Insights Into Influence Of Ice Debris On Mobility Of Rock-ice Avalanches

The hypermobility and devastating results of rock-ice avalanches,one kind of rock avalanches involving ice,snow or firn,are linked to the mechanic behaviour of ice\snow debris.The influence of the varying ice content,bulk friction coefficient and meltwater content was investigated through a series of flume tests and numerical simulation with a self–designed flume,a set of data acquisition system and an open–source software,Mat DEM,based on the discrete element method.The propagation mechanisms of rock-ice avalanches were discussed via analysing the flow characteristics and the ice-water-rock interactions.Some key conclusions are drawn as follows:(1)As ice content increasing,the run-out distances of rock-ice avalanches with low ice contents increase significantly.This implies that a certain amount of ice enhanced the mobility of rock-ice avalanches.While the run-out distances of rock-ice avalanches with high ice contents decrease slightly.The interaction between gravel and ice cubes could not substantially influence the mobility of mixtures because the gravel was embedded in a matrix of ice cubes.(2)The refreezing ice bridge observed in flume tests is relevant to the mixture mobility.Weak refreezing ice bridges,in mixtures with low-medium ice content,are broken during particle rearrangement,dissipating some kinetic energy and thus depressing mixture mobility.Firm refreezing ice bridges,in mixtures with high ice content,embed gravel into ice matrix,thus the basal friction coefficient is lowered and the mixture mobility is enhanced,as a result of the impeded density-driven segregation.(3)As water content increasing,interfacial meltwater lubricates particle corners and enhances the mobility of the mixture with low ice content.Gradually,the cohesion effect of water film or water bridge dissipates more kinetic energy and reduced mixture mobility.(4)A tag value of water content,namely kinetic-retention water content,represents the maximum capacity of mixtures to retain interstitial water.The mass loss along the propagation path occur when the meltwater content reaches this value in rock-ice avalanches.The retarded meltwater on slide surface will be maintained or drained by underlying strata.(5)The granular material presents a combined movement mode of 'granular flow and block sliding' on the icy sliding surface.From the acceleration stage to the early stage of the deceleration,the lower bottom friction makes the granular flow accelerate faster and decelerate more slowly;granular material losses flow characteristic and slides as a block at the end of the deceleration stage.This phenomenon is also observed in pure ice debris moving on coarse sliding surface.