier (Kamb and Engelhardt, 1987), a surging glacier in Alaska, provided motivation for the present research effort. Mini-surges involve waves of high water pressure in the subglacial system that propagate downglacier, in the case of Variegated Glacier at speeds of several hundred meters per hour.…”