15 Mass wasting

Dr. Niranjani Dwivedi

   Pre-requisites

Knowledge of weathering, gravity, slope and simple English language

Keywords

Friction, gravity, heave, impervious, liquefaction, permafrost, shear, strain, stress

Learning Outcome

•  Student will acquire understanding of natural geomorphic processes of surface modification .
•  Student will acquire skill to analyze cause-effect relationship between various environmental and human factors.
• Student will be equipped with knowledge1 to study further hazard management issues.

 1. Mass wasting –Concept and mechanics

1.1. Mass wasting: Concept

1.2. Mechanics of Mass wasting

1.2.1 Modes of movement in mass wasting

2. Factors controlling mass wasting

3. Types of Mass wasting

3.1 Classification of mass wasting

3.2 Major types of mass wasting

3.2.1 Solifluction

3.2.2 Soil creep

3.2.3 Mud flow and earth flow

3.2.4 Slide and slump

3.2.5 Debris Fall and rock fall

3.2.6 Subsidence

3.2.7 Other types

4. Preventing loss due to mass wasting Summary

MASS WASTING

Mass wasting and our life

• Man and mass wasting have a two-way relation.
• Settlements and important infrastructure like dams, railways and roads are threatened by mass wasting.
• Human activity sometimes enhances chances of mass wasting.
• Annual losses in each of the United States, Japan, Italy, and India have been estimated at $1 billion or more.
• Understanding mass wasting process and careful monitoring of vulnerable sites helps in prediction, evacuation and mitigation of mass-wasting related disasters.

    5. Mass wasting: Concept and mechanics

1.1. Mass wasting: Concept

The surface of the earth is everywhere attacked by forces that lead to decomposition and disintegration of rocks. The layer of waste this produces is called regolith. It is unconsolidated matter, and a mixture of rock pieces and fine soil. Lying on a sloping surface, regolith is pulled by the earth’s gravitational force and moves down. This down-slope movement of weathered material under the influence of gravitational influence is defined as ‘Mass wasting’, also termed ‘Mass movement’. Sometimes mass wasting may also involve intact rock beds. According to A. S. Goudie – “A mass movement is the downward and outward movement of slope-forming material under the influence of gravity.” To this definition Encyclopaedia Britannica adds “the rapid or gradual sinking of the Earth’s ground surface in a predominantly vertical direction.” The process has a variety of rates and mechanics of movement; involves particles ranging from minute fine clay to massive rock beds; leads to impacts ranging from insignificant events to large scale disaster, and results in creation of various landforms.

According to W. D. Thornbury the importance of mass wasting as a process of creating surface features was realizes much later. R. P. Sharpe (1938) was the first to pay serious attention to mass wasting process, conditions and types .

1.2. Mechanics of Mass wasting

The process of mass wasting and the all the other details related to it can be understood if we look at the interplay of different forces acting on weathered mass sitting on a sloping surface. Basically it is gravity, which is analyzed into two main components that together control movement of regolith on all slopes (Fig.1.1). These are:

Slide component, also called stress ;

Stick component, also called friction or shear resistance.

(Source: self)

Slide component works in the downslope direction and pulls the rock mass towards foot of a slope, this leads to ‘stress’ or tension between the solid unweathered rock surface and the unconsolidated regolith lying on it. It also exerts stress along bedding planes, joints, crevasses and fractures within a solid rock body. Even the mineral grains within a rock respond to this stress, and internal stress is experienced by the matter.

Stick component works in a direction perpendicular to the slope and creates friction between regolith and slope and at all the other sites that are under stress, mentioned above. The friction created by stick component counteracts the downward pull. Whether a mass would move or not is decided by the critical balance between these forces, which in turn depends primarily on the steepness of slope.

On a slope measuring 30° if gravity exerts force equal to 1kg on a mass, the two components may be calculated as:

Slide component would show force equal to 0.5 kg (1kg sine 30) and Stick component is 0.85kg. (1kg cos 30).

