An assessment procedure for glacial hazards in the Swiss Alps

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Abstract
Glacial hazards such as ice avalanches, glacial lake outburst floods, and debris flows have caused severe damage in populated mountain regions such as the Swiss Alps. Assessment of such hazards must consider basic glaciological, geomorphological, and hydraulic principles together with experience gained from previous events. An approach is presented here to assess the maximum event magnitude and probability of occurrence of glacial hazards. Analysis of magnitude is based on empirical relationships derived from published case histories from the Swiss Alps and other mountain regions. Probability of occurrence is difficult to estimate because of rapid changes in the nature of glacial systems, the low frequency of events, and the high complexity of the involved processes. Here, the probability is specified in qualitative and systematic terms based on indicators such as dam type, geometry, and freeboard height (for glacial lakes) and tendency of avalanche repetition, precursor events, and increased water supply to the glacier bed (for ice avalanche events). The assessment procedures are applied to a recent lake outburst with subsequent debris flow and to an ice avalanche in the Swiss Alps. The results yield reasonable event maxima that were not exceeded by actual events. The methods provide first-order assessments and may be applied in dynamic mountain environments where population and infrastructure growth require continuous evaluation of hazards.Key words: glacial hazards, lake outburst, debris flow, ice avalanche, hazard assessment procedure, probability of occurrence.