Ch. 15 Mass Wasting

Ch. 15 Mass WastingCh. 15 Mass Wasting

““stuff rolls downhill”stuff rolls downhill”

Mass WastingMass Wasting

The downslope movement of rock, regolith, The downslope movement of rock, regolith, and soil under the direct influence of gravity. and soil under the direct influence of gravity.

Does not require a transporting medium. Does not require a transporting medium. It is the process that takes place between It is the process that takes place between

weathering and erosion. weathering and erosion. Combined effects of mass wasting and Combined effects of mass wasting and

running water produce stream valleysrunning water produce stream valleys From slow imperceptible creep to fast moving From slow imperceptible creep to fast moving

avalanches. avalanches.

Why is mass wasting important?Why is mass wasting important?

MW processes represent a significant hazard MW processes represent a significant hazard to people and propertyto people and property

Need to identify where and under what Need to identify where and under what conditions these occurconditions these occur

Avoid construction in areas prone to mass Avoid construction in areas prone to mass wastingwasting

Attempt to prevent mass wastingAttempt to prevent mass wasting

Mass Wasting and landform Mass Wasting and landform developmentdevelopment

For mass wasting to occur, there must be a For mass wasting to occur, there must be a slope angleslope angle Most rapid events occur in areas of rugged, Most rapid events occur in areas of rugged,

geologically young mountainsgeologically young mountains As a landscape ages, less dramatic downslope As a landscape ages, less dramatic downslope

movements occurmovements occur

Controls on Mass WastingControls on Mass Wasting

Gravity is the controlling force. Gravity is the controlling force. Water is a factor. Water is a factor.

Destroys cohesion or internal resistance between Destroys cohesion or internal resistance between particles. particles.

Creates buoyancy for masses of regolith and soil, Creates buoyancy for masses of regolith and soil, thereby reducing the frictional coupling with the thereby reducing the frictional coupling with the underlying substrate. underlying substrate.

Adds considerable weight to the mass of material. Adds considerable weight to the mass of material. Changes the properties of clay; clay becomes Changes the properties of clay; clay becomes

"slick" when wetted. "slick" when wetted.


Adding material to the top of the slope or Adding material to the top of the slope or undercutting the slope at its base can increase undercutting the slope at its base can increase the angle of repose. the angle of repose.

Oversteepening of slopes is a factor. Oversteepening of slopes is a factor. Rock debris is stable at slope angles less than the Rock debris is stable at slope angles less than the

angle of repose. angle of repose. Angles of repose vary between 25 and 40 degrees Angles of repose vary between 25 and 40 degrees

depending on the materials. depending on the materials.


Classification of Mass Wasting ProcessesClassification of Mass Wasting Processes

Classification is based on: Classification is based on: Type of material Type of material

Unconsolidated vs. consolidated (e.g., bedrock) Unconsolidated vs. consolidated (e.g., bedrock) Dry vs. water saturated Dry vs. water saturated

Type of motion Type of motion Fall: Free-fall on steep slopes. Forms talus slopes Fall: Free-fall on steep slopes. Forms talus slopes Slide: Movement along well-defined surface; Slide: Movement along well-defined surface;

material remains fairly coherent. material remains fairly coherent. Flow: Material moves as a viscous fluid, usually Flow: Material moves as a viscous fluid, usually

when saturated with water. when saturated with water. Rate of movementRate of movement

Talus SlopeTalus Slope

SlumpSlump Downward sliding of a mass of rock or Downward sliding of a mass of rock or

unconsolidated material moving as a unit along a unconsolidated material moving as a unit along a curved surface. curved surface.

Slumped material does not travel very fast or very Slumped material does not travel very fast or very far. far.

Crescent-shaped scarps are formed. Crescent-shaped scarps are formed. Water percolating downward and along the curved Water percolating downward and along the curved

surface may promote further instability through surface may promote further instability through lubrication and buoyancy. lubrication and buoyancy.

Commonly occurs on slopes that have been Commonly occurs on slopes that have been oversteepened. oversteepened.

SlumpSlump

SlumpSlumpLa Conchita, La Conchita,

CA 1995CA 1995

Slump, SW MontanaSlump, SW Montana

Rockslide or debris slide Rockslide or debris slide

Downward sliding of blocks of bedrock that have Downward sliding of blocks of bedrock that have broken loose. broken loose.

Among the fastest and potentially most destructive of Among the fastest and potentially most destructive of the mass wasting processes. the mass wasting processes.

Often occurs in areas where the rocks are highly Often occurs in areas where the rocks are highly fractured, particularly if the fracture surfaces or fractured, particularly if the fracture surfaces or bedding planes dip downslope. bedding planes dip downslope.

Often triggered by an earthquake. Often triggered by an earthquake. Examples - Madison River and Gros Ventre rockslidesExamples - Madison River and Gros Ventre rockslides

Gros Ventre RockslideGros Ventre Rockslide

MudflowMudflow

Rapid type of mass wasting that involves a Rapid type of mass wasting that involves a flowage of debris containing a large amount flowage of debris containing a large amount of water. of water.

Most characteristic of semiarid mountainous Most characteristic of semiarid mountainous regions. regions.

