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Geohazards Flashcards

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0
Q

What is a landslide (mass wasting)?

A

What is a landslide (mass wasting)?

It is a downslope movement pf Rock and regality (rocks and soil) due to gravity.

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1
Q

Why are landslides an important part of the erosional process?

A

Why are landslides an important part of the erosional process?Because they:
Create landscapes
Move material from higher to lower
Streams and glaciers pick up material

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2
Q 
Why are landslides an important part of the erosional process?Because they:
Create landscapes
Move material from higher to lower
Streams and glaciers pick up material
How often do landslides occur?
A

Why are landslides an important part of the erosional process?Because they:
Create landscapes
Move material from higher to lower
Streams and glaciers pick up material
How often do landslides occur? All the time

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3
Q

Why are landslides an important part of the erosional process?Because they:
Create landscapes
Move material from higher to lower
Streams and glaciers pick up material
How often do landslides occur? All the timeWhich landslides cause the most fatalities?

A

Ones that occur very quickly and include a lot of fluid

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4
Q

What are the social and financial cost of landslides in the USA?

A

Social: 25-50 deaths
Financial: $1.5 thousand million

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5
Q

What are the death tolls and financial costs like comparatively in developed and developing countries?

A

In developed countries the death toll is lower and the financial cost is higher but in developing countries the death toll is higher and the financial costs are lower

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6
Q

What are some of the reasons for a higher death toll in developing countries?

A 
Higher population density
Lack of zoning laws
Lack of information
Lack of emergency preparation 
Developed countries have knowledge on geology and landslide processes which leads to better planning.
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7
Q

Types of mass wasting processes

What is slope failure?

A

Sudden failure of slope - transport of debris downhill by sliding, rolling, falling or slumping.

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8
Q

Types of mass wasting processes

What is sediment flow?

A

Debris flows downhill mixed with air or water.

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9
Q

Slope failures

Describe a slump

A

Ruptured surface, downward rotation occurred s along a concave upward curved surface.
Each block is relatively undisturbed.
Arcuate scars/depressions.
Isolated or thousands of square metres
Often a result of humans.
Example: San Mateo County (CA) 1997 250,000 tons of rock and soil were moved.

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10
Q

Slope failures

Describe falls

A

There is a talus (accumulation of fallen rocks) slope at the bottom.
A piece of rock is dislodged and falls (single or many).
If it falls by a river it can block the river and the river floods upstream

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11
Q

Slope failures

Describe slides

A

Slabs move down the bedding planes, bed rock parallel to the hill.
Slides down an existing surface (bedding plane, foliation or joint surface).
Piles of talus are common
They differ from slumps as there is no rotation of rocks and regolith and no concave upward shape.

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12
Q

Sediment flows

Describe sediment flows and the two types

A

Occur when sufficient force is applied to rocks and regolith.
1. Slurry flows (20-40% water present)
2. Granular flows (0-20% water present)
They are subdivided further on the basis of velocity which the flow occurs.

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13
Q

Slurry flows

Describe solifluction

A

Slow moving
Moves cm per year
Distinctive lines on hill slopes
Occurs in areas where soil remains saturated for a long time

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14
Q

Slurry flows

Describe debris flows

A

Higher velocity (1m per year to 100m per hour)
Often caused by heavy rain
Can start with a slump and then flow downhill - forms lobes
VERY dangerous

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15
Q

Slurry flows

Describe mudflows

A

Highly fluid, high velocity, mix of water and sediment.
Velocity is more than 1km per hour
Occur after heavy rain where there’s an abundance of u consolidated sediment.
They can be triggered by earthquakes
They can be a result of a volcanic eruption (snow melts and the water mixes) - Lahar - hot!
They travel far and are VERY DANGEROUS!

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16
Q

Granular flows

Describe creep

A

They are very slow, usually a continuous movement of regolith downslope
They occur on almost all slopes (0.01 - 10mm per year)
Evidence - bent trees, offsets in roads and fences
The primary cause is freezing then thawing

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17
Q

Granular flows

Describe Earth flows

A

They are usually associated with heavy rain, travelling several cm a year to 100s of m per day.
Usually active for a long time.

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18
Q

Granular flows

Grain flows

A

Relatively dry material, steep slope (e.g. Sand dune)

Small disturbance can lead to rapid movement.

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19
Q

Granular flows

Describe Debris avalanches

A

High velocity
Large volume
Travel far
Often triggered by earthquakes and volcanic eruptions

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20
Q

What factors influence a slopes stability?

A

Gravity

Water

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21
Q

Describe how gravity influences a slopes stability.

A

Gravity acts downwards, friction acts upwards (see picture) gp acting perpendicular, holds object in place, gt, acting tangential (causes shear stress) pulls the object down.
On a steeper slope the shear stress increases and perpendicular decreases. If gt is greater than friction the object will move.
Shear strength forces resisting
If shear stress is greater than shear strength then there will be movement
If shear strength increases - steeper
If shear strength decreases - lower friction, lower cohesion among particles.

