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Monitoring Changes of Allalingletscher in Connection with the Danger of Large Ice Falls from its Tongue

 

Allalingletscher was mentionned through the last four centuries, especially between 1600 and 1900, because the terminus reached and bloked the river Saaser Vispa several times resulting in the formation of the lake of Mattmark. During this period, this ice dammed lake drained several times and caused serious damages in the Saaser valley. Around 1940, the Allalingletscher was rapidly retreating due to the decay of the thinned-down snout. Between 1954 and 1964 the terminus has been approximately stationary. On 30 August 1965, a huge ice avalanche (2 millions m3 of ice) broke off at the terminus of Allalingletscher and fall down a rock slope towards the Mattmark dam construction site. During this unexpected catastrophic event 88 people were killed.

An intensive glaciological study and monitoring program was undertaken immediately after the catastrophe to safeguard the rescue operation and the construction work as well as to analyse the causes of this catastrophe. These investigations showed that the ice avalanche had occured during a phase of enhanced motion as a result of intensive bed-slip of an even larger ice mass than that which broke off. Although this special condition was necessary for the lower part of the ice mass to slide off, it does not explain why the ice avalanche occured in 1965 and not during other active phases either before or after this date. A particular topography of the bed, combined with an unfortunate mass distribution, has probably played a major role in the catastrophe. In view of a possible repetition of a catastrophic event, a monitoring program to observe the behaviour of Allalingletscher with an automatic camera, aerial photographs and surface velocity and mass balance measurements was initiated. For the case that a large icefall would appear to be imminent, different safety measures have been planned with the power company and the local authorities. Investigations showed that the present road to the Mattmark dam lies outside of the endangered zone, but a large icefall (more than 500'000 m3) can be dangerous for tourists in the immediate vicinity of the endangered zone. It is therefore essential that the endangered area can be closed off and, if necessary, evacuated in time. After 1965 the terminus of Allalingletscher readvanced by 650 m until 1985 and since then it retreated again. Around 1996, the terminus position and the mass distribution was similar to 1965 and a repetition of the slip off event was expected during an active phase. For this reason, different safety measures to avoid damages were installed. After an additional retreat of the terminus by 100 m, on July 31 2000 an ice volume of one million m3 broke off and moved down as an impressive ice avalanche. Because of the safety measures no damages occured. During this event, the detachment was located some 100 m higher up than 1965. Because the glacier snout retreated and thinned-down more than 1965 before breaking off, the recent ice avalanche was only half the size of the catastrophic event of 1965. If the glacier retreat will continue in future, the tongue of Allalingletscher will leave the steep part in a few years and the danger of large ice avalanches will disappear.

 

Keywords: Mattmark catastrophe; ice avalanche; glacier retreat
Contacts: Dr. Andreas Bauder
Commissioned by: Kraftwerke Mattmark, Saas Grund (Valais, Switzerland)
 

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© 2014 ETH Zurich | Imprint | Disclaimer | 11 July 2011
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