SNOW HYDROLOGY (GEOG 4321): AVALANCHES

Instuctor: Mark Williams
Telephone: 492-4794 or 492-8830

Readings

• Avalanche Handbook, chapters 4-7.
• Avalanche Handbook, all appendices.
• Hardy, D., M. W. Williams, and C. Escobar, Near-surface faceted crystals and avalanche dynamics in high-elevation, tropical mountains of Bolivia, Cold Regions Science and Technology, 33 (2-3), 291-302, 2001. pdf file of manuscript text; and figures.
• PPT presentation: Required reading
• Nice examples of pits and slides from Utah Avalanche Center: Suggested reading
• Great overview: Suggested reading
• Mathematical approach to avalanches: Extra reading for those who want more

Avalanche Animations

Avalanche Animation 1

Avalanche Animation 2

Silverton, Colorado, January 04

Avalanche account: read this story!

Avalanche Definitions

• Avalanche area is a location with one or more avalanche paths.
• Avalanche path is a fixed locality within which avalanches move. Distance may vary from 50 meters to more than 2,000 meters.
  • Starting zone is the location where unstable snow failed and began to move. Generally, the avalanche gains mass within the starting zone and accelerates through the starting zone. Slope anlges in starting zones are often 30-45 degrees.
  • Track or zone of transition is the slope below the starting zone that connects the starting zone with runout zone. The avalanche mass generally remains constant. The avalanche no longer accelerates and velocity remains constant. Generally the highest avalanche speeds are attained in the track. The slope angle is generally between 20 to 30 degrees.
  • Runout zone or zone of deposition is the area where deceleration is rapid, debris is deposited, and the avalanche stops. Slope angle often less than 20 degrees.
• Types of Avalanches
  1. Point Release or sloughs are generally composed of loose snow.
     • Snow is cohesionless.
     • Avalanche starts at a point.
     • Spreads downhill laterally, forming an inverted V-shape.
     • Involves only surface snow.
  2. Slab Avalanche is the catastrophic failure of cohesive snow.
     • Powder Avalanches Most or all of the material in the avalanche is suspended above the ground by turbulent eddies.
     • Dry Slabs Cohesive material has no or little liquid water and a "core" of higher density snow in contact with the ground. A hard slab moves as a cohesive unit downslope and a soft slab breaks up immediately into small blocks.
     • Wet Slabs Liquid water present. Much higher friction at the sliding surface, often "plowing" into the soft sliding surface, causing the formation of grooves in the sliding surface and also entraining rocks, dirt, and other materials.

Slab Morphology

• Crown surface. The top fracture surface of the slab. Perpendicular to the slope.
• Crown. Snow remaining on the slope above the crown surface.
• Flank. Sides of the slab, often with a sawtoothed, jagged appearance.
• Bed Surface. Snow remaining below the bottom of the slab. Main sliding surface of the slab. Generally has a smooth appearance because of friction from the sliding slab.
• Stauchwall. Downslope wall of the slab. Often wedge-shaped, usually obliterated by the slab.

Forces

Snow is a unique material. It's a solid which exists close to its melting point. As a result, snow can be elastic, viscous, and brittle. Viscous deformation in snow is similar to the processes that cause folds in mountain systems. Brittle deformation is when the substance breaks, such as a fault in mountain systems. Snow has the ability to deform both viscously and elastically before brittle failure occurs.

Elastic deformation means that when force is applied to the snowpack, it deforms and when the force is removed, the snowpack returns to its original shape. Snow has a limited ability to recover from compression. The strain is relieved by elongation: viscous deformation. Viscous means that force causes the snowpack to deform without breaking and does not return to its original shape when the force is removed. Snow has both those properties. Snow is often termed a visco-elastic substance because of those properties. However, there is a limit to these properties. Snow won't indefinitely elongate, which leads to brittle failure and fractures. When snow does fracture, some of that energy is transmitted through the snowpack elastically. When that happens, the elastic energy can start avalanches on nearby slopes: sympathetic releases.

Like all materials, snow can fracture when loaded in tension, compression or shear. And like most materials, snow has substantial resistance to fracture in compression, BUT IS EASILY FRACTURED IN TENSION!

In COMPRESSION, potential fracture surfaces are pushed together and the snowpack may gain strength. Additionally, forcing grains together "age hardens" the snow, accelerating bonding between crystals. If there's a weak layer within the snowpack, the pack may not glide as a cohesive unit, but SHEAR off along this layer.

Creep and glide accentuate the tensile forces within an inclined snowpack

Creep is when the snowpack moves at different velocities under the influence of gravity. Generally the bottom of the snowpack does not move and motion (and velocity) increase with height in the snowpack. The usual approach to modelling creep behavior is to ap-ply a constitutive law that relates the bulk response of the snow to anapplied stress. For snow on a 36 šslope we expect the shearing component of motion would be about 50% greater than the compressive component. Glide is when the entire snowpack moves under the influence of gravity. Generally, as the surface roughness of the ground decreases, glide increases for a given slope angle.

