Consequences

The inertia related to diffusion of subsurface heat and the retarding effects from latent heat exchange cause global warming-induced permafrost changes to last for very long time periods (Haeberli and Burn 2002; Noetzli et al. 2007). On moderately inclined slopes with abundant fine material of mountains, with a continental climate and widespread permafrost occurrence, increasing active layer depth in degrading permafrost is likely to reduce near-surface soil humidity and, hence, change the living conditions of plants and animals (Etzelmuller et al. 2001). In rugged topography with coarser sediment cover and bedrock, reduction of slope stability is now seen to be the main problem. Steep outer slopes of thick morainic deposits from small glaciers below large rock walls and the fronts of active rock glaciers may be especially sensitive to changes in permafrost conditions (Fig. 14.8). The involved phenomena are, however, rather complex (Zimmermann and Haeberli 1992). The most delicate situation may indeed develop during the transition from conditions with to without permafrost, when the increasing active-layer thickness in the steep slope allows for deeper erosion but permafrost still forms a roughly surface-parallel hydraulic barrier at depth, which inhibits percolation, concentrates precipitation water in a near-surface layer of limited thickness and delivers the so-enhanced subsurface flow directly to the upper parts of the steep slopes.

Concerning large rock falls in steep rock slopes (see. case descriptions by Dramis et al. 1995; Deline 2001), a combination of the factors (i) slope inclination, (ii) geological structure, (iii) permafrost condition, and (iv) topographic history

Fig. 14.8 Debris flow starting zone in a Little Ice Age moraine with marginal permafrost and vanishing avalanche-fed cirque glacier. The resulting debris flow had a volume of about 500,000 m3 and caused heavy damage in the village of Guttannen, Bernese Alps, Switzerland (photo: Flotron AG 2005)

must be considered in each individual case. Among these four primary factors, the ice-related permafrost conditions and topographic history (glacier vanishing with corresponding stress redistribution) are those now subject to the strongest and fastest change (Fig. 14.9; Fischer et al. 2006). In detail, things are again much more complicated than sometimes assumed (Gruber et al. 2004a; Gruber and Haeberli 2007). Lowest stability of bedrock with ice-filled cracks, for instance, does not occur with complete thaw but in "warm" permafrost at temperatures slightly below melting (Davies et al. 2001). With continued permafrost warming, layers at critical temperatures — allowing for ice-rock-water coexistence — will not only extend over larger vertical distances but also to greater depths below surface. The probability of large rock falls must therefore be assumed to slowly but steadily increase.

During the past 20 years in the Alps, periglacial rock falls with volumes exceeding one million m3 and often reaching far below the timberline have occurred at time intervals of a few years. Current research strategies relating to such growing hazards from permafrost areas of cold mountain areas focus on GIS-based spatial definitions of critical factor combinations with rock walls above the timberline, and numerical modelling of flow paths resulting from potential instabilities (Fig. 14.9; Fischer et al. 2006; Noetzli et al. 2006). The goal is to recognize the most critical threats, and to enable early detection, warning and protection (Fig. 14.10) through adequate observation and monitoring. In particular, rock falls into existing lakes, or into lakes which newly form in connection with accelerated glacier shrinkage, have

Fig. 14.9 East face of Monte Rosa and Ghiacciaio del Belvedere, Valle Anzasca, Regione Piemonte, Italy. Most intense rock fall activity in this rock face correlates with warm or marginal permafrost and recently deglaciated surfaces. The detachment zone of an ice avalanche (2005) is marked with a black circle, the one of a rock avalanche (2007) in white (photo: L. Fischer 2004)

Fig. 14.9 East face of Monte Rosa and Ghiacciaio del Belvedere, Valle Anzasca, Regione Piemonte, Italy. Most intense rock fall activity in this rock face correlates with warm or marginal permafrost and recently deglaciated surfaces. The detachment zone of an ice avalanche (2005) is marked with a black circle, the one of a rock avalanche (2007) in white (photo: L. Fischer 2004)

Fig. 14.10 Avalanche protection above Pontresina, Grisons Alps, Switzerland. The retention dam at the bottom of the slope is to protect against snow avalanches and debris flows from marginal permafrost (photo: W. Haeberli 2007)

the potential to produce large flood waves. Such flood waves may trigger devastating and far-reaching debris flows, constituting a serious and still inadequately recognized hazard to people and infrastructure in cold mountain regions.

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