Recent advances in theory and experimen- tation motivate a thorough reassessment of the physics of debris flows. Analyses of flows of dry, granular solids and solid-fluid mixtures provide a foundation for a com- prehensive debris flow theory, and experiments provide data that reveal the strengths and limitations of theoret- ical models. Both debris flow materials and dry granular materials can sustain shear stresses while remaining stat- ic; both can deform in a slow, tranquil mode character- ized by enduring, frictional grain contacts; and both can flow in a more rapid, agitated mode characterized by brief, inelastic grain collisions. In debris flows, however, pore fluid that is highly viscous and nearly incompress- ible, composed of water with suspended silt and clay, can strongly mediate intergranular friction and collisions. Grain friction, grain collisions, and viscous fluid flow may transfer significant momentum simultaneously. Both the vibrational kinetic energy of solid grains (mea- sured by a quantity termed the granular temperature) and the pressure of the intervening pore fluid facilitate motion of grains past one another, thereby enhancing debris flow mobility. Granular temperature arises from conversion of flow translational energy to grain vibra- tional energy, a process that depends on shear rates, grain properties, boundary conditions, and the ambient fluid viscosity and pressure. Pore fluid pressures that exceed static equilibrium pressures result from local or global debris contraction. Like larger, natural debris flows, experimental debris flows of ;10 m 3 of poorly sorted, water-saturated sediment invariably move as an unsteady surge or series of surges. Measurements at the base of experimental flows show that coarse-grained surge fronts have little or no pore fluid pressure. In contrast, finer-grained, thoroughly saturated debris be- hind surge fronts is nearly liquefied by high pore pres- sure, which persists owing to the great compressibility and moderate permeability of the debris. Realistic mod- els of debris flows therefore require equations that sim- ulate inertial motion of surges in which high-resistance fronts dominated by solid forces impede the motion of low-resistance tails more strongly influenced by fluid forces. Furthermore, because debris flows characteristi- cally originate as nearly rigid sediment masses, trans- form at least partly to liquefied flows, and then trans- form again to nearly rigid deposits, acceptable models must simulate an evolution of material behavior without invoking preternatural changes in material properties. A simple model that satisfies most of these criteria uses depth-averaged equations of motion patterned after those of the Savage-Hutter theory for gravity-driven flow of dry granular masses but generalized to include the effects of viscous pore fluid with varying pressure. These equations can describe a spectrum of debris flow behav- iors intermediate between those of wet rock avalanches and sediment-laden water floods. With appropriate pore pressure distributions the equations yield numerical so- lutions that successfully predict unsteady, nonuniform motion of experimental debris flows.

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The physics of debris flows

The mobility of long-runout landslides

Debris flows typically occur when intense rainfall or snowmelt triggers landslides or extensive erosion on steep, debris-mantled slopes. The flows can then grow dramatically in size and speed as they entrain material from their beds and banks, but the mechanism of this growth is unclear. Indeed, momentum conservation implies that entrainment of static material should retard the motion of the flows if friction remains unchanged. Here we use data from large-scale experiments to assess the entrainment of bed material by debris flows. We find that entrainment is accompanied by increased flow momentum and speed only if large positive pore pressures develop in wet bed sediments as the sediments are overridden by debris flows. The increased pore pressure facilitates progressive scour of the bed, reduces basal friction and instigates positive feedback that causes flow speed, mass and momentum to increase. If dryer bed sediment is entrained, however, the feedback becomes negative and flow momentum declines. We infer that analogous feedbacks could operate in other types of gravity-driven mass flow that interact with erodible beds. The mechanisms by which debris flows acquire mass and momentum as they entrain material are unclear. Large-scale experiments suggest that the pore pressure of wet bed sediment increases as the flow moves over the bed, leading to reduced friction and progressive scouring of the base.

