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You have been asked to write an article for a magazine (or newspaper – your choice) about a major mass wasting event (a landslide, mudslide, debris flow, rock fall, etc.), informing the public about the hazards of landslides, why they happen in certain locations, and the types of devastation that may occur as a result.
You may choose from one of the following types of magazine/newspaper genre (audiences) for whom you would like to write:
Your article should discuss the following facts and concepts:
Your article should include the following general information about landslides (see Rubric for guide as to what should be included):
Other Important Information:
Mass wasting is the downslope movement of rock and soil under the
direct influence of gravity.
This is a particularly important geologic
activity in the S.F. Bay Area – we deal with
"landslides" every year during the rainy
season (Oct. -April).
Although these are often small-scale
events — some are very large and are
responsible for thousands of deaths.
Peru (1970) – 20,000 people died during a gigantic
Colombia (1985) – 25,000 people died in the lahar
flows after the eruption of Nevado del Ruiz.
Italy (1960) – a rock slide caused a 90 meter high
wave of water in a mountain reservoir that killed
2,600 people. USGS
I. Slope Stability
One key factor that affects slope stability is
the steepness of the slope.
In A, the pull of gravity is downward.
On a slope, the force of gravity can be
resolved into two components: a component
acting perpendicular to the slope (gp), and a
component acting parallel to the slope (gs).
In B and C, more of
the rock’s weight
(gs) is directed
downslope as the
increases (vector gs
Stephen A. Nelson, Tulane U.
Another important factor in slope stability
is the nature of material – some materials
are inherently stronger (granite v. shale).
The figure shows the slope profile for the
walls of the Grand Canyon. Differences in
the strength of the different rock types
where resistant layers (limestone and
sandstone) form steeper walls than
weaker rocks such as shale.
National Park Service
National Park Service
Although gravity is the controlling
force in mass wasting, other factors
may play an important role in mass
2. Oversteepened slopes
The Role of Water in Mass Wasting
When the pore spaces between grains in a
sediment become filled with water – cohesion
between the grains is destroyed. Water
saturation reduces the internal resistance
(friction?) of materials to flow – the sediments
are less coherent and may flow under the
influence of gravity.
Water adds considerable weight to a package
of sediments – the added weight may be
enough to initiate flow under the influence of
In the S.F. Bay Area, CalTrans and other
government agencies install plumbing systems
in steep slopes to help drain them (prevent
oversaturation) in hopes to prevent slope
failure (ex. Highway 92 near Half Moon Bay). Copyright by Andrew Alden, geology.about.com, reproduced
under educational fair use.
Loss of cohesion in
unconsolidated sediments by
oversaturation from water.
Images by Stephen A. Nelson, Tulane U.
The Role of Oversteepened Slopes in Mass Wasting
Unconsolidated particles assume a stable slope called the angleof
repose. This is the steepest angle at which the material remains stable
(usually varies from 25 to 40°).
If a slope angle is greater than the angle of repose for a particular
material, the slope is oversteepened and is subject to failure.
Many activities are responsible for
oversteepening of slopes:
1. A stream may undercut a valley wall.
2. Pounding waves against the base of a cliff may
undercut the cliff (we are seeing this happen
along the CA coast (Daly City).
3. People commonly oversteepen slopes by
construction of roads and landscaping.
USGS Vetiver Network
The Role of Vegetation in Mass Wasting
Plants protect against erosion and help stabilize slopes – their roots bind soil together.
Mass wasting is enhanced where plants are lacking – by fire, urban development, logging, etc. The image shows multiple landslides resulting from deforestation in Colombia.
In the S.F. Bay Area, CalTrans plants "at risk" slopes in the hope to help stabilize them.
The Role of Earthquakes in Mass Wasting
Earthquakes play a major role in triggering landslides.
Although the conditions for a slope may be favorable for a failure
(slide), it sometimes requires an additional factor to "trigger" the
Earthquakes can provide the "nudge" that is required for an
oversteepened slope to fail.
Remember that the
eruption of Mount St.
Helens was triggered by
a giant landslide. In turn,
the landslide that was
responsible for the
eruption was itself
triggered by an
earthquake due to the
movement of magma
The general types of mass wasting processes and the classification scheme is based upon
1. Type of Material – classification depends
upon whether the material began as
unconsolidated sediments (and soil) or as
2. Type of Motion – Kind of motion may be
described as fall, slide or flow
• Fall – free-fall of detached material. Common
on slopes so steep that there is no
accumulation of material on the slope. Many
falls may result from mechanical weathering
such as frost wedging and the action of tree
• Slide – material remains fairly coherent and
moves along a well-defined surface such as a
joint, fault, or bedding plane.
• Flow – material moves downslope as a viscous
3. Rate of Movement – can be very rapid (>
125 miles/hour) or imperceptibly slow –
ranges from sudden to gradual movement.
II. Classification of Mass Wasting Processes
A fall involves rapid movement of material in free
Rock falls occur when a slab of rock travels
mostly vertically through the air, then after
hitting the ground, rolling and bounding.
