Earthsc 2GG3 - Natural Disasters 2nd half

Mass Wasting

* Gravitational downslope transport of rock, regolith (unconsolidated material), snow, or ice
* Also called mass movement

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Factors Affecting Slope Shape

* Material strength
* Climate
* Process of Formation

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How does **material strength** affect slope shape?

* It is the ability to avoid failure
* Depends on if it is coherent, unconsolidated, or has planes of weakness

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How does **climate** affect slope shape?

* Affects which agents of erosion at work and how fast they are
* Ex; ice, water, wind
* Affects soil and vegetation formation on slopes

How does **process of formation** affect slope shape?

* Glacially carved valleys have a U-shape
* River eroded valleys have a V-shape

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Factors controlling downslope movement of materials

* Slope angle
* Critical angle of repose
* Cohesion and water saturation
* Material strength

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Stable Slope

* Occurs when shear stress is less than shear strength
* σd < σr
* Block stays put (doesn’t move)
* Typically occurs on flatter slopes

Unstable Slope

* Occurs when shear stress is greater than shear strength
* σd > σr
* Block moves (sliding takes place)
* Typically occurs on steeper slopes

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σn - Normal Stress

* Caused by the pull of gravity
* Gravity squeezes the block against the surface of the slope
* Acts perpendicular to the Earth’s surface

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σd - Shear Stress

* The specific component of gravity that pulls block down
* Acts parallel to slope

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σr - Shear strength / Resistance Stress

* Keeps the block from moving
* Depends on friction and cohesion
* Decreases as normal stress decreasing because as surfaces are squeezed together less tightly, friction decreases

Critical Angle of Repose

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* *Factor controlling downslope movement of materials*
* Maximum angle at which sediment particles can stand at without falling

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What does the Critical Angle of Repose depend on?

* Grain size
* Fine grained or solid rock, etc.
* Grain angularity
* Rounded grains < angular grains (can form steeper slopes)
* Moisture content
* Dry sediment typically less stable than damp sediment
* Oversaturated sediment is less stable

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Cohesion

* The attraction between soil particles that is provided by the surface tension of water between the particles

\*Friction prevents grains in dry soils from moving, surface tension by films of water in damp soils prevents movement, in very wet soils the grains are not in contact and basically acts as a fluid--resulting in slope failure

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Material Strength

* *Factor controlling downslope movement of materials*
* Loose, poorly cemented, dry material is weak
* Solid, well-cemented material is stronger
* Areas of weakness form between sedimentary layers & along well developed fractures

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Types of Slope Failures

* Creep + Solifluction
* Slumps
* Translational
* Rotational
* Flows
* Debris
* Mud
* Rockslides + Rockfalls
* Sturzstroms

Creep

* Extremely slow downslope flow of sediment on the surface
* Slowest type of slope failure
* Most loss is damage to buildings and infrastructure, rather than causalities (since it happens so slowly)
* Caused by cycles of freeze-thaw, wetting-drying, and/or warming-cooling
* When ground surface freezes it expands (moves up) then thaws in the summer (moves straight back down due to gravity), as a result there is slight displacement
* Top of sediment/soil moves more than deeper areas, and so this causes objects to rotate/tilt

Solifluction

* Downslope creep driven by sequential freezing and thawing in tundra regions
* Results in a soggy layer of ground above frozen permafrost that then moves downhill
* Similar to **creep**, but layer underneath is permanently frozen
* Top surface turns into mushy globs that move downhill

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Translational Slump

* A landslide that moves along a regular sloping planar surface
* Typically occurs where a block of material is underlain by a weak surface that is basically parallel to the slope
* Planes between sedimentary beds
* Old faults and fractures
* Debris or cohesive mud over underlying bedrock
* Material travels down slope as a cohesive unit
* Once it breaks up it is classified as something else

Rotational Slump

* A landslide in which the mass rotates on a concave failure surface

Flows

* Slope failures involving material that has lost internal cohesion
* Material that has broken part as loose material rolling downhill
* Speed depends on the steepness of the slope and the water content
* Mores saturation results in a faster flow

Mud Flow

* A flow of mud, rock, and water
* Dominated by clay-sized particles
* Includes **lahars**

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Debris Flow

* A slurry of rock, sand, and water flowing downslope
* Water usually makes up less than half of the flow volume

Would happened in Afghanistan, 2014?

