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

Ch. 15 Mass Wasting

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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. - PowerPoint PPT Presentation

Text of Ch. 15 Mass Wasting

  • 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 wetted.

  • 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

  • Talus Slope

  • 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.

  • Slump

  • 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.

  • 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 MountainsDebris 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 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 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 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)