# Experimental Investigation into the Number of Phases in Debris Flows
## 6 Discussion
### Relative Motion of Fluid (Water) and Solids in Debris-Flow Bodies  
Under both gravity and artificial hydraulic and mechanical forcing [76; 93; 94], no relative motion of water or fluid composed of water and fine-grained particles and solids (i.e., phase separation between fluid and solids [28]) occurs in both the experimental debris-flow bodies and static slurries. To be rigorous, water or fluid capable of moving independently of (coarser-grained) solids does not exist or is negligible. This demonstrates that water and solid particles of all sizes in debris flows move together as a single fluid. The sediments in such flows are deposited en masse by freezing [27; 36; 42; 57; 95], and the water drains mainly by evaporation. The \(C_{\rm r}\) values approaching 1 obtained in the escape test indicate the high mobility of the experimental debris flows and deposit geometry (Figure 5), similar to those of natural debris-flow deposits (depositional sheets or flat-topped lobes [20; 96; 97; 98; 99]). The _Solidity_ (\(S\)) values close to 1 of the experimental deposits quantitatively demonstrated the movement as a whole and the en masse deposition of water and solids in the experimental flows (Figure 5). The fact that no fluid or only a negligible quantity of water leaves the experimental deposits, despite the experiment being run on a tilting plane favorable for separation between fluid (if any) and solid phases (Figure 5), demonstrates in a straightforward manner that water (or fluid) independent of (coarser) solids is absent in debris flows. This is in agreement with some previous studies ([PERSON] (1988) [2]; Figure 6c in [PERSON] et al. (2003) [100]; Figure 9 in [PERSON] et al. (2015) [54]; Figures 1b and 6 in [PERSON] et al. (2013) [8]). The covariations of \(P_{\rm e}\) and \(P_{\rm d}\) with \(C_{\rm v}\) and \(d_{\rm max}\) observed in the displacement experiment are easily understood. The relatively higher \(P_{\rm e}\) and \(P_{\rm d}\) values measured in the DYG slurries compared to those in the JG slurries of the same \(d_{\rm max}\) and similar \(C_{\rm v}\) should be associated with lower contents of fines (say \(<\) 0.0625 mm) of the DYG sediments (Figure 1). The small or even negligible discrepancies between \(P_{\rm e}\) and \(P_{\rm d}\) match the incompressibility of water and solids. The dimensionless parameter \(DR\) enables the characterization of the relative motion of water and solids (if present). The low \(DR\) values of 5-18% achieved in this study, and in particular those estimated by considering the durations of real debris-flow surges and whole flow events and their \(d_{\rm max}\) (\(<\)1 and 5%, respectively), show that even under artificial hydraulic forcing, water movement relative to solids in debris-flow bodies is very sluggish or even negligible. This strongly supports the conclusion reached from the escape test that the water and solids in debris flows move as a whole. The experimental slurries before loading roughly corresponded to the scenarios of natural debris flows coming to rest. _PCD_ of only 0.1-1.4% measured at \(t\) = 90 min, together with the results of the escape test and displacement experiment discussed above, show that both the lateral and vertical relative motions of water (fluid) and solids in debris-flow masses are extremely slow, especially with respect to the duration of debris flows. The coupled measurements of \(h\), \(h_{\rm P}\), \(h_{\rm m}\), and _PCD_ (Figures 4 and 8) enabled us to evaluate the response of the tested slurries and the water in them to loading, i.e., to artificial mechanical forcing. The changes in \(h_{\rm m}\) for the loading and observation tubes before and after loading indicate that loading causes plastic deformation of the slurries (i.e., flow of the slurries [1]).