Starving avalanches: Frictional mechanisms at the tails of finite-sized mass movements

The tail of an avalanche is characterized by diminishing flow heights. The decreasing flow heights are due to increased friction and therefore tails are ideally suited to investigate frictional mechanisms in avalanches. Using chute experiments with granular material, we observe two properties of avalanche tails: (1) Coulomb friction m increases in proportion to the decrease in the gravitational work rate W-g; (2) flow heights h are proportional to the square of the basal slip velocity u(0); h proportional to u(0)(2). Another non-steady region can be observed at the front of the avalanche. Although this region is shorter than the tail, we were able to detect a hysteresis of the friction m coefficient as a function of the gravitational work rate. This fact indicates a time dependence of the frictional mechanisms. The results explain why avalanches starve when they are not fed by the intake of additional material at the front.