Mass wasting

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Context: A new study on the high frequency of mass wasting events in the Sedongpu Gully of the Tibetan Plateau since 2017 could be bad signs for India, specifically the country's Northeast..

Details

Mass wasting, also known as mass movement, refers to the movement of rock or soil down slopes under the force of gravity.

Mass movements mechanism.

• These movements transfer the mass of rock debris down the slopes under the direct influence of gravity.
• That means, air, water or ice do not carry debris with them from place to place but on the other hand the debris may carry with it air, water or ice.
• The movements of mass may range from slow to rapid, affecting shallow to deep columns of materials and include creep, flow, slide and fall.
• Gravity exerts its force on all matter, both bedrock and the products of weathering. So, weathering is not a pre-requisite for mass movement though it aids mass movements.
• Mass movements are very active over weathered slopes rather than over unweathered materials.
• Mass movements are aided by gravity and no geomorphic agent like running water, glaciers, wind, waves and currents participate in the process of mass movements.
• That means mass movements do not come under erosion though there is a shift (aided by gravity) of materials from one place to another.
• Materials over the slopes have their own resistance to disturbing forces and will yield only when force is greater than the shearing resistance of the materials.
• Weak unconsolidated materials, thinly bedded rocks, faults, steeply dipping beds, vertical cliffs or steep slopes, abundant precipitation and torrential rains and scarcity of vegetation etc., favour mass movements.

Several activating causes precede mass movements.

• Mass wasting triggers can be categorized into passive and activating causes:

Passive Causes

• Rock and Soil Lithology:Weak or unconsolidated debris is more susceptible to mass wasting.
• Stratigraphy:Thinly bedded rock or alternating weak and strong lithologies.
• Geologic Structures:Faults or other structures that weaken the rock.
• Topography:Steep slopes or cliffs.
• Climate:Frequent temperature swings, freezing and thawing, or heavy rainfall.
• Vegetation:Lack of vegetation increases susceptibility.

Activating Causes

• Undercutting of Slopes:By excavation or erosion.
• Increased Overburden:From structures.
• Increased Soil Moisture:From heavy rainfall.
• Earthquakes:Triggering sudden slope failure.

 

 

• Mass movements can be grouped under two major classes: (i) slow movements; (ii) rapid movements.

Slow Movements

Creep

• Creep is one type under this category which can occur on moderately steep, soil covered slopes.
• Movement of materials is extremely slow and imperceptible except through extended observation.
• Materials involved can be soil or rock debris.
• Depending upon the type of material involved, several types of creep viz., soil creep, talus creep, rock creep, rock-glacier creep etc., can be identified.

Solifluction

• Also included in this group is solifluction which involves slow downslope flowing soil mass or fine grained rock debris saturated or lubricated with water.
• This process is quite common in moist temperate areas where surface melting of deeply frozen ground and long continued rain respectively, occur frequently.
• When the upper portions get saturated and when the lower parts are impervious to water percolation, flowing occurs in the upper parts.

 

Rapid Movements

• These movements are mostly prevalent in humid climatic regions and occur over gentle to steep slopes.
• Movement of water-saturated clayey or silty earth materials down low-angle terraces or hillsides is known as earthflow.
• Quite often, the materials slump making steplike terraces and leaving arcuate scarps at their heads and an accumulation bulge at the toe.
• When slopes are steeper, even the bedrock especially of soft sedimentary rocks like shale or deeply weathered igneous rock may slide downslope.

Mudflow.

• In the absence of vegetation cover and with heavy rainfall, thick layers of weathered materials get saturated with water and either slowly or rapidly flow down along definite channels.
• It looks like a stream of mud within a valley.
• When the mudflows emerge out of channels onto the piedmont or plains, they can be very destructive engulfing roads, bridges and houses.
• Mudflows occur frequently on the slopes of erupting or recently erupted volcanoes.
• Volcanic ash, dust and other fragments turn into mud due to heavy rains and flow down as tongues or streams of mud causing great destruction to human habitations.

Avalanche

• A third type is the debris avalanche, which is more characteristic of humid regions with or without vegetation cover and occurs in narrow tracks on steep slopes.
• This debris avalanche can be much faster than the mudflow.
• Debris avalanche is similar to snow avalanche.

Hazards and Mitigation

• Mass wasting presents significant challenges to civil engineering and infrastructure.
• Historical events like the Oso disaster and the failure of volcanic flanks illustrate the deadly consequences of rapid mass wasting.

Mitigation methods include:

• Afforestation:Planting trees to stabilize slopes.
• Construction of Barriers:Fences, walls, or ditches to contain rockfall.
• Catchment Dams:To contain debris flows.
• Improved Drainage:Reducing soil moisture in source areas.
• Slope Stabilization:Reinforcing slopes to prevent failure.

Land Slides: https://www.iasgyan.in/daily-current-affairs/wayanad-landslides#:~:text=Landslides%20occur%20when%20gravity%20forcespushing,this%20potential%20for%20slope%20failure.

Avalanches: https://www.iasgyan.in/blogs/avalanches

PRACTICE QUESTION Q. What is mass wasting? What are the trigger mechanism for mass wasting? Suggest some mitigation measures. 150 words.