Definition of mass wasting = down slope movement of earth material (solid masses, dry aggregates and slurries) under the influence of gravity.

Image source: http://solidearth.jpl.nasa.gov/PAGES/old_site/landsl02.html

What is the extent of the environmental concern associated with mass wasting? In 2014 in the town of Oso, Washington 43 people were killed and 49 homes were destroyed by a mudflow. It was widely reported in the news, and occasional large and spectacular case histories such as this catch the public's attention for the moment. However, smaller mass wasting events generally escape notice, but in aggreagate are significant and represent a low-grade chronic challenge, as the statistic below suggest.

• average of 25-50 fatalities and $1.5 billion damage per year in U.S..
• numbers much higher elsewhere.
• California and other west coast states have struggled with this in particular.
• More than 315 landslides in Nebraska have been documented. Some have even caused damage, especially in NE part of the state.

" In geology, it would be known as a debris flow. Debris flows amass in stream valleys and more or less resemble fresh concrete. They consist of water mixed with a good deal of solid material, most of which is above sand size. Some of it is Chevrolet size. Boulders bigger than cars ride long distances in debris flows. Boulders grouped like fish eggs pour downhill in debris flows. The dark material coming toward the Genofiles was not only full of boulders: it ws so full of automobiles it was like bread dough mixed with raisins. On its way down Pine Cone Road, it plucked up cars from driveways and the street. When it crashed into the Genofile's house, the shattering of glass made terrific explosive sounds. A door burst open. Mud and boulders poured into the hall. We're going to go, Jackie thought. Oh, my God, what a hell of a way for the four of us to die together." From John McPhee, 1989, Los Angeles Against the Mountains, in The Control of Nature, Publisher -Farrar, Straus, Giroux.

Video on Devore, California mudflow.

Questions to play with:

• What are various types of mass wasting events (classification)?
• What is the largest mass wasting event possible on earth?
• What factors promote slope stability versus slope instability?
• Can future sites of slope failure be predicted?
• Can mass wasting be prevented and/or mitigated?

How they are classified can have legal ramifications as insurance policy sometimes only cover certain types and not others.

Important variables used to classify:

• type of material: rock vs. sediment, grain size distribution (sorted vs. unsorted), clay content and character.
• movement mechanism: parting across a plane, slip along a plane, fluid flow, free falling.
• movement speed: slow creep to catastrophic failure.

Examples of some types we will consider:

• downhill soil creep.
• rock topple, fall or glide.
• earth slumps (termed a rotational landslide in the diagram below): often form in sediment
• earth/rock slides: tend to form in hard rocks, ususally involving slip along a fracture plane within the rock.
• debris flows and mud flows: Colorado example.
• avalanche: important aspect is the runout distance, which for larger avalanches can be larger than most people's intuition would predict.

Diagram from USGS showing some of the different types of mass wasting events. Source: http://pubs.usgs.gov/fs/2004/3072/fs-2004-3072.html

Example of soil creep in metamorphic rocks (phyllites) from the Black Hills.

This image is from the town of Longyear, Norway on the Arctic island of Spitsbergen. These wooden stumps used to be the supports for some older buildings. The buildings are built above the ground to insulate them from the permafrost below. Note how they lean downhill. This is due to soil creep in the active permafrost layer (with abundant freeze-thaw action.

The above image is of a mountain side in Spitsbergen, with over 2000 feet of relief here. Note the conical masses of sediment at the base of the cliff. These are talus cones, and they form by a combination of debris flows and rock falls from the overlying cliffs. Each cone originates from an incision into the mountain side known as a chute. If one looks carefully, a distinct channel with levees exists on the middle and largest talus cone. This is a scar from a more recent mudflow channel. Scars of older mudflow channels are also evident. While it looks steeper here, the talus slope has around a 30 degree slope.

These photos from Yosemite were taken in 2012.

Mass wasting hazard map for Yosemite produced by the USGS. You can see that most of the hazards (the areas in color) are associated with the steep cliffs that lines the glacial valley. It turns out some of the parks facilities were in a particularly high hazard zone, and will be or have been moved. Producing mass wasting hazard maps like this are used for planning and siting purposes. Source: http://pubs.usgs.gov/of/1999/ofr-99-0578/

What is the largest mass wasting event possible on earth?

• Scale variant or invariant behavior, what's the difference? This harkens back to our discussion on the prediciton of earthquakes. If scale invariant, then knowing the behavior over smaller ranges (easier information to get) allows one to make predictions about larger events. However, unlike in the mathematical world where scale invariance can carry on for ever, in the real world there are upper and lower limits over which the phenomena exist. In that the larger events, more infrequent as they are, also have much greater cost associated with them, an important question becomes - what might be the largest landslide possible?
• Human history vs. geologic record? Have we seen in human history all that geology has to offer in terms of size of geologic events? Are smaller events in aggregate or a single larger event of greater risk, greater concern?
• The wave that paved Lanai: there is a single gravel layer with basalt and coral fragments extends up to an elevation of 326 meters up the slope of Lanai. How did it form? There is ongoing debate!
  • Link to USGS report on giant underwater landslides.

