santa tecla, el salvador january, 2001 - centralia … · 1 mass wasting—landscapes in motion...

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Mass Wasting—Landscapes in MotionLandslides can be just

a nuisance…

…or costly (Slumgullion landslide, CO)…

Fig 15.1

Santa Tecla, El

Salvador

January, 2001

…or they can be

deadly (pics by Ed Harp, USGS)

Read more

at:http://landslides.usgs.gov/resear

ch/other/centralamerica.php

Landslide in volcanic

debris triggered by a 7.6

earthquake

1200 missing in Las Colinas

neighborhood of Santa Tecla

GuinsaigonGuinsaigon landslide, 2/17/06landslide, 2/17/06Leyte Is., Leyte Is., PhillipinesPhillipines

Trigger: rainfall,

saturation

characteristicscharacteristics of mass movements?of mass movements?

causescauses of mass movements?of mass movements?

factorsfactors determining areas, timing, conditions of determining areas, timing, conditions of slope stability?slope stability?

how to maphow to map massmass--movement hazards?movement hazards?

how do mass movements sculpt the how do mass movements sculpt the landscape?landscape?

Why study landslides?Why study landslides?

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Landslides are a Landslides are a surficialsurficial process that move process that move

rock and rock fragments downslope. Streams, rock and rock fragments downslope. Streams,

wind, and glaciers all shape surficial wind, and glaciers all shape surficial

landscapes.landscapes.

LandslidesLandslides——what are they? what are they?

& why study them?& why study them?

““LandslideLandslide”” is a generic term for is a generic term for gravitygravity--drivendrivendownslope movement of rock and regolith.downslope movement of rock and regolith.

Falling, flowing, sliding masses of rock and Falling, flowing, sliding masses of rock and regolith are found all across Earth.regolith are found all across Earth.

LandslidesLandslides……so what?so what?

Catastrophic landslides are infrequent, but Catastrophic landslides are infrequent, but sometimes with great losses.sometimes with great losses. Can infrequence Can infrequence lead to a false sense of security?? It seems lead to a false sense of security?? It seems important to better understand them.important to better understand them.

In the U.S. alone, landslides have an impact on In the U.S. alone, landslides have an impact on the order of 1 to 2 billion dollars annuallythe order of 1 to 2 billion dollars annually——not to not to mention injury and loss of life.mention injury and loss of life.

What are the characteristics of mass What are the characteristics of mass

movements?movements?

––Distinguishing the materials in motion:Distinguishing the materials in motion:

Rock, versus debris, versus earthRock, versus debris, versus earth

--RockRock is, of course, rock is, of course, rock

irrespective of sizeirrespective of size

--DebrisDebris is coarseis coarse--grained; 20grained; 20––80% 80%

being >2 mm in sizebeing >2 mm in size

--EarthEarth, 80% or more are <2 mm in size, 80% or more are <2 mm in size

––Types of motionTypes of motion

The criteria of classificationThe criteria of classification

Free fall Free fall –– just as it sounds, material falls through just as it sounds, material falls through

the air, or bounces, and rolls downslopethe air, or bounces, and rolls downslope

Slide Slide –– material (either rock or regolith) that material (either rock or regolith) that

slides along a rupture surfaceslides along a rupture surface

Flow Flow –– continuous movement of rock, regolith, or continuous movement of rock, regolith, or

both that behaves as a highboth that behaves as a high--viscosity liquidviscosity liquid

----Rates of motionRates of motion

Extremely Extremely slow (~1mm/year)… to very rapid (>100 km/hour)

Classification of mass movements

Name

(mechanism)

Sub-type Material Velocity (ft/sec)

FALL

N/A Rock fragments Extremely fast ~

32 ft/sec2

(fastest type of

mass movement)

SLIDE

(landslide)

Rotational (slump)

Translational slide

Unconsolidated sediments

Rock blocks

Slow (< 10-6)

Fast (10-4)

FLOW

Debris avalanche

Debris flow

Mud flow

Earth flow

Solifluction

Rock fragments

Coarse loose sediments

Loose fine sediments (mud)

Loose fine sediments

Soil

Very fast (1-10)

Moderately fast

Fast

Slow

Very slow

There are many table more or less like this. See fig. 15.2 in book

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CreepCreep

Very slow flow is called creep. At left is an example of rock creep, while at right

Fig 15.8we see evidence of earth creep in the

curved tree trunks.

