Size segregation, which is a robust feature of sheared granular mixtures and geophysical mass flow deposits, is found to diminish in the presence of a viscous fluid. We study this inhibitive effect through coupled fluid‐particle simulations of granular flows fully immersed in different ambient fluids. Granular‐fluid mixture flows are modeled according to three distinct flow regimes—free fall, fluid inertial, and viscous—at different angles of inclination. Each flow regime corresponds to distinct flow dynamics and segregation behaviors. We find that segregation is indeed weaker and slower in the presence of an ambient fluid, which is more so as the flow becomes more viscous. The ambient fluid affects segregation in two major ways. First, buoyancy reduces the contact pressure gradients which are needed to drive large particles up, while at the same time reduces the particles' apparent weight. On the other hand, the streamwise drag force substantially changes the flow rheology, specifically the shear rate profile, thereby modifying the segregation behavior in the normal direction. Surprisingly, the fluid drag in the normal direction is negligible regardless of the fluid viscosity and does not affect segregation in a direct manner.

Size segregation is a common feature in geophysical mass flow deposits and is an active process during motion. The presence of interstitial fluids in such flows affect the motion of constituent particles and result in complex segregation behaviors. Effects of the viscosity and density of various interstitial fluids on the rate of particle size segregation are investigated through coupled granular‐fluid simulations of immersed plane‐sheared flows. The segregation rate decreases as the fluid viscosity increases, but remains constant when the viscosity is below certain threshold values. In the low viscosity limit, segregation is affected only by the relative density between the particles and the fluid, and by flow inertial conditions. Analysis of segregation forcing terms based on the mixture theory of segregation reveals that the decrease of segregation rates with the viscosity is due to the increase of fluid drag forces, which effectively weaken the contact stress gradients responsible for driving the large particles to migrate upward. Viscous damping also diminishes velocity fluctuations and thereby inhibits the migration of particles throughout the mixture. The transition in the viscosity dependence is shown to correspond to the transition between granular‐fluid flow regimes. An empirical scaling formula is developed which accounts for the effects of fluid viscosity and the relative density on size segregation immersed in different fluids.

Extreme rainfall events in mountainous environments usually induce significant sediment runoff or mass movements—debris flows, hyperconcentrated flows and stream flows—that pose substantial threats to human life and infrastructure. However, understanding of the sediment transport mechanisms that control these torrent processes remains incomplete due to the lack of comprehensive field data. This study uses a unique field data set to investigate the characteristics of the transport mechanisms of different channelized sediment‐laden flows. Results confirm that sediments in hyperconcentrated flows and stream flows are mainly supported by viscous shear and turbulent stresses, while grain collisional stresses dominate debris‐flow dynamics. Lahars, a unique sediment transport process in volcanic environments, exhibit a wide range of transport mechanisms similar to those in the three different flow types. Furthermore, the Einstein number (dimensionless sediment flux) exhibits a power‐law relationship with the dimensionless flow discharge. Machine learning is then used to draw boundaries in the Einstein number‐dimensionless discharge scheme to classify one flow from the other and thereby aid in developing appropriate hazard assessments for torrential processes in mountainous and volcanic environments based on measurable hydrologic and geomorphic parameters. The proposed scheme provides a universal criterion that improves existing classification methods that depend solely on the sediment concentration for quantifying the runoff‐to‐debris flow transition relevant to landscape evolution studies and hazard assessments.

The motion of fully immersed granular materials, composed of two distinct particle sizes, flowing down rough inclined planes is studied through fluid–particle numerical simulations. We focus on the effect of ambient fluids, as well as their interplay with particle size segregation, on the steady-state kinematic and rheological profiles of the granular-fluid mixture flow. Simulation results are analyzed in the framework of a visco-inertial rheological model, which is first validated in monodisperse flows with a wide range of the ambient fluid viscosity (i.e., from air to water and slurry) and then generalized for size-bidisperse mixtures. It is found that the local effective friction and volume fraction of mixtures with different particle sizes can be approximated from the rheology of single-component flows. While the presence of viscous ambient fluids slows down size segregation (perpendicular to the flow) depending on the mixture composition and flow viscosity, the effective bulk friction is shown to be independent of the state and progress of segregation.

Differently sized particles in sheared granular mixtures undergo size segregation and diffusive remixing, the relative magnitude of which controls the degree to which distinct particle layers form. The presence of viscous interstitial fluids affects individual particle dynamics, resulting in complex segregation and diffusion behaviors. In this study, the effects of different types of fluids (characterized by fluid density and viscosity) on these two processes are investigated, for various flow conditions and material parameters, via coupled numerical simulations of immersed granular shear flows. We observe that both the segregation velocity and the diffusion strength decrease with the fluid viscosity, but these effects occur only when the viscosity exceeds certain threshold values, indicating a transition from viscous to inertial regimes. In the low-viscosity limit where fluid and grain inertia dominate, both segregation and diffusion processes depend on flow conditions and material properties in a manner that is similar to those in dry inertial granular flows. On the other hand, decreasing the relative density between the particles and the fluid slows down segregation but does not significantly affect diffusion. Based on scaling analysis of the simulation data, empirical relationships for the segregation velocity and diffusion coefficient are developed as functions of a modified Stokes number, and are then used to extend an existing segregation-diffusion continuum equation for granular mixtures immersed in different types of fluids.