Here frictional force opposing motion is stronger, hence it will neutralize the weaker slide force and the mass will not move. In contrast, if the slope is steeper, such as 60° then mass movement would happen, because in this case stress will have stronger force, which is directed to the direction of movement (Fig.2). Thus chances of slope failure may be calculated using the following equation where Fs stands for ‘factor of safety’ (Chorley, Schumm and Sugden):

Fs = shear resistance / magnitude of stress

If the value of Fs is 1.0 or more, it indicates that gravity will not be able to move rock debris unless some other factor supports it. If Fs has a value less than 1.0 it indicates that slope is vulnerable to mass wasting. Besides many other factors, the balance between stress and shear resistance depends largely on steepness of slope. The rate and nature of the final movement may be influenced by several other factors that play an important role in the mass-wasting process.

Fig.2

(Source: self)

5.2.1   Modes of movement in mass wasting

Once a mass of regolith is displaced by gravity the weathered material may follow different modes of movements downhill (Fig.3). M. A. Carson and M. J. Kirkby consider three of them most important:

Slide,

Flow, and Heave

Fig. 3

(Source: self)

Slide

In slide movement, motion is maximized along the base of the moving mass. There is a clear and easy to identify surface dividing the mobile upper layer and the intact, stable lower zone. This separating plane is called the shear plane. Usually, the top of the mobile surface is able to keep pace with the rate of motion along the base, but sometimes it may lag behind. Slide can take place in absolutely dry matter, hence it can happen anywhere. However, presence of water facilitates it and induces greater speed.

Flow

In flow mode, material above the shear plane accelerates to the maximum speed at the top, while rate of movement diminishes with increasing depth till it reaches zero along the shear plane. This differential rate of movement within a mass is caused by increasing friction towards the contact zone between regolith and stable rock surface. In this mode water is an essential component of the process. As such it is a feature of humid regions.

Heave

This type of mechanism can move any particle size from a fine clay grain to a large boulder. The rate is mostly very slow, and actual movement imperceptible unless it is measured using sophisticated methods. The ultimate result of change in position of matter can be seen and is taken as evidence of mass wasting activity.

In this mode of movement regolith alternatively experiences swelling or expansion, and shrinking or contraction. Expansion may be a result of moisture absorption, heating or ice crystal formation. As water changes into ice there is about 10% increase in volume of moisture within regolith, causing an upward push. Contraction happens when a moist particle dries up, hot surface cools off or ice crystals melt and convert into liquid. During these expansion-contraction cycle particles rise upwards perpendicular to the original surface as they expand (Points 2, 4, 6 in Fig. 3) and settle down vertically while contraction compels downward movement (Points 3, 5, 7 in Fig. 3). Thus in Fig. 3 a particle originally at point 1 would gradually shift to 2à3à4….till it reaches point 7 at the foot of the slope.

Besides these three modes, sub-varieties of mass wasting may be induced by factors like shape of shear plane and presence of vegetation or boulder obstruction. Examples involving curved and rectilinear shear planes may be given here. Rock debris moving on curvilinear shear plane shows a tendency of difference in velocities at their base and top; mostly speed increases with depth (Fig. 4 & 5).The resulting mass wasting has a characteristic backward rotation. On the other hand a rectilinear shear plane exhibits equal rate of movement throughout the mass (Fig.6).

Fig.4

Maximum movement along shear plane

(Source: self)

Fig.5

(Source: self)

Fig. 6

(Source: self)

Angle of repose

• All loose particles can stabilize in a pile at a certain maximum angle of steepness.
• Any increase in steepness induces slipping of the particles.
• Angle of repose plays a crucial role in talus and scree cone development.

   6. Factors controlling mass wasting

There are several natural and anthropogenic factors that regulate initiation, rate and type of mass wasting. Different views have been expressed in this regard. For example Sharpe (Table-I) groups the controlling factors into:

Passive and Active

Table-I

Causes of Mass Wasting

This classification focuses mostly on inherent characteristics of regolith and its environment. Others (D. J. Varnes 1978) have classified causes of mass wasting with a focus on the forces working on weathered material. From this viewpoint, on the one hand, there are forces that enhance the pulling effect of gravity or the slide component. On the other there are conditions that play a negative role with reference to the stick component, and weaken stability of matter. When a strong slide force combines with a weak stick force, ideal condition for mass wasting is created. The two categories of forces are controlled by several

supporting factors (Table-II):

