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Mass Wasting and HillslopesMass Wasting and Hillslopes
Steep slope
Gd>F
Gentle
slope
Gd< F
Gp
Gp
Gp
Moderate
slope
Gd=F
Boulder
on verge
of moving
Boulder is
stable
BouldermovesdownslopeF
F
F
Gd
Gd
Gd
W
W
W
Gravity overcomes Friction
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Sliding Threshold whenSliding Threshold when gravity componentgravity component ==
friction componentfriction component, both parallel to slope, both parallel to slopeShear Forces are parallel to 2 touching surfaces.
If the slab is about to move, then the
downhill force = resisting force pointing uphill
Downhill force = mass x gravity x sine of dip
FF00
= mg sin (dip)= mg sin (dip) (1)(1) E is the same as the dip
F0 = mg sin()
mg
h
dip
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Your book uses mg = weight "w"Your book uses mg = weight "w"
Downhill force = mass x gravity x sine of dip
FF00
= w sin (dip)= w sin (dip) (1)(1) E is the same as the dip
ShearForceShearForce =F0= w sin()
w
h
dip
Bloom confuses shear force with shear stress
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Stress has units Force/areaStress has units Force/areamass =V V because V has units mass/volume
so F0= Vg sin()
If calculated over a unit area, F0 / a "force per unit area"
F0 / a = s =V(V/a) g sin() but V/a has units lengthssdd'' = gh sin() (2)
when we use the height h as the characteristic length
Equation (2) is the shear stress
mg
h
dip
Notice Vgh is the potential energy per
unit volume
ssdd'' is its component along the slope
pulling downhill
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Role of water for slabsRole of water for slabs
Friction ForceFriction Force is proportional to Normal ForceNormal Force
It is the amount of Force needed to lift the
surfaces apart
Increased water pressure between the surfaces
lifts the upper slab, and it will slip at a lower dip
angle.
mg
h
dip
N
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Water's role for slabs:Water's role for slabs:
BeforeFall
BeforeFall
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Water's role for slabs:Water's role for slabs:
AfterFallAfterFall
Of course, in our
area, winter freezing
causes frost wedging
breaks loose any
remaining bonds
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Classification of slope movementsClassification of slope movements
Slides
FallsSlumps
Flows
(note rotation)
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Slow mass movement indicatorsSlow mass movement indicators
Example: Soil Slump
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Scarp
Scarp
Scarp
CD
CD
Lobe
LobeDF
Soil CreepSoil Creep
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Signs ofSoil CreepSigns ofSoil Creep
Vertical features
exposed in new roadcut
Vertical features (if available) curved near surfaceVertical features (if available) curved near surface
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Creep Typical FeaturesCreep Typical Features
Drunken forestDrunken forest
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SolifluctionSolifluction
Wet and Dry
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Gelifluction: FreezeGelifluction: Freeze
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Gelifluction: ThawGelifluction: Thaw
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Rapid Mass MovementRapid Mass Movement
Flows: mixture moves downslope as aFlows: mixture moves downslope as aviscous fluidviscous fluid
Slumps: move downslope along a concaveSlumps: move downslope along a concaveslip surfaceslip surface
Slides: move downslope along preexistingSlides: move downslope along preexisting
plane of weakness as a single, intact massplane of weakness as a single, intact mass Falls: rock drops from steep slopeFalls: rock drops from steep slope
http://geology.com/news/2008/spectacular-yosemite-rockfall-and-debris-avalanche-photos.shtml
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Rapid Mass MovementRapid Mass Movement
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FlowsFlows
Flows with a high water content are fasterand more dangerous
Debris avalanches- rain- regolith detaches
200 kilometers per hour Lahars
Liquifaction- Quick Clay earthquake -increased pore water pressure - grains
separate - liquefies instantaneously Mudflow swift slurry- heavy rains
Earthflows dry masses of clayey regolith 1-2 meters per hour
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Yungay AvalancheYungay AvalancheSource: Lloyd S. Cluff
Debris Avalanche
Town in Peru
Earthquake dislodgedSlab ice => landslide
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LaharLahar
http://www.massey.ac.nz/~trauma/ishttp://www.massey.ac.nz/~trauma/is
sues/2004sues/2004--1/galley.htm1/galley.htm
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LiquifactionLiquifaction -- Quick ClayQuick Clay
http://cce.oregonstate.edu/geotech/research.html
Caused by Earthquakes
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Mudflow in Sarno, Italy, 1998Mudflow in Sarno, Italy, 1998
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SlumgullionSlumgullion
EarthflowEarthflow San Juan
Mtns, CO
Volcanics
Dams
Lake Fork
of the
Gunnison
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SlidesSlides
Slumps
Mudslides
Rock Slides Avalanche and Debris Slides
www.pdc.org/.../2007/R
io-de-Janeiro/rio.htm
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SlumpingSlumpingand Scarpsand Scarps
in Dorset,in Dorset,
EnglandEngland
These areThese arerotationalrotational
SlumpSlump
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Little Hat Mountain Slump, CALittle Hat Mountain Slump, CA
scarpscarp
Toe,Toe,
no veg.no veg.
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La ConchitaLa Conchita
SlumpSlump
Typical urbanlandslide
Preexisting slide
masses Development to the
edge
Lawsuits
9 houses destroyed
Property valuesdown
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slump scar
Snow Avalanche
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Turtle Mountain Debris SlideTurtle Mountain Debris Slide
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Frost heave, Yosemite NP.Frost heave, Yosemite NP.Glacier Point climbing area.Glacier Point climbing area.
1996 Rockfall1996 Rockfall
162,000162,000--ton granite slab.ton granite slab.
160 mph speed.160 mph speed.Killed several people.Killed several people.
Falls:Falls:
RockfallRockfall
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Angle ofReposeAngle ofRepose
For loose materials, the angle of repose
dictates the maximum steepness a
material can be arranged before it will
move downslope
Bloom claims: coarser materials tend to
have steeper slopes
http://www.quanterra.org/guide/guide1_5.htm
Rockfall Talus Slope
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Slope StabilitySlope Stability
Slope characteristics such as composition,
vegetation, and water content also
influence slope stability.
Haiti is plagued by slides after many trees
were cut down.
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Natural TriggersNatural Triggers
Natural triggers such as:Natural triggers such as:
torrential rainstorms 1967 central Braziltorrential rainstorms 1967 central Brazil Earthquakes 1812 New Madrid, MissouriEarthquakes 1812 New Madrid, Missouri
volcanic eruptions 1980 Mount St. Helensvolcanic eruptions 1980 Mount St. Helens
produce damaging mass movementsproduce damaging mass movements
http://www.terradaily.com/reports/At_least_26_dead_in_China_mudslide_999.html
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