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Sorting of rocks, boulders, and silt/sand-sized particles according to their size is a characteristic feature of debris flow deposits and is an active process during flow which significantly affects the mobility. The degree at which size sorting occurs in debris flows depends on the relative magnitudes of granular processes such as particle size segregation and diffusion. Since debris flows are fluid-saturated phenomena, accurate modelling of size sorting requires the understanding of the influence of fluids on these processes, which have not been systematically studied. Here, we present simulation results and the associated empirical expressions for particle size segregation and diffusion which take into account fluid effects due to buoyant and drag forces. These expressions are developed through scaling analysis of data obtained from coupled granular-fluid simulations of saturated bi-disperse mixtures under simple shear. We further show that using these scaling relationships, an existing segregation-diffusion continuum equation can be extended to model particle sorting in debris flows with various types of fluids.

Debris flows are gravity-driven phenomena common in mountainous regions that are hazardous to downstream facilities. To mitigate the impacts of these disastrous processes, structural countermeasures such as slit dams are constructed in gullies and along mountain slopes. Existing studies on the impact dynamics of debris flows against slit dams typically focus only on the flow characteristics but fail to take the geometry of the structure into account. Here we develop an analytical model, derived from the momentum approach, that allows for the estimation of the runup height and impact load of debris flows on slit dams. The model is validated against discrete element simulations and small-scale flume experiments. It is found that the runup height is controlled by both the Froude number and slit size. The proposed analytical model can predict the runup height well within a certain range of Froude numbers. Results from experiments further reveal that the fontal dynamic pressure is sensitive to the flow properties whereas the peak dynamic pressure is strongly affected by the slit size.

This study presents a field-scale simulation of the Hongshiyan landslide in China. It uses an advanced numerical approach (material point method (MPM)) and a constitutive model (the Drucker- Prager model + [mu](I) rheological relation) for the three-dimensional (3D) simulation. The performance of the developed MPM model is validated with laboratory-scale experimental data on granular collapse before being applied to field-scale analyses. ArcGIS data are used to create a 3D MPM model of the soil body with complicated geometry. Although the developed model can describe the multiple phases of granular flow, it focuses on the runout behavior of the landslide in this work. The landslide is assumed to have occurred suddenly due to an earthquake, and global sudden failure rather than progressive failure is modeled. The MPM simulation results match reasonably well with the measured post-earthquake topography (e.g., deposit height of about 120 m and stretch length of about 900 m in the river) and landslide duration of about 1 min. The velocity of the sliding mass increases rapidly during flow, especially in the first 20 s. The velocity profiles along the depth direction at different locations of the sliding body exhibit an exponential distribution similar to that of a Bagnold-type profile, indicating that the sliding body is fully mobilized. The rate-dependent dissipation parameter j3 used in the model significantly influences the runout behavior (e.g., flow speed, velocity distribution, and deposit shape). Key words: landslide, material point method, ArcGIS, rheological relation. Cette etude presente une simulation a l'echelle du terrain du glissement de terrain de Hongshiyan en Chine. Il utilise une approche numerique avancee (methode du point materiel (MPM)) et un modele constitutif (modele Drucker-Prager + [mu](I) relation rheologique) pour la simulation tridimensionelle (3D). Les performances du modele MPM developpe sont validees a l'aide de donnees experimentales a l'echelle du laboratoire sur le collapsus granulaire avant d'etre appliquees a des analyses a l'echelle du champ. Les donnees ArcGIS sont utilisees pour creer un modele MPM 3D du corps du sol avec une geometrie complexe. Bien que le modele developpe puisse decrire les multiples phases d'un ecoulement granulaire, il se concentre sur le comportement a la fin du glissement de terrain dans ce travail. Le glissement de terrain est suppose s'etre produit soudainement a cause d'un tremblement de terre, et un echec mondial soudain plutot qu'un echec progressif est modelise. Les resultats de la simulation MPM correspondent raisonnablement bien a la topographie mesuree apres le seisme (par exemple, hauteur de depot d'environ 120 m et longueur d'etirement d'environ 900 m dans la riviere) et duree de glissement de terrain d'environ 1 min. La vitesse de la masse glissante augmente rapidement lors de l'ecoulement, en particulier dans les 20 premieres secondes. Les profils de vitesse dans la direction de la profondeur a differents endroits du corps glissant presentent une distribution exponentielle similaire a celle d'un profil de type Bagnold, indiquant que le corps glissant est entierement mobilise. Le parametre de dissipation dependant de la vitesse j3 utilisee dans le modele influe de maniere significative sur le comportement a la sortie (par exemple, la vitesse d'ecoulement, la distribution de la vitesse et la forme du depot). [Traduit par la Redaction] Mots-cles: glissement de terrain, methode du point materiel, ArcGIS, relation rheologique., Introduction On 3 August 2014, a Ms6.5 earthquake with a focal depth of 12 km shocked the margin of the Tibetan plateau in Ludian County, Yunnan Province, China (27.10[degrees]N, 103.33[degrees]E) [...]

