The geomorphological effect of cornice fall avalanches in the Longyeardalen valley, Svalbard

Abstract

The study of snow avalanches and their geomorphological effect in the periglacial parts of the cryosphere is important for enhanced geomorphological process understanding as well as hazard-related studies. Only a few field studies, and particularly few in the High Arctic, have quantified avalanche sedimentation. Snow avalanches are traditionally ranked behind rockfall in terms of their significance for mass-wasting processes of rockslopes. Cornice fall avalanches are at present the most dominant snow avalanche type at two slope systems, called Nybyen and Larsbreen, in the valley Longyeardalen in central Svalbard. Both slope systems are on northwest-facing lee slopes underneath a large summit plateau, with annual cornices forming on the top. High-frequency and magnitude cornice fall avalanching is observed by daily automatic time-lapse photography. In addition, rock debris sedimentation by cornice fall avalanches was measured directly in permanent sediment traps or by snow inventories. The results from a maximum of seven years of measurements in a total of 13 catchments show maximum mean rock debris sedimentation rates ranging from 8.2 to 38.7 kg m⁻² at Nybyen, and from 0.8 to 55.4 kg m⁻² at Larsbreen. Correspondingly, avalanche fan surfaces accreted from 2.6 to 8.8 mm yr⁻¹ at Nybyen, and from 0.2 to 13.9 mm yr⁻¹ at Larsbreen. This comparably efficient rockslope mass wasting is due to collapsing cornices producing cornice fall avalanches containing large amounts of rock debris throughout the entire winter. The rock debris of different origin stems from the plateau crests, the adjacent free rock face and the transport pathway, accumulating distinct avalanche fans at both slope systems. Cornice fall avalanche sedimentation also contributed to the development of a rock glacier at the Larsbreen site during the Holocene. We have recorded present maximum rockwall retreat rates of 0.9 mm yr⁻¹ at Nybyen, but as much as 6.7 mm yr⁻¹ at Larsbreen, while average Holocene rockwall retreat rates of 1.1 mm yr⁻¹ at Nybyen have been determined earlier. As cornice fall avalanches are the dominant type of avalanche in central Svalbard, the related geomorphological effect is assumed to be of significance at periglacial landscape scale. A climate-induced shift in prevailing winter wind direction could change the rockslope sedimentation effectively by changing the snow avalanche activity

Reconstruction of Loch Lomond Stadial (Younger Dryas) glaciers on Ben More Coigach, north-west Scotland, and implications for reconstructing palaeoclimate using small ice masses

“…The longer shaping of such features over several snowmelt seasons under cold‐climate conditions that would have prevailed for some time after deglaciation at higher altitudes is likely to have contributed to the deep incisions. Furthermore, the development of snow cornices along the plateau edge may also have contributed to gully erosion both directly, through subnival fluvial erosion, and indirectly, by ‘pre‐conditioning’ the plateau edge and enhancing fluvial erosion: observations on Svalbard demonstrate that snow cornices promote and trigger denudation (Eckerstorfer et al ., ,b). We have noted similar associations of cold‐based ice connections and gullies elsewhere in a palaeoglaciological context (e.g.…”

Quantifying seasonal cornice dynamics using a terrestrial laser scanner in Svalbard, Norway

“…The steep valley walls descending from the broad plateau summits (approximately 450 m elevation) are characterized in their upper portions by protruding resistant bedrock buttresses and transport couloirs incised by fluvial and gravitational slope processes. The Gruvefjellet slope described in detail by Eckerstorfer et al (2013) consists of a 50-70 m near-vertical bedrock cliff situated under the plateau margin and over a 40-50 • slope that serves as a slab avalanche release area. This broad slope transitions into the transport couloirs which in turn feed extensive avalanche fan deposits downslope.…”

“…Snow cornices are overhanging projections of snow that form due to the deposition of wind-transported snow in the lee of ridgelines or sharp slope inflections (Montagne et al, 1968;Seligman, 1936). Cornices have attracted interest for their hydrologic implications (e.g., Anderton et al, 2004) and as agents of geomorphic change in periglacial environments (Eckerstorfer et al, 2013;Humlum et al, 2007), but they are perhaps best recognized as a snow and avalanche hazard in mountainous terrain (Montagne et al, 1968;Vogel et al, 2012). Cornices pose an avalanche hazard when they fail either as a full cornice failure with the entire cornice detaching from the ground or as a partial failure with a smaller cornice mass separating from the rest of the cornice.…”

Quantifying seasonal cornice dynamics using a terrestrial laser scanner in Svalbard, Norway

“…The steep valley walls descending from the broad plateau summits (approximately 450 m elevation) are characterized in their upper portions by protruding resistant bedrock buttresses and transport couloirs incised by fluvial and gravitational slope processes. The Gruvefjellet slope described in detail by Eckerstorfer et al (2013) consists of a 50-70 m near-vertical bedrock cliff situated between under the plateau margin and over a 40-50° slope that serves as a slab avalanche release area. This broad slope transitions into the transport couloirs which in turn feed extensive avalanche fan deposits downslope.…”

“…Cornices have attracted interest for their hydrologic implications (e.g. Anderton et al, 2004) and as agents of geomorphic change in periglacial environments (Eckerstorfer et al, 2013;Humlum et al, 2007), but are perhaps best recognized as a snow and avalanche hazard in mountainous terrain (Montagne et al, 1968;Vogel et al, 2012). Cornices pose an avalanche hazard when they fail either as a full cornice failure with the entire cornice detaching from the ground or as a partial failure with a smaller cornice mass separating from the rest of the cornice.…”