arXiv subjects

Shiva P. Pudasaini

Publications and source records attributed to Shiva P. Pudasaini.

13 recordsLinked to original sources

arXiv2026

Kinematics of erosion-induced landslide mobility

Erosion-induced landslide mobility is a prominent phenomenon in nature. However, this topic remains poorly understood, even though erosion greatly alters nearly every aspect of landslide mechanics, dynamics, run-out, mobility, deposition structure and the enormous destructive power carried by such catastrophic events. Here, I present a simple, elegant, and exact analytical model describing the kinematics of erosion-induced landslide mobility in terms of a newly constructed mobility controller containing essential forcing mechanisms in erosion. In contrast to the classical perspective, I formally prove that, irrespective of how large the erosion rate is, the erosion rate alone is not sufficient to determine enhanced or reduced mobility; mobility is also greatly regulated by the volume bulking rate, erosion drag or thrust, and flow depth. With this mechanical mobility controller, it is now possible to explain precisely when the mobility of an erodible landslide is enhanced or reduced. I demonstrate that, a mechanically valid, physically founded erosion model must be able to explain both the enhanced and reduced mobility of erosive landslides.

physics.geo-ph↗

arXiv2026

The landslide drag

Drag is one of the most important energy dissipation mechanisms in nature, including landslides and debris flows. To satisfactorily reproduce laboratory or field data in simulating landslides, often empirical relations or convenient numerical values are used for the drag force coefficient. However, this is just a parameter calibration rather than a physical reality. Why should the drag coefficient be a constant for a dynamically evolving landslide? Which drag coefficient represents the physical reality? So, what exactly is the drag remains an open question. As the landslide is a deformable body, the drag-deformation-flow must be interconnected. Empirical drag coefficients lack important dynamical aspects. As the drag coefficient is less likely to be measurable, it must be described with some mechanical models. Yet, there exists no analytical model for the drag coefficient. Here, we postulate that the drag coefficient must be a function of the evolving landslide velocity, as it must contain information constituting the landslide acceleration in relation to the net driving acceleration. We develop an innovative, evolutionary drag coefficient that adjusts automatically during the landslide motion. The drag coefficient is described by a dimensionless acceleration number as it is regulated by the physics and dynamics of the flow. Formal derivation shows that the drag coefficient is the measure of energy inefficiency. This settles down the deliberation on the drag force in landslide dynamics, reshaping the concept of drag. Simulation results highlight the essence, mechanical strength and functionality of the proposed analytical drag as it demonstrates the inherent frictional behaviour of granular debris flows. As the dynamical drag coefficients appeared to be around the often calibrated values, the new drag potentially well reproduces natural event dynamics, but now with clear physical basis.

physics.geo-ph↗

arXiv2025

A multi-phase thermo-mechanical model for rock-ice avalanche

We propose a novel multi-phase thermo-mechanical rock-ice avalanche model. It considers rock, ice and fluid; includes rigorously derived ice melt rate, melting efficiency dependent fluid production rate and a general temperature equation. It explains advection-diffusion of heat including heat exchange across the avalanche, basal heat conduction, production and loss of heat due to frictional shearing and changing temperature, and temperature enhancement due to entrainment. Temperature equation couples rates of thermal conductivity and temperature. Ice melt intensity determines these rates as mixture conductivity evolves, characterizing thermo-mechanical processes. The model includes interfacial mass and momentum exchanges and mass and momentum productions due to entrainment. The latter significantly changes the state of temperature; yet, the former characterizes the rock-ice avalanche. Phase mass and momentum balances and temperature are coupled. New model offers the first-ever complete dynamical solution for rock-ice avalanche with changing temperature and ice melting. We develop an advection-diffusion-decay-source model and its analytical solutions providing novel understanding of temperature evolution. The 2021 Chamoli event simulations with r$.$avaflow (https://www.landslidemodels.org/r.avaflow/) illustrate the functionality of thermo-mechanical rock-ice avalanche model. Four scenarios are considered: variations in ice-melt-efficiency; fraction of ice; ice and rock frictions; governing the process of melting, flow transformation, spreading and mobility. Ice melting designates the motion and explains the rock-ice avalanche mobility: a phenomenal thermo-mechanical play. Essentially different controls of ice and rock frictions on the state of flow mobility are revealed, explaining complex thermo-mechanical processes. This provides a useful method for practitioners and engineers in solving problems associated with rock-ice avalanches.

