Influence of particle size on the mechanism of dry granular run-up on a rigid barrier

Granular flows comprise a wide range of particle sizes. The particle size governs the degree of grain contact and inertial stresses in the flow, thus influencing the mechanism of impact against a rigid barrier. The current commonly adopted design approaches estimating the run-up height are based on the energy principle and the momentum approach. However, both neglect the discrete nature of flows, and do not consider the effects of particle size on the flow regime. In this study, physical experiments using different sizes of monodispersed sand and glass spheres were carried out to investigate the run-up mechanisms on a rigid barrier. Results have shown that the run-up height is not only dependent on the Froude number of the flow before impact, but also on the particle size which principally governs the mechanism of run-up. Inertial flows comprising large particles (the Savage number (N_Sav) > 0.1) profoundly transfer momentum vertically upon impact, resulting in significant grain saltation and high run-up heights. In contrast, frictional flows comprising fine particles (N_Sav <0.1) tend to pile up without significant run-up due to a high degree of contact stresses. This implies that for flows with coarse particles entrained at the front of the flow by way of particle-size segregation, the run-up height is principally influenced by large particles that accumulate at the flow front.