# Experimental Investigation into the Number of Phases in Debris Flows
## 1 Introduction  
Debris flows [1, 2, 3] are known to be mixtures of water and poorly sorted sediments that advance downstream channels or hillslopes. However, this does not define the full range of debris-flow characteristics. Many exciting fields of research remain to be thoroughly explored despite the major advancements made in debris-flow studies over the past 100 years, and the number of phases in debris-flow bodies is an important problem that is yet to be investigated in detail.  
Researchers studying debris flows have traditionally espoused two major opinions on this disputed topic: one-phase (single-phase or mono-phase) and two-phase flows. The one-phase theory [4, 5, 6, 7, 8] emphasizes that debris flows can be considered as homogeneous yield-stress fluids [9, 10, 11]. The two-phase hypothesis argues that debris flows consist of fluid and solid fractions and stresses the importance of fluid and its interaction with solids [12, 13, 14, 15].  
The difference in the understanding of debris flow between the two hypotheses is significant. The former considers that the combination of water and solids results in a new material in which they move together and are deposited en masse, whereas the latter considers fluid and fluid-solid interactions to be important. The most importantdiscrepancy between the two views is whether the relative motion of fluid and solids occurs in debris flows.  
Perhaps some debris flows can superficially appear as two-phase flow, but an irretrievable question of the two-phase assumption is how to distinguish between the solid and fluid components of debris flows, or what constitutes the fluid fraction. The grain-size thresholds separating the sediment belonging to the solid or the fluid phase used in the literature include clean water (i.e., pure fluid or interstitial fluid with a negligible amount of fine solid fraction) [16, 17, 18, 19], 0.002 mm (clay size [20, 21]), 0.0625 mm (sand-silt boundary size [22, 23, 24]), 0.1 mm [25], 2 mm (sand size [12, 26]), \(\sim\)6 mm [24], and 20 mm [26].  
These widely ranging threshold grain sizes (spanning over four orders of magnitude) would seem to indicate the absence of \"solid phase\" in debris flow, because this threshold can be reasonably extrapolated to 30 mm, 40 mm, and up to boulders (\(>\)75 mm in diameter) [21], as long as gradation is continuous (which is the case for typical debris-flow materials [12, 20]). Thus, debris flows including boulders [7, 27, 28, 29, 30] are behaving as \"fluid\". In other words, the high variability in the boundary size that distinguishes fluids and solids indirectly justifies the one-phase view to some degree.  
The number of phases used by investigators dealing with debris flows remains a major concern because it directly influences the identification of debris flows and their deposits, description of kinematics and dynamics, and assessment of geomorphological and hazardous effects [2, 31, 32, 33, 34].  
An inadequate understanding of the number of phases in debris flows is due to a lack of experimental evidence. Here, we experimentally investigate the number of phases within debris-flow masses using the representative sandy (granular, stony, or boulder [35, 36, 24, 37]) and muddy [38, 39, 40, 26, 41] debris-flow materials in China. In addition, only fully developed debris flows (slurry flows [42]) constituting the body between the head and tail of surges (pulses) (Figure 13.6 in [PERSON] (1986) [43]; Figure 10F in [PERSON] et al. (2018) [36]), that is, the main body of debris flows (hereafter referred to as debris flows), were involved in this study.