Pub Date : 2020-12-17DOI: 10.1093/oso/9780198866930.003.0008
F. Louchet
A number of well-established physical and mechanical laws are often improperly applied to snow, which is a very particular medium regarding compacity, viscosity, friction, rupture, etc. Snow is described as a changeable granular and porous material, and analyzed using a combination of statistical and deterministic approaches, with the help of the theory of dynamical systems. Specific models are developed for slab, superficial, and full-depth avalanches. The series of successive physical mechanisms responsible for slab avalanche triggering is now perfectly known, involving weak layer collapse and expansion, in which possible healing may abort the whole process. Loose snow avalanches are reminiscent of Bak’s sand pile model, and full-depth avalanches are modeled in terms of snow-water percolation. The specific arrest mechanisms are analyzed. The present analysis should help in taking wise decisions in the face of unexpected situations. The future of snow avalanches is explored in the context of present climate warming.
{"title":"Summary and Conclusion","authors":"F. Louchet","doi":"10.1093/oso/9780198866930.003.0008","DOIUrl":"https://doi.org/10.1093/oso/9780198866930.003.0008","url":null,"abstract":"A number of well-established physical and mechanical laws are often improperly applied to snow, which is a very particular medium regarding compacity, viscosity, friction, rupture, etc. Snow is described as a changeable granular and porous material, and analyzed using a combination of statistical and deterministic approaches, with the help of the theory of dynamical systems. Specific models are developed for slab, superficial, and full-depth avalanches. The series of successive physical mechanisms responsible for slab avalanche triggering is now perfectly known, involving weak layer collapse and expansion, in which possible healing may abort the whole process. Loose snow avalanches are reminiscent of Bak’s sand pile model, and full-depth avalanches are modeled in terms of snow-water percolation. The specific arrest mechanisms are analyzed. The present analysis should help in taking wise decisions in the face of unexpected situations. The future of snow avalanches is explored in the context of present climate warming.","PeriodicalId":237702,"journal":{"name":"Snow Avalanches","volume":"55 1-2","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132241556","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-12-17DOI: 10.1093/oso/9780198866930.003.0002
F. Louchet
This chapter provides basics of snow structure and topology. As snow is a complex arrangement of ice crystals, themselves found in oodles of geometrical shapes, sizes, and formation mechanisms, we essentially focus on those that are more directly involved in avalanche release. Three snow peculiarities are also outlined. Snow being made of ice, it inherits its particular propensity to melt under external pressure. Since snow cover results from accumulation of snowflakes, it may be considered as a granular material, with quite original properties due to the unusually large grain surface vs volume ratio, and to their related tendency to change shapes and to heal. Snow being also a mixture of ice, air, and water, the topological concept of percolation is of interest to deal with stress distribution in the snow cover, and is briefly discussed.
{"title":"Snow, an Intriguing, Complex, and Changeable Solid","authors":"F. Louchet","doi":"10.1093/oso/9780198866930.003.0002","DOIUrl":"https://doi.org/10.1093/oso/9780198866930.003.0002","url":null,"abstract":"This chapter provides basics of snow structure and topology. As snow is a complex arrangement of ice crystals, themselves found in oodles of geometrical shapes, sizes, and formation mechanisms, we essentially focus on those that are more directly involved in avalanche release. Three snow peculiarities are also outlined. Snow being made of ice, it inherits its particular propensity to melt under external pressure. Since snow cover results from accumulation of snowflakes, it may be considered as a granular material, with quite original properties due to the unusually large grain surface vs volume ratio, and to their related tendency to change shapes and to heal. Snow being also a mixture of ice, air, and water, the topological concept of percolation is of interest to deal with stress distribution in the snow cover, and is briefly discussed.","PeriodicalId":237702,"journal":{"name":"Snow Avalanches","volume":"88 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126071644","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-12-17DOI: 10.1093/oso/9780198866930.003.0007
F. Louchet
We first show why current climate forecasting techniques, based on continuous extrapolations, are unreliable in the case of complex arrangements of interacting entities like the atmosphere–ocean system. By contrast, according to the well-established theory of dynamical systems, the observed present increase of fluctuations (as heat waves, droughts, tornadoes, forest fires) is a warning signal for an impending discontinuous climate tipping. A comparison with paleoclimatic events suggests that the atmospheric temperature would be likely to increase in this case by 6–9 °C in the next few years. In the transient period, the succession of heavy snow-falls and thawing episodes would favor spontaneous full-depth avalanches with larger run-out distances. After tipping into a new equilibrium, significantly warmer temperatures would shift snow-covered areas towards higher altitudes, probably by more than 1000 m, resulting in closure of a number of ski resorts. Glacier retreat and permafrost thawing would also enhance both ice and rock-fall frequency.
