A Contribution to the Prediction of Slush Avalanches

Abstract

Slush avalanches, primarily known from uninhabited arctic and mountainous regions, every year cause damage to man and property in Norway. Seeking objective criteria to identify their hazard zones, and methods for their prediction and control, the Norwegian Geotechnical Institute (NGI) has started a research program on these topics. Thirty-four slush avalanches with known locations of crown surface have been investigated with regard to geomorphic and climatic characteristics. Starting zones, crown surfaces and tracks were classified according to geomorphic conditions important to avalanche release and motion. Weak cohesionless snowpacks of coarse grains, and hard layers or crusts of ice in snow cover or on the ground, are critical conditions for slush avalanche release. Intense rain, falling on cohesionless new snow on these substrata, is the most striking feature of current weather situations. The main starting zones were drainage channels, Sloping bogs, depressions and open fields. Channel-like, scar-like or bowl-like features appeared in the snow cover due to avalanche release. Three main types of release were identified: 1) Sudden release from crown surface, 2) drainage of snow-embanked, water-saturated snowfields through narrow outlets and 3) rapid headward growth from first point of release. Enlargements and confinements along the avalanche tracks were caused by distinctly defined features in the tracks.

Meteorology, Topography and Snowpack Conditions causing Two Extreme Mid‐Winter Slush and Wet Slab Avalanche Periods in High Arctic Maritime Svalbard

“…In general, the snowpack characteristics shown are comparable to the findings of Eckerstorfer and Christiansen (), with a weak depth hoar base, sometimes underlain by a basal ice layer, a soft, coarse‐grained middle part and numerous ice layers. Hestnes () stated that a coarse‐grained snowpack is very prone to flowing and thus favourable for slush avalanche release. We did not find any rock debris in the slush and wet slab avalanche deposits, which suggests that sliding was on an ice layer in both wet avalanche cycles, and none extended to the full depth (Greene et al ., ).…”

“…Mid‐winter slush avalanches have been reported in Iceland, triggered by a combination of snowmelt and rain on snow (Decaulne and Saemundsson, ). Slush avalanches triggered by rain‐on‐snow events in mid winter have also been reported from many locations along the west coast of mainland Norway (Hestnes, ; Hestnes and Sandersen, ; Hestnes et al ., 2004). Snow slush avalanches triggered by mid‐winter rain also occurred in Spain in December 1997 (Furdada et al ., ).…”

“…Nyberg also stated that ‘slush avalanches’ should be used for major events larger than slush flows and slush torrents. Hestnes () showed empirically that slush avalanches are topographically confined to valleys, unlike wet snow avalanches.…”

“…More detailed studies of slush avalanches were carried out by Rapp () in northern Sweden, Nobles () in northern Greenland and Onesti () in central Alaska. The Norwegian Geotechnical Institute devoted much of its scientific efforts to examining slush avalanche occurrences and trigger mechanisms (Hestnes, ; Hestnes and Sandersen, ; Onesti and Hestnes, ) on the mountainous, maritime‐influenced west coast of Norway, where they periodically destroy infrastructure (Hestnes, ). More recently, studies in Iceland (mid‐winter rain‐on‐snow event; Decaulne and Saemundsson, ) and Spain (Furdada et al ., ) have provided detailed descriptions of slush avalanche events, while Jaedicke et al .…”

A low‐angle slushflow in the Kirgiz Range, Kirgizstan

“…In the past decade, research directed specifically at slushflows has accelerated (e.g. Hestnes, 1985;Nyberg, 1985;Onesti, 1985;Clark and Seppalii, 1988;Onesti and Hestnes, 1989).…”