MASS WASTING
SURFICIAL PROCESSES
• Erosion, Transportation, Deposition on the
Earth’s Surface
• Landscapes created and destroyed
• Involves atmosphere, water, gravity
• Agents:
– Mass wasting (gravity), Running water
(streams), glaciers (ice), wind, water waves,
ground water
MASS WASTING
• Masses of debris (mud, sand, gravel)
or bedrock moving downhill
• Landslides and slower movements
• Driven by GRAVITY
Classification of Mass Wasting
• RATE of MOVEMENT
– Extremely slow (~1mm/year) to very rapid
(>100 km/hour)
• MATERIAL
– Bedrock
– Debris- (“soil”, sediment)
Classification of Mass Wasting
• TYPE OF MOVEMENT
– Flow
– Slide
• Translational slide
• Rotational slide (Slump)
– Fall
Classification of Mass Wasting
• TYPE OF MOVEMENT
– Flow
– Slide
• Translational slide
• Rotational slide (Slump)
– Fall
Controlling Factors
• Slope angle- gentle vs steep
• Local relief- low vs high
• Thickness of debris over bedrock- slight vs
great
• Planes of weakness ( in bedrock)
– bedding planes; foliation; joints
– planes at right angle to slope vs parallel to
slope most dangerous
Controlling Factors
• Climatic controls
– Ice- above freezing vs freeze & thaw
– Water in soil- film around grain vs saturation
– Precipitation- frequent but light vs periods of drought
and heavy rainfall
– Vegetation- heavily vegetated vs light or no vegetation
• Gravity
– Shear force- parallel to slope, block’s ability to move
– Normal force- perpendicular to slope, block’s ability to
stay in place due to friction
– Shear strength- resistance to movement or
deformation of debris
The Effect of Slope & Gravity
G=gravity
S=shear
F=friction
N=normal
F
S
G
N
Controlling Factors
• Water
– adds weight
– increased pore pressure in saturated debris
decreases shear strength
– surface tension in unsaturated debris increases
shear strength
• Triggering Mechanisms
– Overloading
– Undercutting
– Earthquakes
Common types of mass wasting
• CREEP
– gentle slopes
– vegetation slows movement
– very slow flow (< 1 cm/year)
• facilitated by water in soil
• or by freeze-thaw in colder climates
– Indicators of creep
• ‘pistol butt’ trees
• leaning tombstones, walls, posts
Solifluction & Permafrost
• Solifluction:
– Flow of water saturated debris over
impermeable material
• Permafrost:
– Ground that remains frozen for many years
Common types of mass wasting
• DEBRIS FLOW
– Motion taking place throughout moving mass
– Includes
• Earthflow
• Mudflow
• Debris Avalanche
Earthflow
– Primarily flow of debris
– may involve rotational sliding
– Scarp above
– Hummocky surface in lower part
– May be slow or fast
– Solifluction
• role of Permafrost in cold climates
Mudflow
– Flow of watery debris
– Occurs where lack of vegetation:
• Dry climates
• Volcanoes
• After forest fires
Debris Avalanche
• Very rapid, turbulent flow of debris
– mud-boulders
• >150 km/hr
• Triggered by
– volcanic eruptions- Mt. St. Helens 1980;
Nevado del Ruiz 1985
– intense rainstorms- Venezuela 1999
– earthquakes- Japan 2000
Rockfalls and Rockslides
• Rockfall
– Bedrock breaking loose on cliffs
– Talus at base of cliffs
• Rockslide
– Bedrock involved
– Sliding along planes of weakness parallel to slope
• Bedding planes; foliation planes; fractures in rock (joints)
Debris Slides and Debris Falls
• Debris fall
– Free-falling mass of debris
• Debris slide
– Debris moving along a well-defined surface
The St. Francis Dam
The dam stood 180 feet high
and 600 feet long
Curved Concrete Structure
On March 12, 1928, after its reservoir
reached full capacity for the first time,
the St. Francis Dam began to leak. At
11:57 PM, the dam collapsed, sending
12 billion gallons of water raging
through the narrow San Francisquito
Canyon into the Santa Clara Valley.
Designed and built two years earlier
by William Mulholland to store water
brought by the Los Angeles Aqueduct
from Owens Valley. Its failure resulted
in a flood which killed over 450 people
and destroyed buildings, bridges,
railroads, and farms. The St. Francis
was only one of 19 dams that
Mulholland had constructed to store
Los Angeles' water supplies.
Preventing Landslides
• Preventing mass wasting of debris
• Preventing rockfalls and rockslides on
highways
L.A. Against the Mountains
• The 1934 flood disaster in Los Angeles basin was
so horrific that Woody Guthrie composed a song
called “Los Angeles New Year’s Flood” to
memorialize the hundred people who were buried
alive, drowned, or never found.
• Light rain began falling on December 30, 1933,
and rapidly intensified to a downpour totaling 7.31
inches in 24 hours.
• By midnight on December 31, 1934, the San
Gabriel Mountains, towering above the Los
Angeles basin, began to discharge massive debris
flows of mud, rocks and trees down dozens of
steep narrow canyons.
• The debris flows reached the basin floor as 20foot walls of water, as they had done for eons.
The Geology of the Great Los
Angeles Basin
• The Los Angeles basin is a group of four alluvial
plains named the San Gabriel Valley, Inland Valley,
San Fernando Valley, and Coastal Plain.
• The plains are surrounded (more or less) by three
mountain ranges named the Santa Monica
Mountains, the San Gabriel Mountains, and the
Santa Ana Mountains.
• The San Gabriel is by far the greatest, with peaks
over 10,000 feet, just 40 or 50 miles inland from
the Pacific Ocean.
The San Gabriel Mountains
• The San Gabriel Mountains orogeny spanned
around 40 million years (25-65 million years ago)
before accelerating in the past 1 million years.
• The San Gabriels are young mountains and are
still rising as rapidly as any mountain range in the
world.
• The San Gabriels rose next to a spectacular
trough plunging six miles below sea level.
• Riddled with faults, the San Gabriels have long
fractured easily and crumbled in the face of Pacific
Ocean storms.
• The San Gabriels continue to disintegrate at one
of the fastest rates in the world, but they are
building up faster than they are disintegrating.
Debris Dam
Fresh
sediment
deposited in
debris
retention
structure
along the
range front
of the San
Gabriel
Mountains.
Recent large landslide covering road in the
foothills of the San Gabriel Mountains
Small, recent shallow landslides in
older scars from previous years, east
of I-5 in Orange County.
Debris flow that initiated from large
landslide above the town of La
Conchita. Debris-flow source is from
large 1995 landslide.