Nice example of these forces on a roof.

Avalanche Motion

FLOWING AVALANCHES
have a core and a dust cloud.

• The core is a relatively high density material composed of snow particles and air locatd at the bottom of the avalanche. The height of the core is usually less than 5 meters. Density of the core is somewhere around 35%.
• The dust cloud is composed primarily of air with suspended snow particles and surrounds the exterior of the core. The height of the dust cloud can be tens of meters. The density of the dust cloud is usually less than 1%, eg air makes up more than 99% of the core.

POWDER AVALANCHES
Have a dust cloud with no core. Often form by falling ice from steep snowfalls.

SHEAR STRESS (tau)

• rho is density
• H is snow depth
• g is gravity
• theta is the slope angle

AVALANCHE VELOCITY (V)

• zeta = coefficient of turbulent friction
• R is the hydraulic radius
• mu is the coefficient of kinetic friction at the bed surface

typical speeds

TYPE	V (m/s)
powder	20-70
flowing	15-60
wet	5-30

AVALANCHE IMPACT PRESSURE (I)
I = rho V²

• rho is the snow density
• v is avalanche velocity

Impact forces as a function of avlanche type

Impact force of an avalanche is determined by its speed, types and concentration of entrained materials, flowing densities and dimensions. Using these parameters we can evaluate the impact forces of different type of avalanches.

Stability Evaluation

Rutschblok Test. After digging a pit and examining the snowpack layers, then isolating a section as wide as a ski, Robert Bland performs the last step in the Rutschblok test. This is but one piece of information in determining snowpack stability, and certainly not the only factor to consider.

CLASS I: STABILITY FACTORS

Current Avalanche Activity

Loading Tests

• Test Skiing
• Explosives
• Hasty Pit
  
• Shovel Shear Test
  
• Stuff block Test
  
• Tap Test
  
• Burp the baby
  
• Rutschblok Test
  

Fracture Propagation

CLASS II: SNOWPACK FACTORS

Past Avalanche Activity

Snowpack Depth

Slope Use

Ram Penetrometer

Snow Temperature

Snowpack Profile

CLASS III: METEOROLOGICAL FACTORS

Surface Conditions

Depth and Water Equivalent of New Snow

Type of New Snow

Density of New Snow

Snowfall Intensity (> 2.5 cm/hr)

Maximum Precipitation Intensity (> 0.5 cm/hr)

Settlement of New Snow

Wind Speed

Wind Direction

Air Temperature

Relative Humidity

Solar Radiation (slope and aspect)

Avalanche Rescue: Hasty Search Technique

1993 Flowchart by Dale Atkins and Lin Ballard.

Hydrologic and Geomorphic Roles of Avalanches

• Required reading (under selected readings in library): Williams and Clow, Hydrologic and biologic consequences of an avalanche striking an ice-covered lake, Proceedings of the Western Snow Conference, 1990.
• Required reading (under selected readings in library): Williams and others, Aquatic ecology as a function of avalanche runout into an alpine lake, International Snow Science Workshop.
• Recommended reading: F. A. de Scally, Hydrologic roles of snow avalanches in High-Mountain environments, pp 69-80, Proceedings of the Western Snow Conference, 1996.

Bibliography

Stability Tests

Rutshblock Test

• Fohn, P, The "rutschblock" as a practical tool for slope stability evaluation. In: Avalanche formation, movement, and effects. IAHS Publ 162, pp 223-228, 1987.
• Fohn, P, Snowcover stability tests and the areal variability of snow strength. In: Proceedings of the 1988 International Snow Science Workshop, Whistler, BC, Canada, pp 262-273, 1988.
• Jamieson, J. B. and C. D. Johnston, Experience with Rutschblocks, Proceedings of the 1992 International Snow Science Workshop, Breckenridge, CO, pp 150-159, 1992.
• Jamieson, J. B. and C. D. Johnston, Rutschblock precision, technique variations and limitations, Journal of Glaciology, V 39, pp 666-674, 1993.

Stuffblock Snow Stability Test

• Birkeland, K. and R. Johnson, The Stuffblock Snow Stability Test Special Report 9623-2836-MTDC, USDA Forest Service, Technology and Development Center, Missoula, Montana, 1996.
• Johnson, R. and K. Birkeland, The stuffblock: a simple and effective snow stability test. In: Proceedings of the 1994 International Snow Science Workshop, Snowbird, UT, pp 518-526, 1994.

Shovel Shear Test

• Shaerer, P, Evaluation of the shovel shear test. Proceedings of the 1988 International Snow Science Workshop, Whistler, BC, Canada, pp 274-276, 1988.