Transcript Mass Wasting - OrdinaryLife.net

Ch. 15 Mass Wasting
“stuff rolls downhill”
Mass Wasting
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The downslope movement of rock, regolith,
and soil under the direct influence of gravity.
Does not require a transporting medium.
It is the process that takes place between
weathering and erosion.
Combined effects of mass wasting and
running water produce stream valleys
From slow imperceptible creep to fast moving
avalanches.
Why is mass wasting important?
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MW processes represent a significant hazard to
people and property
Need to identify where and under what
conditions these occur
Avoid construction in areas prone to mass
wasting
Attempt to prevent mass wasting
Mass Wasting and landform
development
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For mass wasting to occur, there must be a
slope angle
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Most rapid events occur in areas of rugged,
geologically young mountains
As a landscape ages, less dramatic downslope
movements occur
Controls on Mass Wasting
Gravity is the controlling force.
Water is a factor.
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Destroys cohesion or internal resistance between
particles.
Creates buoyancy for masses of regolith and soil,
thereby reducing the frictional coupling with the
underlying substrate.
Adds considerable weight to the mass of material.
Changes the properties of clay; clay becomes
"slick" when wetted.
Controls on Mass Wasting
Adding material to the top of the slope or
undercutting the slope at its base can increase
the angle of repose.
Oversteepening of slopes is a factor.
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Rock debris is stable at slope angles less than the
angle of repose.
Angles of repose vary between 25 and 40 degrees
depending on the materials.
Controls on Mass Wasting
Classification of Mass Wasting Processes
Classification is based on:
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Type of material
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Type of motion
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Unconsolidated vs. consolidated (e.g., bedrock)
Dry vs. water saturated
Fall: Free-fall on steep slopes. Forms talus slopes
Slide: Movement along well-defined surface;
material remains fairly coherent.
Flow: Material moves as a viscous fluid, usually
when saturated with water.
Rate of movement
Talus Slope
Slump
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Downward sliding of a mass of rock or
unconsolidated material moving as a unit along a
curved surface.
Slumped material does not travel very fast or
very far.
Crescent-shaped scarps are formed.
Water percolating downward and along the
curved surface may promote further instability
through lubrication and buoyancy.
Commonly occurs on slopes that have been
oversteepened.
Slump
Slump
La Conchita,
CA 1995
Slump, SW Montana
Rockslide or debris slide
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Downward sliding of blocks of bedrock that have
broken loose.
Among the fastest and potentially most destructive of
the mass wasting processes.
Often occurs in areas where the rocks are highly
fractured, particularly if the fracture surfaces or
bedding planes dip downslope.
Often triggered by an earthquake.
Examples - Madison River and Gros Ventre rockslides
Gros Ventre Rockslide
Mudflow
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Rapid type of mass wasting that involves a
flowage of debris containing a large amount of
water.
Most characteristic of semiarid mountainous
regions.
Tend to follow canyons and gullies.
Lahars are mudflows on the slopes of
volcanoes, often accompanying eruptions.
E.g., Mount St. Helens.
Mudflow
Earth flow
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Downslope movement of water-saturated soil
on hillsides in areas of deep weathering.
Form tongue-shaped masses with well-defined
head scarps.
Moves relatively slowly and may be active for
periods ranging from days to years.
Earth flow
Earth flow near San Francisco, CA
Creep
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Imperceptibly slow downslope movement of
soil and regolith.
Can take place on even gentle slopes and is
extremely widespread.
A primary cause is the alternate expansion and
contraction of surface materials caused by
freezing and thawing or wetting and drying.
Creep
Creep
Creep
Solar powered
landslide monitors
Los Angeles Against the Mountains
Debris Flows in Southern California
Aerial Photo of Pine Cone Rd.
Larger view of Pine Cone Road
Pine Cone Road Topo Map
Alluvial Fan
Alluvial Fan Complex (Bajada)
Satellite Image Southern California
3D image of Los Angeles
Los Angeles Geology
Satellite image of Altadena and San
Gabriel Mountains.
Los Angeles
The Big Squeeze
What causes debris flows in LA?
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San Gabriel Mtns
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Fires
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deeply fractured due to stresses on the rocks caused by faults
rapidly uplifting and weathering
Very steep slopes
Strip vegetation from the slopes
Combustion of chaparral plants leaves wax-like substance about 1 cm
below soil surface. This prevents infiltration of rain and increases runoff
Rain
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LA averages ~ 15 in/yr.
San Gabriels can get extreme rainfall events
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Jan. 1969 - >44 ins. in 9 days
Feb. 1978 – 1.5 ins. In 25 minutes
April 5, 1926 – 1 in. in 1 minute
San Gabriel Mountains
San Gabriel Mountains
San Gabriel Fault
Aerial photo of debris flow scars
Homes on the north side of San Bernardino,
winter of 1980
Home destroyed by a small debris flow
during the winter of 1980
Side view of the home and debris flow path.
Debris flow, La Tuna Canyon, 1984
House and debris flow, Los Angeles, 1978
What can be done?
Deflector wall
Los Angeles County Department of
Public Works debris basins
Can it happen here?
What do you think?
The Debris Flows of
Madison County, VA
JUNE 27, 1995
Location of Madison County
June 27, 1995
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Severe storm triggered hundreds of rock,
debris and soil slides
debris flows inundated areas downslope
causing damage to structures, roads, utilities,
livestock and crops
Rainfall amounts
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as much as 30 inches of rain fell in 16 hours
in the area of maximum storm intensity
probably about 25 inches fell within a fivehour period
Track of the
storm
Time of impact 10:0011:30 EDT
(Home of L. Brown)
Times of impact 11:3011:45 to 1:00 EDT
(Home of R. Lillard)
Time of impact 11:30-12:00 EDT
(Home of J. Crosgrove)