Mass Wasting

When material on a hillside has weathered, it is likely to move downslope because of the pull of gravity. Soil or rock moving in bulk at Earth's surface is called mass wasting. Mass wasting is one of several surficial processes. Other processes of erosion, transportation, deposition - involving streams, glaciers, wind, and ocean waves - are discussed in following chapters.

Landsliding is the best known type of mass wasting. Landslides destroy towns and kill people. While these disasters involve relatively rapid movement of debris and rock, mass wasting can also be very slow. Creep is a type of mass wasting too slow to be called a landslide.

In this chapter, we describe how different types of mass wasting shape the land and alter the environment and what factors control the rapidity or slowness of the process. Understanding mass wasting and its possible hazards is particularly important in hilly or mountainous regions.

Key Points

What is Mass Wasting? What is the principle cause of mass wasting?

What is the roles of water in mass wasting?

What are three major types of slope failure?

Use key words to search for image and video about the process (Slope failure, Slump, Fall, Slide, Topple, wet flow, dry flow, Debris flow, mud flow, Earthflow, Debris avanlance, Debris fall, Sediment flow, Creep, Solifluction, granular flow)

What triggers Mass-Wasting Events?

Different slope design procedure for soil slopes (i.e. avoid problem, reduce driving forces, increase resisting forces) and the application. See table 14.9 in book.

Different slope design procedure for soil slopes (i.e. Stabilization methods, protection methods, warning methods). See Figure 4.16.

Technical Terms

creep

Very slow, continuous downslope movement of soil or debris.

debris

Any unconsolidated material at Earth’s surface.

debris avalanche

Very rapid and turbulent mass wasting of debris, air, and water.

debris fall

A free-falling mass of debris.

debris flow

Mass wasting in which motion is taking place throughout the moving mass (flow). The common varieties are earthflow, mudflow, and debris avalanche.

debris slide

Rapid movement of debris as a coherent mass.

earthflow

Slow-to-rapid mass wasting in which debris moves downslope as a very viscous fluid.

fall

The situation in mass wasting that occurs when material free-falls or bounces down a cliff.

flow

A type of movement that implies that a descending mass is moving downslope as a viscous fluid.

landslide

The general term for a slowly to very rapidly descending mass of rock or debris.

mass wasting

Movement, caused by gravity, in which bedrock, rock debris, or soil moves downslope in bulk.

mudflow

A flowing mixture of debris and water, usually moving down a channel.

permafrost

Ground that remains permanently frozen for many years.

relief

The vertical distance between points on Earth’s surface.

rock avalanche

A very rapidly moving, turbulent mass of broken-up bedrock.

rockfall

Rock falling freely or bouncing down a cliff.

rockslide

Rapid sliding of a mass of bedrock along an inclined surface of weakness.

rotational slide (slump)

In mass wasting, movement along a curved surface in which the upper part moves vertically downward while the lower part moves outward. Also called a slump.

shear force

In mass wasting, the component of gravitational force that is parallel to an inclined surface.

shear strength

In mass wasting, the resistance to movement or deformation of material.

slide

In mass wasting, movement of a relatively coherent descending mass along one or more well-defined surfaces.

solifluction

Flow of water saturated debris over impermeable material.

talus

An accumulation of broken rock at the base of a cliff.

Extra readings

1. Mass wasting is movement of bedrock, rock debris or soil downslope as a mass in response to gravity.

2. Mass wasting is classified on the basis of rate of movement, type of material, and nature movement. Rates vary from slow (< 1 cm/year) to rapid. Types of material include bedrock, unconsolidated debris, and soil. Movements include flow (movement as viscous fluid), slide (movement as coherent mass along defined surface), and fall (free-fall).

3. Slides may be either translational (plane parallel to slope) or rotational (also called slump) (movement along a curved surface).

4. Factors promoting mass wasting include: steep slopes, high local relief, thick debris above bedrock, planes of weakness parallel to hillside, freeze and thaw, saturation of debris with water, long periods of drought with episodes of heavy precipitation, and sparse vegetation.

5. Gravity is the driving force for mass wasting. Shear force is parallel to slope. Shear strength is resistance to movement or deformation. If shear force is greater than shear strength, mass wasting occurs.

6. Water is a critical factor in mass wasting. Small amounts of water actually inhibit mass wasting because surface tension increases shear strength. As the water content increases, the rate of movement increases, for example a change from creep to mudflow.

7. Creep is very slow, continuous downslope movement of soil or unconsolidated debris promoted by water in the soil and cycles of freeze and thaw where shear forces are slightly greater than shear strength.

8. Debris flows are mass wasting taking place as a moving mass, including earthflows, mudflows and debris avalanches. Earthflow is slow or rapid downslope movement of water saturated debris as a viscous fluid. Rotational sliding (movement along a curved surface) (previously called slumping) is commonly associated with earthflow. Solifluction is a variety of earthflow that occurs above permafrost in colder climates. Mudflow is a flowing mixture of debris and water usually moving in a channel. Debris avalanche is a very rapidly moving mass of debris, air and water.

9. Rockfalls are blocks of bedrock that fall freely or bounce down a cliff. A rockslide is a mass of bedrock moving along an inclined surface; a rock avalanche is a larger version of the same process. Debris slides and debris falls are the same process involving movement of a coherent mass of debris rather than bedrock.

10. Landslides can be prevented by recognizing potential problems and proper engineering. Avoiding oversteepening of slopes and undercutting slopes as well as providing adequate drainage for excess water are useful measures.

Questions

What is the principal agent of mass wasting?

What type of mass wasting process which is characterized by movement of material along a curved slip face?

What is water saturated form of mass wasting?

What type of mass wasting associated with volcanoes?

What type of mass wasting/flow that the velocity is between 1 mm/day and 1 km/hour ?

What type of mass wasting that involves with bedrock material?

Hints for above questions: choose one of the following : Rock Avalanche; Gravity; Earthquake; Rockslide; Creep; Lahar; Solifluction; Rotational Slide (Slump); Earthflow; Mud Flow

What is the slowest type of mass wasting process?

What do we call unconsolidated material at the Earth's surface of any size?

Which term refer to A descending mass moving downslope as a viscous fluid?

What is the driving force behind all mass wasting processes

What is the resistance to movement or deformation of debris ? Hint: choose A) shear strength; B) mass; C) shear force; D) density

What do we call Flow of water-saturated debris over impermeable material?

What is nn apron of fallen rock fragments that accumulates at the base of a cliff? Hint: A) soil ; B) ; sediment; C) talus ; D) debris

What are effective in preventing rocks from falling on a roadway?

Slopes that have been stable for many years may sometimes fail catastrophically. What is a common trigger for these failures?

When does a rockfall occur?:

For mass wasting to occur, what forces must become unbalanced?

What is the aspect to classify Mass wasting processes? Or How are mass wasting processes classified?

Write the methods of stabilizing slopes to prevent mass wasting?

In which way that slopes can be destabilized and the potential for mass wasting increased?

What are the triggers of mass wasting events?

Is pyroclastic flow a form of mass wasting?

Where are rock avalanches most common? Hint: A) in the deep sea; B) in areas where glaciers have deposited boulders and other gravel; C) in areas of steep topography where large earthquakes are possible; D) all of these can generate rock avalanches

What is the downslope movement of material under the influence of gravity?