# Experimental Investigation into the Number of Phases in Debris Flows
## 6 Discussion
### Bulk Behavior of Debris Flows  
The flow behavior of the experimental debris flows was fairly similar to that of Bingham materials (Figure 9), which shows that water and solids in debris flows move as a whole and that debris flows can be considered as a one-phase flow. This is because only continuous, homogeneous, isotropic, and monophasic (CHIMP) flows can exhibit Bingham behavior [7; 11; 101].  
Hyperconcentrated (stream) flows are typical two-phase flows of water and sediment [4; 9]. These flows, which often constitute the tail of debris-flow surges (Figure 13.6 in [PERSON] (1986) [43]), are more common than debris flows in mountain basins.  
Here, we prepare a hyperconcentrated mixture of water and sediment using the DYG debris with a maximum grain diameter (\(d_{\rm max}\)) of 30 mm investigated in this study (DYG-30;Figure 1). The solid volume concentration (\(C_{\rm v}\)) of the mixture was 0.59, which is below the lower slurring limit (\(C_{\rm v}^{\rm h}\)) of 0.62. The mixture was tentatively tested rheologically using the large-scale rheometers described in Section 4.4, and the results obtained from the two trials are shown in Figure 10. For comparison, the flow curves of two experimental debris flows (DYG-30-0.64 and DYG-30-0.66; Table 1 and Figure 9) are depicted in this figure.  
It can be seen from Figure 10 that the shear stress (\(\tau\)) for the experimental hyperconcentrated flows, compared to those for the experimental debris flows, displays irregular, large amplitude fluctuations with the rate of shear strain (\(\gamma\)), which is in perfect agreement with those presented in Figure 3b,d in [PERSON] et al. (2020) [53]. The values of \(r^{2}\) achieved by fitting the data points for the two trials with the Bingham model are only 0.48 and 0.03. This indicates that there is no correlation between their \(\tau\) and \(\gamma\), i.e., that two-phase flows of water and sediment do not behave like Bingham fluids and cannot be described using the Bingham model. Various types of rheometers have been used extensively and successfully in debris-flow research [7; 22; 26; 30; 75; 86; 89]. Conversely, the use of rheometers to investigate the two-phase flows of sediment and water is problematic (the coupling connecting the variable-speed motor and impeller of the large rheometer described above is twisted off after only two trials) because of the rapid settlement of solids and subsequent phase segregation [10; 73]. This strongly supports the theory that debris flows can be approximated as monophasic fluids.  
Simultaneously, the comparisons shown in Figure 10 demonstrate more directly that the debris flows behave more like Bingham flows and are far from two-phase flows. In other words, debris flows should be treated as a single flow rather than as a two-phase flow.  
Additionally, the flow processes occurring in mountainous areas, according to [PERSON] and [PERSON] (1987) [1], can be reasonably classified as streamflow, hyperconcentrated flow, debris flow, and granular flow including debris avalanche (rock avalanche, _sturzstrom_,  
Figure 10: Flow curves for experimental hyperconcentrated mixture of \(C_{\rm v}\) = 0.59 compared to those for two experimental debris-flow mixtures of \(C_{\rm v}\) = 0.64, and 0.66, prepared with the same debris (DYG-30, with a lower slurring limit (\(C_{\rm v}^{\rm h}\)) of 0.62, Table 1 and Figure 9), showing that debris flows are close to one-phase Bingham flow, and far from two-phase hyperconcentrated flow. \(r^{2}\) is the coefficient of determination obtained by fitting rheological data using the Bingham model (dashed lines).  
[PERSON] and [PERSON] (2004) [102]). The two-phase hypothesis of debris flows stems from biphasic hyperconcentrated flows, even saturated granular flows, being mistakenly (at least improperly) regarded as debris flows or even used to represent debris flows.