Definition, types and effects of avalanches.ppt

Environmental hazards, geology and geography

1. Avalanches -a warning
http://www.youtube.com/watch?v=6qVwIuznFW0

2. Avalanche
prerequisites
•snow
accumulations
and
•steep topography
Mean snow depth, February (cm)

3. Avalanche fatalities (1998-9)
Kangiqusualujjaq

4. Avalanche facts and figures
(Canada)
•range in size from few 100 m
3
to 100 x 10
6
m
3
.
•most occur in remote mountain areas.
•>1 million events per yr in Canada
•100 avalanche ‘accidents’ (casualties,
property damage) reported per yr.
•Estimated that 1 avalanche in 3000 is
potentially destructive.

5. Avalanche fatalities per year:
North America

6. Source: New Scientist

7. Avalanche deaths, N. America (2002-3)
Activity Fatalities
Skiers 25
Snowmobilers 23
Climbers 5
Snowboarders 4
Hikers 1
Total 58

8. “Avalanches kill
eight in B.C.”
Headline in “The Province”
(Jan. 04, 1998)
“we have a real disaster on our
hands ….this is one
of the worst weekends on
record”
Alan Dennis, Canadian
Avalanche Centre

9. Kootenay avalanches,
Jan. 03, 1998
•6 heli-skiers die in
Kokanee Glacier Park
•2 skiers die on Mt.
Alvin, near New
Denver
•1 snowmobiler dies (4
buried) near Elliot
Lake

10. Avalanches in inhabited
areas (e.g. the Alps)
•On 9th February 1999 in the
afternoon a large avalanche
destroyed 17 buildings on
the edge of Montroc and
killed twelve: vertical drop
2500m to 1300m, horizontal
length 2.25Km, deposit
depth 6m. The map shows
known avalanche paths in
the area, with the 1999
avalanche circled.

11. Juneau, Alaska
(a city at risk)
City
receives
~2.5m of
snow per
year
Mountains
5 -10 m of
snow?

12. Snowfall and avalanche
hazards
More than 70 people died
in the Alps in the winter of
1998-9 as a result of
avalanches resulting from
the heaviest snowfalls in
50 yrs. There was
extensive damage to
property (e.g. Morgex,
Italy), and many tourists
were stranded.

13. Deaths in villages (1998-9)
Kangiquasualujjaq, Qué 9 in school gym
Darband, Afghanistan 70 in village
Gorka, Nepal 6 in village
Le Tour, France 12 in ski resort/village
Galtuer, Austria 20 in ski resort/village
Place Deaths

14. Bruce Tremper Staying Alive in Avalanche Terrain,
(Mountaineer’s Books):
“most avalanches happen during storms but
most avalanche accidents occur on the sunny
days following storms. Sunny weather makes
us feel great, but the snow-pack does not
always share our opinion”.
And elsewhere: People who are most likely to
die are those whose skills at their sport (e.g.
snowboarding) exceed their skill at forecasting
avalanches.
So, some basics…..

15. Avalanche triggers
•Snowstorms dump thick snowpacks
over surface hoar (increased weight)
•Vehicles or skiers increase weight on
pack
•Surface heating (sunshine, warm
airmass) weakens snowpack
•Gravitational creep
•Shaking (seismic, explosives), but rarely
low noise (shouts, aircraft overhead)

16. Avalanche
types and
triggers
from ‘The Province’
Jan. 04, 1998

17. Avalanche types I:
Point-release
•start at a point in loose,
cohesionless snow;
•downslope movement entrains
snow from sidewalls
•in dry snow they are relatively
small
•in wet snow they can be large and
destructive

18. Avalanche types II:
Slabs
•layers of cohesive snow may fail as
a slab
•can be triggered from below
•fracture must occur around the
perimeter (crown, flanks and toe
[or stauchwall])
•depth controlled by depth to
failure plane
crown
flank
toe

19. Slab avalanches
Failures are a result of layered snowpacks

20. Slab avalanches: dry and wet
Dry avalanches move
at 50-200 km/h;
develop powder clouds
Wet avalanches move
at 20-100 km/h;
(denser & slower)
most dangerous!

21. Formation of weak layers in snowpacks
•In calm conditions snow settles as a fluffy, powdery
layer of unbroken crystals (the weak layer). If the
wind speedincreases, a layer of dense broken
crystals settles on top (the slab).
•Cold air over a thin snowpack can create ‘depth
hoar’near the base of the snowpack. Water vapour
sublimates from pores in snow onto ice crystals
(produces a weak layer).
•Surface hoarforms on cold, clear nights. Ice
crystals are large and have weak cohesion.

23. Surface hoar
ice crystals commonly ~10 mm long
Photo: K.Williams

24. Strengthening of surface
hoar layer over time
Avalanches
Graph: Chalmers and Jamieson (2003) Cold Reg. Sci. Tech. 37, 373-381.

25. Snow stability:
Rutschblock test

26. Surface test Bench test
failure plane at depth
Snow stability testing
Images: Landry et al. (2001) Cold Reg. Sci. Tech. 33, 103-121.

27. Effects of slope angle
Point release Slabs
60
45
30
25

28. Avalanche hazard and aspect
Photo: R. Armstrong
leeward? windward?
north-facing? south-facing?
shaded sunny
little T°fluc. large T°fluc.

