Abstract
Small perturbations to a steady uniform granular chute flow can grow as the material moves downslope and develop into a series of surface waves that travel faster than the bulk flow. This roll wave instability has important implications for the mitigation of hazards due to geophysical mass flows, such as snow avalanches, debris flows and landslides, because the resulting waves tend to merge and become much deeper and more destructive than the uniform flow from which they form. Natural flows are usually highly polydisperse and their dynamics is significantly complicated by the particle size segregation that occurs within them. This study investigates the kinematics of such flows theoretically and through small-scale experiments that use a mixture of large and small glass spheres. It is shown that large particles, which segregate to the surface of the flow, are always concentrated near the crests of roll waves. There are different mechanisms for this depending on the relative speed of the waves, compared to the speed of particles at the free surface, as well as on the particle concentration. If all particles at the surface travel more slowly than the waves, the large particles become concentrated as the shock-like wavefronts pass them. This is due to a concertina-like effect in the frame of the moving wave, in which large particles move slowly backwards through the crest, but travel quickly in the troughs between the crests. If, instead, some particles on the surface travel more quickly than the wave and some move slower, then, at low concentrations, large particles can move towards the wave crest from both the forward and rearward sides. This results in isolated regions of large particles that are trapped at the crest of each wave, separated by regions where the flow is thinner and free of large particles. There is also a third regime arising when all surface particles travel faster than the waves, which has large particles present everywhere but with a sharp increase in their concentration towards the wave fronts. In all cases, the significantly enhanced large particle concentration at wave crests means that such flows in nature can be especially destructive and thus particularly hazardous.
Highlights
Large-scale debris flows spontaneously develop wave-like disturbances that move downstream faster than the material flow (Li et al 1983; McArdell et al 2003; Zanuttigh & Lamberti 2007)
Similar roll waves occur in experimental dry granular chute flows (Savage 1979; Forterre & Pouliquen 2003), where they likewise arise from instability of uniform flows
This paper presents small-scale experiments in which a mass of bidisperse granular material consisting of large green and small white spherical glass ballotini (200–250 μm and 75–150 μm diameter, respectively) is released from a hopper and flows down a chute
Summary
Large-scale debris flows spontaneously develop wave-like disturbances that move downstream faster than the material flow (Li et al 1983; McArdell et al 2003; Zanuttigh & Lamberti 2007). Larger particles are initially segregated to the flow surface, where the velocity is highest, and are transported rapidly downstream towards the flow front When this process is combined with frictional differences between the large and small grains there is a very rich variety of behaviour, for example the spontaneous self-channelisation of the flow and the formation of coarse-grained lateral levees (Félix & Thomas 2004; Johnson et al 2012; Kokelaar et al 2014) and lobate finger-like channels (Pouliquen, Delour & Savage 1997; Pouliquen & Vallance 1999; Woodhouse et al 2012; Baker, Johnson & Gray 2016b), which increase the distances that geophysical mass flows travel.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.