Lecture 5 Flashcards
1
Q
What is a landslide?
A
- downslope movement of rock or sediment as a result of gravity
- movement is classified as rapid if it can be detected by eye
2
Q
What variables are used to classify landslides?
A
- mechanism of movement
- type of material
- amount of water present
- speed of movement
3
Q
What are 3 basic mechanisms of movement?
A
- Fall: rock or sediment dropping off the face of a cliff
- Slide: downslope movement along a discrete failure plane
- Flow: movement of particles semi-independently of one another, commonly with the aid of water
4
Q
What is rock fall?
A
- mass movement caused by fall mechanism
- involves rock rolling down a steep slope of falling through the air
5
Q
What is slump?
A
-in this mass movement the failure plane is curved upward
6
Q
What is creep?
A
- mass movement caused by flow mechanism
- speed of movement ranges from a few mm to a metre annually
7
Q
What are forces on slopes?
A
-the stability of a slope is based on the balance between two types of forces:
Driving forces: these move material downslope; they are based on the weight of the material from vegetation, water, etc.
Resisting forces (friction): these oppose downslope movement; they are based on the shear strength of the material
8
Q
What is the factor of safety?
A
-ratio of resisting forces to driving forces
FS=RF/DF
- ratio>1, the slope is stable
- ratio<1, the slope is unstable
- if the factor equals 1 resisting and driving forces are equal
- the forces are determined by relationships among material type, slope and topography, climate, vegetation, and water
9
Q
How does the material type affect the risk of landslides?
A
- the degree of consolidation and the presence of weakness planes can increase the risk of landslides
- slumps are most common in unconsolidated sediment
- slumps are rotational mass movements as opposed to translational mass movements
- translational movements often occur where sediment overlays bedrock; the failure plane is generally at the boundary between the soil and the bedrock
10
Q
How is slope related to driving forces?
A
- steeper slope=greater driving forces
- steepest slopes are associated with rock falls
- moderate slopes are associated with flows
- gentle slopes are associated with creep
- topographic relief: the height of a hill or mountain above the land around it
- dangerous landslides are more likely in areas of high relief
11
Q
How does climate influence landslides?
A
- climate influences amount of water that infiltrates and erodes the soil
- in dry climates, vegetation is sparse, soil is thin, and bare rock is exposed in many areas
- rock falls are more likely in those areas
- in humid climates, soil is thick and rock is generally covered with soil and vegetation
- thus, flows and creep are more likely in those areas
12
Q
What role does water play in landslides?
A
- water saturates soil increasing the likelihood of flows
- following prolonged periods of deep water infiltration slumps can develop
- water can also erode the base of a slope therefore decreasing its resisting force
13
Q
What regions are at risk for landslides?
A
-any location with significant variation in topography
14
Q
What is the Frank Slide?
A
- Canada’s best known landslide
- occurred in 1903 on Turtle Mountain near Frank, Alberta
- landslide killed 76 people, dammed the Crowsnest River creating a lake, and buried 5km of railway
- the exact cause of the landslide is unknown but glaciation mining and heavy snow are likely contributors
15
Q
What are natural service functions of landslides?
A
- landslides can result in the development of new habitats in forests and aquatic ecosystems
- this produces an increase of biodiversity
- landslides can carry sediments that contain valuable minerals which become concentrated at the base of a slope following an event
16
Q
How can landslide hazard be minimized?
A
- recognize where they are most likely to occur
- features indicative of unstable slopes include: cracks on hillside, recessed crest of a valley wall, large boulders or talus at a cliff base, tilted trees, exposed bedrock, an irregular land surface at a slope base
- aerial photos are used to detect some of these features and then hazard maps can be produced
17
Q
How can landslides be prevented?
A
- complete prevention is impossible but certain engineering practices can minimize the hazard:
1. Drainage control: objective is to remove excess water from the soil by pipes and drains
2. Levelling the slope: material from the upper slope can be moved to the slope base with modern technology
3. Slope supports: examples include retaining walls, rock bolts, and metal screens
18
Q
What are landslide warning systems?
A
- tiltmetres: instruments used to detect movement along a slope
- measures very small changes from the horizontal level
- some rock fences along railways in NA are linked to signal systems
- rain gauges on slopes can identify when a precipitation threshold has been reached
19
Q
What is a snow avalanche?
A
- a mass of snow many cubic metres in volume that separates from a snowpack and flows downslope
- rocks, soil, ice, and debris can travel in a similar motion; however the term avalanche is generally reserved for snow
- the intensity of the hazard is dependent on slope steepness, snowpack stability, and weather
20
Q
What are the types of avalanche?
A
- avalanche travelling as a coherent block
- avalance that becomes wider as it travels downslope
- estimated that over 99% of avalanches are not seen by anyone
- it is likely that over 1 million avalanches large enough to kill a person occur annually in Western Canada alone
21
Q
What does snowfall depend on?