Map of the big island of Hawaii. Note the areas identified as slumps. These are places where a submarine 'landslide' has occurred in the past. Those that did not creep, but had some portion that slide suddenly may have very well produced tsunamis. Image source: NOAA - http://www.pifsc.noaa.gov/cred/himap/shoals.php

Image above of submarine landslide forms and deposits off the coast of California. Image source: http://walrus.wr.usgs.gov/socal/cabrillo/tierra/submarine_landslide.html .

• Slope profile steepening:
  • slope toe removal (undercutting) - what can produce undercutting?
  • magmatic inflation (on volcanic slopes, as we learned with Mount St. Helens eruption).
  • angle of repose sedimentation: on the edge of stability.
• Earthquakes: shaking can destabilize a normally stable slope.
• Change in material strength - > weathering and the production of clays (a slippery mineral).
• Water saturation: acts as suspension medium to create slurries (debris flows), promotes weathering, reduces viscosities, reduces friction on slip surface.
• Criteria for slip on a plane and the influence of water:

This is a schematic diagram of a potential slide block produced by the erosion of the slope toe by a river, and it helps elucidate the mechanics of slip. The contact between the yellow sandstone and the underlying granite would be a possible slip plane. The driving force is gravity (the green arrow). It can be resolved into its normal and slip components. As the inclination of the slip plane increases the gravitational force will remain the same, but the resolved normal force diminishes in size, while the shear force grows. Friction can be described as the ratio of normal to shear force that produces movement. Also important is the pore pressure which acts in opposition to the normal force, thereby making it easier to slip. This is why surfaces inclined at only a few degrees can slip - they are weakened by high pore pressures. Some type of question on these mechanics and this diagram will be on the test!

The best predictor of future behavior is past behavior.

Recognition of past occurrences or the right situation:

• geomorphic signature of the mass wasting event
• tree records of movement
• out of place rock
• colluvium
• potential slip planes:
  • bedding planes.
  • joint/fracture surfaces.
  • soil-bedrock contact.
  • active layer in permafrost.

Human activity can induce mass wasting in an area that has been historically stable.

Slope stability analysis - what is strength and what are forces acting on slope material.

Cretaceous Pierre shale: low strength and very prone to mass wasting, widespread in upper midwest.

Can mass wasting be prevented and/or mitigated?

There are a variety of approaches that are used on a regular basis:

• drainage of slope interior.
• rock bolts.
• berm barriers at base of cliff.
• retaining walls (should be drained and anchored).
• rock profiling.
• drainage of lakes to prevent breakout floods once it has occurred.
• slope design.
• zoning.
• debris flow catchment basins.
• insurance.
• monitoring and prediction.

Image source: United States Geological Survey (USGS), 2005. Real-Time Monitoring of an Active Landslide above U.S. Highway 50, California. Retrieved on 25 February, 2006 from http://landslides.usgs.gov/monitoring/hwy50/index.php.

Left: Example of debris flow catchment structure in California. Note that there is an outlet for the water at normal low flows. Right: Emptying debris basin and getting ready for the Image source: http://pubs.usgs.gov/of/2001/ofr-01-0144/

• Huascaran, Peru:
  • occurred in 1970.
  • triggered by earthquake, with source a glacial cirque wall in Mount Huascaran.
  • debris avalanche with 16 km runout distance a source to toe relief of circa 4 km, and speeds in excess of 300 km/hour.
  • movement mechanism?
  • Covered portions of towns of Yungay and Ranrahirca, circa 20,000 fatalities.
  • somewhat smaller avalanche had reached margin of Ranrahirca before in 1962, killing about 3000 people.
  • 
  • Image of the Huascaran avalanche. Source - USGS.
• Gros Ventre, Wyoming.
• Vaiont, Italy:
  • occurred in Oct. 1963.
  • triggered by reservoir construction and filling in previous months.
  • large slide on mountain side slid into reservoir at speed possibly at 90 km/hour, sending a wall of water over the dam down into the town of Longarone below. Wall of water up to 70 meters high.
  • circa 3,000 people killed.
  • had detected creep in mountain side before, but actions were too late.
• Blackhawk, California:
  • occurred in prehistoric time, 135 km east of LA.
  • rock avalanche of interest because of 8 km runout on a slope of 2.5 degrees.
  • modeling suggests it had to move at 120 km/hour.
  • acoustic lubrication is one model for this anomalous behavior.
  • Image to the right is of the slide. Source: http://www.planetary.org/multimedia/space-images/earth/blackhawk-slide.html .
• USGS site on the La Conchita landslide (and its previous history).

USGS mass wasting site.

Mini-field trips:

• Slump along the Keystone trail of Omaha.
• Shell Canyon, Big Horn Mountains of Wyoming earth flows.

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