earth sci 2gg3 lecture notes

8/10/2019 Earth Sci 2GG3 Lecture Notes

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Earth Sci 2GG3 Lecture Five

Landslides & Avalanches:- Terrain Factors

o Slope Angle Avalanches tend to occur on slopes with angles between 25 and 60

degrees Most large avalanches occur between 30 and 45 degrees

o Slope Orientation Leeward slopes

o Other factors: Convex more dangerous than concave slopes Avalanches more common on smooth slopes Vegetation may anchor snow path Gullies or ravines may funnel avalanches, increasing their destructive

force- Geographic Regions at Risk of Avalanches

o British Columbia has the greatest avalanche risko Some along the East Coast (Appalachian Mountains)o West coast has more ski resorts, so we notice these ones a lot more ---- affects

recreational- Impacts of Avalanches

o Human deaths (600 in Canada since mid-1800s)o

Economic losses (destruction, blockage of roads & property damage) Not very relevant compared to other disasters- Links and Natural Service Function

o Link to Natural Hazards: Earthquakes can trigger avalanches Climate chance may increase snowfall --- increase probability of

avalanches occurringo Natural Service Functions

Increase local plan and animal diversity Provides open areas for wildlife

- Human Interaction and Minimizing Risko Increased human interaction with avalanches

Building developments are encroaching into areas prone to avalanches Winter leisure and recreation activates have increased in popularity (net

more people skiing, snowboarding, etc.)o How do we minimize risk?

Locating structures away from areas prone to avalanche


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Engineering structures to deflect snow Reinforcing exposed structures Triggering controlled avalanches with explosives Forecasting

o Minimizing Risk Location of Infrastructure

Risk is determined by determining avalanche frequency, risk anddistribution

Structures Fences, nets, barns and avalanche sheds used for protection

Triggering Explosive charges projected from cannons, fired by artillery, or

dropped from helicopters Forecasting

Based on four types of informationo Observed occurrences of avalancheso Stability and strength tests: shovel, compression and

rutschblock tests (dig a hole, walk seven steps away, thenwalk slowly step by step towards hole and count thestep when it gives away ---- if it gives on third step, thenrisk is 3)

o Snowpack observationso Weather

- 92% if 15 mins, 30% after 35 mins & almost 0% after 2 hours.- 20 people survive

Subsidence and Soil Expansion and Contraction:

Guatemala City, Guatemala- Sinkhole roughly 60 m wide- Three storey building fe ll in (and isnt visible)- Soil got saturated, cavities in the ground below, and this lead to an instantaneous

collapse- Karstic landscape ---- carbonate rocks (limestone) dissolved to form caves /

underground drainage systems

Introduction- Subsidence is a slow or rapid, nearly vertical downward movement of Earths surface


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- Karsto Landscape resulting from the dissolution of limestone, dolostone, marble,

gypsum or rock salt- Soil expansion and contraction results from:

o Changes in the water content of the soilo Freezing and thawing

- Subsidence is not usually life threating, but is one of the most widespread and costlynatural hazards

Karst- Common type of landscape associated with subsidence- Rocks are dissolved by surface water or groundwater- Dissolution produced voids which join to form caves and sinkholes (doline --- scientific

name)

-

A surface pockmarked with a large number of sinkholes is a karst plain

NEED TO KNOW: diagnostic features/landscapes of sinkholes/doline, rocks themselvesdissolved by water

Sinkholes:- Can range from one to several hundred meters in diameter- Two basic types:

o Solution sinkholes Pits formed by dissolution of buried bedrock along planes and fractures

Most common will form slowly over time Like a puddle depression in the ground gets filled in and erodes

what it can (and repeat)o Collapse sinkholes

Collapse of surface or near-surface rock or sediment Cavern/cave underground that gives away, and everything collapses

Cave Systems:- Cave systems are formed when dissolution produces a series of caves- Related to a fluctuating groundwater table- Groundwater seepage will deposit calcium carbonate on the sides, floor and ceiling of

the cave as flowstone, stalagmites and stalactites- Note: once a cave starts, gravity takes over as all, resulting in quicker erosion

Tower Karst, Disappearing Streams & Springs:- Tower karst is created in highly eroded karst regions

o Steep limestone pillars common in humid tropical regions


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- Disappearing streams are streams that flow from the surface into cave openings- Springs are natural discharge of groundwater at the surface

o Vulnerable to contamination

Permafrost- Soil or rock must remain cemented with ice for at least 2 years- More than half of Canada is underlain by permafrost- Continuous permafrost

o Mean annual temperature is less than -5C- Discontinuous permafrost

o Covers 50-90% of the landscape in an areao Mean annual temperature between -4C and -2C

- Sporadic permafrosto Covers less than 50% of the landscape in an areao

Mean annual temperature is between -2C and 0C- The active region thaws in spring and refreezes in fallo When permafrost thaws, it cans create land subsidenceo Extensive thawing creates uneven soil called thermokarst

- Frost-susceptible sediments expand when they freezeo Causes frost heaving

- CLIMATE CHANGEo Melts permafrost, releases methane, heats the planet, melts more permafrost,

releases more greenhouse gasses ---- positive feedback loop- City on Permafrost

o Issues with where to put sewageo Dont want to melt the ground or could face serious problems

Piping- Particles of silt and sand in the subsurface slowly carried by groundwater laterally to a

spring- Caused by groundwater creating tunnels as it percolates through lose sediments- Common in silt and sand sediments- Over time, shallow subterranean tunnels and cavities may develop to produce surface

depressions and ravines

Sediment Compaction- Fine sediments

o Sediment compacts when pore water is removedo Common on river deltaso Flooding replenishes sediment thwarting compaction

