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454 lecture 4

Mass Movements and Hillslopes

Erosion (or lack of) results from balance between internalresistance of materials & magnitude of external forcesacting on them

Evolution of landscapes depends largely on regional slopedevelopment

Mechanics of slope erosion are related to processes of physical weathering – the forces disintegrating rocks alsolower the internal strength of the unconsolidated cover

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Resisting forces

Shear strength

1) overall frictional characteristic, expressed asangle of internal friction, Φ

a) plane friction:

b) interlocking friction:

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2) effective normal stress, δ’,

total normal stress: δ = δ’ + µ

pore pressure can increase or decrease δin unsaturated zone, water molecules attached to surface

particles by tension increase weight of soil (eg. wet sand)in saturated zone, water exerts hydrostatic pressure upward

& supports soil

3) cohesion, c,

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Properties of material change with increasing or decreasingmoisture:

water added to dry soil – voids fill – plastic behavior

more water decreases cohesion – all pores filled

liquid behavior

“Plastic” refers to the way the material responds to stress (force per unit area), in terms of strain (deformation) resultingfrom applied force

stress

strain

y B

y: yield stress (permanent deformation begins)

B: breaking strength (rupture occurs)plasticfailure

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Atterberg Limits:

liquid limit expressed as moisture contentsplastic limit (wt. of contained water/wt. of dry soil)

Range of water contents between two limits is plasticity index

Atterberg limits function of • types of clay minerals • size of particles • history of wetting and drying

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debris flow alongI-70 corridor nearGeorgetown, triggeredby rainfall

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Soil slips along Rt. 287 triggered by rainfall, 8/97

landslide aboveHorsetooth Reservoir,

8/97

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Factors influencing shear stress & resistance in slope materials

1) Factors increasing shear stress (promote failure)removal of lateral support

addition of mass

earthquakesregional tiltingremoval of underlying support

lateral pressure

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Huascaran, Peru (1973 Yungay slide)

Seismically triggered slides

Hebgen Lake landslide,Montana

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2) Factors decreasing shear strength (promote failure)weathering

pore water

structural changes

Gs =

Gs > 1 stable

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Three basic types of mass movements:slides:

flows:

heaves:

leads to seasonal or soil creepvery slow movement of material due to gravity when

cohesion & frictional resistance are spasmodically loweredfunctions in upper few feet of soilevidence includes stone lines, structures, treescaused by swelling & contracting due to wetting/drying or

freezing/thawing

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Slides

failure, crest of sand dune

arcuate soil slips along ridge crest,northern California

slumps on landslide toe, southern Poland

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forest fire & resulting debris flow,Huachuca Mountains, AZ

unburned swale

burned slopeupper channelscoured to bedrock

Flows

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lower reaches of channels& alluvial fan, Huachucas

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Buffalo Creek, Coloradofire, debris flows, floods

1996

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Culebres cut, Panama Canal

debris flow fan, Langtang, Nepal

debris flow, Khumbu, Nepal

debris flow, Idaho

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454 lecture 4failures on dune face

Rio Quijos, Ecuador

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Falls/flows

debris cone, Oi River,Japan

debris cone,Banff National Park, Canada

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Heave

tree response to soil creep, northern Montana

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Classification of Mass Movement Processes

slide heave

flow

wet

dry

fast slow

rockslide talus creepsoilcreep

landslide

river

mudflowearthflow

solifluction

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Vajont dam overtopping, Italy, 1963262 m high; 260 million m3 failure;2,000 casualties

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Mitigation of mass movement hazards

slope stabilization, Japandebris flow monitoring site, Japan

attempted landslideprevention, Seattle

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slope stabilization, Japan

Yoho National Park, Canada

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Individual Grain Movements

Sediment is moved on the surface of slopes by raindrop impact(splash) and by overland flow (wash) – flow is shallow & spreadevenly across slope as uniform sheet

Amount of soil moved by splash depends oni) kinetic energy of raindropsii) type & amount of soil exposed

iii) steepness of slopeparticles are dislodged, detached, dispersed

Sheet wash doesn’t last long

rills

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Amount of soil eroded & transported is balance:

driving resistingvs

erodibility –

Ie, index of erodibilityIe = shear resistance x permeability

Universal Soil Loss Equationerodibility slope length cropping

A = K R L S C Psoil loss rainfall steepness conservation

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Slope angles are not uniformly distributed, but tend to clusterin groups – probably represent stability regimes for slopesformed in particular climatic & lithologic settings

0

10

20

30

40

relative distribution of slope angles

slop

e an

gle

(in d

egre

es)

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Major controls on slope form and evolution are• time• lithology• climate• process

Two contrasting models of slope development focus on process and time

process model:

evolutionary model:

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Influence of lithology on slopes

• coherent, resistant rocks =• more massive bedding =• alternating weak & strong strata =

Resistance of a particular rock type varies with climate (eg.limestone), and resistance depends on whether overlying slopeis controlled by

a) processes of weathering

b) processes of removal

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Influence of climate on slopes1) Slopes in humid temperate regions tend to be

2) Slopes in semiarid/arid regions

convexstraight

concave

cliffdebrisslope plain

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stepped slope profiles, Grand Canyon, Arizona

454 lecture 4Canyonlands, Utah

spheroidal granite weathering & roundedslopes, Missouri

Rt. 125, Colorado

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Slope Development with Time

1) slope decline:

2) slope replacement:

3) parallel retreat:

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4

slope decline slope replacement parallel retreat