Experimental investigation on debris flow resistance and entrainment characteristics: effects of the erodible bed with discontinuous grading

Debris flow deposits in natural channels typically have a wide grain size distribution (GSD). The effects of bed sediment GSD on the basal entrainment rate are neglected in current debris flow erosion models. Field investigations have detected three different vertical graded bedding structures: normal, inverse, and mixed-gradation, characterized by discontinuous gradation sediment and almost without intermediate-sized particles. This study conducted small-scale flume experiments to investigate the debris flow resistance forces and entrainment characteristics by incorporating the effects of discontinuous grading bed sediments. Discontinuous graded bed sediments with varying fine particle content, volumetric water content (VWC), and roundness were designed for comparison. Debris flow resistance in erodible beds generally increased in the group with gravel of larger-sized coarse particle, lower roundness, and higher bed sediment VWC. For discontinuous grading bed sediment, the entrained depths increased in the group with gravel of smaller coarse particle sizes, larger amounts of fine particles, and higher sediment roundness, and decreased with larger VWCs. This abnormality may be attributed to the disproportionately large effects of viscous flow resistance in our small-scale flume tests. The maximum erosion rates of the continuous bed sediment were higher than those of the corresponding discontinuous bed sediment with the same maximum coarse gravel size. This is because, for discontinuous grading bed sediments, localized failure of intermediate-sized sediment grains may cause a large-scale collapse of the solid grain skeleton and enhance basal entrainment rates. A revised formula for calculating the debris flow entrainment rate is proposed by incorporating the kurtosis coefficient, which describes the distribution of discontinuous bed sediments and fine particle content. Our revised formula could facilitate an elaborate estimation of basin erosion and sediment runoff and reveal the development and recession of debris flow fans.