Debris flows occurring in well-vegetated alpine areas usually contain a range of sizes of woody debris. Large woody debris (LWD), which has a retaining effect on further transportation of debris downstream, is mainly distributed in upstream reaches, and the amount of small woody debris (SWD) deriving from LWD increases dramatically midstream and downstream. The Dongyuege (DYG) bouldery debris flow with a sandy-matrix took place in a wildwood area, causing 96 deaths and its clay-sized fraction does not contain typical clay minerals. However, its total travel distance and runout distance in a low-gradient reach (between 2 degrees and 5 degrees) upstream of the depositional fan apex reached 11 km and 3.3 km, respectively. The abundant SWD in the DYG debris flow might have played a crucial role in slurrying, persistence, and the long runout over the low gradient. To understand why this debris flow extended so far, slurrying experiments, pore water escape experiments, and excess pore pressure experiments were performed. Crude debris (CD) collected from the DYG debris flow deposit was used throughout the experiments, the tested materials of which are separated into CD-containing SWD with a maximum grain size (MGS=2mm), purified debris (PD) without SWD with a MGS of 2mm, and SWD <2mm in diameter. In the five slurrying experiments with PD-SWD-water mixtures, as the SWD content was elevated from 0.0 to 2.0wt%, the current velocity of escaping pore water decreased uniformly from 17.2 to 0.9mm/s. When the SWD content was 1.0wt% or greater, the mixtures can be considered as one-phase flows of viscous fluids. The six pairs of pore water escape experiments based on the slurriesremolded with CD and PD, respectively showed that the time needed for pore water to escape from the CD slurries was much greater than those from their PD counterparts. Also, measured was the dissipation rate of the relative excess pore pressure of CD and PD slurries of various densities and volumes, which showed that most of the rates of the PD-slurries were always greater than CD-slurries. Overall, the results show that SWD has a strong influence on the slurrying of the DYG debris without typical clay minerals found in other debris flows, and SWD helps to sustain the high excess pore pressure in the interior of the debris flow mass which resulted in the extended travel distance over such a low gradient. SWD favors the formation and stability of one-phase water-debris mixtures because of its large specific surface area and low density, which makes it able to absorb fine particles and able to be suspended in slurry flows over long timescales. In well-vegetated mountainous areas, SWD should be taken into account in the assessment of debris-flow hazards.
