Diethard Sanders
Features related to snow avalanches and snow glides, Nordkette range (Northern Calcareous Alps)

Geo.Alp 10, 2013, p. 71–92

Abstract
The southern flank of Nordkette range near Innsbruck city (Austria) is sculpted by bedrock-incised ravines that, in their upslope portion, are excavated by snow avalanches (also termed snowflows) into avalanche chutes. The downslope portion of the ravines, in contrast, is mainly shaped by fluvial erosion from ephemeral to perennial creeks. Downslope from the starting zone of most snowflows, the avalanche chutes show: (a) an increasing depth of bedrock incision down to a few tens of meters, accompanied by (b) a progressive change of cross-section from an ‚inverted trapeze‘ to a U-shape, or to a more deeplyincised ‚inverted trapeze‘ shape; farther downslope, the chutes pass into the lower part of the ravines shaped by stream erosion (V-shaped cross-section). Aside of chute incision, full-depth snow avalanches and snow glides produce smaller-scale features of erosion, such as rip-ups a few meters to a few tens of meters in width of soil and scree; these rip-ups may develop into sub-circular to elliptical depressions similar to nivation kettles. Furthermore, the floor of the avalanche chutes is characterized by grooves and striae excavated from soil. Along some of the chutes, bedrock exposures and surface facets of lithoclasts show polish and striation from by-pass of sediment-laden full-depth snowflows. Clasts with avalanche polished-striated (p-s) surface facets can be distinguished from p-s clasts from basal tills of glaciers by a set of criteria. Polished-striated bedrock and clast surfaces are preservable – in declining quality – over a few tens of years at least, and provide a criterion to identify avalanche activity in areas where snowflows are rare, or where snow cover is absent in season. Along Nordkette, features of avalancherelated sediment transport mainly include scree littered within or alongside the chutes, and scree perched on top of boulders. Aside of the lower part of slope drained by streamlets, snow avalanching is the most active geomorphic agent in shaping the chutes. A comparatively common incidence of large, full-depth, sediment-laden snowflows along the Nordkette is related to: (a) the mountain flank dipping with ~50–30° S, maximizing insolation, and (b) the facing of the flank into a major foehn-wind corridor. Rapid warming during foehn lowers the stability of the snow cover and, in case of sustained thawing, of underlying soil and scree. This favours, both, the formation of sediment-laden full-depth avalanches and the excavation of chutes.

keywords: Alps, snow, snow avalanche, avalanche chute


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