physics.geo-ph↗

arXiv2025

The Frictional Brachistochrone

Here, I construct an elegant frictional brachistochrone for a mass point motion of a granular material with the Coulomb frictional energy dissipation that inherently includes the evolving path curvature. The simple model reveals several striking mechanical phenomena. It is applicable to any frictional particle. With increasing friction, the particle path becomes less and less curved until a straight brachistochrone is attained in the limit of sufficiently high friction. The existence of the straight-brachistochrone is phenomenal. Some potential industrial applications of the frictional brachistochrone are considered.

physics.geo-ph↗

arXiv2025

Unified Mechanical Erosion Model for Multi-phase Mass Flows

Erosion poses a great challenge in multi-phase mass flows as it drastically changes flow behavior and deposition pattern by dramatically increasing their masses, adversely affecting population and civil structures. There exists no mechanically-explained, unified multi-phase erosion model. We constitute a novel, unified and comprehensive mechanical erosion rates for solid and fluid phases and demonstrate their richness and urgency. This is achieved by seminally introducing interacting stresses across erosion-interface. Shear resistances from the bed against shear stresses from the landslide are based on consistent physical principles including frictional, collisional and viscous stresses. Proposed multi-phase interactive shear structures are mechanically superior and dynamically flexible. Total erosion rate is the sum of solid and fluid erosion rates which are mechanically extensive and compact. Erosion rates consistently take solid and fluid fractions from the bed and customarily supply to solid and fluid components in the flow. This overcomes severe limitations inherited by existing models. For the first time, we physically correctly construct composite, intricate erosion velocities of particle and fluid from the bed and architect the complete net momentum productions that include all interactions between solids and fluids in the landslide and bed. We invent stress correction, erosive-shear-velocity, super-erosion-drift and erosion-matrix characterizing erosion processes. By embedding well constrained extensive erosion velocities, unified erosion rates and net momentum productions including erosion-induced inertia into mass and momentum balances, we develop a novel, mechanically-explained, comprehensive multi-phase model for erosive mass flows. The new model offers great opportunities for practitioners in solving technical, engineering problems related to erosive multi-phase mass flows.

physics.flu-dyn↗

arXiv2022

Extended landslide velocity and analytical drag

The landslide velocity plays a dominant role in estimating impact force and devastated area. Here, based on Pudasaini and Krautblatter (2022), I develop a novel extended landslide velocity model that includes the force induced by the hydraulic pressure gradient which was neglected by all the existing analytical landslide velocity models. By a rigorous conversion between this force and inertia, I develop two peer systems expecting to produce the same results. However, this contradicts with our conventional wisdom. This raises a question of whether we should develop some new balance equations. I compare the two velocity models that neglects and includes the force induced by the hydraulic pressure gradient. Analytical solutions produced by the two systems are different. The new model is comprehensive, elegant, and yet an extraordinary development as it reveals serendipitous circumstances resulting in a pressure-inertia-paradox. Surprisingly, the mass first moves upstream, then it winds back and accelerates downslope. The difference between the extended and simple solution widens strongly as the force associated with the hydraulic pressure gradient increases, demonstrating its importance. Viscous drag plays an important role in controlling the landslide dynamics. However, no explicit mechanical and analytical model exists for this. The careful sagacity of the graceful form of new velocity equation results in a mechanically extensive, dynamically evolving analytical model for viscous drag, the first of this kind. A dimensionless drag number is constructed. Contrary to the prevailing practices, I have proven that drags are essentially different for the expanding and contracting motions, an entirely novel perception. Drag coefficients are close to the often used empirical or numerical values. But, now, I offer an innovative, physically-founded analytical model for drag in mass flow simulation.

