Rockfall at Fox Glacier, New Zealand: a Hazard Analysis using Structure from Motion and Spatial Modelling

Abstract

As glaciers retreat, downwasting of the ice increases the instability of surrounding rock slopes. In addition, ice thinning can lead to surface morphology changes that can result in a progressively concave transverse profile. This short-term paraglacial process may lengthen the run-out distance of potential rockfalls. The Fox glacier is a temperate maritime glacier undergoing high annual precipitations and warm temperatures, thus it is one of the most climate sensitive of the South-Alps. Current rapid retreat appears to be associated with an escalation of rockfall activity, particularly in the terminus region where walking-tracks are located. Glacier-related tourism is a key point in the valley development, consequently the lack of safe-access displays a dual issue in terms of economy and of risk management. The area was therefore relevant to develop a rockfalls assessment using a physics-3D model (RockFall Analyst, Lan et al 2007) with a high resolution digital elevation model (DEM) acquired by Structure from Motion (SfM). The analysis improves the last 2D-hazard profiles by spatial modelling of the rockfall trajectories and taking into account the characteristics of rock behaviour on ice. The simulations show blocks travelling further onto the glacier than the last profiles. We identify potential rockfall run-out on hazard maps thus providing a first assessment tool for local guide for working in this dynamic environment. Ongoing research on precipitation data are carring out. Futur researches with geophysical investigations on the talus slope at seasonal scale could considerably refine the understanding of the linkage between paraglacial processes and rockfall occurence. Numerous alpine glaciers are retreating and become prone to natural hazards, thus resulting in the increase around the word of management between risk and glacier tourism.