Study Guide For EESA05, Chapter 5

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19 Jul 2010
Chapter 5: Snow Avalanches
5.1 Introduction to Snow Avalanches
- Avalanches are rapid down slope movements of snow, ice, rock, or soil
- Snow avalanches are masses of snow, generally more than few cubic m in volume that separate from the
intact snowpack and slide or flow down slope
- Avalanche can travel as coherent block, deforming and fragmenting to some degree, or it may rapidly
disaggregate into small particles that move independently of one another
- they are commonbut 1 in 3000 is potentially destructive to people and property
- Driven by gravity; differ from landslides in the failure and transport mechanism and the physical properties of
the failed material
Snow Climatology
- probability of snow falling and accumulating in an area depends on the season and on such geographic factors
as latitude, altitude and proximity to an ocean or large body of freshwater
- snow accumulates when temperatures are at or below freezing
- length of the snow season depends on latitude and altitudes; snow may fall and remain on the ground for only
a few weeks at low elevations in mid-latitudes or for almost 12 mos at high latitudes or in high mts
- snow fall is rare S of 35 N latitude and N of 40 S latitude, except in high elevations
- west coast of N hemisphere, continents are devoid of snow to much higher latitudes
- usually heavy snow falls occur around large lakes
- snow belts are source of much of the precipitation that falls as snow
- amt of snow on ground depends on many factors, most important of which are slope of land, elevation,
amount of snowfall, and winds
- snow accumulates on slopes less than about 45 degrees; it sluffs away on steeper slopes
- temperature decreases with altitude
- slabs are snow buildups that are unstable
- amt of snow on ground can differ considerably over short distances
Avalanche Initiation
- point-release avalanches begin with failure of a small amt of loose fluffy snow and grow as they move down
o sliding snow causes failure in adjacent snowpack, producing a distinctive, widening- trough
o occur after heavy snowfall
o thick, loose snow is unstable, both because the snow crystals have little time to bond
- layers of cohesive snow can fail as a slab, leaving steep crown, lateral and toe scraps and a smooth basal
failure plane
- slab avalanche begins with fracturing of the snowpack along a weak layer at depth
o gravity causes the snowpack to creep down slope, with the top of the snowpack moving faster than the
o if a weak layer is present, it may deform, or shear, more than the layers above and below it
o weak horizon slowly deforms over hrs or days and followed by sudden failure
o initial point failure propagates along the weak surface, causing a slab to break away along bounding crown
and lateral fractures
Weak Layers
- slab avalanche requires a buried weak layer and an overlying stronger slab
- new snow that has not had time to bond to the layer below, especially if it is light and powdery, is susceptible
to sliding; compacted snow less likely to move than light powdery snow
- snow that is above the level of boulders and plants on a slope has natural objects to anchor it and is therefore
more dangerous than thinner snow
weak layers in snowpack form in 3 ways:
1) wind
- blowing snow can build up on sheltered lee slopes and wind can stabilize the snowpack on other slopes
- wind slab is a body of thick, poorly bonded snow deposited by wind on a slope
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