Plasticity and geophysical flows: A review

The objective of this review is to examine how the concept of plasticity is used in geophysical fluid dynamics. Rapid mass movements such s snow avalanches or debris flows involve slurries of solid particles (ice, boulder, clay, etc.) within an interstitial fluid (air, water). The bulk ehavior of these materials has often been modeled as plastic materials, i.e., a plastic material yields and starts to flow once its stress state has ignificantly departed from equilibrium. Two plastic theories are of common use in fluid dynamics: Coulomb plasticity and viscoplasticity. These heories have little in common, since ideal Coulomb materials are two-phase materials for which pore pressure and friction play the key role in the ulk dynamics, whereas viscoplastic materials (e.g., Bingham fluids) typically behave as single-phase fluids on the macroscopic scale and exhibit viscous behavior after yielding. Determining the rheological behavior of geophysical materials remains difficult because they encompass coarse, rregular particles over a very wide range of size. Consequently, the true nature of plastic behavior for geophysical flows is still vigorously debated. n this review, we first set out the continuum-mechanics principles used for describing plastic behavior. The notion of yield surface rather than ield stress is emphasized in order to better understand how tensorial constitutive equations can be derived from experimental data. The notion f single-phase or two-phase behaviors on the macroscopic scale is then examined using a microstructural analysis on idealized suspensions of pheres within a Newtonian fluid; for these suspensions, the single-phase approximation is valid only at very high or low Stokes numbers. Within his framework, the bulk stress tensor can also be constructed, which makes it possible to give a physical interpretation to yield stress. Most of he time, depending on the bulk properties (especially, particle size) and flow features, bulk behavior is either Coulomb-like or viscoplastic in imple-shear experiments. The consequences of the rheological properties on the flow features are also examined. Some remarkable properties of he governing equations describing thin layers flowing down inclined surfaces are discussed. Finally, the question of parameter fitting is tackled: ince rheological properties cannot be measured directly in most cases, they must be evaluated from field data. As an example, we show that the oulomb model successfully captures the main traits of avalanche motion, but statistical analysis demonstrates that the probability distribution of he friction coefficient is not universal. 2006 Elsevier B.V. All rights reserved.