Glaciological and Hydraulic Investigations in Connection with the Danger of Ice Avalanches falling in a recently formed Proglacial Lake

Tongue of Triftgletscher and proglacial lake in Summer 2003 (click to enlarge)

Triftgletscher is located above the Gadmental (Bernese Alps, Switzerland). The glacier bed of the glacier snout is characterized by an important overdeepening. During the ongoing glacier retreat in recent years, a lake was formed at the terminus. This lake will increase in size and volume and will finnaly reach a water volume of approximately 6 mio m³ by the year 2007. When the glacier terminus will have further retreated behind the present lake, the glacier surface and bed slope will be 35 degrees steep, forming a hanging glacier tongue with an ice volume between 4 and 6 mio m³ above the lake. There will be a considerable danger that ice avalanches will fall into the lake and generate dangerous flood waves.

Glacier flow modelling showed that large ice avalanches (several mio m³ of ice) are very unlikely. Nevertheless, an ice avalanche comparable with the well studied events at the Allalingletscher in 1965 and 2000 (1-2 mio m³ of ice) cannot be completely excluded. If such an event occurs at Triftgletscher, the flood descending in the Gadmental could not be neglected, and appropriate mitigation measures need to be planned.