29. start
zone
track
run-out
zone

30. Effects of clearcutting
in mountainous terrain.
A wet slab avalanche
was generated from a
clearcut block on a 37°
slope at Nagle Creek,
BC (1996). It split into
six separate avalanche
paths, which destroyed
$400K of timber

31. Avalanche forecasting
•Wind speed:
hazard increases if wind >25 km/h.
•Snowfall forecast:
<0.3 m snow depth -no hazard.
>1.0 m -major risk.
•Temperature change:
hazard increases if T >0°C.

32. Avalanche forecasting:
(Centre for Snow Studies, Grenoble, France)
SAFRAN
CROCUS
MEPRA
Predicts average weather for
23 zones in Alps;
Predicts snowpack changes;
(errors tend to accumulate)
Predicts snow stability
3-phase model

33. Protecting settlements
In Switzerland and some parts of US
‘red zones’have avalanche return
intervals <30 yrs or large avalanches
(impacts >30 kPa) <300 yrs.Building is
prohibited in these areas.
In ‘blue zones’the upslope walls of a
building must be reinforced or include
a deflecting wedge.

34. Avalanche
protection
structures
(snow nets)
~5 m high

35. Andermatt,
Switzerland.
Village protected
by fences to hold
snowpack, and
forest (cutting
forbidden by
C13th by-law)

36. Protecting
transportation
corridors:
e.g
Coquihalla Hwy.

37. Protecting highway links
Boston Bar (Coquihalla Highway)
•71 avalanche paths producing ~100 events / yr.
•RI varies from < monthly to ~25 yrs.
•Forecasts from 5 weather stations (4 in alpine)
•Defences:
-snowsheds (#5 shed cost $12M)
-raised highway; deflection dams; check dams
-use of artillery and ropeways to initiate
controlled events

38. Will global
warming reduce
the avalanche
hazard in
temperate alpine
areas?
Data from
Switzerland show
that snowpacks in
the 1990’s were
significantly thinner
than in any decade
since the 1930’s.
Natural variation or
global warming?
Laternser and Schneebeli (2003) Int. J. Climatology 23, 733-750.
above
below

39. Will global warming reduce the avalanche hazard in
temperate alpine areas?
Scott and Kaiser(2003?) Amer. Met.Soc Conference; pdf 71795.
Below normal Above normal

40. Ice avalanches*
•On September 21, 2002 the terminus of the Kolka
Glacier in the Caucasus Mountains collapsed, and
some 4 M m
3
of ice swept 20 km down-valley,
killing ~100 people and burying a village. A similar
event occurred in the same valley in 1902.
Kolka Glacier
avalanche
debris
*cf. Mt.Yungay, Peru (1970)

41. Subsidence and local ground failure
= vertical displacement of
the ground surface
D, v
Vertical
displacement
Velocity
slight
large
slow fast
sinkholes
expansive
soils
surface
loading
before
after

42. Subsidence and local ground failure
Expansive soils
Sinkholes:
•associated with soluble rocks -carbonates and
evaporites plus mining activities
•annual cost ~$10M in North America
Subsidence:
•associated with tectonics, surface loading,
agricultural drainage and fluid extraction
•annual cost ~$100M in North America
•associated with smectite clays and frost-
heaving
•annual cost >$1000M in North America

43. •Characterized by rapid surface collapse
e.g. New Mexico (1918) a sinkhole 25m wide by 20 m
deep formed in a single night.
•Individual holes small, but may be locally numerous
•Collapse behaviour unpredictable; often triggered
by heavy rain, which causes loading of soil and
sinkhole collapse (e.g. in Pascoe Co., Florida., twice
as many sinkholes are reported in wet season vs. dry
season)
Sinkholes

44. Sinkholes
Occur in soluble
carbonatesor evaporites
Relative
solubility
limestone dolomitegypsum halite
1 1150 7500

45. Stage 1 -
Cavern
formation
Stage 2 -
Sinkhole
formation

46. Large sinkhole, central Florida
House for scale

47. Sinkhole formation in halite,
Dead Sea
Dead Sea
halite
fresh
water
sinkholes collapse
above halite caverns
**

48. 1912 survey of one land section in Indiana,
showing numerous sinkholes

49. Subsidence and local ground failure
•Effects -damage to urban and
suburban infrastructure
•Detection -e.g. GPR and ER (see
next slide)
•Mitigation -non-intensive land uses
on affected land to minimize
hazard

50. Sinkhole detection
(ground-penetrating radar imagery)
soil
limestone
sinkhole

51. Sinkhole detection:
electro-resistivity techniques

52. Global distribution of vertisols

53. Vertisol profile
Note blocky
structure and
uniform black
upper horizons

54. Vertisols -‘Gilgai’

55. Vertisols-dry season
shrinkage and cracking

56. Vertisols -‘Slickensides’

57. Smectite clay minerals = expansive soils
Graphic: www.smianalytical.com
H
2O

58. Damage to buildings on
expansive soils
Farm buildings, Idaho House, Texas

59. How significant is the problem?
•Expansive soils are the #1 cause of
structural damage to buildings and
urban infrastructure (roads, sidewalks,
pipelines) in the US.
•Annual losses ~ US-$2 -$7 G
(probably x2 the amount associated
with all other natural hazards!)

60. Future problems:
e.g. Dallas, TX
•Expansive soils (= ‘low
urbanization potential’) are
predominant on the
interfluves of the plains of
north Texas.
•Suburban construction is
increasingly moving onto
these soils in as low and
medium risk soils reach their
development capacity (>50%
of new construction on these
soils in some counties).
Source: Williams (2003) Environmental
Geology 44: 933-938