Factors that increase stress,

Factors that  reduce shear strength

Table- II

Factors Affecting Stress and Shear Components

The above two tables make it clear that interaction between several factors leads to conditions favouring slope failure. Broadly speaking, on the one hand, regolith has some inherent favourable qualities – it becomes heavy, slippery, non-cohesive and ready to move, while on the other hand some external factors play a vital role, such as if foothill area supporting upper slope and stabilizing it is removed, or violent tremors destroy the shear strength and dislodge weathered material from the slope. Both the categories of such factors lead to the same result, that is, gravity succeeds in pulling down the weathered surface layer. Two or more conditions may combine in some cases and cause mass wasting of exceptional intensity. For example, the April 2015 Nepal earth quake triggered several incidents of mass wasting in the area. Village Langtang suffered from a two to three km-wide avalanche. The Trishuli river valley of Nepal witnessed several landslides.

Sub marine landslides

• Slump, mud flow, and landslides keep happening all along shorelines.
• They carve network of canyons in off shore belts.
• Undersea cables are damaged as sediment travels with great velocity here.
• 1929 Grand Banks Earthquake (North East America) caused submarine landslide displacing 200 km3 sediment and snapped 12 submarine transatlantic telegraph cables

    7. Types of Mass wasting

•  3.1 Classification of mass wasting Different classifications of mass wasting have been suggested, founded on different bases. Arthur Bloom discusses a ‘Descriptive Classification of Mass Movement’; mode of movement and nature of weathered material involved are the bases of his classification. Another classification of mass wasting is suggested by Carson and Kirkby. Their classification takes into account moisture conditions, along with rate and mechanics of movement of weathered matter (Fig 7).

Fig.7

(Source: self, based on Chorley et al)

The following three mechanisms of movement are the primary basis in this classification of mass wasting:

1. Heave
2. Flow and
3. Slide

   In this classification, the three basic categories replace one another as climatic condition change from extreme dry to extreme wet moisture regimes. Ultimately mass wasting is replaced by fluvial processes (warm regions) or glacial processes (cold regions). Particle size involved ranges here from soil to talus to rocks. Depending on availability of moisture and nature of regolith, sub-types acquire specific characteristics and are identified as several separate classes of mass wasting. Thus the three basic mechanisms lead to several subtypes of the process (Fig.7 and 8).

Fig.8

(Based on Sharpe)

   3.2 Types of mass wasting

3.2.1 Solifluction

It can be translated as ‘soil flow’. As the name indicates it is a sub-type of flow movement. In this type surface is covered with water saturated regolith. As water content increases soil cover changes to a soggy matter and loses cohesive strength. This weakens friction or stick component, enabling gravity to move the weathered layer on the slope.

Presence of an impervious sub-surface layer and ample soil moisture are vital for this process. It can happen both in warm and cold climatic regions. In warm regions moisture is provided by precipitation or surface run off. Here, the sub-surface impervious layer may be a bed of slate or schist or any other such hard rock. In cold regions water is supplied as meltwater during spring season, when all surface snow and ice starts melting. The impervious sub-surface layer here is the vast deep frozen ground (permafrost) that neither warms up nor thaws. To differentiate processes in these two climatic regimes A. L. Washburn (1967) suggested usage of ‘gelifluction’ for the process in cold-climatic regions. A. L. Bloom also uses this term in his literature.

Solifluction/gelifluction is a slow process as compared to the other types. Mostly it covers extensive areas. The whole of Siberian tundra and Greenland hillsides are an evidence of its regional scale. Ground influenced by this type of mass wasting is broken into gently sloping, terrace-like features, which are sometimes bound by vegetative growth (Fig.9).

Fig. 9

Garland-like solifluction form in the Swiss National Park, at an altitude of 2’300m. (Source: https://commons.wikimedia.org/wiki/File:Solifluktion_Girlande.JPG)

3.2.2 Soil creep

This type of mass wasting is present almost everywhere on the earth’s surface, because first, it requires minimal support from steepness of surface and second, it can involve any type of debris. Third, it also does not need moisture, hence can happen in all types of climates. It follows mechanics of heave (see fig. 3).

Areas experiencing soil creep show several symptoms that are easy to identify – trees with downward bending lower trunks (Fig.11), tilt in exposed rock beds, broken fences and tilted electricity poles are some examples.

