lec 13 masswasting - napa valley college · process of erosion erosion: materials transported by...
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UPCOMING EXAM• BRING PENCIL And A SCANTRON!
• Let me know if you are interested in taking the optional comprehensive final.
• No programmable calculators or other electronic devices. No dictionaries.
• Covers Chapters Geologic Time, Glaciers, Shorelines, Mass Movement, Streams
• Bring blank paper in case you need to work out a problem (NO WRITING IN THE TEST BOOKLET).
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The 1995 La Conchita landslide, southern California, U.S.A.
Mass MovementLandslides,
Rockfalls, Debris Flows, avalanches and sinkholes.
1999 Rockfall
Newfoundland
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Landslide destroyed road near Hwy 17 Scott’s Valley, CA
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The Toll
Landslide deaths in the U.S. have been estimated at 25-50 people per year.
Every year around the world hundreds of people are killed by
mass wasting events (landslides, debris flows, and rockfalls).
Infrastructure/property damage is a global occurrence, but a large
portion of events are around the Pacific rim---that includes California.
Every year California suffers property damage and/or loss of life due to landslides. Landslides are California’s most common thus costliest natural disaster.
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Before the Yungay Avalanche
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After the Yungay Avalanche ~25,000 killed 1970 Peru
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Process of Erosion Erosion: materials transported by wind, water, or gravity and
deposited somewhere else. With mass wasting we will be focusing
generally on gravity being the transportation force and water
(liquid or ice) facilitating the process.
Mass movement (also known as mass wasting) is the down slope movement of
earth materials under the influence of gravity. The detachment and movement of
earth materials occurs if the stress imposed is greater than the strength of the
material to hold it in place.
Landslides, rockfalls, debris flows and avalanches all have one thing in common,
they occur on slopes. The angle of the slope can vary greatly and in some cases
the slope angle can be very shallow.
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Terms you need to know.
• Rockfall
• Rockslide
• Slump
• Debris Flow
• Creep
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External triggers
• Earthquake shaking, especially in steep
topography, with saturated slopes, or in
saturated sand (liquefaction)
• Volcanic eruptions create piles of loose
material (e.g., ash), destroy vegetation,
melt snow/ice, cause mountain collapse
• Heavy rainfall and melting snow,
especially if continuous over days/weeks
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Moisture and rate of motion
characteristics.
The slower events (creep) generally result only in property damage.
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Why are there so
few landslides in
the Sierras?
A. There are few steep
slopes there.
B. The trees hold the land
in place.
C. Soil was removed by
glaciers.
D. There is no rain, only
snow.
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Soil Formation
Rock chemically breaks down through the process called weathering.
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California Coast Range--Factors that
create slope instability.
• Slope—Many steep slopes!
• Water—A lot of wet weather and storms
(especially in Northern California)!
• Geologic materials, Geologic structures, and
Weathering—Much of the rock is weak due to
being highly fractured, folded and faulted, and
weathered and some are just weak and slippery
(e.g. serpentinite). Soils or unconsolidated
sediments present on slopes.
• Vegetation?
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Serpentinite mélange. Tectonically-rounded
blocks of serpentinite, surrounded by a
sheared-out, foliated paste of crushed
serpentinite.
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Factors that determine slope stability
• Slope?
• Water?
• Geologic materials?
• Geologic structures?
• Weathering?
• Vegetation?
The California Coast Range (and coast) regularly experiences mass wasting events.
The majority are rotational slumps but debris flows are common as well.
Most mass wasting events in California are triggered by wet storms.
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Driving force (gravity) > resisting strength of rock/regolith
Components of resisting strength? 1.Friction
2.Cohesion
Figure 15.13
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View of the mudslide area in the town of La Conchita.
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Shear strength is a measure if the resistance of earth materials to be moved.
Angle of repose is the steepest angle from horizontal that an unconsolidated
material can maintain. Grain shape strongly effects the angle as does size
distribution. The angle of repose of dry sand is around 33 degrees.
The material that the slope is composed of is important for several reasons:
-- plays a role in how steep of a slope can be maintained and also how high a cliff
can be.
-- the type of slope failures (mass wasting events) that may occur.
-- the magnitude of these events.
~33 degrees for dry sand
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The material that the slope is composed of dictates:
-- how steep of a slope can be maintained.
-- how high a cliff can be.
-- the type of slope failures (mass wasting events) that occur.
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Steep slopesvs. shallow slopesis controlled by the rock type.
Grand Canyon formation through the
Colorado River cutting down into the
bedrock. Canyon widening through mass
wasting events.
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Consolidated materials:(tends to fall or slide)
• Cemented sediments = sedimentary rock.
• Crystalline materials: igneous, metamorphic, and some sedimentary rocks (evaporites).
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Bedding Planes
Safer
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Rock Discontinuities
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Masses of rock often are full of
joints and fractures. columnar joints
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Columnar joints
Devil’s Postpile, CA.
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Napa area
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Water and Water Ice
Why does ice float on liquid water?
Why is this important?
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Water exerts pressureon particles and bedding surfaces which acts a lubricant on frictionalsurfaces.
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Water and Water Ice
The density properties of
water are very unique and
have broad ranging
implications in geologic
and ecologic processes.
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Mechanical Weathering:
Frost Wedging
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Rockfalls
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SF Telegraph Hill
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1996 Happy Isles rockslide. Photo by David Walter
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Happy Isles Rockfall
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Granite Slide – Just outside Yosemite, CA
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Talus and talus slopes
Talus is rock debris that is piled
up (generally at the angel of
repose) at the base of cliffs.
