Visualization of dominant stress-transfer mechanisms in experimental debris flows of different particle-size distribution

Abstract

Physical modelling of debris flow in a small-scale flume has been carried out to investigate the internal stress-transfer mechanisms within unsteady, saturated, and segregating granular free-surface flows. Measurements of the internal velocity fields within model flows were obtained via planar laser-induced fluorescence and particle image velocimetry. Normalized velocity profiles taken at a section over the flow duration were found to essentially collapse onto a single curve, the shape of which was dependent on the particle-size distribution. While all flows exhibited internal basal slip and shear, for tests on well-graded materials that are most representative of debris flows, the shear rate was found to reduce towards the surface to near-zero, exhibiting near plug-flow. Dimensional analysis shows that particles of different size within these flows experienced different dominant stress-transfer mechanisms -frictional, collisional or viscous. Rapid grain-size segregation therefore is both due to and results in different modes of stress transfer within a single flow. This means that in a segregating and hence, stratified system, different flow regimes will act concurrently at microscale and mesoscale. Results highlight the complexity of debris flows, so that it may be undesirable to ascribe a single microscale constitutive behaviour throughout, and further calls into question the concept of flow regimes for debris flows based on bulk measurements.Key words: debris flow, dimensionless number, flow regime, plane laser-induced fluorescence, flume model tests.Résumé : La modélisation physique de l'écoulement des débris dans un canal à petite échelle a été réalisée pour étudier les mécanismes de transfert de contraintes internes au sein des flux à surface libre granulaires instables, saturés et séparés. Les mesures des champs de vitesse internes au sein des flux modèles ont été obtenues par vélocimétrie à fluorescence induite laser plane a par Image de particule. Les profils de vitesse normalisés pris à une section sur la durée d'écoulement ont été trouvés à se replier essentiellement sur une seule courbe, dont la forme était dépendante à la distribution de la taille des particules. Alors que tous les flux ont exposé un glissement basal interne et de cisaillement, pour les essais sur des matériaux bien classés qui sont les plus représentatifs des flux de débris, le taux de cisaillement a été trouvé à se réduire vers la surface près de zéro, présentant presque un écoulement piston. L'analyse dimensionnelle montre que des particules de taille différente au sein de ces flux ont connu différents mécanismes de transfert de contrainte dominante -de frottement, collisionnel, ou visqueux. La ségrégation rapide à taille de grain est donc à la fois en raison de résultats et dans différents modes de transfert de contrainte dans un seul flux. Cela signifie que dans un système de ségrégation et donc, stratifié, les différents régimes d'écoulement agissent simultané-ment à l'échelle micro et méso. Les résultats mettent en...

Spatial and temporal evolution of an experimental debris flow, exhibiting coupled fluid and particulate phases

“…The collapse of the profiles onto one indicates the similarity of the flow over this time frame, as also shown in other gravity-driven granular flows, e.g. [3,26,41]. The characteristics of experimental flows that are comparable to the current work are summarised in Table 1.…”

“…The distinct shape of the velocity profiles for different experimental parameters lead to the definition of four different granular flow regimes that are relevant to debris flow bodies-immature, mature, plug flow and solid bed flow. These definitions have been frequently used to classify experimental debris flows conducted in more recent years [26,28,41]. Experimental debris flows are also often classified by directly comparing the internal velocity profiles with analytical profiles that are derived from simplified mathematical models with an assumed rheology [4].…”

“…Experimental debris flows are also often classified by directly comparing the internal velocity profiles with analytical profiles that are derived from simplified mathematical models with an assumed rheology [4]. This simple comparison can reveal whether the flow is dominated by granular or viscous-type behaviour [26,28,41]. It is important to note that analytical velocity profiles are derived from steady flow conditions.…”

Velocity profiles and basal stresses in natural debris flows

“…In a recent study, Sanvitale and Bowman () found that well‐graded Duran glass particles flowing down a small‐scale flume form a plug flow type with high shear rates at the bottom and low velocity gradient at the top. Similarly, velocity profiles for natural sediment with a high content of fines showed the highest shear concentrated at the base in large‐scale rotating drum flows (Kaitna et al ., ).…”

Experimental study on the coefficient of restitution of grain against block interfaces for natural and engineered materials

“…First is through rheological approach and the other method is through coupled DEM/CFD analysis. In debris flows at the microscale, the particle dynamics involve exchange of momentum due to inertial granular collisions, friction between grains, viscous shear and solid fluid interactions (Iverson, 1997, Sanvitale andBowman, 2016). In general, the mobilized blocks during flows/rock falls move down the slope due to gravity with a subsequent impact and rebound (Ritchie, 1963, Paronuzzi, 1989).…”