physics.geo-ph↗

arXiv2022

P-Index

I propose the P-Index that genuinely constitutes a well-defined, compact author citation metric.

cs.DL↗

arXiv2022

Dispersive landslide

Considering the non-hydrostatic mass flow model (Pudasaini, 2022), here, we derive a novel dispersive wave equation for landslide. The new dispersive wave for landslide recovers the classical dispersive water waves as a special case. We show that the frequency dispersion relation for landslide is inherently different than the classical frequency dispersion for water waves. The wave frequency with dispersion increases non-linearly as a function of the wave number. For dispersive landslide, the wave frequency without dispersion appears to heavily overestimate the dispersive wave frequency for higher wave number. Due to the dispersion term emerging from the non-hydrostatic contribution for landslide, the phase velocity becomes a function of the wave number. This gives rise to the group velocity that is significantly different from the phase velocity, characterizing the dispersive mass flow. The dispersive phase velocity and group velocity decrease non-linearly with the wave number. Yet, the group velocity is substantially lower than the phase velocity. We analytically derive a dispersion number as the ratio between the phase velocity and the group velocity, which measures the deviation of the group velocity from the phase velocity, provides a dynamic scaling between them and summarizes the overall effect of dispersion in the mass flow. The dispersion number for landslide increases rapidly with the wave number, which is in contrast to the dispersion in water waves. With the definition of the effective dispersive lateral stress, we prove the existence of an anti-restoring force in landslide. We reveal the fact that due to the anti-restoring force, landslides are more dispersive than the piano strings. So, the wave dispersion in landslide is fundamentally different than the wave dispersion in the piano string. Our model constitutes a foundation for the wave phenomenon in dispersive mass flows.

physics.geo-ph↗

arXiv2022

A Non-Hydrostatic Multi-Phase Mass Flow Model

Modeling mass flows is classically based on hydrostatic balance equations. However, if momentum transfers scale similarly in slope parallel and flow depth directions, then the gravity and acceleration can have the same order of magnitude effects, urging for a non-hydrostatic model formulation. Here, we extend existing single-phase Boussinesq-type models by developing a new non-hydrostatic model for multi-phase mass flows consisting of solid and fine-solid particles and viscous fluid (Pudasaini & Mergili, 2019). The new model includes enhanced gravity and dispersion considering various interfacial momentum transfers. We outline new contributions in the non-hydrostatic Boussinesq-type multi-phase gravity waves emerging from phase-interactions. We present a general, well-structured framework of multi-phase mass flows with enhanced gravity and dispersion, setting a foundation for comprehensive simulations. We discuss situations where non-hydrostatic, dispersive effects are more pronounced for such flows. Reduced models demonstrate the importance of non-hydrostatic contributions. Simplified analytical solutions reveal how the new dispersive model can be reduced to non-dispersive ones, yet largely generalizing existing models. We postulate a novel, spatially varying dissipative force, called the prime-force, which physically controls the dynamics and run-out of the mass flow in a precise way. Practitioners and engineers may find this force very useful in technical applications. This illuminates the need of formally including the prime-force in momentum equations. A simple dispersion equation is derived highlighting the essence of dispersion on mass flow dynamics. Dispersion produces a wavy velocity field about a reference state without dispersion, the first of this kind for avalanching debris mass. Dispersion intensity increases energetically as the solid volume fraction or friction decreases.