Talus slopes are rockfall deposits.
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Talus cones on the north shore of Ifjorden, Svalbard, Norway.
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TALUS SLOPES – ROCKY MOUNTAINS
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TALUS SLOPES – ROCKY MOUNTAINS
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TALUS SLOPES – DEATH VALLEY
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HWY 1 Landslide California Rockslide
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Rockslide/Landslide
Ferguson Slide on California State Highway 140
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Massive landslide deposit.
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A massive landslide scar and deposit.
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A massive landslide scar and deposit.
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Home destroyed by the April 1998 El Niño-triggered Anzar Road landslide,
San Benito County, California, U.S.A. (Schuster et al., 1998).
Weakly or unconsolidated material:(will slide, slump and often flow)
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Factors that determine slope stability
Water: a small amount
increases cohesion but a large
amount reduces friction;
weight of water increases
driving force too.
Water saturation of materials
on a slope is the primary
cause of landslides.
Earthquakes can cause
liquefaction of
unconsolidated materials thus
reducing the friction between
grains.
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Roots can mechanically weather bodies of rock, but
roots often hold rocks and soil in place on a slope.
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Vegetation often stabilizes slope. Slopes are very susceptible to failure after fires
remove vegetation. Failure generally occurs in the wet winter storms that follow
the fire season.
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Paseo Del Mar,
San Pedro, CA
(Southern California)
November 2011
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Slump
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Parts of a earth slump
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Large landslide along
the coast between Fort
Funston and John Daly
Blvd. (Daly City)—what
are the parts?
Old coastal highway
scarp
previous cliff toptoe?
body
Fort FunstonSan Andreas fault
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LANDSLIDE – TENESSEE VALLEY BEACH – MARIN, CA
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Slumping and Scarps in Dorset, England
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The head of a rotational slip in the Black Hills of North Dakota. (Source: Image
provided by the National Geophysical Data Center, NOAA).
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Earth flow
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Seattle, Washington
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Clay bed
prevented
drainage
due to being
impermeable,
thus overlying
strata became
saturated.
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The Gross Ventre Slide – Sheep Mountain
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Rock Slide of 1959
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Landslide scar
Quake Lake
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Evidence for massive
prehistoric landslide
that would have
triggered a great
tsunami.
Molokai
Landslide debris
Hawaiian Islands
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http://www.mbari.org/volcanism/Hawaii/HR-Landslides.htm
Debris from enormous landslides off Oahu and
Molokai extends hundreds of kilometers
(map generated from sonar data © 2001 MBARI)
Pali above Kane'ohe Bay on Oahu: the
slide-scarp of the Nuuanu submarine
landslide
(photo © 2004 J.B. Paduan)
Evidence for huge submarine landslides in Hawaii
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Major landslide deposit around Hawaii.
Landslides that enter water can cause a tsunami!
As the big island grows, it’s unstable flanks collapse forming major landslide deposits.
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K’ilueau Volcano Giant rift on Hawaii has people
wondering if this is going to be
a massive landslide.
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Describe a hillside and its environment that is most susceptible to mass movement.
Be thorough!
Foliation, Bedding, Joints parallel to slopeSlope is steep
Slope consists of unconsolidated materialSlope has lots of water (poor irrigation above)Slope gets a lot of traffic (animal or human)
Lots of weight on top of slopeBase of slope undercut
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What are some things that can act as triggers for mass movement?
Be thorough!
Undercut highway slope (humans)Build atop hillside (or dump load)Stream or waves undercut hillside
EarthquakeLarge Explosion
Storms
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La Conchita1995
The town was built on a 10,000 year
old landslide. Unconsolidated
sediments formed steep hills around
the town. Orchards at the top of the
hills were irrigated. Wet weather
oversaturated the slopes and fissures
formed. On March 4th 600 tons of mud
destroyed 9 homes.
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View of the mudslide area in the town of La Conchita.
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Motorist are stranded between two mudslides on Highway 101 next to the community of La Conchita between Ventura and Santa Barbara.
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La Conchita residents clear a path for water drainage on Fillmore Avenue after mudslides Monday morning.
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A motor home is stuck in the mud on Pacific Coast Highway along the Rincon.
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A helicopter surveys damage from a mudslide near La Conchita along Highway 101 Monday morning.
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The Santa Clara River passes under the Harbor Blvd. bridge just south of the Ventura city limits. The ocean, top right, is filled with debris.
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A work truck is stuck in the mud on Highway 101 at La Conchita Monday morning
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Vehicles are stuck in the mud on Pacific Coast Highway along the Rincon.
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Debris Flows
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Schematic of typical hillslope setting for debris flows. Debris is earth material generally greater than coarse sand size. A
debris flow occurs when the water content of the soil is sufficient for the material to flow like a viscous fluid. Debris flows
usually travel down existing stream channels. Over geologic time debris can accumulate to form a fan-shaped deposit at
the toe of the slope.
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December 1999 debris-flow damage to the city of Caraballeda, north coast of Venezuela. (Photo by
L.M. Smith, Waterways Experiment Station, U.S. Army Corps of Engineers).
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British Columbia debris flow during a one-night flood in a small stream.
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Upstream view of concrete crib-type check dam with low-flow center section in
southern California. (photograph by Los Angeles County Flood Control District).
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Mudflow in Sarno, Italy, 1998
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Rio Nido in Sonoma County. El Nino induced mudslide 1998
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Creep
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Burrowing rodents and wormssimilarly play a part in movingmaterials downhill.
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