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22
Q

What is the safety factor and how do you calculate it?

A 
Safety factor, Fs, is 
Shear strength/shear stress
Fs < 1.0 actively unstable
1< Fs < 1.3 conditionally unstable 
Fs > 1.3 stable
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23
Q

Describe how water influences a slopes stability

A

If the sand is dry (unconsolidated grains) there is an angle of repose (steepest angle at which grains remain stable, controlled by friction)
Slightly wet sand creates a wall
Sand that is too wet has no angle of repose.

Fluid pressure. Friction is reduced and shear strength is reduced leading to slope failure.

Liquefaction. Loose sediment over saturated with water, grain to grain contact is lost.
Result from earthquakes , lots of water being added and snow infiltration

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24
Q

What are major triggers of landslides

A

Earthquakes
Heavy rainfall, sudden snowmelt.
Volcanic eruptions
Other:
Slope modification - human or natural, changed slope angle, mass wasting event can restore slope to angle of repose
Undercutting - river undercuts bank then falls
Fire - removal of vegetation, region is less fixed over time
Added mass - waste material, mining tailings, structures water leakages.
Minor shocks e.g. Heavy trucks, trees blowing, human-made explosions

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25
Q

Explain how heavy rain and volcanic eruptions can cause landslides

A 
Heavy rainfall: 
Changes in hydrological conditions
Reduces grain to grain contact 
Saturated rock and regolith (increasing weight) 
Volcanic eruptions: 
Can trigger earthquakes
Can melt snow or craters lakes empty
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26
Q

How many landslides occur in triggered events?

A

After trigger it might be zero or thousands

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27
Q

How long does it take for a landslide to occur after a trigger?

A

It ranges from minutes to weeks

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28
Q

How big an area can a triggered landslide cover?

A

From m2 to km2

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29
Q

Give some examples of landslides and their triggers

A

Umbria, Italy, rapid snowmelt, 1997 1st January, 2000 km2 area, 4233 landslides.
North ridge California, 17/01/1994, 10,000 sq km2 area, 11,111 landslides triggered by earthquake.

30
Q

Landslide inventory

Where are landslides likely to occur in the future?

A

They’re likely to occur in the same place

31
Q

Describe some features of hazard assessment of mass wasting

A

It can be extremely hazardous with extensive loss of life and property
Most areas prone to hazards can be recognised, slopes can be stabilised or avoided, warning systems can be put in place to minimise the hazard.
Slope map
Geological map
Landslide map
Landslip hazard map

32
Q

What are the hazard maps

A

Slope map
Geological map
Landslide map
Landslip hazard map

33
Q

What can hazard maps be used for?

A

Planners can use hazard maps to make decisions

In hindsight hazardous conditions were obvious

34
Q

What are the barriers o making hazard maps in all prone areas?

A

The priorities of the government (spend money elsewhere)
Is data available
Not enough expertise

35
Q

Can you predict when a landslide will happen?

A

You can’t do predictions for landslides only probabilities

You can get a short term prediction of location, time and magnitude

36
Q

What are the issues with short term predictions?

A

Earthquake triggered events are hard to predict
Slope destabilisation and undercutting triggered events require constant attention
Volcanic eruptions are easier to predict than earthquakes
Hydrological conditions - but with heavy precipitation even the forecast can’t tell which slope will be the most vulnerable

37
Q

What are landslide warnings

A

Areas that are not normally wet are wet
New cracks and unusual bulges
A sudden decrease in creek water levs

38
Q

Prevention and mitigation of mass wasting slope assessment

A 
All slopes are susceptible 
Engineering techniques 
- shortcrete
- debris chutes (to channel flows)
- rock fall sheds
- drainage pipes 
- over steepened slopes could be graded 
- rapidly lower levels of water in human-made reservoirs
- land use planning
- monitor high risk slopes
39
Q

Volcanoes!

How many volcanoes are there with recorded historic eruptions?

A

700

40
Q

What percentage of the world population live Worthing 100km historically active volcanoes?

A

9%

41
Q

What % of fatalities have been caused by secondary effects (since 1600 AD) ?

A

40%

42
Q

What is the estimated financial cost of volcanic eruptions in property damage to the USA?

A

$16,000,000,000

43
Q

What is the social cost of an eruption?

A

Stress - evaluation
Disruption of communities
Psychological suffering

44
Q

What factors influence the degree of a hazard?

A 
Distance from volcano eruption products travel
Velocity of eruption products
temperature of eruption products
Length of warning - precursory activity
Frequency of occurrence
45
Q

What factors influence social effects?

A 
Nature - degree
Extent of resultant death, injury
Reactions of community leaders to hazard and aftermath
Assistance from overseas
Perception
46
Q

What are volcanic ‘pulses’?

