avy

avy - Transcript

Snow
Avalanches
what is an avalanche?
?a falling mass of snow and/or ice
?a mass-wasting process
?analogous to debris flows or mudslides
?a natural hazard - threat to life and property
?an expression of earth system complexity
why do we care?
?recreation
?ski areas
?backcountry
?transportation
?highways
?railroads
?communities/structures
?avalanches are cool
Westwide Avalanche Network
outline
?overview of:
?avalanche interests
?accident statistics
?avalanche formation factors
?current research
ski area avalanche management
?snow compaction
?skier traffic
?boot packing
?intentional avalanche
release
?explosives
?ski cutting
?cornice management
Westwide Avalanche Network
backcountry recreation
?no active control
?who gets caught –
trends in avalanche
statistics
?avalanche accidents
?forecasting
?education
?be your own snow
expert
Westwide Avalanche Network
us avalanche fatalities by winter
us avy fatalities by state
us avy fatalities by age group
fatalities by activity
recent trends
?recent explosion in
backcountry use
?technological advances
?ski gear
?snowmobile power and design
?“Extreme” hype
?availability of avalanche
education
Gallatin National Forest Avalanche Center
fatalities by state
fatalities by age group
fatalities by activity
avalanches and transportation
?highways and railways
?control and reduction
?high cost of link closure
?potential for disaster
transportation
?active control
WSDOT
www.gaz-ex.com
WSDOT
?passive control
www.eng.ucalgary.ca
www.eng.ucalgary.ca
avalanche hazard to communities
?primarily a European issue
?higher alpine population density
?resort development in US
types of avalanches
?loose snow (point release)
?slab
?soft slab
?hard slab
?distinction based on snow cohesiveness
?can be wet or dry snow
slab avalanche terminology
Starting zone
Track
Runout zone
slab avalanche
terminology
?crown face
?bed surface
?flanks
?stauchwall
avalanche formation factors
?terrain
?weather
?snowpack
?humans
?good news: the snowpack is stable the majority of the time
terrain
?Is the terrain capable of
producing an avalanche?
terrain
?factors to consider:
?slope angle
?slope size and consequences
?slope shape
?vegetation and trees
?runout
?aspect with respect to wind
?elevation
terrain
Is this avalanche terrain?
slope angle
60°
45°
30°
25°
0°
?NOTE:
referring to the
steepest part of
the slope
slope shape
?convexities and concavities
zone of
tension
zone of
compression
Grav
ity’s
infl
uenc
e
snow
slab

vegetation and
trees
?indicators of
avalanche activity
?tree “flagging”
?secondary growth
?trees can anchor
snow

vegetation and trees
?ground cover affects:
?effective snow depth
?heat transfer ~
snow metamorphism
in basal layers
vegetation change
?implications of
?climate change
?timber cutting
?creation of starting zones
?forest fires
?removes ground cover, thins trees
?large avalanche events
terrain trap

