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.