Particle size segregation is ubiquitous in granular systems with differently sized constituents but is found to diminish in the presence of viscous ambient fluids. We study this inhibiting effect through coupled fluid-particle numerical simulations. It is found that size segregation is indeed slower in the presence of fluid and this effect becomes more significant as fluid viscosity is increased. Direct calculation of segregation forcing terms reveal that the ambient fluids affect segregation in two major ways: buoyant forces reduce contact pressures, while viscous dissipation diminish particle-fluctuation driven kinetic pressures, both of which are necessary in driving large particles up. Surprisingly, the fluid drag in the normal direction is negligible regardless of the fluid viscosity and does not directly affect segregation.

Debris now is generally composed of a wide range of solid panicles and viscous pore fluid. It flows al a high velocity down a slope channel. Interactions between solid and fluid phases affected by multiple parameters govern the rheological properties of debris flows. A dimensional analysis for a systematic study of the governing parameters is presented. Multiple dimensionless numbers with clear physical meanings are critically reviewed. The applications of field data to the studying of natural debris flows are demonstrated. Specific values of dimensionless numbers for classifying flow regimes of large-scale natural debris flows are obtained. Compared with previous small-scale physical model tests, this study shows that the contact friction between particles dominates in natural debris flows. In addition, the solid inertial stress due to particle collisions and the pore-fluid viscous shearing stress play key roles in governing the dynamic properties of debris flows. The channel width as a confinement to the flows can affect the solids discharge per unit width significantly. A dimensionless number related to pore-fluid pressure dissipation is also found that can satisfactorily distinguish between surge flows and continuous flows in the field. Key words: debris flow, dimensional analysis, dimensionless number, flow regime, channel confinement, classification. Un ecoulement de debris est generalement compose de particules solides et d'un fluide interstitiel visqueux, et se deplace a grande Vitesse en suivant des canaux en pente. Les proprietes rheologiques des ecoulements de debris sont gouvernees par les interactions entre les phases fluides et solides, qui elles sont affectees par de multiples parametres. L'article presente une etude systematique des parametres par analyse dimensionnelle. Plusieurs nombres sans dimension ayant des correspondances physiques bien etablies sont revises. L'application de donnees de terrain lors de l'etude d'ecoulements de debris naturels est demontree. Des valeurs specifiques ont ete obtenues pour les nombres sans dimension utilises pour classifier les regimes d'ecoulement des ecoulements de debris naturels a grande echelle. Comparativement aux modeles physiques precedents sur une petite echelle, cette etude montre que la friction de contact entre les particules domine dans le cas d'ecoulements de debris naturels. De plus, la contrainte d'inertie du solide due aux collisions entre les particules, et la contrainte de cisaillement visqueuse du fluide interstitiel, jouent des roles importants dans l'etablissement des proprietes dynamiques des ecoulements de debris. La largeur du canal qui confine l'ecoulement peut affecter de facon significative la decharge de solides par unite de largeur. Un nombre sans dimension relie a la dissipation des pressions des fluides interstitiels a aussi ete determine afin de dislinguer correctement sur le terrain les ecoulements en surpression des ecoulements continus. Mots-cles : ecoulement de debris, analyse dimensionnelle, nombre sans dimension, regime d'ecoulement. confinement par un canal, classification. [Traduit par la Reduction], Introduction Debris flows occur when masses of poorly sorted sediment, agitated and saturated with water, surge down slopes in response to gravitational attraction (Iverson 1997). Debris flows differ from rock [...]

The mechanisms of air entrainment and velocity distribution at the upper, lower and side air-water interfaces downstream of chute aerators remain unclear. This study reports a series of experiments involving various approach-flow features and geometric parameters of aerators. Analytical solutions are developed for the air concentration profiles in the air-water layers, and it is shown that the solutions agree well with the experiments. Empirical equations are established for the turbulent diffusion of bubbles and the diffusion rate of the aeration layer. The turbulent diffusion coefficient of the lower nappe is found to be approximately 10 times and 1.2-1.8 times larger than those of the upper and side nappes, respectively. The diffusion rate of the aeration layer is the highest at the lower nappe and lowest at the upper nappe, gradually decreasing along the cavity. The velocity distribution indices are found to be different for the upper, lower and side nappes. [ABSTRACT FROM AUTHOR]