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GSC 1620 Chapter 8 Mass Movements (Wasting). Mass Wasting zMass Wasting – downhill movement of Earth materials under the influence of gravity zAnnually

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  • GSC 1620 Chapter 8Mass Movements (Wasting)

  • Mass WastingMass Wasting downhill movement of Earth materials under the influence of gravityAnnually in the U.S. average mass wasting property damages exceed 4 billion dollars and typically 50 or more people are killedLandslide is a colloquial term used by most people to describe all types of mass wasting (see slides)

  • Venezuela, 1999~30,000 deaths fromrain induced debris flows

  • Venezuela, 1999Note the largeboulders moved by the debris flow

  • Mass WastingThe adjacent photos show the effects of mass wasting in Laguna Beach, CA (6/1/05)The average price of homes in this community is 1.75 million dollars!

  • Mass WastingGravity force of attraction between two bodies; the magnitude of this force is dependent only on the masses of the bodies and their separation distance (see slide)The Earths gravity pulls objects toward its center

  • BADistanceGravity Force = Constant x (Mass A x Mass B) Distance2

  • BackgroundOn sloped surfaces, the force of gravity can be conceptually subdivided into the shear force (stress) and normal force (see slides)

  • GravityShear stressNormal forceShear strength (friction)If the shear stress actingon the slope exceeds theslopes normal force plusshear strength, whatshould happen to the slope materials? Mass waste or not?

  • GravityNormal forceShear stressShear strength (friction)Notice how the shearstress increases and thenormal force decreasesas the slope angle increases

  • Angle of ReposeAngle of repose maximum slope angle at which particles remain stable; for loose materials this angle typically varies from 25 40 degreesIf the slope materials angle of repose is exceeded by natural or human forces, portions of the slope should mass waste (see slides)

  • Solid rockVertical cliffGravel slopeSandClayey soilIn unconsolidated (loose) slope materials, larger and moreangular particles have a higher angle of repose

  • SlopesNatural slopes should be viewed as landforms that have attained equilibrium with their surrounding conditions; if that equilibrium is disturbed by natural or human forces the slope may mass waste to reestablish its equilibrium

  • Other Mass Wasting FactorsThe amount of water in the slope materials and the type and distribution of slope vegetation also affect slope stability (see slides)

  • Role of Water in Slope Stability

  • The more completelythe surface is coveredby vegetation and themore interlocking and denser the root system, the more stable a slope of unconsolidatedmaterials.

    The removal of vegetation during land development or fire sets the stage for mass wasting.

  • Mass Wasting TriggersMost mass wasting events are triggeredHuman and natural triggers existCommon human triggers:Slope oversteepeningVegetation removal Addition of excessive weight to slopeAddition of excessive water to slope (see slide)

  • Mass Wasting TriggersFour typical natural triggers:1) Prolonged and/or intense rainfall, or rapid snow melting, that saturates the slope materials

  • Mass Wasting Triggers2) Earthquakes violent ground shaking induces ground failure(5/17/2008)FOX News

  • Mass Wasting Triggers3) Volcanic eruptions remember lahars?

  • Mass Wasting Triggers4) Stream or wave undercutting water erosion removes support for the overlying slope materials

  • Mass Wasting Classification

  • Rate: potentially quite fast; perhaps100 miles per hourRate: generally slow; a few feet toperhaps tens of feet per day

  • Rate: potentially quite fast; perhaps100 miles per hourRate: generally slow; a few feet toperhaps tens of feet per day

  • Californian slumps and flows19952005 renewed mass wasting claims ten lives

  • Rate: quitefast; up toabout 125miles per hourWave undercutting of slope

  • RockfallBlocks

  • Waveundercutting

  • Sediment Fall

  • Many subcategories: debris, mud, earth, lahar, snow avalanche,creepChaotic, turbulentinternal motionRate: vast range; extremely slow (inchesper year) toperhaps 600miles per hourAll fast-moving, extensive travel distance(tens of miles) flows possess ahigh degree of lubrication fromwater and/or air

  • California Mudflow

  • In 1970, a verypowerful earthquakein Peru triggered a debrisavalanche that buriedmost of the towns ofYungay and Ranrachircaand killed about 20,000 people. Estimatedmaximum speed offlow: 600 miles perhour!

  • Effects of lahar andlahar-inducedstream floodingfrom 1980 eruptionof Mount St. Helens

  • SnowAvalancheWhy have deathsfrom snow avalanchesin North America been increasing?

  • Avalanche Video

  • Mass Movements: Five Main Types

  • Mass Wasting TypesMass wasting movement rates vary from exceedingly slow to exceedingly fastDont be fooled even the very slow mass wasting processes can threaten human development over long time periods (see slides)

  • Rate: maybe12 inchesper year

  • Submarine SlumpsMass Wasting Also Occurs Beneath Water

  • Why should care about submarine mass wasting?What are some of the possible effects?

  • Recognition of Mass Wasting HazardsThe steeper the slope the higher the riskThe less vegetated the slope the higher the riskKnowledge of the subsurface geology and local/regional geologic and climatic conditions helpfulGeologically recent mass wasting events are often recognized by slope scars sections of oversteepened slopes mostly devoid of vegetation (see slides)

  • 1925

  • Recognition of Mass Wasting Hazards1925, WyomingCNN.com; 5/26/08

  • Mass wasting slope scar

  • Slope scars

  • Human InterventionHumans have used various methods to attempt to reduce the risk of mass wasting including: slope retention structures; mass wasting diversion structures, slope drainage; slope reduction; and slope stitching (see slides)None of these methods is failsafe!

  • Concreteblock retentionstructure

  • Retention structuresmade of rock rubbleencased in wire mesh

  • Mass wasting diversion structure builtto divert snow avalanches, Canadian Rockies

  • (SlopeDrainage)

  • Drain Pipes

  • Slope Reduction

  • Figure 8.30ASlope Stitching

  • U.S. Geological Survey (USGS) Landslide Warning SystemConstructed in response to 1982 rain-triggered landslides that killed 25 in the San Francisco, CA area and caused $66 million damageThe USGS method is a mathematical prediction of slope stability based on: rainfall intensity and rainfall total, storm duration, slope angle and slope stability characteristics (e.g., soil/rock type, slope fluid content)

  • U.S. Geological Survey (USGS) Landslide Warning SystemThe mathematical relationships between the variables are determined from observations of previous landslidesFirst test of model: February, 1986; of ten landslides with known occurrence times, eight took place when forecast

  • Other WorkIn addition, ground tilt meter and GPS measurements can be used to assess precursor changes to slope stabilitySlope pore water pressures can also be measured remember, the higher the slope material water pore pressure the greater the risk of mass wasting (see slides)

  • Slopestabilitymonitoringsite

  • Woodway, WA

  • Other WorkAttempts to improve these methods continue see picture describing the USGS and US National Forest Service experimental flume and the use of smart rocksThen review U.S. landslide potential map

  • U.S. Landslide PotentialUnited States Geological Survey

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