Geohazards Flashcards
What is a landslide (mass wasting)?
What is a landslide (mass wasting)?
It is a downslope movement pf Rock and regality (rocks and soil) due to gravity.
Why are landslides an important part of the erosional process?
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
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?
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
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?
Ones that occur very quickly and include a lot of fluid
What are the social and financial cost of landslides in the USA?
Social: 25-50 deaths
Financial: $1.5 thousand million
What are the death tolls and financial costs like comparatively in developed and developing countries?
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
What are some of the reasons for a higher death toll in developing countries?
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.
Types of mass wasting processes
What is slope failure?
Sudden failure of slope - transport of debris downhill by sliding, rolling, falling or slumping.
Types of mass wasting processes
What is sediment flow?
Debris flows downhill mixed with air or water.
Slope failures
Describe a slump
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.
Slope failures
Describe falls
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
Slope failures
Describe slides
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.
Sediment flows
Describe sediment flows and the two types
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.
Slurry flows
Describe solifluction
Slow moving
Moves cm per year
Distinctive lines on hill slopes
Occurs in areas where soil remains saturated for a long time
Slurry flows
Describe debris flows
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
Slurry flows
Describe mudflows
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!
Granular flows
Describe creep
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
Granular flows
Describe Earth flows
They are usually associated with heavy rain, travelling several cm a year to 100s of m per day.
Usually active for a long time.
Granular flows
Grain flows
Relatively dry material, steep slope (e.g. Sand dune)
Small disturbance can lead to rapid movement.
Granular flows
Describe Debris avalanches
High velocity
Large volume
Travel far
Often triggered by earthquakes and volcanic eruptions
What factors influence a slopes stability?
Gravity
Water
Describe how gravity influences a slopes stability.
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.
What is the safety factor and how do you calculate it?
Safety factor, Fs, is Shear strength/shear stress Fs < 1.0 actively unstable 1< Fs < 1.3 conditionally unstable Fs > 1.3 stable
Describe how water influences a slopes stability
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
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
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
How many landslides occur in triggered events?
After trigger it might be zero or thousands
How long does it take for a landslide to occur after a trigger?
It ranges from minutes to weeks
How big an area can a triggered landslide cover?
From m2 to km2
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.
Landslide inventory
Where are landslides likely to occur in the future?
They’re likely to occur in the same place
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
What are the hazard maps
Slope map
Geological map
Landslide map
Landslip hazard map
What can hazard maps be used for?
Planners can use hazard maps to make decisions
In hindsight hazardous conditions were obvious
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
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
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
What are landslide warnings
Areas that are not normally wet are wet
New cracks and unusual bulges
A sudden decrease in creek water levs
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
Volcanoes!
How many volcanoes are there with recorded historic eruptions?
700
What percentage of the world population live Worthing 100km historically active volcanoes?
9%
What % of fatalities have been caused by secondary effects (since 1600 AD) ?
40%
What is the estimated financial cost of volcanic eruptions in property damage to the USA?
$16,000,000,000
What is the social cost of an eruption?
Stress - evaluation
Disruption of communities
Psychological suffering
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
What factors influence social effects?
Nature - degree Extent of resultant death, injury Reactions of community leaders to hazard and aftermath Assistance from overseas Perception
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
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
What is a volcano?
A place where molten rock (magma) rises from the interior and issues on to surface as lava
What is the difference between lava and magma?
Magma is underground and lava is on land
Describe a divergent zone
Convection currents swirl away from each other
Plates are moving apart (rifting)
New lithosphere is being generated
Mostly submarine
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
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
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)
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
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
If the magma is lower viscosity what does this mean?
That it is quicker and more fluid
What is the VEI?
Volcanic explosivity index
Measures how high the plume goes
Volume of ejecta
Frequency
What can volcanoes cause
Volcanic phenomena!
Gases can cause acid rain
Pyroclastic flow
What is tephra?
The general term for fragments of volcanic rock and lava
- blasted into the air by explosions
- carried up by hot gases
What is tephra fall?
The weight of ash exceeds the strength of roof material
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
What percentage of volcano related deaths were volcanic gases responsible for in the 20th century?
3%
What are volcanic gases?
CO2 - unconsciousness and asphyxiation SO2 H2S HCI HF H2SO4
Describe the features of a lava flow
Little threat to life
Slow moving (10km per hour)
Deaths are rare
Damming (not good) threat
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
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
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
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
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
What is a tsunami?
Any sudden, non-meteorologically-induced impulse in the water regardless of the size
(Nuclear, volcanoes, earthquakes, landslides, asteroids)
What are the three main aspects of a tsunami?
Generation
Propagation
Land
Generation
What are the potential mechanisms for generating a tsunami?
Earthquake Landslide Volcanic eruption Meteorite impact Nuclear explosion