Tend to follow canyons and gullies. Tend to follow canyons and gullies. Lahars are mudflows on the slopes of Lahars are mudflows on the slopes of

volcanoes, often accompanying eruptions. volcanoes, often accompanying eruptions. E.g., Mount St. Helens. E.g., Mount St. Helens.

MudflowMudflow

Earth flow Earth flow

Downslope movement of water-saturated soil Downslope movement of water-saturated soil on hillsides in areas of deep weathering. on hillsides in areas of deep weathering.

Form tongue-shaped masses with well-defined Form tongue-shaped masses with well-defined head scarps. head scarps.

Moves relatively slowly and may be active for Moves relatively slowly and may be active for periods ranging from days to years. periods ranging from days to years.

Earth flowEarth flow

Earth flow near San Francisco, CAEarth flow near San Francisco, CA

Creep Creep

Imperceptibly slow downslope movement of Imperceptibly slow downslope movement of soil and regolith. soil and regolith.

Can take place on even gentle slopes and is Can take place on even gentle slopes and is extremely widespread. extremely widespread.

A primary cause is the alternate expansion and A primary cause is the alternate expansion and contraction of surface materials caused by contraction of surface materials caused by freezing and thawing or wetting and drying. freezing and thawing or wetting and drying.

CreepCreep

CreepCreep

CreepCreep

Solar powered Solar powered landslide monitorslandslide monitors

Los Angeles Against the MountainsLos Angeles Against the Mountains

Debris Flows in Southern CaliforniaDebris Flows in Southern California

Aerial Photo of Pine Cone Rd.Aerial Photo of Pine Cone Rd.

Larger view of Pine Cone RoadLarger view of Pine Cone Road

Pine Cone Road Topo MapPine Cone Road Topo Map

Alluvial FanAlluvial Fan

Alluvial Fan Complex (Bajada)Alluvial Fan Complex (Bajada)

Satellite Image Southern CaliforniaSatellite Image Southern California

3D image of Los Angeles3D image of Los Angeles

Los Angeles GeologyLos Angeles Geology

Satellite image of Altadena and San Satellite image of Altadena and San Gabriel Mountains.Gabriel Mountains.

Los AngelesLos Angeles

The Big SqueezeThe Big Squeeze

What causes debris flows in LA?What causes debris flows in LA? San Gabriel Mtns San Gabriel Mtns

deeply fractured due to stresses on the rocks caused by faultsdeeply fractured due to stresses on the rocks caused by faults rapidly uplifting and weatheringrapidly uplifting and weathering Very steep slopesVery steep slopes

FiresFires Strip vegetation from the slopesStrip vegetation from the slopes Combustion of chaparral plants leaves wax-like substance about 1 cm Combustion of chaparral plants leaves wax-like substance about 1 cm

below soil surface. This prevents infiltration of rain and increases runoffbelow soil surface. This prevents infiltration of rain and increases runoff RainRain

LA averages ~ 15 in/yr.LA averages ~ 15 in/yr. San Gabriels can get extreme rainfall eventsSan Gabriels can get extreme rainfall events

Jan. 1969 - >44 ins. in 9 daysJan. 1969 - >44 ins. in 9 days Feb. 1978 – 1.5 ins. In 25 minutesFeb. 1978 – 1.5 ins. In 25 minutes April 5, 1926 – 1 in. in 1 minuteApril 5, 1926 – 1 in. in 1 minute

San Gabriel MountainsSan Gabriel Mountains

San Gabriel MountainsSan Gabriel Mountains

San Gabriel Fault

Aerial photo of debris flow scarsAerial photo of debris flow scars

Homes on the north side of San Bernardino, Homes on the north side of San Bernardino, winter of 1980winter of 1980

Home destroyed by a small debris flow Home destroyed by a small debris flow during the winter of 1980during the winter of 1980

Side view of the home and debris flow path.Side view of the home and debris flow path.

Debris flow, La Tuna Canyon, 1984Debris flow, La Tuna Canyon, 1984

House and debris flow, Los Angeles, 1978House and debris flow, Los Angeles, 1978

What can be done?What can be done?

Deflector wall

Los Angeles County Department of Los Angeles County Department of Public Works debris basinsPublic Works debris basins

Can it happen here?Can it happen here?

What do you think?What do you think?

The Debris Flows of The Debris Flows of Madison County, VAMadison County, VA

JUNE 27, 1995JUNE 27, 1995

Location of Madison CountyLocation of Madison County

June 27, 1995 June 27, 1995

Severe storm triggered hundreds of rock, Severe storm triggered hundreds of rock, debris and soil slides debris and soil slides

debris flows inundated areas downslope debris flows inundated areas downslope causing damage to structures, roads, utilities, causing damage to structures, roads, utilities, livestock and crops livestock and crops

Rainfall amountsRainfall amounts

as much as 30 inches of rain fell in 16 hoursas much as 30 inches of rain fell in 16 hours in the area of maximum storm intensity in the area of maximum storm intensity

probably about 25 inches fell within a five-probably about 25 inches fell within a five-hour period hour period

Track of the Track of the stormstorm

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)

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Ch. 15 Mass Wasting