http://www.geo.brown.edu/geocourses/geo292/papers/old/Coussot-and-Meunier1996debrisflows.pdf

Recognition, classification and mechanical description of debris flows

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

/pdf/plasticity-and-geophysical-flows-a-review-4bos64lahx.pdf

Plasticity and geophysical flows: A review

The postglacial Quaternary colluvial systems in western Norway are arrays of steep fans, often coalescing into aprons, developed along the slopes of valley sides and fjord margins. The coarse debris, derived from weathered gneissic bedrock and its glacial-till mantle, varies from highly immature to mature. The depositional processes are mainly avalanches, ranging from rockfalls and debrisflows to snowflows, but include also waterflow and debris creep. The mechanics and sedimentary products of these processes are discussed, with special emphasis on snow avalanches, whose role as an agent of debris transport is little-known to sedimentologists. The subsequent analysis of sedimentary successions is focused on colluvial-fan deltas, which are very specific, yet little-studied, coastal depositional systems. The stratigraphic variation and depositional architecture of the colluvial facies assemblages, constrained by abundant radiometric dates, are used to decipher the signal of regional climatic changes from the sedimentary record. The stratigraphic data from two dozen local colluvial successions are compiled and further compared with other types of regional palaeoclimatic proxy record. The analysis suggests that the colluvial systems, although dependent upon local geomorphic conditions, have acted as highly sensitive recorders of regional climatic changes. The study as a whole demonstrates that colluvial depositional systems are an interesting and important frontier of clastic sedimentology.

Postglacial colluvium in western Norway: depositional processes, facies and palaeoclimatic record

Quantifying changes in Earth's ice sheets and identifying the climate drivers are central to improving sea level projections. We provide unified estimates of grounded and floating ice mass change from 2003 to 2019 using NASA's Ice, Cloud and land Elevation Satellite (ICESat) and ICESat-2 satellite laser altimetry. Our data reveal patterns likely linked to competing climate processes: Ice loss from coastal Greenland (increased surface melt), Antarctic ice shelves (increased ocean melting), and Greenland and Antarctic outlet glaciers (dynamic response to ocean melting) was partially compensated by mass gains over ice sheet interiors (increased snow accumulation). Losses outpaced gains, with grounded-ice loss from Greenland (200 billion tonnes per year) and Antarctica (118 billion tonnes per year) contributing 14 millimeters to sea level. Mass lost from West Antarctica's ice shelves accounted for more than 30% of that region's total.

/pdf/pervasive-ice-sheet-mass-loss-reflects-competing-ocean-and-tcqo98hkp5.pdf

Pervasive ice sheet mass loss reflects competing ocean and atmosphere processes.

Results taken from 270 publications on rates are summarized, and collated with those from 149 publications reviewed previously (Young, 1969, 1974). The data are classified by major climatic zone, normal or steep relief, and consolidated or unconsolidated rocks. Representative rates and their ranges are given for soil creep, solifluction, surface wash, solution (chemical denudation), rock weathering, slope retreat, cliff (free face) retreat, marine cliff retreat, and denudation, the last being compared with representative rates of uplift. Solifluction is of the order of 10 times faster than soil creep, but both cause only very slow ground loss. Solution is an important cause of ground loss for siliceous rocks, on which it may be half as rapid as on limestones. Total denudation, brought about mainly by surface wash, reaches a maximum in the semi-arid and probably also the tropical savanna zones. Acceleration of natural erosion rates by human activities ranges from 2–3 times with moderately intense land use to about 10 times with intensive land use (and considerably higher still where there is recognized accelerated soil erosion). Where there is active uplift, typical rates are of the order of 10 times faster than denudation, although in some high, steep mountain ranges these may approach equality.

Rates of surface processes on slopes, slope retreat and denudation

A numerical computer model, based on the finite differencing of the Navier-Stokes fluid equations, is used to simulate snow-avalanche flow. In order to verify and calibrate the numerical model, snow-flow tests 0.20 m deep with flow velocities between 0-18 m/s were conducted. Data concerning position, velocity, and flow depth versus time were collected and compared to model runs on the computer. The frictional force on moving snow is investigated and found to be modeled by a term that is proportional to the square of the flow velocity.

T. E. Lang

Papers

Modeling of Snow flow

Experiments on mechanics of flowing snow

Snow–Avalanche Impact on Structures

A failure criterion for snow

Acoustic Emission in Snow at Constant Rates of Deformation