The Basal Boundary Condition of an Alpine Glacier: a Hydro-Mechanical Approach, Unteraargletscher, Switzerland

 

Unusual flow behaviour of glaciers occurs mainly at the ice-bed interface whereby the basal water pressure distribution is of crucial importance for such processes. The pressure itself is controlled by the amount of water input into and the drainage capacity of the subglacial hydrological system. The major portion of the water is drained through englacial or subglacial conduits which are formed during the ablation period. The geometry of such conduits is governed by two opposing processes: enlargement by melting due to frictional heat and creep-closure by ice deformation. Thus, such a conduit, also known as Röthlisberger channel, changes continuously its geometry due to the large temporal variability of glacial discharge. Therefore, the probability to observe steady-state conditions is very low and to describe its dynamical behaviour we have to consider its transient evolution. Tracer experiments provide the means to investigate hydrological systems where direct access is difficult. Repeated injections at small temporal intervals enable us to monitor changing flow conditions.

Series of tracer tests were undertaken at Unteraargletscher, Switzerland, over a number of diurnal discharge cycles during the ablation season 2000. Dye injections into a moulin were repeated at intervals of a few hours. Records of dye concentration reveal pronounced diurnal variations in water flow velocity. Furthermore, the velocity-discharge relationship is strongly non-linear. A time-dependent model of subglacial water flow that accounts for the ability of a Röthlisberger channel to adjust its size to the hydraulic conditions, is used to interpret the observed variations in flow velocity and to infer the pressure distribution along the channel.