* Heavy rain turned a slump into a mudflow, burring part of a village
* 2,000 deaths

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Rio De Janeiro (Brazil) & Slope Failure Risk

* Highly weathered Sugarloaf mountains surround the city
* Poor communities built on material where landslides are common
* 8 million poor Brazilians face mudflow risk
* Wealthy communities build homes on stable ground

Vargas Tragedy

* Several debris + mudflows over a 2 day period caused by heavy rainfall
* Town was built on debris from past flows
* The ground was very flat
* 30,000 deaths, 75,000 left displaced
* Highest deathtoll landslide

Quick Clay

* Water-saturated mud deposited in salty water
* Composed of clay flakes with large pore spaces between
* Highly unstable
* Also known as **Leda Clay**
* Found in the St. Lawrence, Saguenay, and Ottawa Valleys

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Lemieux Landslide (June 20, 1993)

* The headscarp retrogressed 680m into level ground above the river bank
* 2.8 million tons of clay and silt liquefied and flowed into the South Nation River valley, damming off the river

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Rockslide

* Sudden movement of rock and debris down a non-vertical slope
* Very little water

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Vaiont Slide

* October 9th, 1963
* Southern alps of Northeast Italy
* Longarone (specifically)
* Dam was built in order to get hydroelectric power
* Water table rose into the mountains, and wetted surfaces (which would normally be dry) and thus it become unstable
* Eventually a massive rock slide occurred, displacing the water over the dam downstream, while upstream a Tsunami formed
* 2,500 lives lost

Rockfalls

* A slope failure that occurs when a mass free-falls down a cliff or steep slope
* Slope is typically vertical

Frost Wedging

* Splitting of rock through pressure exerted when water freezes
* Freezing water expands by 9.2%
* Coastal areas which oscillate around the freezing point are prone to this

Sturzstroms

* Also known as *Rock Avalanches*
* Extremely rapid downslope movement of large volumes of rock and debris
* Largest and most destructive landslides
* Typically begin as rock slides, but breaks up, entraining air & water
* Travels velocities as high as 100-300km/hr
* Can travel distances up to 20 times their vertical fall
* May flow as a fluid composed of rock fragments suspended in air (**fluidization**)

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Frank, Alberta Disaster

* Started as a translational slide and then gained speed and developed into a sturzstorm
* Event lasted less than 100 seconds
* Slope failure occurred along fractured limestone planes
* Based on the speed and distance of the sturzstrom, it is thought to have traveled on a cushion of compressed air
* Completely buried the town, crossed a river, and even surged up 120m on another slope

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Hazard Mapping & Assessment

* GIS software can help identify areas of past and potential failures by mapping attributes that contribute to slope failures such as;
* Slope
* Bedrock type (lithology)
* Sediment texture + depth
* Vegetation
* Drainage
* Historic slope failures

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Ways to Reduce / Mitigate Risk of Mass Wasting

* Add vegetation
* Roots stabilize the ground
* Divert river (such as away from cliff) to have a new course that won’t produce mass wasting
* Retaining wall
* Blocks debris
* Lower reservoir of water, to a level below the surface which can’t be saturated because when it is saturated it poses a potential slope failure
* Drain pipes
* To help lower water table
* Metal screening
* Holds material to slope
* Debris basin
* Helps catch debris, so the debris doesn’t move into an area where it can harm others
* Rock bolts
* To bolt joints together
* Installation of detection devices, such as acoustic flow monitors
* However, only gives couple minutes of warning… not enough time to evacuate people downslope
* Prohibit building at the base of mountains and in dangerous areas along steep slopes
* Zone hazardous area as open space for parks, golf courses, and agriculture

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Subsidence

* Relatively slow downwards movement of land
* Typically at rates of cm/yr
* The setting of the ground

Collapse

* Rapid movement of land
* Ranging from cm/hr to m/s of material disappearing almost instantaneously
* A lot more dangerous than subsidence

Karst

* Topography resulting from dissolution of limestone rocks by water

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Sinkholes

* Found in Karst terrain
* A ground depression caused by collapse into an underground cavern

