Flow regimes and basal normal stresses in rock–ice avalanches by experimental rotating drum tests

Abstract

Rock–ice avalanches have been observed to travel longer distances than rock avalanches, and their flow regime may vary as ice melts. Several reports on the deposition characteristics, mobility, and hazard assessment of rock–ice avalanches have been published. However, thus far, no systematic investigation on the flow regime evolution and basal stress characteristics of rock–ice avalanches have been conducted. In this study, flow regime is characterized by several dimensionless spaces bounded by parameters (Froude number, Savage number, Friction number, Mass number, and Reynolds number). To examine the mechanisms controlling basal stress, a 12.56-cm2 load plate was used to measure the basal normal stress in a 1.5-m-diameter 30-cm-wide vertically rotating drum in a temperature-controlled laboratory. Results demonstrate that the flow regimes of rock–ice avalanche, dominated by short-term collision or long-term friction, considerably vary due to the influence of initial conditions (ice content, grain size, flow velocity, and meltwater). Although rock–ice avalanches under experimental and natural conditions substantially differ, the dominant stress mechanism is the same as that in the movement process. Other findings show that the average normal stress profiles vary significantly with the ice content and meltwater volume. Furthermore, the mean stress (σ¯), maximum stress (σ1%), and standard deviation (σsd) demonstrate the close relationship between fluctuations, static loads of flow, and grain interactions. Grain agitation in the rock–ice avalanche is caused by intense collisions depending on grain size, velocity, and fluid matrix at the grain scale level.