Rock avalanche mobility is controlled by the characteristics of material in the path of the flow

Rock avalanche mobility is one of the most intriguing aspects of landslide behaviour.  It has long been recognised that this type of landslide tends to travel faster and further than would be expected from simple friction parameters.  There has been a huge amount of debate as to the causes of this behaviour, with multiple theories having been proposed, some of which have been quite exotic.  To date no consensus position has emerged, and the debate rages on.

In a new contribution to the discussion on rock avalanche mobility, a paper in the journal Engineering Geology (Aaron and McDougall 2019) examines 45 well constrained rock avalanche case histories to better understand the controls on their runout.  Their work used a semi-empirical modelling approach to back analyse the mobility of the landslides, testing a series of hypotheses centred on the idea that rock avalnche mobility is controlled, at least in part, by interaction with the basal materials that they encounter along their path.  This is perhaps most easily imagined in the context of flowing over glacial ice, such as in this beautiful example:

The Lamplugh Glacier rock avalanche via Paul Swanstrom.

In this case, flowing over a very low friction ice surface might be expected to increase rock avalanche mobility.  The same might also occur where the landslide encounters loose, saturated materials, but perhaps to a lesser degree.

The authors conclude that for many of these landslides their behaviour can be best modeled by assuming that the frictional rheology / resistance of the landslide changes in the transition from the source zone to the runout path. In general where the rock avalanche ran across bedrock, the resistance to motion was found to be high, but where the path traversed ice or a saturated substrate the resistance to motion was found to be much lower.

The results are a really interesting contribution to the debate. This matters as high mobility rock avalanches can have a devastating impact on communities and infrastructure, and unsurprisingly there is increasing interest in trying to forecast likely behaviour.  As the authors state:

“…forecasts must parameterize the relationship that governs the basal resistance the rock avalanche experiences during emplacement…this choice must account for the expected shear characteristics of the path material”

Reference

Aaron, J. and McDougall, S. 2019.  Rock avalanche mobility, the role of path material. Engineering Geology. https://doi.org/10.1016/j.enggeo.2019.05.003