6.2: Classification of Mass Wasting

Overview of Classification

It is important to classify slope failures so that we can understand what caused them, learn how to mitigate their effects, and communicate clearly. 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), and
• The rate at which it moved.

The type of motion is the most important characteristic of a slope failure, and there are three different types of motion: if the material drops through the air, vertically or nearly vertically, it’s known as a fall, if the material moves as a mass (without internal motion within the mass), it’s a slide, and if the material has internal motion, like a fluid, it’s a flow. Unfortunately, it’s not normally that simple. Many slope failures involve two of these types of motion, some involve all three, and in many cases it’s not that easy to tell how the material moved. The types of slope failure that we’ll cover here are summarized in Table 6.1.1.

Table \(\PageIndex{1}\): Classification of Slope Failures Based on Type of Material and Type of Motion

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), but some can be faster
Rock avalanche	A rock body that slides and then breaks into small 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 cases mixed with ice	Flow (although sliding motion may also occur)	Very slow (mm/y to cm/y)
Slump	Thick deposits (m to 10s of m) of unconsolidated sediment	Motion as a unit along a curved surface (rotational sliding)	Slow (cm/y to m/y)
Mud flow	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 mud flow, but typically faster)	Fast (m/s)

Rock Fall

Rock fall describes the abrupt movement of rock which become detached from steep slopes or cliffs. 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. If you’ve ever hiked along a steep mountain trail on a cool morning you might have heard the occasional fall of rock fragments onto a talus slope as the sun melts the ice, releasing rock fragments that had been wedged out the night before.

A typical talus slope, near to Keremeos in southern BC, is shown on Figure \(\PageIndex{1}\). In December 2014 a large block of rock split away from a cliff in this same area. It broke into smaller pieces, which fell and tumbled down the slope and crashed into the road, smashing the concrete barriers and gouging out large parts of the pavement.

Figure 6.2.1 Left: A Talus Slope Near to Keremeos, BC, Formed by Rock Fall from the Cliffs Above. Right: The Results of a Rock Fall onto a Highway West of Keremeos in December 2014.

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 plane 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 steep slope. A good example is the Downie Slide north of Revelstoke BC, which is illustrated on Figure \(\PageIndex{2}\). In this case a massive body of rock is very slowly sliding down a steep slope along a plane of weakness that is parallel to the slope.^[1]

Figure \(\PageIndex{2}\): The Downie Slide, A Sackung, on the Shore of the Lake Revelstoke Reservoir (Above the Revelstoke Dam). The head scarp is visible at the top and a side-scarp along the left-had side.

A rock slide is typically a “translational slide”, meaning that the part of the rock that is moving down the slope without rotating—it is translating. Please don’t confuse “translational” with “transitional”. As we’ll see below, a “rotational slide” (or slump) is when the material (typically unconsolidated sediments) moves as a single rotating unit along a curved surface.

Rock Avalanche

If a rock slide starts moving quickly the rock is likely to break into many small pieces, and at that point it can become a rock avalanche, in which the large and small fragments of rock move in a fluid manner supported by cushion of air within and beneath the moving mass. The 2010 slide at Mt Meager (west of Lillooet), is a rock avalanche ( Figure \(\PageIndex{3}\)).^[2]

Figure \(\PageIndex{3}\): The 2010 Mt. Meager Rock Avalanche, Showing Where the Slide Originated (top center of image). It then raced down a steep narrow valley, into the valley of Meager Creek, and then out into the wider valley of the Lillooet River at the bottom of the image.

Creep

The very slow—mm/y to cm/y—movement of soil or other unconsolidated material on a slope is known as creep. Creep, which normally only affects the upper several centimeters of loose material, is typically a type of very slow flow, but in some cases sliding may take place. Creep can be facilitated by freezing and thawing, because particles get lifted perpendicular to the surface by the growth of ice crystals within the soil and are then let down vertically by gravity when the ice melts. The same effect can be produced by frequent wetting and drying of the soil.