In mountainous areas, heaps of weathered matter collects at the base of slopes as talus and scree cones due to soil creep (Fig.10). In these cones, the frontal margin of cone migrates forward as soil creep continues and cones keep growing in size. Similar to soil creep is rock creep, which involves large boulders and rock chunks..

Fig. 10

North shore of Isfjorden, Svalbard, Norway

(Source: https://commons.wikimedia.org/wiki/File:TalusConesIsfjorden.jpg)

Fig.11

(Source: self)

3.2.3Mud flow and earth flow

The mechanics of earth flow and mud flow are similar to any other flow type of mass weathering; however, both have certain distinguishing features. Mud flow is large-scale, channelized movement while earth flow may happen on any surface in a localized manner.

Mud flow is confined to valleys and is a rapid movement. Arid regions have the most ideal conditions for it. Here seasonal streams cut valleys in mountainous areas. For most of the year, or sometimes for several years, the valleys stay dry and receive large amount of weathered debris from slopes. When eventually rains come, they are usually torrential and last for a short while (a general characteristic of precipitation in hot arid areas). As water is channelized and moves down the valleys, it keeps on incorporating accumulated rock debris from the valley floor. Absence of vegetative cover in these regions further supports this process. This mass has great velocity, erosive and transporting capacity. Movement stops under two conditions only – one, the matter becomes thick in consistency and is no longer fluid; two, flow reaches flat foothill area. At the terminal point all material brought down is left as a mound or in a fan shaped feature, called alluvial fan.

Eliot Blackwelder (1928) considered mud flow an important geomorphic agent in arid and semi-arid regions. He explained long distance transportation of huge rocks in these regions as the work of mud flow.

3.2.4 Slide and slump

Both these types follow slide mechanics and their rate of movement is fast (Fig. 7 and 8). For initial movement presence of water or ice as lubricant is not needed but rate of movement accelerates if regolith is wet. The distinction between these two is marked by difference in their shear planes. Slide has a rectilinear shear plane (Fig. 6) while slump has a concave upward, or spoon-shaped shear plane. Slide is again classified according to nature of weathered material. Debris slide, rock slide, mud slide and landslide are some examples.

Landslide is the most devastating, sudden and rapid slide movement (Fig.12). It is capable of transporting and burying settlements and destroying the ecosystem for a long time (Vedio1). It happens on slopes that have a combination of all positive stress and negative resistance factors, but the actual movement begins when it is triggered by some sudden change like heavy rain or earthquake. Mountains experience most landslides during rainy seasons. Rainwater increases weight of regolith and also lubricates it. In cold regions snow plays a similar role. Sometimes frozen rock mass may begin to slide, but as the heat of friction melts ice crystals and water content increases, the movement transforms into flow type.

Fig. 12

The Mameyes Landslide, Puerto Rico, which buried more than 100 homes, was caused by extensive accumulation of rains

(Source: – https://commons.wikimedia.org/wiki/File:Mameyes.jpg#/media/File:Mameyes.jpg)

Concavity of shear plane in slump results in backward tilt of slumping blocks (Fig.5 and 13). The mass also breaks down in blocks and creates a terraced surface (Fig. 14). The foot of the slope has a mound called ‘Toe’. The area from where the movement has started is marked by a steep scar (Fig. 13).

Fig. 13

Slump in Mupe Bay, England

(Source: https://commons.wikimedia.org/wiki/File:Mupe_Bay_cliffs.jpg)

Fig. 14

Lake Garda, Northern Italy, road cutting and removing support triggered a slump

(Source: https://commons.wikimedia.org/wiki/File:Limone_sul_Garda_Hotel_001.JPG)

3.2.5 Debris Fall and rock fall

Velocity in fall type ranks highest among all mass wasting types. Movement in fall type is vertical or almost vertical, and long cliff faces are an ideal site for it. The motion may start as slide, but may change to topple and terminate as fall. Slopes that have experienced glaciation are susceptible to rock falls (Arthur Bloom 1998), because unloading and frost action separate large rocks from the main slope; later, these blocks may be shaken off by triggers like earthquake tremors.

Falling rock pieces pose danger to lower slopes, especially transport links and settlements. To manage this risk, rocks have been covered by steel net along the Sion-Panvel highway (Fig. 15).

