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133 Natural Hazards and Society > Slopes > Flashcards

Slopes Flashcards

(78 cards)

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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
Froude and Petley, 2018 land slide hotspots
Incresae in frequency most prevalent in South East Asia
26
Characteristics of landslides
Volume - single boulder (km3) Velocity - cm/y 100s m/s Horizontal travel distance ( small 2x fall
27
Types of movement
fall/topple slide spread flow
28
Type of material
rock soil snow/ice
29
Falls and topple
Detachment ( rotation) Common( rapid, variable volume) Disruptive
30
Slides
The movement on a shear surface Are rotational or translational
31
Topple in the UK
Oxwich Bay, 2009 and 2018
32
Rotational slides in the UK
Holbeck Hall, Scarborough , 1993 Transformed into mud flow
33
Translational slides in the UK
Burton Bradstock 2012 Borunemouth 2016
34
Failures in weak material - spreads and eartrh flows
liquefraction gentle slopes
35
Failures in weak material example
Alaska 1964
36
Sensitive quick clays
Marine clays Na+ binding with silicates - causing salt leaching sig loss of strength 30 times less resistant
37
Sensitive quick clay example
Rissa , Norway 1978
38
Loess weak material example
Derbyshire 2001
39
Type of flows
Earthflow Debris flows Rock/debris avalanches( coarse)
40
Earthflow
rapid slow( creep )
41
Earthflow example
Oso, March 2014 Washington,USA 43 deaths Rain Past activity , logging
42
Debris flow characteristics
Mixture of water and sediment -rapid flow channelised surging flow denser- transporting larger particles than stream flows
43
How does debris flow
Particles are suppored by = -buoyancy ( stream flow, denser fuild -grain grain collisions matric strength -with intense rainfall or rapid snowmelt on steep slopes
44
Lahars
Associated with volcanic activity or re-working volcanic debris (tephra)
45
What happened in Venezuela Dec 1999
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
Rock and debris Avanlanches
Km's movement in mins Volume to 10s km3 Long runout 10x fall Recurrence ( twice per decade ) Sturzstoms
47
Rock and debris Avanlanches
Km's movement in mins Volune to 10s km3 Long runout 10x fall Recurrence ( twice per decade ) Sturzstoms
48
Hope Slide, BC
48x106 m3 Jan 1965 4 killed 1100 m relief 70-80 deposists potential trigger- earthquakes?
48
Hope Slide, BC
48x106 m3 Jan 1965 4 killed 1100 m relief
49
What happened in the Philippines feb 2006
Guinsaugon rockslide - debris avalanche 15-20 x 10^6 100-140km/h 1112 killed
50
Cause of the 2006 event
steep slopes weak rocks along the fault zone deep troipical weathering Orientation of rock structures
51
triggers of the 2006 event
Rainfall ( 500 mm in 3 days Earthquake M 2.6 Precursors - river dried up ( cracks in ground )
52
Snow avalanches
masses of snow that rapidly decend steep slopes contains soil, vetegation and ice
53
Two types of snow avalanches
Loose snow avalanches Slab avalanched
54
Loose snow avalanches
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
Loose snow avalanche characteristics
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
Snow slab avalanches
Release of a cohesive slab over an extended plane of weakness. Analogous to planar failure of rock slopes.
57
AVALANCHE FACTORS
Temperature ▪ Slope gradient ▪ Aspect ▪ Wind direction ▪ Terrain ▪ Vegetation ▪ Snowpack conditions
58
Avalanche triggers
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
Snow slope stability equation
S = τf (σ, x, t)/ τ (x, t)
60
S
snow stability
61
t
time
62
x
slope in question
63
τf
snow strength
64
σ
normal stress
65
τ
shear stress
66
Theoretically...
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
Landslide hazards
Impact Burial Structural failure Secondary /multiple hazards
68
Secondary /multiple hazards
transformations displacement of water failiure of landslide dams
69
What is the Lodalen Norway event
1905 Ramnefjell Rockforll of 50,000m3 from 500m Wave of 40 m above lake 61 deaths
70
What happned in 1936
Rockfall 1 M m3 from 800m Wave 74 m high 74 deaths conclusion = potential for repeat events
71
Main aspects of avalanche hazard mitigation
Awareness Avoidance Event modification ( engineering structural development ) Vulnerability modification
72
Awareness
Forecasting Hazard Maps Landslide prediction
73
Avoidance
non-structural methods ( land use restrictions temporary evacuation and artifical triggering
74
Event modification
Structural measures to divert and retard and starting zone structures design to prevent avalanche initiation or forest management
75
Slope stabalisation
slope grading support ( concrete , walls,vegetation) PRotection ( nets shelters) Drainage
76
Debris flows mitigation
barriers to divert Regualr dam and basin checks Engineered channels Planning
77
Vulnerability
Planning ( non-conflicting use) Forecasting ( monitoring and warning education)

133 Natural Hazards and Society (10 decks)

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