Positive feedback and momentum growth during debris-flow entrainment of wet bed sediment

The conceptual problems arising in the definition and measurement of temperature in non-equilibrium states are discussed in this paper in situations where the local-equilibrium hypothesis is no longer satisfactory. This is a necessary and urgent discussion because of the increasing interest in thermodynamic theories beyond local equilibrium, in computer simulations, in non-linear statistical mechanics, in new experiments, and in technological applications of nanoscale systems and material sciences. First, we briefly review the concept of temperature from the perspectives of equilibrium thermodynamics and statistical mechanics. Afterwards, we explore which of the equilibrium concepts may be extrapolated beyond local equilibrium and which of them should be modified, then we review several attempts to define temperature in non-equilibrium situations from macroscopic and microscopic bases. A wide review of proposals is offered on effective non-equilibrium temperatures and their application to ideal and real gases, electromagnetic radiation, nuclear collisions, granular systems, glasses, sheared fluids, amorphous semiconductors and turbulent fluids. The consistency between the different relativistic transformation laws for temperature is discussed in the new light gained from this perspective. A wide bibliography is provided in order to foster further research in this field.

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Temperature in non-equilibrium states: a review of open problems and current proposals

The role of convention in various definitions of clock synchronization and simultaneity is investigated. We show that two principal methods of synchronization can be considered: system internal and system external synchronization. Synchronization by the Einstein procedure and by slow clock transport turn out to be equivalent if and only if the time dilatation factor is given by the Einstein result (1−v
2)1/2. An ether theory is constructed that maintains absolute simultaneity and is kinematically equivalent to special relativity.

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A Test Theory of Special Relativity: I. Simultaneity and Clock Synchronization

This paper explores the interesting problem of projectile motion without the vacuum idealization. Particular attention is paid to golf ball trajectories with and without lift. No lift trajectories with linear and quadratic drag are considered first. Then, trajectories with lift and linear drag are investigated. Projection angles for maximum range are determined for all these cases. Computer solutions are used throughout, with a Runge–Kutta routine used for all cases except for the well‐known closed solution for the no lift, linear drag projectile.

Maximum projectile range with drag and lift, with particular application to golf

The Aharonov-Bohm effect is relatively little known, but it poses fundamental questions in quantum theory. It concerns physical effects on charged particles in field-free regions. The present paper provides an over-all critical review and commentary on the literature of the Aharonov-Bohm effect. The two fundamental questions raised by the effect concern the role of the force concept in quantum mechanics, and the localizability of physical effects principle. A recent letter from Professor Bohm concerning his evaluation of the significance of the effect is included in the Appendix.

Aharonov-Bohm Effect—Quantum Effects on Charged Particles in Field-Free Regions

Johann Bernoulli's brachistochrone problem is now three hundred years old. Bernoulli's solution to the problem he had proposed used the optical analogy of Fermat's least-time principle. In this analogy a light ray travels between two points in a vertical plane in a medium of continuously varying index of refraction. This solution and connected material are explored in this paper.

https://iopscience.iop.org/article/10.1088/0143-0807/20/5/301/pdf

Johann Bernoulli's brachistochrone solution using Fermat's principle of least time

This paper is a historical and critical review of an ether theory which starts with rod contraction and clock retardation, and a comparison between this ether theory and the special theory of relativity. The two theories are generally equivalent since they both lead to the Lorentz transformation equations. The possibility of an experimental difference between the two theories may lie in a one-way experiment, and this subject is briefly explored with the tentative conclusion that so far it seems that no one has made a specific proposal for such a crucial one-way experiment nor has anyone conclusively demonstrated the impossibility of such a test.

The Rod Contraction-Clock Retardation Ether Theory and the Special Theory of Relativity

Sadi Carnot was one of the giants of physics. Although he used the incorrect caloric theory of heat in his Reflexions, nevertheless he may rightfully be considered to be `the Founder of the Second Law of Thermodynamics'.

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Herman Erlichson

Papers

Maximum projectile range with drag and lift, with particular application to golf

Aharonov-Bohm Effect—Quantum Effects on Charged Particles in Field-Free Regions

Johann Bernoulli's brachistochrone solution using Fermat's principle of least time

The Rod Contraction-Clock Retardation Ether Theory and the Special Theory of Relativity

Sadi Carnot, `Founder of the Second Law of Thermodynamics'