weather
?Is the weather affecting the snow stability?
?Precipitation (snow or rain)
?Wind
?Temperature
precipitation
?addition of mass to the snowpack
?rate of addition is important
?stress vs. strength
stress
stren
gth
gravity
?
compression
?
wind
?snow redistribution
?change in snow density
?cornices
wind
wind
temperature
?changes in temperature can affect snow
stability
?change during storms
?rapid warming
?metamorphism
effects
snowpack
?“Can the snowpack avalanche?”
?snow stability evaluation
?weak layer
?slab
stability
stability
?force balance
?increase stress
?decrease strength
?stress (?) vs. strength
? = m*g*sin ?
stres
s
stren
gth
gravity
?
compression
?
stability evaluation
?observe signs of weakness
?recent avalanching
?collapsing or “wumpfing”
?propagating cracks
?evaluate structure of snowpack
?are weak layers present?
?is there a slab?
?test the stability of the snowpack
?stability tests
stability tests
?strength/stress
balance between
slab and weak
layer
Chris Landry
www.pmru.org Gallatin National Forest Avalanche Center
weak layers
?surface hoar
?surface deposition
?faceted crystals
?internal snowpack metamorphism
?depth hoar (sugar snow)
?near-surface facets
surface hoar
?frozen dew
?sublimation
?feathery crystal form
?often 3-4 cm in length
?strong in compression
?weak in shear
faceted snow
?depth hoar:
?generally in basal layer
?sugary consistency
?near-surface facets
?formed at surface
?can be found anywhere in
the snow column
?strong in compression
?weak in shear
growth of faceted snow
?requires strong temperature gradient
?typically > 10°C/m
?t.g. induces vapor pressure gradient
?H2O vapor moves from high to low v.p.
vapor
cold
low v.p.
warm
high v.p.
faceted snow
variation in snow properties
?system complexity produces variability
?over space
?over time
?snow is thermodynamically active
?can exist in 3 phases in snowpack
?sensitive to small environmental changes
?change can be rapid
spatial variation
?wind
?sun
?temperature
?trees
?…
temporal
variation
?change can be rapid
human factor
?“Can you make an objective
assessment of the avalanche
danger?”
?in the vast majority of avalanche
accidents, the avalanche was caused
by the victim or a member of the
victim’s party
human factors
?decision making
?routefinding/travel
habits
?emotions and logic
?preparedness
?education
?other concerns:
?sales product
(ski hill)
?transportation
delays
?real estate
location
?ego
HENDERSON MOUNTAIN,
COOKE CITY, MT
February 4, 1992




“Any rapid change in the thermal or
mechanical state of the snowpack is a
precursor to avalanching.”
- Ed LaChapelle
putting it all together
slab avalanche ingredients
slope
slab weak layer
trigger
rescue
?if you are caught in an avalanche,
your best hope is your partners
?other options:
?self-rescue
?organized rescue
?time of burial
is critical
types of rescue
?companion rescue means:
?safe travel
?proper gear and training
?practice
burial time
burial depth
current research
? microscale:
? crystal bond development
? 3D tomography
? mesoscale
? spatial patterns of stability
? change in stability over
time
? macroscale
? remote sensing of snow
properties
? study plot ~ starting zone
relationships
avalanche forecasting
?US forecast centers
?forecasters use:
?weather
?snowpack
?terrain
…to produce danger ratings
avalanche forecasts
Travel in avalanche terrain
should be avoided and
travel confined to low
angle terrain well away
from avalanche path
run-outs.
Extremely unstable slabs
certain on most
aspects and slope
angles. Large,
destructive avalanches
possible.
Widespread natural or
human triggered
avalanches certain.
EXTREME
(black)
Travel in avalanche terrain is
not recommended.
Safest travel on
windward ridges of
lower angle slopes
without steeper terrain
above.
Unstable slabs likely on a
variety of aspects and
slope angles.
Natural and human triggered
avalanches likely.
HIGH
(red)
Be increasingly cautious in
steeper terrain.
Unstable slabs probable on
steep terrain.
Natural avalanches possible.
Human triggered
avalanches probable.
Considerable
(orange)
Use caution in steeper
terrain on certain
aspects (defined in
accompanying
statement).
Unstable slabs possible on
steep terrain.
Natural avalanches unlikely.
Human triggered
avalanches possible.
MODERATE
(yellow)
Travel is generally safe.
Normal caution is
advised.
Generally stable snow.
Isolated areas of
instability.
Natural avalanches very
unlikely. Human
triggered avalanches
unlikely.
LOW
(green)
...WHAT TO DO... ...WHERE... ...WHY... ...WHAT...
Recommended Action
in the Backcountry
Degree and Distribution
of Avalanche
Danger
Avalanche Probability
and Avalanche
Trigger
Danger Level (& Color)
United States Avalanche Danger Descriptors
summary
?avalanches:
?are part of a complex, rapidly changing, earth
surface system
?affect recreation, transportation, and civic
interests
?are functions of terrain, weather, and
snowpack conditions
further information
?avalanche classes:
?CAIC
?Silverton Avalanche School
?AAA
?forecasts
?www.avalanche.org
?science
?US Forest Service National Avalanche Center