Meteorology and snowpack structure associated with avalanche hazard, Porter Heights, Canterbury.

• Main Author: Allen, Simon Keith
• Language: en
• Published: University of Canterbury. Department of Geography 2009
• Online Access: http://hdl.handle.net/10092/2778

With an increase in use of New Zealand's alpine terrain, there is a growing need to understand processes and phenomena associated with snow avalanche hazard. This research investigates snowpack structure and meteorological influences associated with avalanching at Porter Heights Ski Area and overviews the management practices employed in response to this hazard. Data sources are derived from meteorological and snowpack observations dating back to 1977, coupled with comprehensive field studies in 2003. Weak faceted and mixed crystal forms were found to comprise on average over 40 % of the early season snowpack on sunny and shaded slope aspects, although they could persist on shaded slopes throughout the season, contributing to spring avalanche events. The growth of both lower pack depth hoar, and near-surface facets were strongly influenced by synoptic airflows. Forty-eight hours after the onset of a cool southeast airflow, the temperature gradients in a shallow snowpack ranged from -0.36 °C cm-1 near the surface, to -0.17 °C cm-1 deeper in the pack. A warmer northwest flow induced surface melting, and dramatically altered the thermal regime of the snowpack. Thick layers of rounded grains indicated the importance of equilibrium growth and wind redistribution of snow. Wind loading is most extensive on slopes leeward to the prevalent, strong, west to northwest winds, and slab formation in this terrain is regularly controlled using hand placed explosives and ski cutting techniques. Density measurements centred on a median value of 185 kg/nr' suggested the significance of decomposing new snow in forming surface slabs, with the largest snowfalls at Porter Heights occurring during east to southeast storm events. While this study has cemented a comprehensive understanding of meteorology and snowpack structure at Porter Heights, backcountry avalanche forecasting in the neighbouring terrain will further benefit from larger scale, collaborative future research initiatives.