Slope-break Collisions: Comment on “Insight Into Granular Flow Dynamics Relying on Basal Stress Measurements: From Experimental Flume Tests” by K. Li et al.

Numerical simulations show that the positive correlation observed in laboratory experiments by Li et al. (2022) between an increase of grain size and particle agitation, on the one hand, and an increase of granular flow mobility, on the other hand, is not a valid cause-and-effect relationship. In other words, their mobility differential is not caused by a different energy dissipation rate that results from a different grain size content. Instead, the flows stop because of a head-on collision with the horizontal flume at the bottom of a steep 40º incline. Essentially, the slope-break jams the granular movement. Indeed, a combination of laboratory experiments and numerical simulations demonstrated that the mobility of unhindered dense granular flows increases as grain size and clast agitation decrease. Consequently, there is no evidence that the high mobility of large natural rock avalanches is due to an increase of particle agitation.