Mass Wasting and HillslopesMass Wasting and Hillslopes
Steep slopeGd > F
Gentle slopeGd< F
Gp
Gp
Gp
ModerateslopeGd= F
Boulderon vergeof moving
Boulder isstable
BouldermovesdownslopeF
F
F
Gd
Gd
Gd
W
W
W
• Gravity overcomes Friction
W = mgF0 = Gd
Sliding Threshold when Sliding Threshold when gravity componentgravity component = = friction componentfriction component, both parallel to slope, both parallel to slope
• Shear 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) is the same as the dip
F0 = mg sin(α)
αmg
h
dip
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) is the same as the dip
Shear ForceShear Force = F0 = w sin(α)
αw
h
dip
Aside: Bloom confuses shear force with shear stress.Stress = Force / unit areaStress units are, e.g. Newtons/m2
or pounds force/ inch2 aka psi
That said, we will skip the issue by staying with Forces
Role of water for slabs Role 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.
• Proportionality constant c
αmg
h
dip
N
Friction Coefficient c?Friction Coefficient c?
• Ff uphill = N x constant “c”
• Notice N = mg cos
When it slips, F0 = Ff = N x constant
Then
• F0 = mg sine mg cos x c
• so c = sine cos
ExampleExample
• Suppose the rock slips at = 30o
• sine 30o = 0.5
• Cosine 30o = 0.866
• c = Sine 30o /cosine 30o
• c = 0.5/0.866 = 0.577
αmg
h
dip
N
Water's role for slabs: Water's role for slabs:
Before FallBefore Fall
Water's role for slabs: Water's role for slabs:
After FallAfter Fall
Of course, in ourarea, winter freezingcauses frost wedging,breaks loose any remaining bonds
Classification of slope movementsClassification of slope movements
Slides
FallsSlumps
Flows
(note rotation)
Slow mass movement indicatorsSlow mass movement indicators
Example: Soil Slump
Scarp
Scarp
Scarp
CD
CD
Lobe
LobeDF
Soil CreepSoil Creep
Signs of Soil CreepSigns of Soil Creep
Vertical featuresexposed in new roadcut
Vertical features (if available) curved near surfaceVertical features (if available) curved near surface
Creep Typical FeaturesCreep Typical Features
““Drunken forest”Drunken forest”
SolifluctionSolifluction
Soil saturated with water, soggy mass flows downhill
When soil moisture cannot flow deeper, trapped in soil
Gelifluction: Freezing lifts particles, Gelifluction: Freezing lifts particles, thaw drops them further downhillthaw drops them further downhill
Gelifluction: ThawGelifluction: Thaw
Rapid Mass MovementRapid Mass Movement
• Flows: mixture moves downslope as a Flows: mixture moves downslope as a viscous fluidviscous fluid
• Slumps: move downslope along a concave Slumps: move downslope along a concave slip surfaceslip surface
• Slides: move downslope along preexisting Slides: move downslope along preexisting plane of weakness as a single, intact mass plane 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
Rapid Mass MovementRapid Mass Movement
FlowsFlows
• Flows with a high water content are less viscous, faster and more dangerous– Debris avalanches- rain- regolith detaches 200
kilometers per hour– Lahars– Liquefaction- Quick Sand due 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
Mixture moves downslope as a viscous fluidMixture moves downslope as a viscous fluid
Yungay AvalancheSource: Lloyd S. Cluff
Debris Avalanche
Town in PeruEarthquake dislodgedSlab ice => landslide25000 killed
May 31, 1970 Ancash Earthquake
LaharLahar
http://www.massey.ac.nz/~trauma/http://www.massey.ac.nz/~trauma/issues/2004-1/galley.htmissues/2004-1/galley.htm
Liquefaction - Quick Clay or SandLiquefaction - Quick Clay or Sand
Asphalt Parking Lot
Caused by EarthquakesCaused by EarthquakesSediment not compacted is like “pick-up-sticksSediment not compacted is like “pick-up-sticksSeismic waves increase fluid pressure, force grains Seismic waves increase fluid pressure, force grains apart, structures above resting on water, they sink in. apart, structures above resting on water, they sink in.
Mudflow in Sarno, Italy, 1998Mudflow in Sarno, Italy, 1998
Slumgullion Slumgullion EarthflowEarthflow
• San Juan Mtns, CO
• Volcanics• Dams
Lake Fork of the Gunnison
Earthflows dry masses of clayey regolith1-2 meters per hour
SlidesSlides
• Slumps: special case, weakness is curved
• Mudslides
• Rock Slides
• Avalanche and Debris Slides
www.pdc.org/.../2007/Rio-de-Janeiro/rio.htm
Slides: move downslope along preexisting plane of weakness as a single, intact mass Slides: move downslope along preexisting plane of weakness as a single, intact mass
Slumping Slumping with visible with visible Scarps in Scarps in Dorset, Dorset, EnglandEngland
These are These are rotationalrotational
SlumpSlump
Little Hat Mountain Slump, CALittle Hat Mountain Slump, CA
scarpscarp
Toe, Toe, no veg.no veg.
La Conchita La Conchita SlumpSlump
• Typical urban landslide, after heavy rains– Preexisting slide
masses– Development to the
edge of existing– Lawsuits– 9 houses destroyed– Property values down
slump scar
Snow Avalanche
Turtle Mountain Debris SlideTurtle Mountain Debris Slide
http://blogs.agu.org/mountainbeltway/2012/08/17/turtle-mountain-and-the-infamous-frank-slide/
East limb limestones at steep angle
Locals mining coal seamunder thrust fault
April 1903
Frost heave, Yosemite NP.Frost heave, Yosemite NP.Glacier Point climbing area.Glacier Point climbing area.1996 Rockfall1996 Rockfall162,000-ton granite slab.162,000-ton granite slab.160 mph speed.160 mph speed.Killed several people.Killed several people.
Falls: Falls: RockfallRockfall
Angle of ReposeAngle of Repose• For loose materials, the angle of repose
dictates the maximum steepness a material can be arranged before it will move downslope
• Bloom claims: p 189 lower right to 190 “The angle of the talus is a function of fragment size and angularity ….”
http://www.quanterra.org/guide/guide1_5.htm
Rockfall Talus Slope
An ExampleAn Example
• These talus cones illustrate the characteristic steep slopes. Talus, due to its large grain size, has a steep angle of repose.
Talus cones from Glacier National Park in Canada.
http://capone.mtsu.edu/cdharris/GEOL100/erosion/talus-creep.htm
Angle of Repose depends on particle size and shape?Angle of Repose depends on particle size and shape?
Is this right? Should we believe this? Do an experiment.What is your null hypothesis?
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.
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
Human TriggersHuman Triggers
• excessive irrigationexcessive irrigation
• clear-cutting of steep slopesclear-cutting of steep slopes
• slope oversteepening or overloadingslope oversteepening or overloading
• mining practices mining practices
can also cause mass movement.can also cause mass movement.