based on the relative flow depth and friction coefficients

The friction coefficients of debris flows over a rigid bed from several previous experiments were compiled in a preliminary investigation on the classification of phase transitions in debris flows. The collected friction coefficients were compared to the theoretical values of the friction coefficients in the relationship with the relative flow depth on the basis of sediment particle size (h/d) under various conditions. The friction coefficients of debris flows with h/d values less than 20 agreed closely with the theoretical value for boulder debris flows derived from the constitutive equations, while the friction coefficients with h/d values in the range 1000–10,000 agreed roughly with the theoretical value for turbulent water flows. The friction coefficients with h/d values of 30–300 exceeded the theoretical value for both debris and turbulent water flows. These intermediate debris flows were observed in experiments involving turbulent mud flows. However, a review of these experiments revealed that they may have included debris flows in which the turbulent structure was not well developed, and could be considered as debris flows in transition from laminar to turbulent flows. In some of the transitional debris flows, an interface dividing the flow structure into an upper turbulent-flow layer and a lower debris-flow layer was observed as reported for sediment-laden flows. The friction coefficient for transitional debris flows was modeled considering the shift of this interface. The model was able to explain the value for transitional debris flows, inferring that phase transition in debris flows from laminar to turbulent flows is induced by the shift of the interface