Santa Tecla, El Salvador January, 2001 - Centralia … · 1 Mass Wasting—Landscapes in Motion...
Transcript of 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.
15.2 What causes mass movements?15.2 What causes mass movements?
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.
15.2 What causes mass movements?15.2 What causes mass movements?
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.
15.2 What causes mass movements?15.2 What causes mass movements?
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.
15.2 What causes mass movements?15.2 What causes mass movements?
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.
15.2 What causes mass movements?15.2 What causes mass movements?
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.
15.3 What factors determine slope stability?15.3 What factors determine slope stability?
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.
15.3 What factors determine slope stability?15.3 What factors determine slope stability?
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.
15.3 What factors determine slope stability?15.3 What factors determine slope stability?
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.
15.3 What factors determine slope stability?15.3 What factors determine slope stability?
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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15.3 What factors determine slope stability?15.3 What factors determine slope stability?
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.