Experimental study of debris-flow entrainment over stepped-gradient beds incorporating bed sediment porosity

Abstract

The incorporation of bed sediment by a natural debris flow can cause significant growth in its size and destructive potential. In this study, the erosion features of debris flows in channels with stepped-gradient beds were determined based on field observations and flume experiments. Field investigations revealed that channel sections with abrupt bed slope variations were eroded more intensely than others. A series of small-scale flume tests were conducted to study the pore pressure variations in erodible bed sediment and the basal entrainment behaviour of debris flows in channels with stepped-gradients. Reference tests on a uniform bed slope demonstrate that inflection points in the bed longitudinal profile greatly alter the debris mixture flow pattern and accelerate entrainment. Pore pressure generation in the bed sediment supports debris flow entrainment by reducing basal friction. Peak pore pressures at concave sites were approximately two times higher than those observed at convex sites, likely due to large changes in debris flow dynamics and intense interactions between the flow mixture and bed sediment. Low bed sediment porosities will increase the interlocking of grain particles, promote the formation of clusters and enhance the strength of the grain skeleton, suppressing basal entrainment. In the flume tests, lower peak pore pressures were observed as bed porosity decreased, possibly due to fewer connecting channels for pore fluid pressure to transmit in the compacted bed sediment. Dimensionless analysis indicates that pore pressure fluctuations in the bed sediment were mainly generated by the overriding debris flow. A new entrainment formula of debris flow over stepped-gradient beds is proposed based on flume tests and theoretical analyses, which may aid future debris flow modelling and hazard assessments.