Triggers tend to be heavy rain, frost wedging,
earthquakes, animals and people. Stephen A. Nelson, Tulane U.
Although falls are a hazard, they are
an important process that helps to
widen valleys and erode mountains
over geologic time.
Tom Trujillo, Associated Press (SFGate)
The above image shows the damage at
Curry Village after a rockfall.
Lloyd DeForest, USGS
Slidesoccur when blocks of
bedrock and/or soil break loose
and slide down a slope – these are
fast and destructive types of mass
Rockslides occur where layers
are inclined or where joints or
fractures are parallel to the slope.
These are most commonly
triggered by heavy rains or snow
melts which "lubricate" the slide
Herbythyme, Wikimedia Commons
On July 10, 1996, a giant rockslide
occurred in Yosemite. The left image
shows the detachment zone of the
rockslide and the impact areas (C/D, B).
In addition, an air blast from the impact
flattened 10 acres of forest. The blast
area is shown in the lower left region.
The right figure shows a
close-up of the blast zone.
Rock fragments were
embedded in trees for
hundreds of feet into the
forest. UCB Seismological Lab
The Chaos Jumbles in Mt. Lassen
NP were formed by a huge
rockslide that occurred ~300
years ago when the volcanic peaks
of the Chaos Crags collapsed. The
slide had a lot of horizontal
motion because the debris rode
on a bed of compressed air.
A slump is a downward sliding mass of rock or unconsolidated
sediment as a unit along a curved surface — this is very common in the
S.F. Bay Area.
A slump commonly occurs because a slope has been oversteepened.
A crescent-shaped scarp is created and the block's upper surface is
sometimes tilted backwards.
The slump may occur as a
single mass or as a series
Water can percolate down
through the fracture and
cause more movement.
Slumps on Mars.
A flow is a relatively rapid type
of event involving a flow of
soil &/or rock &/or mud
containing a large amount of
Because of their fluid
properties, debris flows follow
canyons and stream.
Clatskanie, OR PUDUSGS
Two main types of occurrences of
1.In semiarid regions – cloudburst
or snowmelt creates a sudden flood
that entrains a lot of soil because
there is no vegetation to anchor
the surface material. The result is a
lobe- or tongue-shaped fan of well-
mixed soil, mud and rock.
2.Lahars – occur on
stratovolcanoes. Eruptions suddenly
melt snow and glaciers that release
a tremendous amount of material –
ex. Mount St. Helens.
Debris flows are very
common in the Coast Range
The images show
Mt. Kazbek in S.
Russia before and
after the recent
collapse of the
Kolka Glacier that
triggered a debris
flow that traveled
more than 15
small villages killing
dozens of people.
Oct. 3, 2001 Sept. 27, 2002
The long, dark grey streak running upward through the center of the
scene shows the gorge that was overrun by the debris flow. The deep
reds show vegetated land surfaces, grey areas are bare rock, and white
shows ice-covered lands.
Earth flows are common on
hillsides in humid areas, during
times of heavy precipitation.
When the soil saturates, the
material may break away, leaving a
scar on the slope and forming a
tongue- or teardrop-shaped mass
downslope -common in the S.F.
The materials are most commonly
rich in clay and silt.
Since earth flows are viscous, they
move slower than debris flows
and may remain active for a long
period of time.
There are several ways to try to
prevent landslides. Most prevention
methods employ various engineering
controls to minimize the hazard and
stabilize a slope.
If a landslide occurs, mitigation means
reducing the effects or the intensity of
After a landslide occurs, the first task is
to remove the landslide material and
stabilize the slope.
Most methods of mitigation overlap
with preventive measures.
La Conchita, CA
III. Landslide Prevention and Mitigation
1. Vegetation. Planting vegetation is
particularly effective in stabilizing
slopes that consist of sediment. The
roots bind the loose sediment and
may penetrate to the underlying rock
to anchor the sediment. Vegetation
is deep roots is more effective.
Vegetation also helps stabilize slopes
by absorbing water from the soil Sathiyam TV
Ach Consulting Engineers
2. Retaining Walls. The purpose
behind a retaining wall is to
strengthen an oversteepened slope.
Retaining walls are especially
common along roadsides where a flat
or level surface has been cut into a
slope for the roadway.
Orange County Transportation Authority
3. Controlling Water. We saw
that water plays an important
role in mass wasting. Various
engineering controls are
employed to “dewater” a slope
to increase its stability in wet
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4. Terracing. Terracing involves a series of benches on a hillside.
Frequently, retaining walls are also used to stabilize the steepened
portions of the slope.
Terracing is effective where it is not feasible to build a single large
Gerald Tyner, CSULBSFGate
Earth Stability Limited
5. Rock Bolts. Rock slides and falls are
common where the slope consists of
fractured rock. Rock bolts are used to
anchor fractured rock to more massive
Rock bolts are installed by drilling a hole
through the slope then inserting and
anchoring the bolt.
Rock bolts are frequently installed with
flexible metal mesh to help stabilize the
slope and prevent material from falling.
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