Impact Dynamics of Boulder-Enriched Debris Flow on a Rigid Barrier

Abstract

Boulders entrained in debris flow induce high impact forces on a rigid barrier. In current design practice, the concentrated load from boulders is estimated using the Hertz equation with a load reduction factor (Kc). Separately, the distributed load from the debris is estimated using the hydrodynamic equation. The existing design practice is simply adding the estimated loads using the two equations. The interaction between debris flow and boulders during the impact process is neglected. In this study, physical tests were conducted using a newly developed 28-m-long flume to shed light on the impact dynamics of debris flows with and without boulders on an instrumented rigid barrier. Contrary to existing design practice in which the boulder and debris impact loads are added together, the debris provided a cushioning effect to attenuate the impact force of the boulders. This cushioning effect was governed by a reflection wave with a length scale LR/d (where d = boulder diameter), which serves to cushion thickness on impact. LR/d from 0.4 to 2.0 can reduce the impact load by up to 80% compared with existing design practice (Kc=0.1).