Avalanches form of one nature’s deadliest hazards, with the momentum of flowing rocks and debris endangering human environments and physical landscapes. The destructive ability of these avalanches is enhanced through the formation of a flow-front containing large, angular particles. These coarse grains act like a dam resisting the motion of more mobile material flowing behind, and cause the flow to break up into a number of channels that allow the flow to travel further at higher velocities. Similar to the shaking of a cereal box, an avalanche sorts the larger particles to the top, where the higher velocities carry them forwards. These particles may be overrun and resegregated back upwards, creating a recirculating motion just behind the front of the avalanche.
Mathematicians at The University of Manchester have conducted unique moving-bed flume experiments and simulations which reveal that, whilst most large particles recirculate quickly in this zone, a few large particles recirculate very slowly as they pass through regions of many small particles. Exact mathematical solutions for both the particle concentration field and the individual particle trajectories successfully capture this new behaviour for the first time. The insight provided by this work will help inform better predictive avalanche models in the future.