Mass Wasting

Mass Wasting

Definition

The geomorphic process by which soil, regolith, and rock move downslope under the force of gravity.

Explanation

Mass wasting occurs on both terrestrial and submarine slopes and has been observed on Earth, Mars, Venus, and Jupiter's moon. When the gravitational force acting on a slope exceeds its resisting force, slope failure (mass wasting) occurs. The slope material strength and cohesion and the amount of internal friction between material help maintain the stability of the slope and are known collectively as the slope shear strength. The steepest angle that a cohesionless slope can maintain without losing its stability is known as its angle of repose. When a slope possesses this angle, its shear strength perfectly counterbalances the force of gravity acting upon it.

Types of Mass Wasting

Ultimately the classification results into the following types of mass wasting:

1. Creeps.
2. Slides.
3. Slumps.
4. Flows.
5. Falls.

1. Creeps

• Creep is a long term mass-wasting process on a downhill.
• In this process, the combination of small movements of soil or rock occurs in different directions over time.
• Creep is directed by gravity gradually downslope.
• The steeper the slope, the faster the creep.
• The creep makes trees and other shrubs bend and curve to reach the sunlight.
• These often trigger landslides because the dirt underneath is not very strong.
• The trees most of the time die out because of lack of water and sun, and these rarely happen in wet climates.
• Caused by freezing and then thawing, or hot and then cold temperature, it causes surface soils to move up then down.
• Creep happens at a rate that is not noticeable to the naked eye, and it also happens in the tropical regions.

2. Slides

• Slides are the sudden downhill movement of masses of rock or sediment or soils.
• Slides are also called Translational Slides, as they involve only straight movement along a direction.
• Rockslides and debris slides result when rocks or debris slide down a pre-existing surface, such as a bedding plane, foliation surface, or joint surface.
• Piles of the talus are common at the base of a rock slide or debris slide.
• Slides differ from slumps in that there is no rotation of the sliding rock mass along a curved surface.

     Landslides

Landslide is a general term used to describe the downslope movement of soil, rock, and organic materials under the effects of gravity and also the landform that results from such movement.

Landslides can be classified into different types on the basis of the type of movement and the type of material involved

The material in a landslide may be either a rock or soil (or both).

4. Slump

• The term slump refers to the slipping of coherent rock material along the curved surface of a decline.
• Slumps are also called as Rotational Slides due to the type of movement involved.
• These are special category slides wherein the downward rotation of rock or regolith occurs along a concave upward curved surface.
• The upper surface of each slump block remains relatively undisturbed, as do the individual blocks.
• Slumps leave arcuate scars or depressions on the hill slope.
• Slumps may be isolated or may occur in large complexes covering thousands of square meters.
• They often happen due to human activities.
• Slumps are common along the roads where slopes have been over steepened during construction.
• They are also common along the river banks and sea coasts, where erosion has undercut the slopes.
• Heavy rains, tectonic disturbances, tremors, and earthquakes can also trigger slumps.

5. Flows

• Movement of soil and regolith resembling a fluid behavior is called a flow.
• These include avalanches, mudflows, debris flows, earth flow, lahars, and sturzstroms.
• Water, air, and ice are often involved in enabling this fluid-like motion of the material.

Sediment Flows

Sediment flows occur when sufficient force is applied to rocks and regolith that they begin to flow downslope.

A sediment flow is a mixture of rock, and/or regolith with some water or air.

They can be broken into two types depending on the amount of water present as Slurry flows and granular flows.

Slurry Flows

Slurry Flows- are sediment flows that contain between about 20 and 40% water. As the water content increases above 40%, the slurry flows grade into streams. Slurry flows are considered as water-saturated flows.

Granular Flows

Granular Flows - are sediment flows that contain between 0 and 20% water. Granular flows are possible with little or no water.

Fluid-like behavior is given to these flows by mixing with air. Granular flows are not saturated with water.

Each of these classes of sediment flows can be further subdivided on the basis of the velocity at which flowage occurs.

Debris Flows

Debris Flows occur at higher velocities than solifluction.

The velocities may range between 1 meter/hr to 100 meters/hr.

It often results due to heavy rains causing saturation of the soil and regolith with water.

They sometimes start with slumps and then flow downhill forming lobes with an irregular surface consisting of ridges and furrows.

Mudflows

Mudflows are very different type of mass-movements.

These are a highly fluid, high velocity mixture of sediment and water that has a consistency ranging between soup-like and wet concrete.

They move at velocities greater than 1 km/hr and tend to travel along valley floors.

These usually result due to heavy rains in areas where there is an abundance of unconsolidated sediment that can be picked up by the streams.

Thus after a heavy rain streams can turn into mudflows as they pick up more and more loose sediments on the way.

Mudflows can travel for long distances over gently sloping stream beds.

Because of their high velocity and long distance of travel they are potentially very dangerous types of mass-movements.

Earth flows

Earth flows are usually associated with heavy rains and move at velocities between several cm/yr and 100s of m/day.

They usually remain active for long periods of time.

They generally tend to be narrow tongue-like features that begin at a scarp or small cliff.

Grain flows

Grain Flows are usually formed in relatively dry material, such as a sand dune, on a steep slope.