A 
Products provide 
-construction materials
- abrasive and cleaning agents 
- raw materials, chemical and industrial uses 
Soil
- fertile 
- material breaks down 
Geothermal 
- heat can beat groundwater (geothermal heat warms more than 70% of Iceland homes) 
- minerals
47
Q

Where do volcanoes occur?

A

Divergent (pull apart) (extensional) plate boundaries
Convergent (compressional) (pushes down beneath one another)
Intra plate locations
Away from boundaries e.g. Hawaii

48
Q

What is a volcano?

A

A place where molten rock (magma) rises from the interior and issues on to surface as lava

49
Q

What is the difference between lava and magma?

A

Magma is underground and lava is on land

50
Q

Describe a divergent zone

A

Convection currents swirl away from each other
Plates are moving apart (rifting)
New lithosphere is being generated
Mostly submarine

51
Q

Describe convergent zones

A

Ocean lithosphere subducts beneath land
Sediments wig H2O, CO2 dragged down
Impurities lower the melting temperature of the rock
Magma forms and rises Duce to lower density
Surface volcanoes form about 200km from subduction zone

52
Q

Describe intraplate zones

A

Plate moves over a hot spot and the magma pushes through
Plate moves onwards and keeps going over the hotspot
Eg Hawaii

53
Q

What are the different types of magma?

A 
Magma - molten rock beneath surface
Lava - magma erupted 
Magma a major elements are O, Si, Al, Fe, Ca, Mg, Na and minor chemicals
Magma consists of: 
- Liquid portion 
- Solid portion 
- Solid rocks 
- Dissolved gases (volatiles)
54
Q

What type of magma does a divergent zone have?

A

Basaltic magma
Relatively low silica content (<45-55%) high eruption temperature (1050-1200C) and low bolo time content (gases)
Low viscosity

55
Q

What type of magma does a convergent zone have?

A

Andesitic and rhyolitic magma
More silica (due to partial melting of crust)
Medium to low temperature (650-1000C) higher volatile content
Contains lots of dissolved gases
Very explosive, medium viscosity

56
Q

If the magma is lower viscosity what does this mean?

A

That it is quicker and more fluid

57
Q

What is the VEI?

A

Volcanic explosivity index
Measures how high the plume goes
Volume of ejecta
Frequency

58
Q

What can volcanoes cause

A

Volcanic phenomena!
Gases can cause acid rain
Pyroclastic flow

59
Q

What is tephra?

A

The general term for fragments of volcanic rock and lava

  • blasted into the air by explosions
  • carried up by hot gases
60
Q

What is tephra fall?

A

The weight of ash exceeds the strength of roof material

61
Q

If a jet plane passes through a volcanic ash cloud (volcanic tephra) what happens?

A

Windshield damage
Ash accumulation
Abrasion of ran blades
Remedying and deposition of ash in engines

62
Q

What percentage of volcano related deaths were volcanic gases responsible for in the 20th century?

A

3%

63
Q

What are volcanic gases?

A 
CO2 - unconsciousness and asphyxiation 
SO2
H2S
HCI
HF
H2SO4
64
Q

Describe the features of a lava flow

A

Little threat to life
Slow moving (10km per hour)
Deaths are rare
Damming (not good) threat

65
Q

Describe the features of Pyroclastic flow

A

High density mix of hot gases and ashes, cinders etc
>80km per hour
Contains rock fragments
High temperature
Most common DIRECT cause of death in eruptions during last 400 years

66
Q

Describe the features of a Lahar

A

Water and ash
Hyper concentrated volcanic mud flows
High density flows up to 75% solids
Very erosive, pick up material as they flow downhill

67
Q

What is probabilistic forecasting?

A

The probability of having an event of given size or bigger occurring in any given year (or hour or month)
For many natural hazards probability or frequency of occurrence is inversely proportional to its magnitude
Frequency occurrence is (see sign) (1/magnitude)
Smaller ones occur a lot more frequently than large ones

68
Q

Give an example of an eruption

A

Mt pinatubo eruptions - june 1991
Major landscape changes
Build houses and grain stores on stilts to avoid getting buried by next lahars

69
Q

What is the difference between risk assessment and risk perception?

A

Risk assessment - quantitative or qualitative valuation of significance of risk

Risk perception - subjective judgment that people make about the characteristics and severity of a risk

70
Q

What is a tsunami?

A

Any sudden, non-meteorologically-induced impulse in the water regardless of the size

(Nuclear, volcanoes, earthquakes, landslides, asteroids)

71
Q

What are the three main aspects of a tsunami?

A

Generation
Propagation
Land

72
Q

Generation

What are the potential mechanisms for generating a tsunami?

A 
Earthquake 
Landslide
Volcanic eruption
Meteorite impact 
Nuclear explosion

Geography (7 decks)

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