{"title":"Snow and Avalanches in a Climate Warming Context","authors":"F. Louchet","doi":"10.1093/oso/9780198866930.003.0007","DOIUrl":"https://doi.org/10.1093/oso/9780198866930.003.0007","url":null,"abstract":"We first show why current climate forecasting techniques, based on continuous extrapolations, are unreliable in the case of complex arrangements of interacting entities like the atmosphere–ocean system. By contrast, according to the well-established theory of dynamical systems, the observed present increase of fluctuations (as heat waves, droughts, tornadoes, forest fires) is a warning signal for an impending discontinuous climate tipping. A comparison with paleoclimatic events suggests that the atmospheric temperature would be likely to increase in this case by 6–9 °C in the next few years. In the transient period, the succession of heavy snow-falls and thawing episodes would favor spontaneous full-depth avalanches with larger run-out distances. After tipping into a new equilibrium, significantly warmer temperatures would shift snow-covered areas towards higher altitudes, probably by more than 1000 m, resulting in closure of a number of ski resorts. Glacier retreat and permafrost thawing would also enhance both ice and rock-fall frequency.","PeriodicalId":237702,"journal":{"name":"Snow Avalanches","volume":"8 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129146635","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-12-17DOI: 10.1093/oso/9780198866930.003.0004
F. Louchet
Starting zone sizes are shown to obey statistical laws, named “power laws”, stating that the recurrence time of an event of a given size increases in a precise proportion with its size. Extrapolation of such laws fitted on small-sized events allows a determination of recurrence times for big and uncommon events. The key role of the weak layer (WL) failure is illustrated by “Propagation Saw Tests” (PST), showing that the collapse of a WL zone of a few decimeters may act as a switch, triggering a very large scale spontaneous WL failure. However, the consequences of such a collapse may be damped down by sintering of broken WL grains. We analyze bridging indexes, often used to estimate WL resistance to collapse under loading. We define a new bridging index, extending the usual one to the case of elastic bending, and we discuss the validity domains of both of them.
{"title":"Slab Avalanche Release: Data and Field Experiments","authors":"F. Louchet","doi":"10.1093/oso/9780198866930.003.0004","DOIUrl":"https://doi.org/10.1093/oso/9780198866930.003.0004","url":null,"abstract":"Starting zone sizes are shown to obey statistical laws, named “power laws”, stating that the recurrence time of an event of a given size increases in a precise proportion with its size. Extrapolation of such laws fitted on small-sized events allows a determination of recurrence times for big and uncommon events. The key role of the weak layer (WL) failure is illustrated by “Propagation Saw Tests” (PST), showing that the collapse of a WL zone of a few decimeters may act as a switch, triggering a very large scale spontaneous WL failure. However, the consequences of such a collapse may be damped down by sintering of broken WL grains. We analyze bridging indexes, often used to estimate WL resistance to collapse under loading. We define a new bridging index, extending the usual one to the case of elastic bending, and we discuss the validity domains of both of them.","PeriodicalId":237702,"journal":{"name":"Snow Avalanches","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133191058","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-12-17DOI: 10.1093/oso/9780198866930.003.0006
F. Louchet
This chapter is dedicated to two geometrically extreme and opposite cases of snow avalanches, respectively qualified as loose snow and full-depth ones. A general feature is the absence of a weak layer, which eliminates any basal crack initiation by collapse. The former type consists of superficial flows of low-cohesion snow grains, whereas the latter essentially involves the whole snow cover gliding on the bare ground. Superficial avalanches are often released on high-angle slopes, gradually gathering new snow grains as they propagate in fan-like shapes. The smallest ones are known as “sluffs”. Their release mechanism is reminiscent of Bak’s sand pile described in appendix A. Different types of full-depth avalanches can be defined in terms of water percolation through the snow cover or at the snow/ground interface. They correspond to different release processes. Their flow and arrest mechanisms are also analyzed in terms of grain sintering in a granular fluid.