(Fig. 15)

(Source: https://commons.wikimedia.org/wiki/File:Sion_Panvel_Highway_Rock_Netting.jpg and https://commons.wikimedia.org/wiki/File:Rockfall.jpg#/media/File:Rockfall.jpg)

3.2.6 Subsidence

This type of mass wasting does not involve a shear plane or horizontal displacement. Surface layers move almost vertically downwards when they lose support from beneath. Sub-surface weakness may be caused by several factors, for example:

Mining,

Excess withdrawal of underground water,

Removal of rocks by carbonation in Karst regions, Removal of sub-surface fluid lava.

3.2.7 Other types

Besides the above mentioned types of mass wasting, there are other examples with minor variations in characteristics. Some of these are:

Avalanche – Any sudden and desaterous landslide may be called avalanche (Bloom 2003). It is a phenomenon of humid regions (Thornbury, 1969). It involves weathered rock mass, (that may have ice), both slide and flow mechanics, and rapid movement. If a slope experiences repeated debris avalanche, clear channels are cut, called ‘debris chute’.

Rockglide and topple – Large rock blocks are moved if the surface beneath them is lubricated or is softened to move plastically. The surface blocks simply ride over the mobile underlying material. Destabilized while gliding, these blocks tend to fall, called toppling.

Spreading – This involves multiple blocks. The mechanism of this movement is similar to rock glide, but it is a lateral movement. Cambering is an example of this type. Cambering takes place in glacial environment, where sediment moves from hill sides towards valley, and carries along sedimentary blocks.

Liquefaction – Solid surface (like sand and clay) is shaken during earthquake tremors and grain compaction within rock is loosened. This allows clay-rich rocks to behave like plastic matter, causing a typical spread movement known as ‘liquefaction’. It can uproot buildings from their foundations and move them.

Predicting landslides with acoustics

• A new type of sound sensor system has been developed to predict the likelihood of a landslide.
• It consists of a network of sensors buried across the hillside or embankment that presents a risk of collapse.
• The sensors, acting as microphones in the subsoil, record the acoustic activity of the soil.
• Noise rates, created by inter-particle friction, are proportional to rates of soil movement and so increased acoustic emissions mean a slope is closer to failure.
• Once a certain noise rate is recorded, the system can send a warning, via a text message, to the authorities responsible for safety in the area.

Source:http://www.usnews.com/science/articles/2010/10/25/new-acoustic-early-warning-system-for-landslide-prediction)

8. Preventing loss due to mass wasting

Owing to its vast reach and damaging effects, mass wasting forms an important part of geomorphology, especially applied geomorphology. To prevent social and economic loss, methods have been developed to observe and collect data on sites that are threatened by immediate movement.

A world map of landslide susceptibility has been prepared with the help of satellite images of soil, slope and deforested areas (Fig.16). This is connected to a satellite-aided global network program ‘Nowcast’, which is proposed to identify and warn of imminent landslides (Fig.17). Other local level methods are also developed, for example, ‘Slide Minder’ is a Wireline extensometer that can monitor slope displacement and transmit data remotely via radio or Wi-Fi (Fig. 18).

Fig.16

(Source: http://earthobservatory.nasa.gov/IOTD/view.php?id=7783)

Fig. 17

(Source: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120010510.pdf)S

Fig.18

(Source: https://commons.wikimedia.org/wiki/File:SlideMinder_Extensometer.png)

Summary

The surface of the earth is constantly impacted by forces whose action levels it down. Mass wasting is one of them. It is important because it works in all climatic regions, on all types of slopes and all surfaces. Gravitation of the earth is the sole force controlling all mass wasting incidents. However, temperature oscillations, freeze-thaw cycles, and moisture in form of rain or snow support this process in many ways.

Different types of mass wasting are grouped into categories on the basis of how a weathered mass moves: slide, flow and heave are identified as three basic mechanisms. These three modes further have several sub-groups, mainly on the basis of nature of regolith involved and rates of mass movement.

Each type has a role in formation of some or the other landform, including general lowering of surface.

The process of mass wasting is vitally important for us, because it destabilizes the surface on which man performs all his actions. Methods to measure vulnerability of areas, and means to predict or prevent a mass movement are developed and employed to safeguard against calamities.

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