Rock fallRock fall

Fig 15.3

A famous recent fall event. New Hampshire’s “Old Man

of the Mountain” succumbed to the forces of nature in

May 2003.

Falls are very Falls are very

fast but fast but

limited in limited in

scopescope

Often damage roadwaysOften damage roadways

Rarely kill people other than climbersRarely kill people other than climbers

Topanga boulder Topanga boulder picpic byby

Damian Damian DovarganesDovarganes

Associated Press Associated Press (Jan. 10, 2005)(Jan. 10, 2005)

A planar slide surface in California.

Fig 15.5

Material slides down the steeply inclined

bedding planes and into the ocean. This roadcloses frequently

due to rock slides.

Rupture surfaces

often follow a

preexisting

orientation that is

prone to weakness,

like the sedimentary

bedding planes

shown here.

Rupture surfacesRupture surfaces

–– Slides move along a rupture surfaceSlides move along a rupture surface

–– May be a planar surface like a bedding plane, or a May be a planar surface like a bedding plane, or a

rock joint or faultrock joint or fault

–– Can also be a curved surfaceCan also be a curved surface

Commonly on unconsolidated soil or regolithCommonly on unconsolidated soil or regolith

The mass rotates along the curved breakThe mass rotates along the curved break

Such a rotational slide is called a slumpSuch a rotational slide is called a slump

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ShallowShallow--translational landslidetranslational landslide——Orting WAOrting WA SlumpSlump--earthflowearthflowFig 15.5

An earth slump in NY.

Note in the inset below

the back-tilted trees

and pavement which

shows rotational

movement.

“backward rotational”

La Conchita Slump-

earthflow, Calif.

Failed hillside at Chehalis, WashingtonFailed hillside at Chehalis, Washington

Caused by 1996 storms Caused by 1996 storms (photo by Bob Schuster, USGS)(photo by Bob Schuster, USGS)

Fig. 8-3, p.189

the main features of a rotational landslide

Characteristic features:

Fig 15.6

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Earthflow in the Sutter ButtesEarthflow in the Sutter Buttes

Earthflow in the Sutter Buttes

http://web.umr.edu/~rogersda/professional_experience/aldercrest-

banyon_ls.htm

1999 1999 AldercrestAldercrest--BanyonBanyon

Landslide, Kelso, WA. Landslide, Kelso, WA. J. J. David RogersDavid Rogers

Debris avalancheDebris avalanche

Fig 15.9

Debris avalanche in Nicaragua. Debris avalanches are

mixtures of materials that flow rapidly downslope

though a debris

avalanche behaves

as a fluid mass, it

generally contains

very little water.

gravity

Mount St. Helens May 18, 1980, seconds after the Mount St. Helens May 18, 1980, seconds after the beginning of the eruption.beginning of the eruption. Photo by Gary Rosenquist Photo by Gary Rosenquist from Bear Meadow, ~11 km northeast of the volcano.from Bear Meadow, ~11 km northeast of the volcano.

Hummocky terrain of the Mount St. Helens 1980 Hummocky terrain of the Mount St. Helens 1980 debrisdebris--avalanche deposit as seen from the avalanche deposit as seen from the Hummocks Trail, about 9 km from the volcano. Hummocks Trail, about 9 km from the volcano.

UnzenUnzen Volcano, Japan, Volcano, Japan, showing path of 1991 showing path of 1991 pyroclastic flow and pyroclastic flow and surge that killed 39 surge that killed 39 including Maurice and including Maurice and KatiaKatia KrafftKrafft and and vulcanologist Harry vulcanologist Harry Glicken. Glicken.

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Madison Canyon, Montana, 1959

Fig 15.1

An earthquake-triggered landslide dammed lake.

Glacier Lake, WA, south of Packwood, circa AD 1500?