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Cavern

* Found in karst terrain
* A large natural underground cave or tunnel

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Karst Topography Formation

1. Cave form just below water table, due to dissolution


1. Water is most acidic right at top of water table, and thus the most dissolution occurs here
2. Water table continues to sink--old caves empty, allowing **speleothems** to grow
3. Caves collapse, creating sinkholes/depressions in terrain
4. New caves get bigger

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Speleothems

* Form as a result of mineral deposition in karst caves
* Includes; columns, stalactites, and stalagmites


* Ca(HCO3)2 = CaCO3 + CO2 (lost to air) + H2O (evaporates)
* Water percolates through fractures in limestone and then drips from stalactites to stalagmites

Columns

* Type of speleothem
* Formed when a stalactite and stalagmite meet each other

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Stalactite

* Type of speleothem
* Hangs from the top of cavern

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Stalagmite

* Type of speleothem
* Grows from bottom of cavern

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Collapse Sinkholes

* Formed when weak rock overlaying a cavern collapses
* Most dangerous kind of sinkhole

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Dissolution Sinkhole

* Depression formed from dissolution of rock at the ground surface
* Rain water dissolves surface… slow process!

Cover-subsidence Sinkhole

* Depression formed when loose sediment at the surface washes into caverns/voids below

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Climate Factors for Karst Formation

* Rainfall
* Water is the driving force behind dissolution
* Temperature
* Limestone & dolostone dissolve more readily in cold water
* Vegetation
* Plants can make water more acidic
* Rocks dissolve more readily in acidic conditions
* Soil Type
* Influences the permeability of land surface

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Guatemala, 2010 Disaster

* Sinkhole caused by floodwaters as a result of tropical storm Agatha
* 20m wide, 30 stories deep

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What can cause subsidence?

* Extraction of groundwater
* Can also cause irreversible permanent reduction in aquifer porosity, permeability, and storage
* Extraction of oil
* Drying of peat

How does the **extraction of groundwater** lead to subsidence?

* When groundwater is extracted, pore spaces collapse, and the ground subsides

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Groundwater Pumping in Santa Clara, California

* Too much groundwater was extracted during the early days, and as a result the land subsided by many meters
* Reductions in pumping and increases in recharge has now raised water levels and so subsidence has nearly stopped

What was subsidence in Sacramento-San Joaquin Valley caused by?

* Groundwater pumping for crop irrigation
* Decomposition of organic matter in the sediments from agricultural activities

\*Some parts of this region settled 8 meters in just 50 years

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Subsidence in Mexico City

* Caused by the groundwater extraction to support their large population
* Has caused buildings to warp (since some areas are subsiding faster than others)

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Land Subsidence in Mekong Delta (Vietnam)

* Caused by the increase in groundwater use
* Delta will become more vulnerable to flooding
* Salinization of the surface and groundwater
* Puts pressure on agriculture

Permafrost

* Ground that remains frozen (below 0 deg) for at least 2 consecutive years

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Interstitial Ice

* Ice that crystallizes in pores between grains of sediment
* Helps hold sediment together

Segregated Ice

* Lenses of pure ice developed in permafrost sediment

Ice Wedges

* Wedge-shaped accumulation of ice in permafrost regions
* Formed from freeze-thaw cycles

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Permafrost Melting

* Melting of interstitial and segregated ice
* Causes land subsidence, slumping, and mudflows

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Oil Pipeline in Northern Alberta

* Constructed across discontinuous permafrost
* To keep oil flowing, the pipeline was heated and this heat melted the underling permafrost resulting in a lot of water

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Thermokarst

* Karst-like landscape in permafrost terrain
* Caused by the melting of permafrost
* Has nothing to do with limestone

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What causes the formation of **Thermokarsts?** (Besides permafrost melting)

* Warming climate
* Erosion along coastlines + rivers, where ice-rich sediment is exposed
* Heating of the ground surface
* Vegetation removal

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Thermokarst Lakes

* Circular, oval or square lakes formed by thermokarst melting

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Tuktoyaktuk, Northwest Territories

* Community that is the most northerly point in Canada accessible by road
* Highest point is only 25m above sea level
* Has extensive flooding and land loss caused by;
* Rising sea levels
* Coastal erosion (1m/yr)
* Thermokarst activity
* Nearby is **Pelly Island**
* Fastest eroding island in the world

How can houses stop sinking as a result of permafrost melting?

