Glacier hazards

Since the Alps are among the most densely populated high mountain areas in the world, Switzerland is specially affected by glacial and periglacial hazards. On the other hand, however, it also has an extensive and well-recognized tradition in investigating such processes. In fact, the Swiss Federal Government undertakes continued and considerable efforts in this field. Following the ice-avalanche catastrophe of Mattmark in 1965, a Working Group for Glacier Hazards had been established by the Swiss Government (Bundesrat) for a number of years into the 1970-es. As a result, a systematic inventory of historical events in Switzerland was compiled, analyzed and applied (Haeberli and others, 1989). After a number of projects related to glacier hazards, the Swiss National Research Programme 31 'Climate Change and Natural Catastrophes' comprised a specific scientific project on ice melting and natural hazards (Haeberli et al. 1999). A number of monitoring and modelling studies related to single hazardous situations have been performed in connection with recent catastrophes or imminent hazard situations (cf. for instance, Haeberli and others, 2001 and in press; Kääb, 2000; Margreth and Funk, 1999). At present, an approach for area-wide modelling of glacier hazard potentials with respect to an integrated and adequate information base for planning and detailed monitoring is being developed (cf., Huggel and others, 2002 and submitted).
Glacier hazards stored in the database include ice avalanches, debris flows from outbursts of subglacial water reservoirs and periglacial lakes, rock falls and debris flows in and from glacial environments. If available quantitative measures of the magnitude of events are stored in the database (categories reservoir volume, outburst flood or ice break-off volume, maximum discharge).

Involved magnitudes are highly variable. For ice avalanches, the volume of ice breaking off ranges between tens or hundred thousand to 4.5 million cubic meters. Lake outburst volume is recorded from tens thousand to a maximum of 20 millions of cubic meters of water. Maximum discharge from lake outbursts range between about a hundred to 8000 cubic meters per second and , in case of outbursts from subglacial water reservoirs, can reach up to 200 cubic meters per second. In high moutain regions with larger glacier dimensions, such as the Himalayas, stored water volumes in glacier lakes reach over 100 millions of cubic meters, and peak-discharge values of more than 10,000 cubic meters per second have been reported.

Geographic distribution
The main concentration of historical events of glacier catastrophes is in the southern Swiss Alps, Canton Valais (see figure). This is due to the large glacierized area and particularly steep terrain. Another important region is found in the central Swiss Alps, i.e. in the Bernese Alps. The eastern Swiss Alps have experienced only a limited number of events.

Figure geographic distribution (click to enlarge)



Seasonal variability

The below figure shows the distribution of different kinds of glacier hazards over the year. Outoutbursts from glacier lakes occur in a period from May to October with a clear peak in July/ August. Ice avalanches, however, can take place during the whole year depending on the geometry of the rupture plane and subglacial temperature conditions (permafrost). In fact, winter events of ice avalanches are especially dangerous since the runout distance can be enlarged due to reduced friction of the snow surface and large snow avalanches can be triggered. Ruptures of subglacial water reservoirs, also termed water pockets, show a similar distribution during the year as lake outbursts. Their peak in summer months is related to the collection of water underneath the glacier after the onset of the melt season. From June on, in certain cases the water may reach a sufficiently high pressure to open subglacial channels to form catastrophic drainage (Björnsson, 1992; Walder and Costa, 1996). Hazards from glacier length variations are more on a medium- to long- term basis and generally not associated with a certain time of the year (except for very fast moving glacier events such as surges or surge-type accelerated movements; Haeberli and others, in press). Major glacier advances have repeatedly threatened and destroyed human installations in the past. An important hazard often related to glacier length variations are combinations of the above-mentioned hazards, e.g. a glacier advancing or retreating over a ridge, thus producing an ice avalanche which then could trigger the outburst of a lake.

Figure seasonal variability (click to enlarge)