Spatial and temporal variability of snowpack stability in the Craigieburn Valley, New Zealand

Abstract

Spatial variability of snowpack stability has been a topic of considerable contemporary research in the Northern Hemisphere, yet relatively few studies have examined the issue in New Zealand. In order to address this gap in knowledge, the present study has two objectives: (1) to analyse the avalanche terrain of Craigieburn Valley using a geographic information system; and (2) to investigate the spatial and temporal variability of snowpack stability within Craigieburn Valley. To facilitate the achievement of these objectives, 35 consecutive days were spent ‘on-mountain’ at Craigieburn Valley, between 22 June and 26 July 2012. This enabled valuable observations of the weather, and associated implications for the seasonal snowpack of the area, to be obtained prior to snowpack stability testing. Compression tests (CTs) and extended column tests (ECTs) were performed on a 36°, easterly aspect slope at 1,810 m a.s.l. within the Craigieburn Valley ski area boundary, and on a uniform 37°, southerly aspect slope at 1,650 m a.s.l. within the Craigieburn Valley backcountry. In addition, the backcountry testing location was roped off, enabling observations of an undisturbed snowpack to be made.

Over the course of early Austral winter 2012, Craigieburn Valley experienced both significant snowfall and rainfall events. 125 cm of snowfall accumulated as a result of a single storm in early-June, yet 70 mm and 106 mm single storm rain events were recorded in late-June and mid-July, respectively. The primary snowpack weakness observed was persistent weak layers of faceted crystals. The development of persistent weak layers occurred as a result of a relatively thin snowpack depth (approximately 80 cm), relatively high snowpack temperature gradients of up to 2.6°C per 10 cm, and the presence of a significant crust in the snowpack, due to rainfall which occurred in late-June. Terrain analyses showed that 94% (93 ha) of primary avalanche terrain (slope angle between 30° and 45° in the Craigieburn Valley backcountry is of southerly, south-westerly and south-easterly aspect, compared to just 25% (38 ha) of the Craigieburn Valley ski area.

Considerable variability of snowpack stability was observed on the undisturbed uniform slope in the Craigieburn Valley backcountry. As such, stability testing from a single snowpit could not reliably represent the snowpack stability of an entire slope. The proportion of stability ratings that were representative of the expected slope stability varied between 0% when CT results were used, to 30% when ECT results were used. In addition, very unstable stability test results were obtained, despite an expectation that the slope had good stability. The implications of these results is that more than one snowpit should be carried out on a single slope in order to improve the reliability of stability test results obtained. Temporal changes in snowpack stability were also observed. Given a lack of adverse weather conditions, overall slope stability increased. However, the spatial variability increased, and furthermore, very poor stability results were still obtained.

A key theme to emerge from the present study is that considerable spatial variability of snowpack stability may be observed in the early winter months, particularly on aspects of a southerly-component. Given the desire of Craigieburn Valley Ski Club management to expand the current ski area boundary, this presents challenges regarding hazard mitigation; as snowpack weaknesses tend to persist for longer on aspects of a southerly-component. The findings presented in this study offer improvements to the understanding of avalanche terrain within Craigieburn Valley. Combined with insights into the variability of snowpack stability, the understanding of potential avalanche hazard in the Craigieburn Valley is improved, which may contribute to the ongoing mitigation of avalanche hazard in the area. In addition, due to a relative paucity of previous research, the present study offers a valuable contribution to the knowledge of spatial and temporal variability of snowpack stability in the New Zealand setting.