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

S

G N

F

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 DamThe dam stood 180 feet high and 600 feet longCurved 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 20-foot 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.