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133 Natural Hazards and Society > Slopes > Flashcards

Slopes Flashcards

1
Q

Worldwide statistics

A

(Increaing) several 1000 deaths pa

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2
Q

UK statistics

A

> 18,000 features known
Many dormant/fossil

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3
Q

What is the slope failure?

A

Downward movement under the influence of gravity

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4
Q

Movement of soil/rock

A

Landslides

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5
Q

Movement of snow/ice

A

snow avalanche

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6
Q

Components of slope failure

A

Source/head/Scarp
Body/Chute/Toe/Fan

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7
Q

What is slope stability

A

The balance of =
Driving force ( gravity) on weight and slope angle)

Resistance of slope materials on cohesion , cement and friction

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8
Q

What is the Mohr–Coulomb failure criterion

A

Used to describe the strenght of soils and rocks

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9
Q

How does failure occur

A

Critically oriented plane when gravitational shear stress/ plane exceeds the shear strength

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10
Q

Equation

A

τf = c + σ tanφ

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11
Q

τf

A

sheer strength

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12
Q

σ

A

effective normal stress on the failure plane

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13
Q

c

A

cohesion angle

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14
Q

φ

A

friction angle

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15
Q

Factors contributing to slope failure

A

Steep slopes
Loading
Weak materials
Weathering
Trigger

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16
Q

Landslide triggers

A

Water
Seismic activity
human activity

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17
Q

Landslide earthquake correlation

A

The magnitude and frequency links

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18
Q

Few landslides =

A

M<4

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19
Q

M9 triggers

A

landslides up to 500km+ from epicentre

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20
Q

Distances correlate with…

A

focal depth

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21
Q

Areas correlate with …

A

magnitude

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22
Q

What was the Wenchuan earthquake

A

M7.9
triggered.56,000 landslides

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23
Q

What was the Alaska earthquake

A

1964
M9.2
large masses of material landed on glaciers

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24
Q

Human contributions to landslide hazards

A

Modification of slopes:
loading,cutting,drainage

Artificial slopes:tips
Land degration, logging
Triggers
Encroachment

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25
Q

Froude and Petley, 2018
land slide hotspots

A

Incresae in frequency most prevalent in South East Asia

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26
Q

Characteristics of landslides

A

Volume - single boulder (km3)

Velocity - cm/y 100s m/s

Horizontal travel distance ( small 2x fall

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27
Q

Types of movement

A

fall/topple
slide
spread
flow

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28
Q

Type of material

A

rock
soil
snow/ice

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29
Q

Falls and topple

A

Detachment ( rotation)
Common( rapid, variable volume)
Disruptive

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30
Q

Slides

A

The movement on a shear surface
Are rotational or translational

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31
Q

Topple in the UK

A

Oxwich Bay, 2009 and 2018

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32
Q

Rotational slides in the UK

A

Holbeck Hall, Scarborough , 1993

Transformed into mud flow

33
Q

Translational slides in the UK

A

Burton Bradstock 2012
Borunemouth 2016

34
Q

Failures in weak material - spreads and eartrh flows

A

liquefraction
gentle slopes

35
Q

Failures in weak material example

A

Alaska 1964

36
Q

Sensitive quick clays

A

Marine clays
Na+ binding with silicates

  • causing salt leaching
    sig loss of strength
    30 times less resistant
37
Q

Sensitive quick clay example

A

Rissa , Norway 1978

38
Q

Loess weak material example

A

Derbyshire 2001

39
Q

Type of flows

A

Earthflow
Debris flows
Rock/debris avalanches( coarse)

40
Q

Earthflow

A

rapid
slow( creep )

41
Q

Earthflow example

A

Oso, March 2014
Washington,USA
43 deaths
Rain
Past activity , logging

42
Q

Debris flow characteristics

A

Mixture of water and sediment
-rapid flow
channelised
surging flow
denser- transporting larger particles than stream flows

43
Q

How does debris flow

A

Particles are suppored by =
-buoyancy ( stream flow, denser fuild
-grain grain collisions
matric strength

-with intense rainfall or rapid snowmelt on steep slopes

44
Q

Lahars

A

Associated with volcanic activity or
re-working volcanic debris (tephra)

45
Q

What happened in Venezuela Dec 1999

A

Rainstorms induced thousands of landslides along the Cordelia de la Costa Vargas ( northern Venezuela

Land relief = steep mountains with settlements on the alluvial fans.