Red and maroon dots show landslides or landslide-dammed lakes.

landslide deposit

Spider Lake, SE Olympic Mountains

Day Lake

Mass Wasting—Landscapes in Motion

Columbia R

iver

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Parts of maps 79 (above) and 88 (right) of

Lewis and Clark (Moulton, 1983) showing

the area of the “Quicksand River” (Sandy

River) and nearby areas along the Columbia

River. These maps also show camps used

by the party between Oct. 28, 1805 and

April 6, 1806.

Troutdale WA Bridge of the Gods at Cascade Locks

“Landslip above the Cascades”, Paul Kane, 1847, Stark Museum of ArtWatercolor on paper

Ponding to 320 ft:

Backwater to Umatilla R. confluence

Volume: 20 billion cubic meters

Fill in 30-70 days (1996 and 2001)

Celilo Falls

Ponding to 240 ft:

Backwater to John Day R. confluence

Volume: 7 billion cubic meters

Fill in 10-25 days (1996 and 2001)

FlowsFlows

Fig 15.7

Flows are much like they sound. Rock and/or regolith

moving as an uncohesive mass much as a liquid. In (a)

below we see wet sand and gravel flowing as a slurry,

while in (b), this debris flow is made of rock, wood, and

other materials entrained as it flowed.

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Van Trump debris flow going over Comet Falls at Van Trump debris flow going over Comet Falls at

Mount RainierMount Rainier Stevens Creek debris flowStevens Creek debris flow——Columbia Columbia

GorgeGorge

debris flows Grand Canyon

Electron Mudflow ~A.D. 1502Electron Mudflow ~A.D. 1502--

OrtingLahars, or

volcanic debris flows

Mount Rainier from SunriseMount Rainier from Sunrise

The summit collapsed to generate

the Osceola Mudflow about 5,000 yr

B.P. The Columbia Crest cone grew

in the crater formed by the collapse,

one of the world’s 20 largest lahars.

The Osceola buried a native

American site in Enumclaw.

Enumclaw artifacts, Gerald Hedlund photo

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Red dots show buried trees near Mount Rainer

http://motion.wr.usgs.gov/

Mount Rainier Animation Project

Complex flowsComplex flows

Fig 15.10

This started as a large curved-base slide (a slump), but materials

broke up as it rotated and slid, becoming a flow.

15.1 What are the characteristics of mass

movements?

Mass Mass movements are downslope are downslope movement of rock, regolith, or both due movement of rock, regolith, or both due solely to gravity.solely to gravity.

The three characteristics used to describe mass

movements are

1) the material (rock, debris, earth),

2) the type of movement (fall, slide, flow)

3) the velocity (fast, slow).

Further notesFurther notes

Falls free fall down steep slopes.

Slides move along surfaces of rupture, planar or

curved.

Flows behave as a viscous fluid even though no

water may be present.

Even more notesEven more notes

Landscape features such as scarps, surface

cracks, tilted/bent trees, bumpy topography, and

talus provide clues to the occurrence and type of

mass movement.

Some mass-movement events may involve more

than one process. Rock and regolith initially

separate along a rupture surface, but the slide-

displaced materials may fall or flow.

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15.2 What causes mass movements?15.2 What causes mass movements?

Fig 15.11

Under the conditions below, a brick on a board will slide if tilted far enough.

What does this mean? 1) Increasing slope favors motion or slope instability.

Next, we may sand the board and find it moves more easily. 2) Smoother

surfaces favor motion and slope instability, and if we wet the board, it moves

easier still; 3) the presence of

water favors motion and slope

instability.

So slope angle, smoothness of

surface, and presence of

water all seem to enhance the

tendency to slide down a

slope.


Fig 15.12

Gravity is the driving force: on any slope there is a

component of gravity parallel to the sloping surface and

this part increases as slope increases. Thus, the steeper

the slope, the more downward force.


