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Natural Disasters > Lecture 5 > Flashcards

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
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2
Q

What variables are used to classify landslides?

A
  • mechanism of movement
  • type of material
  • amount of water present
  • speed of movement
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3
Q

What are 3 basic mechanisms of movement?

A
  1. Fall: rock or sediment dropping off the face of a cliff
  2. Slide: downslope movement along a discrete failure plane
  3. Flow: movement of particles semi-independently of one another, commonly with the aid of water
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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
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5
Q

What is slump?

A

-in this mass movement the failure plane is curved upward

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6
Q

What is creep?

A
  • mass movement caused by flow mechanism
  • speed of movement ranges from a few mm to a metre annually
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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

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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
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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
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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
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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
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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
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13
Q

What regions are at risk for landslides?

A

-any location with significant variation in topography

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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
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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
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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
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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
Q

What is a slab?

A

-large sections of snow path crack off

26
Q

What is the danger zone for avalanches?

A
  • 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
Q

Describe avalanche motion

A
  • 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
Q

What triggers avalanches?

A
  • 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
Q

Describe the avalanche path

A

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
Q

How does orientation of slope affect avalanche?

A
  • 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
Q

What regions are at risk for avalanches?

A
  • snowpack of at least 50cm is required for avalanche
  • in NA deep snowpacks are most common in rocky mountains
32
Q

What are the effects of avalanches?

A
  • 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
Q

Describe the Chilkoot avalanche

A
  • 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
Q

What are natural service functions of avalanches?

A
  • 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
Q

Describe human interaction with avalanches

A
  • humans encroach on areas prone to avalanches
  • as tourism and recreation have increased in the rockies and alps, deaths from avalanches have increased
36
Q

How can avalanche risk be minimized?

A
  • 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
Q

What is a splitting wedge?

A
  • 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
Q

What are the 3 major strenght and stability tests used to assess snowpack?

A
  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
Q

What are the survival rates for avalanche rescue?

A
  • 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
Q

What are avalanche survival aids?

A
  • 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
Q

Which structure allows avalanches to travel over a road or railway?

A

avalanche shed

Natural Disasters (11 decks)

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