Avalanche Speed Research Articles

On the computation of parameters that model snow avalanche motion

This paper explores the computational problem of finding suitable numbers to use in a two-parameter model of snow avalanche dynamics. The two parameters are friction, μ, and a ratio of avalanche mass to drag, M/D. Given a path profile, and a maximum avalanche speed, then it is possible to compute unique values for u and M/D. If only the path profile and the stopping position are known, then it is possible to compute tables of pairs {μ, M/D} which can be tested as predictors of avalanche speeds. To generate these tables it is convenient to scale M/D in multiples of the total vertical drop of the path. The computations were tested on 136 avalanche paths. Values of {μ, M/D} were stratified, and certain values were rejected as unrealistic.

Seismic and Impact–Pressure Monitoring of Flowing Avalanches

Abstract Continuous records have been made during the passage of dry–snow avalanches of both seismic signals, which allows the avalanche speed to be estimated, and impact pressures on load cells with surface areas of 645 and 6 450 mm2. The impact pressure recordings show an initial peak followed by a base pressure. The observed initial and base pressures vary strongly within avalanches and from one avalanche to another, but, on average, they can be correlated with the frontal speed and the density of the deposited avalanche snow. It is concluded that well–developed dry–snow avalanches have an unsteady wave motion similar to the slug flow observed in ultra–rapid flow of water, and that they consist of three stratified components: dense flowing snow at the bottom, light flowing snow, and powder snow.

Seismic and Impact–Pressure Monitoring of Flowing Avalanches

AbstractContinuous records have been made during the passage of dry–snow avalanches of both seismic signals, which allows the avalanche speed to be estimated, and impact pressures on load cells with surface areas of 645 and 6 450 mm2. The impact pressure recordings show an initial peak followed by a base pressure. The observed initial and base pressures vary strongly within avalanches and from one avalanche to another, but, on average, they can be correlated with the frontal speed and the density of the deposited avalanche snow. It is concluded that well–developed dry–snow avalanches have an unsteady wave motion similar to the slug flow observed in ultra–rapid flow of water, and that they consist of three stratified components: dense flowing snow at the bottom, light flowing snow, and powder snow.

Application of Numerical Transient Fluid Dynamics to Snow Avalanche Flow. Part I. Development of Computer Program Avalnch

AbstractA two-dimensional, transient fluid-dynamics computer code has been modified for specific application to the avalanche-runout problem. This code, called AVALNCH, permits the separation of path geometry effects from such flow factors as friction and viscosity. The longitudinal profile of the avalanche path is divided into cells, 10 to 20 m long, each of which can be assigned specific values for slope gradient, surface friction, and internal kinematic viscosity. The program gives average avalanche speed cell-by-cell down the path and the location and depth of avalanche debris. Internal kinematic viscosity and surface friction were modeled on an avalanche path of simple geometry and were found to be of about equal significance in predicting runout distance. Additionally, surface friction is represented by an exponentially increasing function as speed decreases in the runout zone, in order to model observed avalanche terminal-motion characteristics. Program AVALNCH is reduced to a basic algorithm that is efficient to run, and contains the essential mechanics to model avalanche flow accurately. The most pressing need is for more physical data to permit the matching of program output to observed results under a variety of conditions.

Application of Numerical Transient Fluid Dynamics to Snow Avalanche Flow. Part I. Development of Computer Program Avalnch

AbstractA two-dimensional, transient fluid-dynamics computer code has been modified for specific application to the avalanche-runout problem. This code, called AVALNCH, permits the separation of path geometry effects from such flow factors as friction and viscosity. The longitudinal profile of the avalanche path is divided into cells, 10 to 20 m long, each of which can be assigned specific values for slope gradient, surface friction, and internal kinematic viscosity. The program gives average avalanche speed cell-by-cell down the path and the location and depth of avalanche debris. Internal kinematic viscosity and surface friction were modeled on an avalanche path of simple geometry and were found to be of about equal significance in predicting runout distance. Additionally, surface friction is represented by an exponentially increasing function as speed decreases in the runout zone, in order to model observed avalanche terminal-motion characteristics. Program AVALNCH is reduced to a basic algorithm that is efficient to run, and contains the essential mechanics to model avalanche flow accurately. The most pressing need is for more physical data to permit the matching of program output to observed results under a variety of conditions.

Debris flows originating from soil slips during rainstorms in Southern California

Debris flows generated during rainstorms present a greater risk of death and injury to southern California residents than all other kinds of slope failure combined. During the years 1962-1971, twenty-three people in the greater Los Angeles area died from being buried or struck by debris flows, all of which probably originated as soil slips. Soil slips and debris flows are recurring major natural geomorphological processes in the region. Soil slips are reconstituted into debris flows when the initial movement (sliding failure) of slabs of soil and wedges of ravine fill causes remoulding of the saturated moving mass into viscous, debris-laden mud, which then flows down available drainage courses. This change of state results in a marked reduction in resistance to shear, permitting masses to accelerate down the same slopes on which, only moments earlier, the slabs of soil mantle had barely overcome the resistance to sliding. Many flows reach avalanche speeds and do not begin to deposit significant amounts of detritus until they reach lower gradients far from their sources. The deposits form steep ‘alluvial’ fans at the mouths of short, steep drainage basins tributary to broad valleys, and form debris trains in and along narrow trunk canyons below the mouths of short, steep tributaries. The exceptional storm period of January 18th to 26th, 1969, was accompanied by thousands of soil slips and provided an unusual opportunity to determine the times of occurrence of numerous debris flows, establish their origin from soil slips, and compare the times of those events with rainfall records from an extensive network of continuously recording rain gauges and a sequential set of radar weather maps. An empirical association between soil slips and rainfall suggests that a 10 inch antecedent rainfall is required to bring most of the colluvial soil of the area to field capacity, and 0.25 inch-per-hour is the minimum rate at which surface infiltration exceeds subsoil drainage for most of the colluvial soils of the area so that pore pressures are raised in a zone above the less permeable parent materials. Soil moisture of this level also appears to be sufficient so that most of the colluvial soils of the area will become at least partly liquid when disturbed.