Fig 15.13

Friction and cohesion are resisting forces to gravity. Friction is an opposing

force when two surfaces are in contact that increases with the “roughness” of

contacting surfaces. Friction also varies with slope angle, as it is a function of

mass and gravity. Cohesion is an attractive force between particles at the

atomic level. Loose sand or gravel have little cohesion, but clay particles, with

charged surfaces, have high cohesion.


Fig 15.14

Movement occurs when the driving force (downslope part of

gravity) is greater than the resisting forces (friction and cohesion).

For any sloped system there is a critical angle that, if exceeded,

will result in movement.


Fig 15.15

On a rocky slope, usually there are faults, joints, bedding planes,

or some combination thereof. The fractured rock is stable if the

friction along joints and bedding planes, and cohesive forces

between blocks, is greater than the downslope component of

gravity. When some part of this system changes, it becomes

unstable and mass movement results.


Fig 15.16

In the case of regolith, much of the resisting force is in the cohesion and friction

between particles, as well as the friction and cohesion between the regolith and

the bedrock surface below. When particles lose contact with one another, the

bulk resistive force is effectively gone and movement results.

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15.2 What causes mass movements?

Mass movements occur if the gravitational force is

stronger than the resistive strength of the rock or

regolith.

The resisting strength is the sum of all frictional

resistance to movement and the cohesion between

minerals in the rock or regolith.

The magnitude of the gravitational force directed parallel

to the slope is greater at steeper angles.

Fig 15.17

15.3 What factors determine slope stability?15.3 What factors determine slope stability?

Water: small amounts

increase cohesion,

but add too much and

it causes particles to

lose contact with one

another reducing both

friction and cohesion.

Angle of repose: this is

the maximum angle of

a stable slope as

determined by the

friction, cohesion, and

particle shape.


Fig 15.18

Change of slope:

Both natural and artificial

processes can alter slope. As

streams (or construction) cut

into a bank, slope is

increased, thereby increasing

the downslope forces.

Eventually the driving force

becomes too great and failure

occurs.


Fig 15.19

Joints, faults, bedding planes, or any planar feature that interrupts cohesion is a

risk for slope instability. Water will preferentially travel down such features,

weakening them by reducing resisting forces and by weathering the minerals

therein. Chemical and physical weathering play roles in slope destabilization.


Fig 15.20 Vegetation:

Roots penetrate regolith and

tend to bind particles together

as well as absorb water. They

are a stabilizing force on slopes.

Here we see the result of

vegetative loss due to fire and

the resultant mass movement.


Fig 15.21

Differing climates often have different vegetative regimes. Here in

a temperate mid-latitude forest, we have dense tree cover and

thick regolith. As a result, slopes tend to be fairly stable. Mass

movement occurs, but generally it is slow-velocity types such as

creep, slumps, and flows.

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Fig 15.21

However, in an arid region the lack of vegetation due to available

moisture lends to unstable slopes. Weathered fragments fall or

wash away quickly with nothing to bind them. Mass movements

are typically falls and slides.

15.3 What factors determine slope stability?

Abundance of water

The composition and texture of materials

The presence and orientation of planar features in

the rock that may form rupture surfaces

The steepness of the slope, the amount of

weathering, and vegetation.

15.4 When do mass movements occur?15.4 When do mass movements occur?

Fig 15.22

Peru, 1970: earthquakes trigger debris avalanches around Nevado Huascaran. The mixture of regolith, rock, and ice buried the town of Yungay, killing 18,000.

15.4 When do mass movements occur?15.4 When do mass movements occur?

Fig 15.24

A lahar in the Philippines in the wake of the eruption of Mt. Pinatubo (c. 1991).

A lahar is a mixture of debris and ash saturated with water either from rain or

from rapidly melted snow and ice from atop the volcano. Lahars such as thisburied whole cities,

and in total they

deposited a volume in

the Philippines that

would cover

Washington D.C. to a

depth of three stories.

15.5 How do we know ... how to map mass-movement

hazards?

Fig 15.27

A flowchart that

addresses different

aspects of the

landscape. This is the

sort of tool that hazard

managers use to

assess the potential of

landslides.

santa tecla, el salvador january, 2001 - centralia … · 1 mass wasting—landscapes in motion...

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