Unraveling driving factors for large rock–ice avalanche mobility

Large rock–ice avalanches have attracted attention from scientists for decades and some of these events have caused high numbers of fatalities. A relation between rock slope instabilities in cold high mountain areas and climate change is currently becoming more evident and questions about possible consequences and hazard scenarios in densely populated high mountain regions leading beyond historical precedence are rising. To improve hazard assessment of potential rock–ice avalanches, their mobility is a critical factor. This contribution is an attempt to unravel driving factors for the mobility of large rock–ice avalanches by synthesizing results from physical laboratory experiments and empirical data from 64 rock–ice avalanches with volumes >1x10⁶ m³ from glacierized high mountain regions around the world. The influence of avalanche volume, water and ice content, low‐friction surfaces, and topography on the apparent coefficient of friction (as a measure of mobility) is assessed. In laboratory experiments granular ice in the moving mass was found to reduce bulk friction up to 20% while water led to a reduction around 50% for completely saturated material compared with dry flows. Evidence for the effects of water as a key driving factor to enhance mobility was also found in the empirical data, while the influence of the ice content could not be confirmed to be of much relevance in nature. Besides liquefaction, it was confirmed that mobility increases with volumes and that frictional surface characteristics such as flow paths over glaciers are also dominant variables determining mass movement mobility. Effects of the topography along the flow path as well as channeling are assumed to be other critical factors. The results provide an empirical basis to roughly account for different path and flow characteristics of large rock–ice avalanches and to find appropriate ranges for friction parameters for scenario modeling and hazard assessments. Copyright © 2011 John Wiley & Sons, Ltd.