14 hydro landslides_forstudents
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Transcript of 14 hydro landslides_forstudents
Interlude F & Chapter 16
Landscapes,Hydrology, and Mass Movements
Landscapes,Hydrology, and Mass Movements
Interlude F & Chapter 16
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
Interlude F & Chapter 16
Interlude F & Chapter 16
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
Interlude F & Chapter 16
Shaping the Earth Surface
Interlude F & Chapter 16
Describing the Earth Surface
• Relief• Elevation difference between 2 points.
Interlude F & Chapter 16
Describing the Earth Surface
Contour maps:
Interlude F & Chapter 16
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
Time
Interlude F & Chapter 16
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
Interlude F & Chapter 16
Interlude F & Chapter 16
Hydrologic CycleWater plays major role in surface erosion & deposition.
Hydrologic cycle- describes never ending water flow between various reservoirs
Interlude F & Chapter 16
Hydrologic Cycle
The reservoirs:Average time spent in each reservoir:
Interlude F & Chapter 16
Extraterrestrial (water) flow!?!?Mars Other?
Enceladus (Saturn moon)
Interlude F & Chapter 16
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
Interlude F & Chapter 16
Interlude F & Chapter 16
Intro to Mass Movements
• Most humans consider Earth to be• Earth’s surface, however, is mostly
unstable • Due to weathering/erosion/gravity
Interlude F & Chapter 16
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
Interlude F & Chapter 16
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
Interlude F & Chapter 16
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)
Interlude F & Chapter 16
Types of Mass WastingCreep, solifluction, rock glaciers (not shown) slumpingFlows (mud) avalanches, falls (rock)
Interlude F & Chapter 16
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
Interlude F & Chapter 16
Types of Mass Wasting
Creep tilts trees, gravestones, and walls
Interlude F & Chapter 16
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
Interlude F & Chapter 16
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
Interlude F & Chapter 16
Types of Mass WastingMud & debris flows – h20 rich mass movement
Mudflow- slurry of water and fine sediment
Debris flow- mudflow with large rocks
Interlude F & Chapter 16
Types of Mass WastingLahar – a volanic mud or debris flow
volcanic ash from recent/ongoing eruption
water from heavy rain or melted glacial ice
Interlude F & Chapter 16
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
Interlude F & Chapter 16
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.
Interlude F & Chapter 16
Types of Mass Wasting
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
Interlude F & Chapter 16
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
Interlude F & Chapter 16
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
Interlude F & Chapter 16
Types of Mass Wasting
Submarine mass movements. • Gigantic submarine slope failures are widespread
• Large than land-based failures • Important process sculpting adjacent land• Create catastrophic tsunamis
Interlude F & Chapter 16
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
Interlude F & Chapter 16
Interlude F & Chapter 16
Weakening the Surface • Mass movements require earth materials
• Fragmentation & weathering.
Interlude F & Chapter 16
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
Interlude F & Chapter 16
1. Downslope forces (Fd) = gravity
Weight of earth materials
2. Resisting forces (Fr) = material strength
3. Cohesion friction
Slope Stability
Interlude F & Chapter 16
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
Interlude F & Chapter 16
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
Interlude F & Chapter 16
Failure Triggers• Destabilizing event usually triggers slopes failure • Triggers are natural & anthropogenic:
• Shocks or vibration• Changes in slope characteristics • Changes in slope strength
Interlude F & Chapter 16
Failure Triggers• A triggering event is not necessary for movement
• Slope materials slowly weaken over time • Gravity
• Result: movements often
Interlude F & Chapter 16
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
Interlude F & Chapter 16
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
Interlude F & Chapter 16
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
Interlude F & Chapter 16
Failure Triggers• Changes in slope strength
• Weathering – creates weaker regolith• Vegetation – stabilizes slope. Removing it..
• Shows excess water removal• Destroys
Interlude F & Chapter 16
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
Interlude F & Chapter 16
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
Interlude F & Chapter 16
• Action can reduce mass movement hazards• Revegetation – adding plants has 2 positive effects
• Removes water • Roots
Prevention
Interlude F & Chapter 16
Prevention• Action can reduce mass movements hazards
• Slowing or eliminating undercutting- increases stability • Removing erosion agent at slope base• Reducing effect of erosion agent
Interlude F & Chapter 16
Prevention• Engineered structures.
• Rock staples – rods drilled into rock to hold steep face
Interlude F & Chapter 16
Prevention
• Engineered structures.• Avalanche sheds – structures that shunt avalanche snow• Controlled blasting – surgical removal of dangerous rock