A
- snowfall accumulation depends on latitude, altitude, and proximity to bodies of water
- temperature decreases with altitude therefore high mountains have permanent snow cover
- snow accumulates on mountain slopes that are at angles of less than 60 degrees
- if the angle is too steep nothing can accumulate
22
Q
What is a point-release avalanche?
A
- these begin as an initial failure after a heavy snowfall
- the sliding snow then causes more failures in the adjacent snowpack causing the trough to widen
23
Q
What is a slab avalanche?
A
- these occur when a snowpack fractures along a weak layer parallel to the surface
- these avalanches move as cohesive blocks leaving behind a scarp
- more dangerous than point release
- they are the most dangerous
24
Q
What is slough?
A
-sliding loose snow
25
What is a slab?
-large sections of snow path crack off
26
What is the danger zone for avalanches?
- slopes between 30 and 45 degrees
- as snow falls it builds up layer upon layer which bond
- if the bond is weak new snow falls on unstable foundation
- slope angle is most important terrain factor for avalanche formation
- most occur between 25 and 60 degrees
- below 25, snow does not easily slide
27
Describe avalanche motion
- rapidly moving avalanches (speeds over 35 km/h) often generate clouds of powdered snow
- fastest avalanches have been measured at speeds of 200km/h
- some avalanches are powerful enough to climb opposing slopes (in a valley)
28
What triggers avalanches?
- most avalanches occur soon after snowstorms
- some may occur when daytime heating from the Sun warms the upper part of the snowpack
- avalanches that cause injuries or fatalities are often triggered by people
- some avalanches are triggered intentionally with explosives
29
Describe the avalanche path
Start Zone: the area where the snowpack first fails
Track: the area along which the avalanche accelerates and reaches max velocity
Run-out zone: the area of deceleration and snow deposition
30
How does orientation of slope affect avalanche?
- deposits of snow on leeward slopes can consist of interlayered strong and weak layers
- slopes facing the Sun are more prone to daytime avalanches during clear weather
- other factors include smoothness of slope, amount of vegetation, and topography of the slope itself
31
What regions are at risk for avalanches?
- snowpack of at least 50cm is required for avalanche
- in NA deep snowpacks are most common in rocky mountains
32
What are the effects of avalanches?
- most avalanche deaths occurred in late 1800s and early 1900s in canadian history
- in total over 600 people have died from avalanches in Canada
- avalanches cause millions of dollars in economic losses in BC each year due to closed highways
- damage to forests is evident each year but property damage is relatively minor
33
Describe the Chilkoot avalanche
- this disaster occurred in 1898 and remains one of the worst avalanches in NA history
- an avalanche spread over the Chilkoot Trail causing 60 deaths
- the trail was heavily used at that time by people heading to Yukon during the Klondike Gold Rush
- the Chilkoot trail extends from Alaska to BC and is the easiest route through the mountains
34
What are natural service functions of avalanches?
- ecological disturbance
- increase local plant and animal diversity
- maintain open areas in otherwise forested regions
- more diverse than having an entire forest for animals to graze on the mountain side
- can serve as an important habitat zone for certain plants and animals
35
Describe human interaction with avalanches
- humans encroach on areas prone to avalanches
- as tourism and recreation have increased in the rockies and alps, deaths from avalanches have increased
36
How can avalanche risk be minimized?
- buildings in hazardous areas within a specific recurrence interval may require special engineering
- this may include reinforced walls or deflection structures
- in avalanche start zones fences or nets can be installed to support the snowpack
- splitting wedges on the sides of buildings can force an avalanche around the structure
- avalanche sheds allow avalanches to travel over roads or railways without disruption to traffic
- controlled triggers are used to force avalanches to occur in order to prevent buildup of the snowpack (explosives)
37
What is a splitting wedge?
- splits the snow around building to protect it (concrete triangular walls)
- mounds and berms can be used to slow and deflect avalanches away from populated areas
38
What are the 3 major strenght and stability tests used to assess snowpack?
1. Compression Test: a vertical force is placed on the top of the snowpack to detect weak layers
2. Shovel test: it assesses the strength by isolating a column of snow and applying force on the uphill side
3. Rutschblock test: a skier pushes and jumps on a column of snow to detect cohesion of the snowpack
39
What are the survival rates for avalanche rescue?
- motion of snow itself kills about 25% of victims
- survival depends on the length of time the person is buried and the burial depth
- over 90% survive if rescued within 15 minutes, 30% within 35 minutes, and 0% within 2 hours
- buried victims die of a combination of suffocation and hypothermia
- less than 10% of victims survive burial in more than 1.5m of snow
- best chance of survival depends on effective search by other members of the group rather than waiting for help
- chances of finding buried victime increase when everyone in group carries standard avalanche survival aids
40
What are avalanche survival aids?
- avalanche cord: a 10m rope that drags behind a person while skiing snowboarding or snowshoeing
- avalanche transceiver: a portable device that emits a radio signal to assist in finding the location of a victim
- avalanche dogs: they can detect human scent rising through the snow and can quickly cover large areas
41
Which structure allows avalanches to travel over a road or railway?
avalanche shed
