Calving of Tidewater Glaciers

In this project the dynamics of tidewater glaciers and the involved processes such as calving and basal sliding. This is done by two different approaches.

The first observational part of this study is focussed on Hansbreen, a 16 km long tidewater glacier situated in South Spitsbergen. A detailed analysis of a 16 year time series of front position and geometry was used to identify and investigate the important processes and factors that control calving and changes in front position between 1982 and 1998 and on a seasonal scale. The observed abrupt retreat in 1990 is found to be related to a basal depression in the terminus region. The additionally observed seasonal variations of the front positions are mainly due to variations of the calving rate and not due to seasonal variations in ice velocity. Observations of Hansbreen further indicate that melting at the water line may play an important role by triggering the process of calving during periods of slow front position changes.

The flow behaviour of Hansbreen is studied in detail during two field investigations performed in summer 1998 and 1999 in cooperation with J. Jania from University of Silesia, Poland. Temporal variations of the flow of Hansbreen were measured on time scales from hours to seasons. During the melting season, short events of strongly increased surface velocities were observed, rather than generally enhanced 'spring' velocities. These speed-up events are related to periods of strongly increased water input to the glacier, due to rainfall or enhanced surface melt during a föhn weather situation. The close correlation between water pressure recorded in a moulin and the observed surface velocities suggests that the speed-ups are caused by a strong increase of basal water pressure which leads to enhanced basal sliding. The speed-up was associated to a short peak of the longitudinal strain, which may lead to additional fracture in the terminus region and therefore affect the process of calving. The observed short-term velocity variations and associated processes on Hansbreen are very similar to those observed on landbased valley glaciers. This suggests that the relevant mechanisms and physical processes that control the flow and its temporal variations are the same.

Spatially, a strong increase of the flow towards the calving front was observed on Hansbreen. It could be shown that increased basal sliding due to decreased effective pressure towards the front is responsible for the increase in flow. Using a glacier flow model including a water pressure dependent sliding law the observed velocity pattern, especially the frontal acceleration, could be well reproduced.

In a second part of the thesis the dynamics of tidewater glaciers is approached by numerical modelling. A time dependent numerical flow model for tidewater glaciers has been developed which solves the full equations for the stress and velocity fields and includes a water pressure dependent sliding law. A calving criterion is implemented which removes at each time step the part of the glacier that is thinning below a critical height above buoyancy. In contrast to previous modelling approaches, the calving rate is an output quantity of the model and not prescribed.

Model experiments are performed for a synthetic but typical tidewater glacier with a basal depression in the terminus region. With these model experiments the qualitatively described mechanisms and concepts suggested to explain changes in front positions are examined and the importance of different factors and processes, such as basal topography, calving and basal sliding on the dynamics of tidewater glaciers is investigated.

The linear relation between calving rate and water depth proposed on empirical grounds is qualitatively reproduced by the model for the situation of a slowly retreating or advancing terminus, but not for situations of rapid changes. Length changes of tidewater glaciers, especially rapid changes, are dominantly controlled by the basal topography and are to a minor degree a direct reaction to a mass balance change. Rapid changes in terminus positions preferably occur in places where the bed slopes upwards in ice flow direction. In the present model calculations thinning due to a change in mass balance is only the triggering process of a rapid retreat through a basal depression. The results also confirm the suggested cycles of slow advance and rapid retreat through a basal depression by different authors (Meier and Post, 1987). We conclude that accurate information on the near terminus bed topography is required for reliable predictions of terminus changes due to climate changes.

The glacier dynamics model is additionally applied to Hansbreen to analyse the observed retreat from 1982 to 1998. A seasonal calving rate, estimated from the observed front positions, is additionally prescribed in the model. The model calculations show that the process of buoyancy-induced calving is controlling the abrupt retreat of Hansbreen through the basal depression. The modelled location of the jump in retreat is found to be independent of the assumed mass balance and the prescribed seasonal calving rate and is fixed to the position where the basal depression is located. Thus, the abrupt retreat of Hansbreen is mainly an effect of basal topography in the terminus region and not a direct response
to a change in mass balance or climate.