On the Connection between Debris Flow Activity and Permafrost Degradation: A Case Study from the Schnalstal, South Tyrolean Alps, Italy

ABSTRACTThe possible influence of permafrost degradation on the formation of debris flows in an area of the South TyroleanAlps, Italy, was examined by comparing debris flow activity since 1983 with the modelled contemporary permafrostdistribution. The study focused on the spatial congruence of new initiation zones and potentially marginalpermafrost, which should be especially sensitive to climatic change and is presumed to be currently degrading. Theresults show that distinct changes in the spatial position of debris flow initiation areas mainly occurred at elevationsabove this marginal zone. Consequently, the changes detected in debris flow activity do not appear to have beeninfluenced by atmospheric warming‐induced degradation of permafrost. However, a link may exist to the thickeningof the active layer caused by the melting of a glacier. Copyright © 2011 John Wiley & Sons, Ltd. KEY WORDS : periglacial debris flows; mapping; permafrost degradation; spatial modelling INTRODUCTIONAreasofmountainpermafrostareexpectedtobesignificantlyaffected by current climate warming (Haeberli and Gruber,2009). Mountainous areas are also subject to increasinglyintense use and development (Haeberli et al., 1997). Thus,the possible increase in hazard potential−especially in theEuropean Alps−has generated considerable research on thelinks between atmospheric warming, permafrost degradationand slope instability (e.g. Schlyter et al., 1993; Davies et al.,2001; Harris et al., 2001; Kneisel et al., 2007; Noetzli et al.,2007;Allen etal.,2010;Keileretal.,2010).Thepresentstudyfocuses on debris flows, which are common mass‐wastingprocesses in the changing alpine environments and thus posesignificant hazards to life and infrastructure.Recent progress in measurement techniques and model-linghasimprovedourknowledgeoftherelationshipbetweenpermafrost and rock face stability (e.g. Harris et al., 2009),but the possible role of climatically induced permafrostdegradation in the initiation of debris flows is not yet wellunderstood. One reason for this is an inadequate compre-hension of the evolution of permafrost beneath moderatelyinclined slopes where debris and snow cover act as complexinterfaces between the atmosphere and the subsurface(Luetschg et al., 2004; Gruber and Haeberli, 2009). Thiscreates a significant challenge for modelling the thermalcondition and spatial distribution of permafrost. Further-more, permafrost in unconsolidated materials generally hashigh ice contents, which retard potential thawing by theuptake of latent heat (Noetzli et al., 2007). Permafrost soilsthus may respond to climate forcing over several decades tocenturies (cf. Haeberli, 1992) and the effects of permafrostdegradation on debris flows may be difficult to detect withinone or two decades. Finally, debris flows themselves arehighlycomplexgeomorphicprocesses.Ourunderstandingoftheir initiation, resulting from the temporal and spatialconcurrence of several highly variable factors includingdebris availability and the occurrence of transient triggeringevents, and their overall sensitivity to changes in climaticparameters, is still incomplete (cf. Rebetez et al., 1997;Zimmermann et al., 1997; Jomelli et al., 2004, 2007). Yet,climatically induced thawing of alpine permafrost isexpected to significantly affect the hydraulic and geotech-nical properties of perennially frozen unconsolidated