* By being belt on stilts, buildings can isolate their heat from the underlying permafrost
* This is what they do in northern communities, such as those found in Nunavut

Most common natural disaster

* Floods

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Weather systems that cause flooding

* Tropical cyclones
* Monsoon season
* Severe isolated thunderstorms
* Atmospheric rivers

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Flash flood

* A flood which occurs when a large amount of precipitation falls in a very short period of time (within 6 hours)
* Commonly occurs because the ground cannot absorb water in time
* **High intensity & short duration**

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Slow-Onset Flood

* Occurs when a large amount of precipitation falls steadily over a long time period and takes weeks to months to subside
* Can be seasonal or regional floods
* **Low intensity & long duration**

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Formation of a stream

* Overland flow eventually forms a channel and water from adjacent land feeds into it. This channelized water is now able to drain faster compared to overland flow

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Inputs & Outputs to Streams

* Melted snow adds water
* Swamps and puddles collect water on flat land; water drains into the stream
* Some water infiltrates the substrate and becomes groundwater (which flows underground)
* Sheetwash flows over land into the stream
* Rain / snow falls directly into the stream
* Some water entering the stream flows through soil first
* Groundwater enters the stream via springs

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Watershed

* The land area providing water to a specific drainage network

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Drainage Network

* An array of streams that drain into the same, larger (trunk) stream

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What is unique about the Amazon river watershed

* It is the largest in the world

What is unique about water in the **Great Basin?**

* It does not drain to the sea

Continental Divide

* Divides different watersheds

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Stream Gradient

* The slope of the river channel
* Typically decreases downstream until it reaches the **base level / mouth**

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Base Level

* An elevation that a stream cannot erode past
* Controlled by level of the body of water which the stream discharges into

Bankfull

* When the water level in a river is equal to the height of the banks
* Flooding occurs when water rises above this level
* Most rivers do not flow every year, and so they do not reach this stage often

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What can the bankfull width and depth tell us?

* The bankfull discharge and the capacity of the channel

Floodplain

* Broad, flat area next to a stream that becomes either partly or completely covered during a flood


* It is risky to build on a floodplain!

Types of loads streams can carry

* Dissolved
* Suspended
* Bed

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Dissolved Load

* Dissolved species such as ions (calcium, potassium, etc.) that are carried by streams

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Suspended Load

* Finer particles, such as clay, silt, and fine sand, carried in suspension

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Bedload

* Heavier sediment in a stream that is moved along the streambed/floor (rocks)

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Main difference between a fast and slow flowing stream

* A faster flowing stream can carry larger grain sizes then slower ones

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What does the volume of sediments carried by a stream depend on?

* Flow volume and velocity
* During a flood, flow volume & velocity increases, so more sediment is transported

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Natural Levees

* As a stream spills over its flood plain, it moves from a deep channel with high velocity, to a shallow broad floodplain with low velocity
* Biggest grain sized material fall out first right next to river channel, while finer material is carried further away from channel
* Overtime this forms a ‘wall’

Alluvial Fan

* Fan-shaped deposit of sand and gravel
* Occurs at the mouth end of a mountain canyon, where the stream gradient flattens at the main valley floor (steep to flat slope)

Delta

* Accumulation of sediment deposited by a river at its entrance into a basin

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Permanent Stream

* Stream which flow all year
* Below the water table
* Found in temperate environments

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Ephemeral Stream

* Stream which only flows for part of the year
* Above the water table
* There is nothing feeding this channel on a day to day basis
* Found in dry desert environments

Meandering River

* A river with a single channel and high to moderate sinuousity

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Cutbanks

* Form on the outside bend of meanders
* Form where stream velocity is the highest and water erodes into the bank

Point Bars

* Formed on the inside bend of meanders
* Form where stream velocity is the slowest on so deposition of sediment occurs

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Oxbow Lake

* Form as a result of a meander loop becoming too sinuous, that a neck cut off occurs, splitting the river into two
* A straight river channel is formed as well as an isolated oxbow lake

What is happening with the **Clark Fork River**?

* It gradually eroded the cutbank until it undercut houses that were originally built 10m back from the river

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