The influence of fine sediment on the fluidity of debris flows based on the equilibrium concentration and friction coefficient

The fluidity of debris flows containing a wide range of grain sizes is higher than that of debris flows containing fully sorted sediment. This is due to the phase shift of fine sediment in the debris flow. In the phase shift, fine sediment is mixed with pore water and behaves as a fluid phase, although sediment generally behaves as a solid phase. The pore fluid density and representative particle diameter increase with the percentage of fine sediment, which behaves as the fluid phase (the percentage of the phase shift). Studies have estimated the percentage of the phase shift from the change in the equilibrium concentration or friction coefficient. However, the two methods have been applied separately, and no study has examined whether the estimation of phase shift using the equilibrium concentration agrees with that obtained from the friction coefficient. Therefore, we conducted debris flow flume experiments over an erodible bed to apply the two methods and compare the estimations of the phase shift. Five sediment grain size distributions were used, and the equilibrium concentrations, deposition slopes, and flow depths were measured. The pore fluid densities were calculated so that the theoretical equilibrium concentrations and friction coefficients derived from the constitutive equations for debris flow agreed with the experimental equilibrium concentrations and friction coefficients. The pore fluid densities of some grain size distributions calculated using the equilibrium concentration disagreed with those calculated using the friction coefficient. This disagreement was affected by the grain size distribution. Therefore, the grain size distribution should be considered when estimating the influence of the phase shift.