“stuff rolls downhill” Ch. 15 Mass Wasting “stuff rolls downhill”

Mass Wasting 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? 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 For mass wasting to occur, there must be a slope angle 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. 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. 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

Talus Slope

Slump 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 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.

Mudflow

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

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.

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? San Gabriel Mtns deeply fractured due to stresses on the rocks caused by faults rapidly uplifting and weathering Very steep slopes Fires 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 LA averages ~ 15 in/yr. San Gabriels can get extreme rainfall events 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 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 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 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)