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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
What are major triggers of landslides
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
25
Explain how heavy rain and volcanic eruptions can cause landslides
``` 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 ```
26
How many landslides occur in triggered events?
After trigger it might be zero or thousands
27
How long does it take for a landslide to occur after a trigger?
It ranges from minutes to weeks
28
How big an area can a triggered landslide cover?
From m2 to km2
29
Give some examples of landslides and their triggers
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
Landslide inventory Where are landslides likely to occur in the future?
They're likely to occur in the same place
31
Describe some features of hazard assessment of mass wasting
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
What are the hazard maps
Slope map Geological map Landslide map Landslip hazard map
33
What can hazard maps be used for?
Planners can use hazard maps to make decisions | In hindsight hazardous conditions were obvious
34
What are the barriers o making hazard maps in all prone areas?
The priorities of the government (spend money elsewhere) Is data available Not enough expertise
35
Can you predict when a landslide will happen?
You can't do predictions for landslides only probabilities | You can get a short term prediction of location, time and magnitude
36
What are the issues with short term predictions?
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
What are landslide warnings
Areas that are not normally wet are wet New cracks and unusual bulges A sudden decrease in creek water levs
38
Prevention and mitigation of mass wasting slope assessment
``` 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
Volcanoes! | How many volcanoes are there with recorded historic eruptions?
700
40
What percentage of the world population live Worthing 100km historically active volcanoes?
9%
41
What % of fatalities have been caused by secondary effects (since 1600 AD) ?
40%
42
What is the estimated financial cost of volcanic eruptions in property damage to the USA?
$16,000,000,000
43
What is the social cost of an eruption?
Stress - evaluation Disruption of communities Psychological suffering
44
What factors influence the degree of a hazard?
``` Distance from volcano eruption products travel Velocity of eruption products temperature of eruption products Length of warning - precursory activity Frequency of occurrence ```
45
What factors influence social effects?
``` Nature - degree Extent of resultant death, injury Reactions of community leaders to hazard and aftermath Assistance from overseas Perception ```
46
What are volcanic 'pulses'?
``` 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
Where do volcanoes occur?
Divergent (pull apart) (extensional) plate boundaries Convergent (compressional) (pushes down beneath one another) Intra plate locations Away from boundaries e.g. Hawaii
48
What is a volcano?
A place where molten rock (magma) rises from the interior and issues on to surface as lava
49
What is the difference between lava and magma?
Magma is underground and lava is on land
50
Describe a divergent zone
Convection currents swirl away from each other Plates are moving apart (rifting) New lithosphere is being generated Mostly submarine
51
Describe convergent zones
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
Describe intraplate zones
Plate moves over a hot spot and the magma pushes through Plate moves onwards and keeps going over the hotspot Eg Hawaii
53
What are the different types of magma?
``` 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
What type of magma does a divergent zone have?
Basaltic magma Relatively low silica content (<45-55%) high eruption temperature (1050-1200C) and low bolo time content (gases) Low viscosity
55
What type of magma does a convergent zone have?
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
If the magma is lower viscosity what does this mean?
That it is quicker and more fluid
57
What is the VEI?
Volcanic explosivity index Measures how high the plume goes Volume of ejecta Frequency
58
What can volcanoes cause
Volcanic phenomena! Gases can cause acid rain Pyroclastic flow
59
What is tephra?
The general term for fragments of volcanic rock and lava - blasted into the air by explosions - carried up by hot gases
60
What is tephra fall?
The weight of ash exceeds the strength of roof material
61
If a jet plane passes through a volcanic ash cloud (volcanic tephra) what happens?
Windshield damage Ash accumulation Abrasion of ran blades Remedying and deposition of ash in engines
62
What percentage of volcano related deaths were volcanic gases responsible for in the 20th century?
3%
63
What are volcanic gases?
``` CO2 - unconsciousness and asphyxiation SO2 H2S HCI HF H2SO4 ```
64
Describe the features of a lava flow
Little threat to life Slow moving (10km per hour) Deaths are rare Damming (not good) threat
65
Describe the features of Pyroclastic flow
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
Describe the features of a Lahar
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
What is probabilistic forecasting?
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
Give an example of an eruption
Mt pinatubo eruptions - june 1991 Major landscape changes Build houses and grain stores on stilts to avoid getting buried by next lahars
69
What is the difference between risk assessment and risk perception?
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
What is a tsunami?
Any sudden, non-meteorologically-induced impulse in the water regardless of the size (Nuclear, volcanoes, earthquakes, landslides, asteroids)
71
What are the three main aspects of a tsunami?
Generation Propagation Land
72
Generation | What are the potential mechanisms for generating a tsunami?
``` Earthquake Landslide Volcanic eruption Meteorite impact Nuclear explosion ```

Geography (7 decks)

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