200 mm rainfall followed by a major storm

Debris flows and flash floods on alluvual fans inundated costal communities

20,000-20,000 deaths

46
Q

Rock and debris Avanlanches

A

Km’s movement in mins

Volume to 10s km3

Long runout 10x fall

Recurrence ( twice per decade )

Sturzstoms

47
Q

Rock and debris Avanlanches

A

Km’s movement in mins

Volune to 10s km3

Long runout 10x fall

Recurrence ( twice per decade )

Sturzstoms

48
Q

Hope Slide, BC

A

48x106 m3
Jan 1965
4 killed
1100 m relief
70-80 deposists
potential trigger- earthquakes?

48
Q

Hope Slide, BC

A

48x106 m3
Jan 1965
4 killed
1100 m relief

49
Q

What happened in the Philippines feb 2006

A

Guinsaugon rockslide - debris avalanche

15-20 x 10^6
100-140km/h
1112 killed

50
Q

Cause of the 2006 event

A

steep slopes
weak rocks along the fault zone

deep troipical weathering

Orientation of rock structures

51
Q

triggers of the 2006 event

A

Rainfall ( 500 mm in 3 days
Earthquake M 2.6
Precursors - river dried up ( cracks in ground )

52
Q

Snow avalanches

A

masses of snow that rapidly decend steep slopes

contains soil, vetegation and ice

53
Q

Two types of snow avalanches

A

Loose snow avalanches
Slab avalanched

54
Q

Loose snow avalanches

A

From a point of coesionless surface layer of dry/wet snow, it is released.

The inital failure is analogous to the rotation of landslide but occurs within a small volume

55
Q

Loose snow avalanche characteristics

A

saltation’, entrainment of air and snow, flow, turbulence,
▪ gaining velocity, ‘powder’, mixing with air – buoyant clouds
▪ fanning out from point release
▪ Speeds up to 300 km/h

56
Q

Snow slab avalanches

A

Release of a cohesive slab over an extended plane of weakness.

Analogous to planar
failure of rock slopes.

57
Q

AVALANCHE FACTORS

A

Temperature
▪ Slope gradient
▪ Aspect
▪ Wind direction
▪ Terrain
▪ Vegetation
▪ Snowpack conditions

58
Q

Avalanche triggers

A

Artificial triggering
Localized rapid near-surface loading by people or
explosives

▪ Gradual uniform loading duesnowfall

▪ No-loading situation that changes
snowpack properties,
surface warming (natural
triggering or spontaneous
release).

59
Q

Snow slope stability equation

A

S = τf (σ, x, t)/ τ (x, t)

60
Q

S

A

snow stability

61
Q

t

A

time

62
Q

x

A

slope in question

63
Q

τf

A

snow strength

64
Q

σ

A

normal stress

65
Q

τ

A

shear stress

66
Q

Theoretically…

A

unstable conditions will occur when the stability index S approaches 1.

Strength and load vary spatially and temporally within the snowpack
–> this means that the application
of this critical stress concept for snow slope failure is not straightforward, and snow stability
depends on scale.

Schweizer et al. (2003) – Reviews of Geophysics

67
Q

Landslide hazards

A

Impact
Burial
Structural failure
Secondary /multiple hazards

68
Q

Secondary /multiple hazards

A

transformations
displacement of water
failiure of landslide dams

69
Q

What is the Lodalen Norway event

A

1905
Ramnefjell
Rockforll of 50,000m3 from 500m
Wave of 40 m above lake
61 deaths

70
Q

What happned in 1936

A

Rockfall 1 M m3 from 800m
Wave 74 m high
74 deaths
conclusion = potential for repeat events

71
Q

Main aspects of avalanche hazard mitigation

A

Awareness
Avoidance
Event modification ( engineering structural development )
Vulnerability modification

72
Q

Awareness

A

Forecasting
Hazard Maps
Landslide prediction

73
Q

Avoidance

A

non-structural methods ( land use restrictions temporary evacuation and artifical triggering

74
Q

Event modification

A

Structural measures to divert and retard and starting zone structures design to prevent avalanche initiation or forest management

75
Q

Slope stabalisation

A

slope grading
support ( concrete , walls,vegetation)
PRotection ( nets shelters)
Drainage

76
Q

Debris flows mitigation

A

barriers to divert
Regualr dam and basin checks

Engineered channels

Planning

77
Q

Vulnerability

A

Planning ( non-conflicting use)
Forecasting ( monitoring and warning
education)

133 Natural Hazards and Society (10 decks)

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