  Understanding the physics of phase-separation between solid and fluid phases
as a mixture mass moves down slope is a long-standing challenge. Here, we
propose an extension of the two phase mass flow model (Pudasaini, 2012) by
including a new mechanism, called separation-flux, that leads to strong
phase-separation in avalanche and debris flows while balancing the enhanced
solid flux with the reduced fluid flux. The separation flux mechanism is
capable of describing the dynamically evolving phase-separation and levee
formation in a multi-phase, geometrically three-dimensional debris flow. These
are often observed phenomena in natural debris flows and industrial processes
that involve the transportation of particulate solid-fluid mixture material.
The novel separation-flux model includes several dominant physical and
mechanical aspects such as pressure gradients, volume fractions of solid and
fluid phases and their gradients, shear-rates, flow depth, material friction,
viscosity, material densities, topographic constraints, grain size, etc. Due to
the inherent separation mechanism, as the mass moves down slope, more and more
solid particles are transported to the front and the sides, resulting in
solid-rich and mechanically strong frontal surge head, and lateral levees
followed by a weaker tail largely consisting of viscous fluid. The primary
frontal solid-rich surge head followed by secondary fluid-rich surges is the
consequence of phase-separation. Such typical and dominant phase-separation
phenomena are revealed for two-phase debris flow simulations. Finally, changes
in flow composition, that are explicitly considered by the new modelling
approach, result in significant changes of impact pressure estimates. These are
highly important in hazard assessment and mitigation planning and highlight the
application potential of the new approach.
