14 hydro landslides_forstudents

transcript

Interlude F & Chapter 16

Landscapes,Hydrology, and Mass Movements

Outline• Shaping the Earth surface

-uplift vs. erosion-describing the surface: relief, contours-factors controlling landscape development

• Hydrologic cycle

• Mass movements-Basics-Various types-Submarine mass movements

• Weakening the surface-Slope stability-Failure surfaces and triggers-Link to plate tectonics-Hazard and prevention

Shaping the Earth Surface

• Result of competition between uplift and erosion.• Adding or subtracting elevation.

• Driven by tectonics:• Uplift- vertical surface motion upwards• Subsidence-vertical motion downwards• Driven by surface processes:• Erosion- surface lowering by mass removal• Deposition- surface rising by mass addition

Shaping the Earth Surface

Describing the Earth Surface

• Relief• Elevation difference between 2 points.

Describing the Earth Surface

Contour maps:

Factors Controlling Landscape Development

Agents of transport & erosion• Water, ice wind

Elevation distribution• Relief, slope angle

Climate• Mean temp and precipitation amount

Life• Ecology/ biota can weaken/stabilize the surface

Exposed material type (substrate)• Erodibility

Hydrologic CycleWater plays major role in surface erosion & deposition.

Hydrologic cycle- describes never ending water flow between various reservoirs

Hydrologic Cycle

The reservoirs:Average time spent in each reservoir:

Extraterrestrial (water) flow!?!?Mars Other?

Enceladus (Saturn moon)

Intro to Mass Movements

• Most humans consider Earth to be• Earth’s surface, however, is mostly

unstable • Due to weathering/erosion/gravity

What is a Mass Movement?• Downslope motion of earth materials by gravity • Type of natural hazard

• Natural landscape process

These hazards can produce catastrophic losses

Intro to Mass Movements• Mass movements are important to rock cycle• Initial step in sediment transportation• Significant landscape change agent• All slopes are unstable, they change continously • Mass movements are often

Types of Mass WastingClassified based on 4 factors:

1. Material type (rock, regolith (loose debris), snow, ice)

2. Rate of movement (fast, intermediate or slow)

3. Nature of moving mass (cloud, slurry, or distinct blocks)

slurry-liquid with suspended soilds

4. Surroundings (subaerial or submarine)

Types of Mass WastingCreep, solifluction, rock glaciers (not shown) slumpingFlows (mud) avalanches, falls (rock)

Types of Mass WastingCreep –slow downhill movement of regoith

Due to expansion and contraction

via wetting and drying, freezing and thawing

Grain moves:

perpendicular to slope upon expansion

Verically by gravity upon contraction

Types of Mass Wasting

Creep tilts trees, gravestones, and walls

Types of Mass WastingSolifluction –slow downhill movement of tundra

melted permafrost slowly flows over deeper, frozen soil

generates hillsides with solifluction lobes

Rock glaciers – mix of rocks fragments and ice

rocks added faster than ice accumulation

Behave like glacial ice, flowing slowly downhill

Types of Mass WastingSlumping – sliding of regolith as coherent blocks

Slip occurs along a spoon-shaped failure surface

Variety of sizes and rates of motion

Distinctive features

head scarp, bulging toe

Types of Mass WastingMud & debris flows – h20 rich mass movement

Mudflow- slurry of water and fine sediment

Debris flow- mudflow with large rocks

Types of Mass WastingLahar – a volanic mud or debris flow

volcanic ash from recent/ongoing eruption

water from heavy rain or melted glacial ice

Types of Mass WastingLandslides – movement down nonvertical slope

• Rock slide – consists of rock only• Debris slide – mostly reoglith

Movement down failure surface is quick

Debris can move 300 km/hr on cushion of air

Landslide Case Study• Vaiont Dam disaster – illustrates need to evaluate geology when

siting structures• Built in 1960 in a deep gorge in Italian Alps. • Limestone over shale dipped toward valley center. • On 10/9/63, 600 million tons of limestone fell into the reservoir. • A wave crested the dam, destroyed villages, and killed 2,600.

Avalanches – turbulent clouds of debris and air • Snow avalanche – over-steepened snow detaches• Debris avalanche – rock and dust fragment • Move up to 250 km/hr on a cushion of air

Types of Mass Wasting• Rock & debris falls – vertical freefall

• Bedrock/regolith that falls rapidly• Block impact, fragment and keep moving• Talus blocks pile up at slope base

Types of Mass WastingSubmarine mass movements. • 3 types – based on degree of disintegration

1. Submarine slumps – coherent blocks break and sip

2. Submarine debris flows – moving material breaks apart

3. Turbidity currents – sediement moves as a turbulent cloud

Usually well-perserved

Submarine mass movements. • Gigantic submarine slope failures are widespread

• Large than land-based failures • Important process sculpting adjacent land• Create catastrophic tsunamis

Weakening the Surface • Mass movements require earth materials

• Fragmentation & weathering.

Weakening the Surface• Slope stability is a trade-off between 2 forces:

1. Downslope force – gravitational pull

2. Resisting force – material properties that resist motion

• Movement occurs when downslope forces win

1. Downslope forces (Fd) = gravity

Weight of earth materials

2. Resisting forces (Fr) = material strength

3. Cohesion friction

Slope Stability

Slope Stability• Loose material assumes an “angle of repose”.• Maximum stable angle • Due to material properties

Particle size, shape, surface roughness• Typical repose angles:• Fine sand:35• Coarse sand: 40• Angular pebbles: 45

Failure Surfaces• Weak subsurface layers can initiate motion• Failure surface- types include

• Saturated sand/clay layers• Joints parallel to surface• Weak sedimentary bedding (shale)• Metamorphic foliation

Failure Triggers• Destabilizing event usually triggers slopes failure • Triggers are natural & anthropogenic:

• Shocks or vibration• Changes in slope characteristics • Changes in slope strength

Failure Triggers• A triggering event is not necessary for movement

• Slope materials slowly weaken over time • Gravity

• Result: movements often

Failure Triggers• Shocks & vibrations. • Vibrations decrease material friction • On unstable slope, downslope force takes over• Many sources of vibration are common:

• Motion of heavy machinery, vehicles

Failure Triggers

• Vibrations cause saturated sediments to liquefy• Quick clay – pore water slurries clay flakes when shaken• Saturated sand – fluidized by increase in pore pressure

Failure Triggers

• Changes in characteristics can destabilize a slope• Angle – steepening slope beyond angle of repose• Loading –add weight to top of slope

• Water – as rain or via humans

Failure Triggers• Changes in slope strength

• Weathering – creates weaker regolith• Vegetation – stabilizes slope. Removing it..

• Shows excess water removal• Destroys

Link to Plate Tectonics

• Tectonic processes influence mass movements • Create uplift – topo and relief leads to steep slopes• Fragment crust – joints and faults ease disintegration• Generate earthquakes – trigger

Identifying Slope Hazards • Geologic mapping can identify risk regions• Past failures • Currently unstable slopes:

• Cracked and bulging ground• Measureable changes in surveyed land features

GPS can detect slow movements

• Action can reduce mass movement hazards• Revegetation – adding plants has 2 positive effects

• Removes water • Roots

Prevention

Prevention• Action can reduce mass movements hazards

• Slowing or eliminating undercutting- increases stability • Removing erosion agent at slope base• Reducing effect of erosion agent

Prevention• Engineered structures.

• Rock staples – rods drilled into rock to hold steep face

Prevention

• Engineered structures.• Avalanche sheds – structures that shunt avalanche snow• Controlled blasting – surgical removal of dangerous rock

14 hydro landslides_forstudents

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