Ch. 15 Mass Wastingstuff rolls downhill
Mass WastingThe 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 valleysFrom slow imperceptible creep
to fast moving avalanches.
Why is mass wasting important?MW processes represent a
significant hazard to people and propertyNeed to identify where and
under what conditions these occurAvoid construction in areas prone
to mass wastingAttempt to prevent mass wasting
Mass Wasting and landform developmentFor mass wasting to occur,
there must be a slope angleMost rapid events occur in areas of
rugged, geologically young mountainsAs a landscape ages, less
dramatic downslope movements occur
Controls on Mass Wasting Gravity is the controlling force. Water
is a factor. 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
Controls on Mass WastingAdding 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. 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: Type of material Unconsolidated vs.
consolidated (e.g., bedrock) Dry vs. water saturated Type of motion
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
SlumpDownward 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.
SlumpLa Conchita, CA 1995
Slump, SW Montana
Rockslide or debris slide 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 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.
Earth flow 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 near San Francisco, CA
Creep 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.
Solar powered landslide monitors
Los Angeles Against the MountainsDebris Flows in Southern
Aerial Photo of Pine Cone Rd.
Larger view of Pine Cone Road
Pine Cone Road Topo Map
Alluvial Fan Complex (Bajada)
Satellite Image Southern California
3D image of Los Angeles
Los Angeles Geology
Satellite image of Altadena and San Gabriel Mountains.
The Big Squeeze
What causes debris flows in LA?San Gabriel Mtns deeply fractured
due to stresses on the rocks caused by faultsrapidly uplifting and
weatheringVery steep slopesFiresStrip vegetation from the
slopesCombustion of chaparral plants leaves wax-like substance
about 1 cm below soil surface. This prevents infiltration of rain
and increases runoffRainLA averages ~ 15 in/yr.San Gabriels can get
extreme rainfall eventsJan. 1969 - >44 ins. in 9 daysFeb. 1978
1.5 ins. In 25 minutesApril 5, 1926 1 in. in 1 minute
San Gabriel Mountains
San Gabriel Mountains
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
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 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 amountsas much as 30 inches of rain fell in 16 hoursin
the area of maximum storm intensity probably about 25 inches fell
within a five-hour period
Track of the storm
Time of impact 10:00-11:30 EDT (Home of L. Brown)
Times of impact 11:30-11:45 to 1:00 EDT (Home of R. Lillard)
Time of impact 11:30-12:00 EDT (Home of J. Crosgrove)