Mass Movements

Mass movement- The down slope movement of earth material under the influence of gravity.

Figure 10.3 Key role in mass movement is gravity

In order for the material to move down slope, it must overcome 1.Friction –surface resistance to relative motion. 2.Inertia – tendency of an object at rest to remain at rest.

Internal Causes of Slope Failure Inherently weak earth materials

1.Clay – most earth failures in the soil zone are due to clay.

Clay layer in a sedimentary rock beach cliff

2.Pyroclastic materials – Light colored tuff from Santorini produced 3500 year ago in Minoan eruption.

3.Shale – sedimentary rock made of clay and silt particles

Bedding planes, friend or foe?

Internal problems caused by water 1. Adds weight to a slope 8.34 lb/gallon 54 gal. = lb! 54 gal. barrel

2.Saturation increases the pore-water pressure Low pore water pressure High pore water pressure

3.Dissolves the cementing material in rock

4.Causes “piping”, the subsurface erosion of earth material

The Vaiont Dam Failure Vaiont, Italy 1963 Setting:Dam on river in a glacially carved canyon.

Some Factors Leading to Slope Failure 1.Valley is a syncline 2.Cavernous weathering of limestone rock 3.Valley downcutting by river 4.Slope saturation - water from reservoir and heavy rains.

Massive landslide and ~3,000 dead.

Falls Occurs when one or more rocks fall down steep slopes.

Rockfall, South Island, New Zealand

Moonlight Beach, Encinitas

Yosemite ,000-ton rock mass fell.

Yosemite rockfall 1996

A common cause of rock fall is frost wedging. Talus cones

Slide Occur when a more or less coherent mass moves down slope.

Rotational slide- slump 1.Curved failure plane. 2.Head moves down, toe moves up and out.

Don’t move far or fast

In California, triggered by heavy winter rain. Pacific Palisades, CA

Another cause of slumps – road cuts. Elks Lodge near Lewiston, Idaho

Translational slide – landslide 1.Mass moves down and out. 2.Moves far and fast. 3.Primary causes: heavy rains or earthquakes.

Landslide on Big Sur Coast

Madison River Canyon Landslide 1959

Madison River Canyon from top of slide

View from debris dam and visitors center

Flow Mass loses coherence and moves as a viscous fluid.

Earthflow- Downslope movement of a fluid layer of mainly soil and weathered rock material that is saturated with water.

Earthflows often begin as slumps. Usually, like slumps, they do not move very fast.

Road cut caused slump/earthflow

Debris flow- Downslope movement of a fluid mixture of mud and larger rock debris. Venezuela 1999

Key differences between earthflows and debris flows: 1.A debris flow moves faster; meters/second. 2.A debris flow carries larger rock material.

Debris flows can carry large boulders because their density is 1.5 – 2.0 times that of water. 74

Causes of debris flows: 1.Most commonly heavy rains or rapid snow melt. 2.Volcanic eruptions – a lahar is a type of debris flow. 3.Earthquakes

Debris flow Alps 2004

Debris flow after rains in an area previously hit by wildfires in Utah.

Debris flow in San Bernardino after 1980 wildfires

Amago Creek debris flow after Oct fires

Debris Avalanche- A very fast moving debris flow occurring on steep slopes.

La Conchita 1995 slump/earthflow 2005 debris avalanche

Debris avalanches O4-4 ziw&feature=fvwrel

Small mass movements usually move horizontally less than twice their vertical distance of fall. With very large mass movements they may move up to 25 times the distance of their vertical fall.

Sturzstroms- Long-run out debris flows. Blackhawk sturzstrom 300 million m 3 Drop.75 mi.Flow 5.6 mi.

Nevado Huascaran

1970 subduction zone earthquake 100 million m 3 of rock, ice glacial sediments, and water. Speed mph Devastated area 8.7 mi 2 18,000 deaths

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Landslide Thistle, Utah 1983

Landslide 1903 in Frank, Alberta