- Collapsible sediments


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o Loess (wind-blown sediments) and some stream deposits in arid regions areloosely bound or are water-soluble

o Infiltrating water weakens bonds, causing sediment to compact- Organic sediments

o Wetland soils may contain large amounts of organic matters and watero When water is drained or soil is decomposed, these soils compact

Expansive Soils- These soils expand during wet periods and shrink during dry periods- Common in clay, shale and clay-rick soil contain smectite- Can produce desiccation rocks- Tilting and cracking of blocks of concrete and wavy bumps in asphalt can cause

structural damage

Earthquakes and Deflation of Magma Chambers- Can lower the ground surface over large areas- Coastal subsidence can cause flooding- The outer coasts of Vancouver Island, Washington and Oregon have repeatedly been

lowered by earthquakes- Magma uplifts the volcano during an eruption- The chamber empties after an eruption, surface subsides

Regions at Risk- Landscapes underlain by soluble rocks, permafrost or easily compacted sediment

- Soils that contain abundant smectite clay are susceptible to shrinking and swelling soils- Soils containing sit are susceptible to frost heaving

Effects of Subsidence and Soil Expansion and Contraction- Sinkhole formation

o Can cause considerable damage, highways, homes & sewage lineso Triggered by fluctuations in river table

Water level lowers, cavern ceiling collapses = sinkhole- Groundwater Use and Contamination

o Caves provide direct connections between surface water and groundwatero Groundwater can be vulnerable to pollutiono The water table can significantly lower during droughts

- Permafrost Thawo Melting of permafrost has caused roads to cave in, airport runways to fracture,

railroad tracks to buckle and buildings to crack, tilt or collapse- Coastal Flooding and Loss of Wetlands

o Along the Mississippi Delta, this has contributed to the sinking of New Orleans


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o Wetlands that protect the city from surges are disappearing- Soil Volume Changes

o Swelling of expansive soils and frost heavingo Causes billions of dollars in damage annually in North America

Links to Other Natural Hazards- Subsidence can be an effect of earthquakes, volcanic activity and climate change

o Climate chance can add to the drying of soils and the altering of the groundwatertable

- Subsidence may cause floodingo Over-pumping of groundwater

- Water Supplyo Karst regions contain the worlds most abundant water supply

- Aesthetic and Scientific Resourceso

Caves and karst landscapes are scenic areas that attract tourists and provideresearch for scientists- Unique Ecosystems

o Some animal species can only live in caveso Caves also provide shelter for other animals

Human Interaction with Subsidence- Withdrawal of Fluids

o Pumping oil, natural gas or groundwater decreases fluid pressure, causing rocksand sediment to subsidence

- Underground Miningo Coal mine structures have collapsedo Water is used to dissolve and pump out salt, leaving behind cavitieso Flooding in salt mines can also cause sinkholes

- Permafrost Thawo Poorly insulated buildings directly on frozen groundo Burial of warm utility lines

- Restricting Deltaic Sedimentationo Construction of dams, levees or canals

- Draining Wetlandso Soil is drained for agriculture and settlemento Extraction of peat for horticulture

- Landscaping on Expansive Soilso Poor landscaping practiceso Adding or removing plant changes water levels, contributing to shrinking and

swelling soils


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Minimizing Subsidence Hazards- Restricting Fluid Withdrawal

o Preventing oil and groundwater extractiono Injection wells add water when oil is pumped

- Regulating Miningo Prohibit mining in settled areas

- Preventing Damage from Thawing Permafrosto New engineering for buildings and pipelines on permafrost

- Reducing damage from Deltaic Subsidenceo Removing or breaching levees could re-establish marshes

- Stopping the Draining of Wetlandso Proper water management of existing marshes and swamps

- Preventing Damage from Expansive Soilso Design of subsurface drains, rain gutters & foundationso

Construct buildings on a layer of compacted fill

Perception of and Adjustments to Subsidence Soil Hazards- Perception of Subsidence and Soil Hazards

o Few people are aware of the extent of these natural hazardso People who live in dramatically affected areas are more aware than others

- Adjustments to Subsidence and Soil Hazardso Geologic and soil mapping

Helps in predicting and avoiding areas when subsidence might occuro Surface features

Cracks, hummocky grounds & closed depressions are signs of subsidenceo Geophysical surveys

Knowledge of subsurface environment is essential

River Flooding

Introduction- Stream and rivers are part of the hydrologic cycle- Surface flow (runoff) finds its way to streams- Streams are tributaries of rivers- A region drained by a single stream is called a drainage, watershed, river basin or

catchment- The gradient of a river is determined by calculating its drop in elevation over distance

o Greatest in headwaters, decreases downstream and is lowest at the river mouth,which is its base level (base level = sedimentation and erosion rate are the same)

Water erodes only as low as the local water table level


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Hamilton --- base level is Lake Ontario (70 m above sea level)o The slope of a river is shown on a longitudinal profileo The valley of a river is steeper-sided and narrower in its headwaterso The floodplain is the flat surface adjacent to the channel

Earth Material Transported by Rivers- Rivers move a tremendous amount of material- Bed load

o Particles of sand and gravel that slide, roll and bounce (saltation) along thebottom of a channel in rapidly moving water

- Suspended loado Silt and clay particles that are carried in the water

Accounts for nearly 90% of the total load of most riverso Dissolved load

Ions that are carried in solution in the water

River Velocity, Discharge, Erosion and Deposition- Amount of erosion and deposition depends on stream area (A), velocity (V) and

discharge (Q)- Discharge is the volume of water flowing through a cross section of a river per unit time

(Q = V*A)- Changes in area lead to changes in velocity

o Narrow channel have higher velocity wide ones- When a river slows, it deposits sediment creating an alluvial on land or a delta in water

earth sci 2gg3 lecture notes

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