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CLASSIFICATION OF MASS WASTING

CLASSIFICATION OF MASS WASTING

It is important to classify slope failures so that we can understand what causes them and learn how to mitigate their effects. The three criteria used to describe slope failures are:

  • The type of material that failed (typically either bedrock or unconsolidated sediment)
  • The mechanism of the failure (how the material moved)
  • The rate at which it moved

The type of motion is the essential characteristic of slope failure, and there are three different types of motion:

  • If the material drops through the air, vertically or nearly vertically, it is known as a fall.
  • If the material moves as a mass along a sloping surface (without internal motion within the mass), it is a slide.
  • If the material has internal motion, like a fluid, it is a flow.
image
Major types of mass wasting and their physical characteristics from the USGS.

Unfortunately, it is not typically that simple. Many slope failures involve two of these types of motion, some involve all three, and in many cases, it is not easy to tell how the material moved. The types of slope failure are summarized below.

Failure Type Type of Material Type of Motion Rate of Motion
Rock fall Rock fragments Vertical or near-vertical fall (plus bouncing in many cases) Very fast (>10s m/s)
Rock slide A large rock body Motion as a unit along a planar surface (translational sliding) Typically very slow (mm/y to cm/y), some can be faster
Rock avalanche A large rock body that slides and then breaks into smaller fragments Flow (at high speeds, the mass of rock fragments is suspended on a cushion of air) Very fast (>10s m/s)
Creep or solifluction Soil or other overburden in some small cases, mixed with ice Flow (although sliding motion may also occur)

 

 

Very slow (mm/y to cm/y)
Slump Thick deposits of unconsolidated sediment Motion as a unit along a curved surface (rotational sliding) Slow (cm/y to m/y)
Mudflow Loose sediment with a significant component of silt and clay Flow (a mixture of sediment and water moves down a channel) Moderate to fast (cm/s to m/s)
Debris flow Sand, gravel, and larger fragments Flow (similar to a mudflow, but typically faster) Fast (m/s)

ROCK FALL

Rock fragments can break off relatively easily from steep bedrock slopes, most commonly due to frost-wedging in areas where there are many freeze-thaw cycles per year. When hiking along a steep mountain trail on a cool morning, one might have heard the occasional fall of rock fragments onto a talus slope. This happens because the water between cracks freezes and expands overnight, and then when that same water thaws in the morning sun, the fragments that had been pushed beyond their limit by the ice fall to the slope below.

Rock fall in Jalalabad, Afghanistan.

ROCK SLIDE

A rock slide is the sliding motion of rock along a sloping surface. In most cases, the movement is parallel to a fracture, bedding, or metamorphic foliation plane, and it can range from very slow to moderately fast. The word sackung describes the very slow motion of a block of rock (mm/y to cm/y) on a slope.

Rock slide

ROCK AVALANCHE

If a rock slides and then starts moving quickly (m/s), the rock is likely to break into many small pieces, and at that point it turns into a rock avalanche, in which the large and small fragments of rock move in a fluid manner supported by a cushion of air within and beneath the moving mass.

Goodell Creek rock avalanche, November 25, 2008.

CREEP OR SOLIFLUCTION

The very slow, millimeters per year to centimeters per year, movement of soil or other unconsolidated material on a slope is known as creep. Creep, which generally only affects the upper several centimeters of loose material, is typically a very slow flow, but in some cases, sliding may take place. Creep can be facilitated by freezing and thawing because particles are lifted perpendicular to the surface by the growth of ice crystals within the soil, and then let down vertically by gravity when the ice melts. The same effect can be produced by frequent wetting and drying of the soil. In cold environments, solifluction is a more intense form of freeze-thaw-triggered creep.

Creep is most noticeable on moderate-to-steep slopes where trees, fence posts, or grave markers are consistently leaning in a downhill direction. In the case of trees, they try to correct their lean by growing upright, and this leads to a curved lower trunk known as a “pistol butt.”

image
Soil creep below Morgan’s Hill. Image taken by Derek Harper.

SLUMP

Slump is a type of slide (movement as a mass) that takes place within thick unconsolidated deposits (typically thicker than 10 m). Slumps involve movement along one or more curved failure surfaces, with downward motion near the top and outward motion toward the bottom. They are typically caused by an excess of water within these materials on a steep slope.

MUDFLOWS AND DEBRIS FLOWS

When a mass of sediment becomes completely saturated with water, the mass loses strength, to the extent that the grains are pushed apart, and it will flow, even on a gentle slope. This can happen during rapid spring snowmelt or heavy rains, and is also relatively common during volcanic eruptions because of the rapid melting of snow and ice. (A mudflow or debris flow on a volcano or during a volcanic eruption is a lahar.) If the material involved is primarily sand-sized or smaller, it is known as a mudflow.

If the material involved is gravel-sized or larger, it is known as a debris flow. Because it takes more gravitational energy to move larger particles, a debris flow typically forms in an area with steeper slopes and more water than does a mudflow. In many cases, a debris flow takes place within a steep stream channel, and is triggered by the collapse of bank material into the stream. This creates a temporary dam, and then a significant flow of water and debris when the dam breaks.

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Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Kategoria: Moje artykuły | Dodał: kolo (2019-04-08)
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