Segregation, runout and deposition in debris flow

Abstract

Debris flow is one of the most destructive natural hazards due to its high mobility and long runout. The prediction of its runout distance, deposit area, and impact pressure on structure are of great importance to hazard mitigation (e.g. the design of debris-resisting barriers). The deposit morphology of debris flow is largely affected by particle-scale mechanisms, such as size segregation. Segregation leads to boulder-rich front and self-formed lateral levees that enhance runout distance. From the macroscopic perspective, rheological properties are controlling parameters for depositional characteristics. Due to the multi-phase and multi-scale nature of debris flow, this thesis employs both continuum and discrete approaches to add in-depth understanding to the processes of segregation, runout and deposition.

Size segregation is studied using discrete element method (DEM). A parametric study shows how size ratio, slope angle, volume concentration and inter-particle friction affect the progress and maximum extent of segregation. Statistical analyses link DEM data to the state-of-the-art continuum theory of segregation, which confirms the theoretical consideration that the excess pressure taken by large particles drives their upward movements. A migration mechanism is proposed for large particles: unlike small particles dropping through voids without enduring contacts, large particles undergo more endurable contacts within crowded and anisotropic contact networks; they are subject to shear due to the surface-driven velocity gradient, and are expulsed with necessary rotations. Moreover, since non-slip condition is crucial to the modelling of segregation with DEM, a roughness parameter Ra is proposed to characterise base roughness. A phase transition from slip (Ra < 0.51) to non-slip (Ra > 0.62) regimes is observed with this parameter.

Runout and deposition are studied in computational fluid dynamics (CFD) with homogeneous viscoplastic models, which are more relevant to debris flow with high contents of water and fines. Focus is laid on the effect of rheological parameters, i.e. yield stress and viscosity. Numerical results, which are validated against small-scale flume experiments, show that the rapid deposition of debris flow is due to relatively low viscosity, resulting in elongated deposit morphology; high yield stress leads to thick deposits and steep edges. Runout scaling from laboratory to field is established taking yield stress and viscosity into consideration.

To understand fluid–particle interactions in debris flow, a numerical framework coupling DEM and CFD is developed, incorporating the volume of fluid (VOF) scheme to track the dynamics of free fluid surface. With the coupled CFD–DEM model, good agreement is achieved in the simulation of a three-phase (i.e. air, water, solid) dambreak experiment. This model will exploit the strengths of both DEM and CFD to explore the interplay between granular segregation and fluid rheology. Future work will focus on developing constitutive models that bridge micro-mechanics with multi-phase continuum models. It is expected that better predictions can be achieved on flow velocity, front depth, deposit morphology, and impact forces from individual boulders, which are crucial for safer and more economic design of debris-resisting barriers.