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

Mid‐winter wet avalanche cycles in High Arctic Svalbard occurred during January 2010 and March 2011, allowing studies of slush and wet slab avalanche deposits. Both cycles represented extreme events in magnitude and frequency and were caused by the passage of low‐pressure atmospheric systems with positive air temperatures, high wind speeds and 100‐year record monthly rainfall. Slush avalanches were confined to river‐cut gorges, with low starting‐zone inclinations, and deposits consisting of flow lobes and levées. Wet slab avalanches were not confined topographically, started anywhere on open mountain slopes and displayed tongue‐shaped debris deposits. During both of the two wet avalanche periods analysed, snowpack conditions favoured the release of slush avalanches, as the snowpack consisted of a coarse‐grained middle section above a water‐impermeable ice layer. Such snowpack conditions are typical for central Svalbard. The resulting slush and wet slab avalanches were extreme in their size and runout distances, crossing frequently used snowmobile tracks at 20 locations and posing a threat to traffic and infrastructure. Four additional potential large‐scale slush avalanche periods were identified from analysis of the meteorological record from Longyearbyen (1912–2011). They cluster in the mid to early 1990s, with comparable meteorological conditions to the January 2010 and March 2011 wet avalanche cycles. It is concluded that the frequency and duration of low‐pressure weather systems are the dominant controls on wet snow avalanches, and that mean snow season air temperature (October–May) is of little importance. Copyright © 2011 John Wiley & Sons, Ltd.