A small disturbance sends the dry unconsolidated grains moving rapidly down slope.

6. Falls

• Falls are major categories of mass-wasting.
• Falls are further categorized into two types as rock falls or debris falls.
• Rockfalls occur when a piece of rock on a steep slope becomes dislodged and falls down the slope.
• A rockfall may be a single rock or a mass of rocks, and the falling rocks can dislodge other rocks as they collide with the cliff.
• A fall, including rockfall, is where regolith cascades down a slope but is not of sufficient volume or viscosity to behave as a flow.
• Falls are promoted in rocks that are characterized by the presence of vertical cracks or joints.
• Debris falls are similar, except they involve a mixture of soil, regolith, vegetation, and rocks.
• Falls also occur due to the undercutting action of water as well as undercutting of waves near coastal regions.
• They usually occur at very steep slopes such as a cliff or escarpments.
• The rock material may be loosened by tremors or earthquakes, rain, plant-root wedging and expanding ice.

Controls of Mass Wasting

1. Gravity
2. Water
3. Time 
4. Slope Angle
5. Climate
6. Vegetation

1. Role of gravity

• Mass wasting is fully related to gravity. Creep, slides, and flows all of these processes share one thing in common, namely, that they are caused by the incessant downward pull of gravity, which moves all loose slope material downwards.
• Gravity-driven mass wasting processes are a subset of larger set of processes that transport weathered and unweathered earth materials.
• The trigger for mass-movement may come due to micro-seismic activities, tectonic dislocations and shaking of the ground by neo-tectonic forces.
• Such incidences may be common in places where there is an active fault on a hill slope.

2. Role of Water

• Water has hydraulic pressure. When water enters into the soil, it fills the pore spaces of soils and weathered zones.
• Due to this accumulation of water molecules on the regolith and soils, the weight increases.
• It also starts moving down through the pores.
• Water can create slippery phases between the bedrock and the overburden.
• The amount of water in rock or soil influences slope stability.
• Large quantities water from melting snow or heavy storms greatly increase the likelihood of slope failure.
• The additional weight that water adds to a slope can be enough to cause mass movement.
• The mass-movement may occur after any slow and steady rainfall.
• Throughout the world, the water-induced mass movements are more in number. Rainfall is the major triggering mechanism.

3. Role of Time

• Physical and chemical weathering can weaken slope materials decreasing resisting force. This causes the rock to become very weak and mass wasting occurs

4. Slope angle

• Opposing a slope shear strength is the force of gravity.
• Gravity operates vertically but has a component acting parallel to slope, thereby causing instability.
• All slopes are in a state of dynamic equilibrium, which means that you are constantly adjusting new conditions.
• The greater a slope angle, the greater the chance for mass wasting
• A number of processes can over steepen a slope. One of the most common is undercutting by stream or wave action.
• This removes the slope's base, increases the slope angle, and thereby increases the gravitational force acting parallel to the slope.
• Excavations for road cuts and hillside building sites are another major cause of slope failure.
• Grading the slope too steeply, or cutting into its side, increases the stress in rock or soil until it is no longer strong enough to remain at the steeper angle and mass wasting ensues.

5. Weathering and Climate

• Mass wasting is more likely to occur in loose or poorly consolidated slope material than in solid bedrock.
• As soon as solid rock is exposed at the Earth's surface, weathering begins to disintegrate and decompose it, thereby reducing its shear strength and increasing its susceptibility to mass wasting.
• The deeper the weathering zone extends, the greater the likelihood of some type of mass movement. 

6. Role of Vegetation

• Vegetation affects slope stability in several ways. By absorbing the water from a rainstorm, vegetation decreases water saturation of a slope material and the resultant loss of shear strength that frequently leads to mass wasting.
• The removal of vegetation by either natural or human activity is a major cause of many mass movements.
• Removal of trees may affect the binding mechanism between overburden and the bedrock.
• Roots of trees have the ability to keep the mass in-tact on a slope.

Effects of Mass Wasting

• The combined effects of mass wasting and running water produce stream valleys, which are the most common and conspicuous of Earth's landforms.
• If streams alone were responsible for creating the valleys in which they flow, the valleys would be very narrow features.
• Most river valleys are much wider than they are deep, is a strong indication of the significance of mass-wasting processes in supplying material to streams.
• Mass movements affect the following elements of the environment. 
• The topography of the earth's surface, particularly the morphologies of mountain and valley systems, both on the continents and on the ocean floors The character quality of rivers and streams and groundwater flow.
• The forests that cover much of the earth's sub-aerial surface.
• Habitats of natural wildlife that exist on the earth's surface, including its rivers, lakes, and oceans.

Importance of mass movements

• Mass-wasting is an important part of the erosional process, as it moves material from higher elevations to lower elevations where transporting agents like streams and glaciers can then pick up the material and move it to even lower elevations.
• Mass-wasting processes are occurring continuously on all slopes; some act very slowly, others occur very suddenly, often with disastrous results.
• Any perceptible down-slope movement of rock or regolith is often referred to in general terms as a landslide.
• Landslides, however, can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs.
• As human populations expand and occupy more and more of the land surface, mass-wasting processes become more likely to affect humans.
• The table below shows the impact of mass-wasting processes on human life over the last century.