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

Abstract

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.

Towards an Indirect Determination of the Mass-balance Distribution of Glaciers using the Kinematic Boundary Condition

“…More than 6000 velocity vectors were determined. Because surface velocities on Unteraargletscher are known to vary on a number of different time scales (Iken et al 1983, Gudmundsson 1999a, the velocity vectors in Fig. 5 represent an annual average velocity distribution, but not necessarily actual velocities at some given point in time.…”

Seasonal changes in basal conditions at Briksdalsbreen, Norway: the winter–spring transition

“…In particular, it is thought to drive the seasonal reorganization of the subglacial drainage system from a winter to summer regime (e.g. Willis et al 1990;Gordon et al 1998;Mair et al 2001Mair et al , 2002 and trigger variations in glacier dynamics (Iken et al 1983;Iken & Bindschadler 1986;Kavanaugh & Clarke 2001;Harper et al 2002;Mair et al 2003;MacGregor et al 2005). Proglacial stream discharge and turbidity are also often affected, as this event increases the amount of water and sediment evacuated from the bed of the glacier (Kavanaugh & Clarke 2001;Mair et al 2003;Harper et al 2005).…”

“…Willis et al 1990;Nienow et al 1996Nienow et al , 1998Gordon et al 1998) and glacier dynamics (e.g. Iken et al 1983;Iken & Bindschadler 1986;Mair et al 2001Mair et al , 2002Harper et al 2002;MacGregor et al 2005). In the summer, 'steady-state' conditions resume.…”

Modelling ice–bed coupling during a glacier speed‐up event: Haut Glacier d'Arolla, Switzerland

“…A sudden influx of large amounts of water to the glacier can result from a variety of sources, including extended periods of strong surface melting, intense rainstorms and the release of water from englacial, subglacial or ice-marginal storage. It has been argued that these circumstances can lead to the backup of water in the subglacial drainage system and an increase in water pressure at the bed (Iken et al, 1983). A glacier speed-up event may originate locally where the water pressure reaches a level high enough to cause substantial ice-bed decoupling and enhanced basal motion (Raymond and Malone, 1986).…”