Impact force of granular flows on walls normal to the bottom: slow versus fast impact dynamics

Abstract

The devastating effects of natural hazards due to the propagation of mass flows, such as landslides, debris flows, and avalanches, can be avoided, or at least reduced, by placing protective barriers or catching dams in the runout zones. Such structures can store the whole mass and finally stop the flow before it may reach vulnerable infrastructures. Their design requires the modelling of the runup of granular flows on rigid walls and the induced impact force. In this study, careful attention is paid on how the incoming flow regime (either slow or fast flow) that takes place before the impact with the wall can drive the prevailing process at stake during the impact of the flow with the wall. Slow flows produce gentle pile-up of the mass behind the wall with gradually varied streamlines, while faster flows give rise to a granular jump traveling upstream. Two different analytic solutions are proposed and checked against recent small-scale laboratory tests by Ashwood and Hungr (2016), who investigated both slow and fast impact dynamics of granular flows against a wall. This allows to clarify the intertwinned relation between the incoming flow regime and the induced impact force, thus providing crucial information for the geotechnical engineers in charge of the design of mitigation structures against mass flows and mountain hazards.