Snow avalanche phenomena on the eastern side of the Craigieburn Range, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The role of terrain, meteorological and snowpack factors contributing to snow avalanche occurrence were investigated on the eastern side of the Craigieburn Range. Data on which this analysis was based were collected in the winters of 1979 to 1981. The majority of slab avalanching occurred on lee slopes with large bowl shaped starting zones of 33 to 36 degrees inclination while frequency of avalanching was related to path size factors. Theoretical models adequately predicted avalanche runout distance on the majority of paths. However, these models proved only partially satisfactory for valley side paths with steep vegetated runout zones. An empirical model developed for predicting avalanche runout based on measured terrain characteristics performed well. Threshold values for avalanching were established for precipitation, temperature and wind related variables. A discriminant analysis model for distinguishing dry, wet and non-avalanche days was established using maximum six hour precipitation intensity, variation in wind direction, total global radiation and two hour mean temperature. An independent test of this model produced acceptable results. Synoptic weather types dominated by vorticity advection processes and characterised by cyclonic circulation were the most frequent storm types producing snowfall in the study period. However, systems typified by a mixture of warm advection and vorticity advection processes with well developed troughs, associated fronts, high rates of thermal advection and combinations of stability and instability produced the greatest amounts of snow and snow avalanching. A three phase snowpack development model was proposed and considered in relation to snow avalanching. The magnitude of positive and negative snowpack temperature gradients exceed those generally required for recrystallisation processes. Temperature gradient snow crystals were observed at the base and mid to upper parts of the snowpack. Fracture line profile, shovel test and shear frame analyses revealed that partially metamorphosed new snow undergoing equitemperature metamorphism, especially clusters of rimed needles, and snow in the early stages of temperature gradient metamorphism, were characteristically weak in shear. Three categories of potential sliding layers for avalanche release are presented. Consideration of snowpack and meteorological information indicates that the Craigieburn Range snow avalanche climate approximates most closely that of one transitional between a maritime and continental situation.