Phase classification of laboratory debris flows over a rigid bed based on the relative flow depth and friction coefficients

Abstract

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.

Basal interstitial water pressure in laboratory debris flows over a rigid bed in an open channel

“…Even in a laboratory setting, accurate measurements of interstitial water pressure in an open channel are difficult, because the collisions of sediment particles with the pressure gauges, especially pitot tubes, can influence the measurements. When using other types of water pressure gauges, it may also be difficult to measure the interstitial water pressure, as laboratory debris flows are usually tested in an open channel up to 10 m long (Hotta and Miyamoto, 2008). This results in a flow depth of several cm at most, so that a very sensitive pressure gauge is required to measure the interstitial water pressure.…”

Subaerial sediment-water flows on hillslopes

“…(2) the grain-size distribution of the solid component; and (3) the physical and chemical interactions between the solid component particles (Costa, 1984;Hotta and Miyamoto, 2008). This explains the gradation in hillslope processes from slides to flows depending on water content, mobility and evolution of the movement.…”

Influence of fine sediment on the fluidity of debris flows

“…Although sediment particles in the boulder debris flows, themselves, move 58 laminar motion, the pore fluid should be turbulent because of the strong shear induced by the 59 sediment particles. Hotta and Miyamoto (2008) pointed out that the friction coefficient in 60 turbulent sediment flows was comparable with that of clear water; thus, the fine sediment 61 contributes to the fluidity, even when the mass density of the interstitial fluid is increased 62 without increasing the viscosity. Using the same concept, Nishiguchi et al (2011) modeled 63 debris flows with mixed grain sizes and large flow depths in which fine sediment was involved 64 in the interstitial water; by doing so, they were better able to predict the run-out of large debris 65 flows.…”

“…These equations have been validated experimentally by comparing the theoretical and 43 experimental velocity distributions (Takahashi, 1977;Egashira et al, 1989;Itoh and Egashira, 44 1999) and the flow resistance (Arattano and Franzi, 2004;Hotta and Miyamoto, 2008). Debris 45 flows have been classified into "boulder debris flows" or "stony debris flows", and the 46 interparticle stresses induced by particle-to-particle collisions and particle friction in the flow 47 have been considered.…”

“…where d is the particle size, c is the volumetric sediment concentration, and K(c) is an equation 119 expressed as a function of c. Refer to Hotta and Miyamoto (2008) for more detailed 120 information on K(c).…”