Creep is most noticeable on moderate to steep slopes where trees, fence posts or gravestones are consistently leaning in a downhill direction (Figure \(\PageIndex{4}\)). 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” (or “j-shaped tree trunk”). Creep can take place on nearly flat surfaces.

Figure \(\PageIndex{4}\): A Hillside with Pistol-Butt Trees as Evidence of Persistent Creep

Slump

Slump is a type of slide (movement as a mass), that takes place within thick unconsolidated deposits (typically greater than 10 m). Slumps involve movement along a curved surface, with downward motion near to the top and outward motion towards the bottom (Figure \(\PageIndex{5}\): ). They are typically caused by an excess of water within the materials on a steep slope.

Figure \(\PageIndex{5}\): A Depiction of the Motion of Unconsolidated Sediments in an Area of Slumping

An example of a slump in the Lethbridge area, Alberta, is shown on Figure \(\PageIndex{6}\). This feature has likely been active for many decades and moves a little more whenever there are heavy spring rains and significant snow-melt runoff. The toe of the slump is failing because it has been eroded by the small stream at the bottom.

Figure \(\PageIndex{6}\): A Slump Along the Banks of a Small Coulee Near to Lethbridge, Alberta. The main head-scarp is clearly visible at the top, and a second smaller one is visible about one-quarter of the way down. The toe of the slump is being eroded by the seasonal stream that created the coulee.

Mud Flows and Debris Flows

When a mass of sediment becomes completely saturated with water, to the extent that the grains are pushed apart, the mass will lose strength and flow, even on a gentle slope. This can happen during rapid spring snow melt or heavy rains and is also relatively common during volcanic eruptions because of the rapid melting of snow and ice. If the material involved is primarily sand-sized and smaller it is known as a mud flow, such as the one shown on Figure \(\PageIndex{7}\). If the material involved is a mixture of sizes, including gravel-sized and 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 a steeper slope and more water than does a mudflow. A typical debris flow is shown on Figure \(\PageIndex{8}\). This event took place in November 2006 in response to very heavy rainfall. There was enough energy to move large boulders and to knock over large trees.

Figure \(\PageIndex{7}\): A Slump (left) and an Associated Mudflow (center), Near to Lethbridge, Alberta.

Figure \(\PageIndex{8}\): Effects of a Debris Flow Within a Steep Stream Channel Near to Buttle Lake, BC., November 2006

As already noted, to understand a slope failure we need to be able to determine what type of material moved, what type (or types) of motion were involved, and how quickly it moved. The type of motion is the most important of these, and so Figure \(\PageIndex{9}\) is provided here to help you clearly understand how things moved in different types of slope failure.

Figure \(\PageIndex{9}\): Types of Slope-Failure Motion. A fall is typically a rock fall. A flow can be creep, mudflow, debris flow, or rock avalanche. A translational slide is typically a rock slide. A rotational slide is typically a slump.

Media Attributions

• Figure \(\PageIndex{1}\): Photos by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{2}\): Image from Google Earth.
• Figure \(\PageIndex{3}\): 2010 Mount Meager Landslide by Tim Gage, 2014, CC BY SA 2.0, via Wikimedia Commons, https://commons.wikimedia.org/wiki/F..._landslide.jpg
• Figure \(\PageIndex{4}\): Photo by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{5}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{6}\): Photo by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{7}\): Photo by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{8}\): Photo by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{9}\): Steven Earle, CC BY 4.0

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1. Kalenchuk, K. S., Hutchinson, D. J., Diederichs, M. S., and Moore, D. (2012). Downie Slide, British Columbia, Canada. In Clague, J. J. & D. Stead (Eds.), Landslides: Types, Mechanisms and Modeling (p. 345-358). Cambridge University Press. ↵
2. Guthrie, R. et al., (2012). The 6 August 2010 Mount Meager rock slide-debris flow, Coast Mountains, British Columbia: characteristics, dynamics, and implications for hazard and risk assessment. Natural Hazards Earth System Science, 12(5), 1277–1294, https://doi.org/10.5194/nhess-12-1277-2012. ↵