physics.flu-dyn↗

arXiv2021

The Mechanics of Landslide Mobility with Erosion

Erosion, as a key control of landslide dynamics, significantly increases the destructive power by rapidly amplifying its volume, mobility and impact energy. Mobility is directly linked to the threat posed by an erosive landslide. No clear-cut mechanical condition has been presented so far for when, how and how much energy the erosive landslide gains or loses, resulting in enhanced or reduced mobility. We pioneer a mechanical model for the energy budget of an erosive landslide that controls its mobility. A fundamentally new understanding is that the increased inertia due to the increased mass is related to an entrainment velocity. With this, the true inertia of an erosive landslide can be ascertained, making a breakthrough in correctly determining the mobility of the erosive landslide. Outstandingly, erosion velocity regulates the energy budget and decides whether the landslide mobility will be enhanced or reduced. This provides the first-ever explicit mechanical quantification of the state of erosional energy and a precise description of mobility. This addresses the long-standing question of why many erosive landslides generate higher mobility, while others reduce mobility. By introducing three key concepts: erosion-velocity, entrainment-velocity and energy-velocity, we demonstrate that erosion and entrainment are essentially different processes. Landslides gain energy and enhance mobility if the erosion velocity is greater than the entrainment velocity. We introduce two dimensionless numbers, mobility scaling and erosion number, delivering explicit measure of mobility. We establish a mechanism of landslide-propulsion providing the erosion-thrust to the landslide. Analytically obtained velocity indicates that erosion controls the landslide dynamics. We also present a full set of dynamical equations in conservative form which correctly includes the erosion induced net momentum production.

physics.geo-ph↗

arXiv2021

The Landslide Velocity

Proper knowledge of velocity is required in accurately determining enormous destructive energy of a landslide. We present the first physics-based general analytical landslide velocity model that incorporates internal deformation and external forces: net driving force and viscous resistant. The model stands as a novel non-linear advective-dissipative system where classical Voellmy and inviscid Burgers' equation are specifications. Non-linear advection and external forcing fundamentally regulate the motion which substantially enhances our understanding of a coherently deforming landslide. Since analytical solutions provide fastest, cost-effective and best rigorous answer to the problem, we construct several new/general exact analytical solutions covering wider spectrum of landslide velocity. New solutions bridge existing gap between negligibly and massively deforming landslides. This provides a novel, rapid and consistent method for efficient coupling of different types of mass transports. Mechanism of landslide advection, stretching and approaching to steady-state has been explained. Shifting, up-lifting and stretching of velocity field stem from forcing and advection. Our solution describes fascinating breaking wave and emergence of landslide folding. This happens as the solution simultaneously introduces domain propagation, velocity up-lift and non-linear advection. Domain translation and stretching depends on net driving force and viscous drag controls shock wave generation, wave breaking, folding and velocity. Landslide dynamics are architectured by advection and reigned by system forcing. Analytically obtained velocities are close to observed values, constituting a new foundation of landslide velocity. This provides the practitioners with key information in instantly/accurately estimating impact force that is important in delineating hazard zones and mitigation of landslide hazards.

physics.geo-ph↗

arXiv2016

A mechanical model for phase-separation in debris flow

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.

physics.geo-ph↗

arXiv2016

A mechanical erosion model for two-phase mass flows

Erosion, entrainment and deposition are complex and dominant, but yet poorly understood, mechanical processes in geophysical mass flows. Here, we propose a novel, process-based, two-phase, erosion-deposition model capable of adequately describing these complex phenomena commonly observed in landslides, avalanches, debris flows and bedload transport. The model is based on the jump in the momentum flux including changes of material and flow properties along the flow-bed interface and enhances an existing general two-phase mass flow model (Pudasaini, 2012). A two-phase variably saturated erodible basal morphology is introduced and allows for the evolution of erosion-deposition-depths, incorporating the inherent physical process including momentum and rheological changes of the flowing mixture. By rigorous derivation, we show that appropriate incorporation of the mass and momentum productions or losses in conservative model formulation is essential for the physically correct and mathematically consistent description of erosion-entrainment-deposition processes. We show that mechanically deposition is the reversed process of erosion. We derive mechanically consistent closures for coefficients emerging in the erosion rate models. We prove that effectively reduced friction in erosion is equivalent to the momentum production. With this, we solve the long standing dilemma of mass mobility, and show that erosion enhances the mass flow mobility. The novel enhanced real two-phase model reveals some major aspects of the mechanics associated with erosion, entrainment and deposition. The model appropriately captures the emergence and propagation of complex frontal surge dynamics associated with the frontal ambient-drag with erosion.

physics.flu-dyn↗