{"title":"Superficial and Full-Depth Avalanches","authors":"F. Louchet","doi":"10.1093/oso/9780198866930.003.0006","DOIUrl":"https://doi.org/10.1093/oso/9780198866930.003.0006","url":null,"abstract":"This chapter is dedicated to two geometrically extreme and opposite cases of snow avalanches, respectively qualified as loose snow and full-depth ones. A general feature is the absence of a weak layer, which eliminates any basal crack initiation by collapse. The former type consists of superficial flows of low-cohesion snow grains, whereas the latter essentially involves the whole snow cover gliding on the bare ground. Superficial avalanches are often released on high-angle slopes, gradually gathering new snow grains as they propagate in fan-like shapes. The smallest ones are known as “sluffs”. Their release mechanism is reminiscent of Bak’s sand pile described in appendix A. Different types of full-depth avalanches can be defined in terms of water percolation through the snow cover or at the snow/ground interface. They correspond to different release processes. Their flow and arrest mechanisms are also analyzed in terms of grain sintering in a granular fluid.","PeriodicalId":237702,"journal":{"name":"Snow Avalanches","volume":"159 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133968348","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-12-17DOI: 10.1093/oso/9780198866930.003.0003
F. Louchet
The main mechanical and physical quantities and concepts ruling deformation, fracture, and friction processes are recalled, with particular attention paid to the simplicity of the analysis, but without betraying the scientific validity of the arguments. We particularly discuss the difference between between elastic and plastic deformation, and quasistatic and dynamic loadings, essential in avalanche triggering mechanisms. The physical origin of Griffith’s rupture criterion that rules both fracture nucleation and propagation, and the transition between brittle and ductile failure processes, is thoroughly discussed. We also explain the physical meaning of the classical Coulomb’s friction law, showing why it can hardly apply to a non-conventional porous, brittle, and healable solid like snow.
{"title":"Deformation, Fracture, and Friction Processes","authors":"F. Louchet","doi":"10.1093/oso/9780198866930.003.0003","DOIUrl":"https://doi.org/10.1093/oso/9780198866930.003.0003","url":null,"abstract":"The main mechanical and physical quantities and concepts ruling deformation, fracture, and friction processes are recalled, with particular attention paid to the simplicity of the analysis, but without betraying the scientific validity of the arguments. We particularly discuss the difference between between elastic and plastic deformation, and quasistatic and dynamic loadings, essential in avalanche triggering mechanisms. The physical origin of Griffith’s rupture criterion that rules both fracture nucleation and propagation, and the transition between brittle and ductile failure processes, is thoroughly discussed. We also explain the physical meaning of the classical Coulomb’s friction law, showing why it can hardly apply to a non-conventional porous, brittle, and healable solid like snow.","PeriodicalId":237702,"journal":{"name":"Snow Avalanches","volume":"23 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115606313","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-12-17DOI: 10.1093/oso/9780198866930.003.0005
F. Louchet
The chapter starts with a review of a few unfounded arguments sometimes used to account for snow slab instability, and often resulting from the application of mechanical laws that are invalid in a granular, brittle, and healable material like snow. Statistical aspects are investigated using a two-threshold cellular automaton, one for basal instability, and the second one for crown crack opening. The results reproduce the power-law size distribution of starting zone sizes mentioned in chapter 4, and validate a “4-step” triggering scheme made of successive initiation and expansion events for both the basal crack and the crown crack. The possible sintering of collapsed weak layers is then analyzed. It is shown to flow as a slurry for shear strain rates less than a predetermined threshold, or to sinter in the opposite case, which provides a “joker” to any successful “4-step” scheme, turning an incipient avalanche into a simple “whumpf”.
{"title":"Slab Avalanche Modeling","authors":"F. Louchet","doi":"10.1093/oso/9780198866930.003.0005","DOIUrl":"https://doi.org/10.1093/oso/9780198866930.003.0005","url":null,"abstract":"The chapter starts with a review of a few unfounded arguments sometimes used to account for snow slab instability, and often resulting from the application of mechanical laws that are invalid in a granular, brittle, and healable material like snow. Statistical aspects are investigated using a two-threshold cellular automaton, one for basal instability, and the second one for crown crack opening. The results reproduce the power-law size distribution of starting zone sizes mentioned in chapter 4, and validate a “4-step” triggering scheme made of successive initiation and expansion events for both the basal crack and the crown crack. The possible sintering of collapsed weak layers is then analyzed. It is shown to flow as a slurry for shear strain rates less than a predetermined threshold, or to sinter in the opposite case, which provides a “joker” to any successful “4-step” scheme, turning an incipient avalanche into a simple “whumpf”.","PeriodicalId":237702,"journal":{"name":"Snow Avalanches","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"127647244","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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