Landslide Avalanches and Mud-flows

In this chapter, we aim to study all types of mass movements which are hazardous to human life and livelihood. These include landslides, Avalanches, rockfalls, Mud-flow and earth flows. We have already read these topics in our Geography textbook.

Let’s study these from the Disaster point of view.

Mass Movements

The term Mass Movement refers to the transfer of mass of rock debris down the slope under the direct influence of gravity.

On the basis rate of movement, the mass movements can be grouped under three categories:

1. Slow Movement
2. Rapid Movement
3. Very Rapid Movement

Slow Movement:

When the soil/rock has a small amount of water saturation, slow imperceptible movement is observed over a long period of time.

• Creep: Creep is an extremely slow and imperceptible movement of Materials such as soil or rock debris. It can occur on moderately steep, soil-covered slopes. For example, slow creep in Joshimath. This is hazardous to livelihood, but not disastrous enough to take lives.
• Solifluction: It is the flow of water-lubricated soil mass or fine-grained rock debris over an impermeable material. 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 Movement

It occurs when water-saturated debris moves at a rapid pace

Mudflow:

In the absence of vegetation and heavy rainfall, the soil in the landscape can flow like a stream of mud within the valley (as seen in the diagram above).

• Thick layers of weathered material get saturated with water and either slowly or rapidly flow down along definite channels. They can be destructive when emerge out of channels.
• Fly ash, Volcanic ash, dust and other debris can also cause mudflow. Areas with frequent or recent eruptions are susceptible.
• For example, In April 2020, Reliance Power’s Ultra Mega Power Project (UMPP) in the Sasan area of Singrauli fly ash dyke collapsed. The flood of the toxic ash slurry from the collapsed dyke located in the adjoining Harhawa village washed away six people.

Earthflow:

Movement of water-saturated clayey or silty earth material down low-angle terraces or hillsides. Accumulation of slump takes place at the toe.

Very Rapid Movement:

It involves relatively dry material.  It includes Landslides along with other very rapid mass movements such as slumps, debris slides, rockfalls and rockslides.

Avalanche: Humid regions with/without vegetation cover and in narrow tracks on steep slopes. Flow much faster. It can also be considered a mass movement.

Classification of Very rapid mass movements:

These are relatively rapid and perceptible movements. The materials involved are relatively dry. The size and shape of the detached mass depends on the nature of discontinuities in the rock, the degree of weathering and the steepness of the slope. Depending upon the type of movement of materials several types are identified in this category.

1. Slump: It is the slipping of one or several units of rock debris with a backward rotation with respect to the slope over which the movement takes place.
2. Debris slide: Rapid rolling or sliding of earth debris without backward rotation of mass is known as debris slide. Debris fall is nearly a free fall of earth debris from a vertical or overhanging face.
3. Rockslide: The sliding of individual rock masses down bedding, joint or fault surfaces is rockslide. Over steep slopes, rock sliding is very fast and destructive. Slides occur as planar failures along discontinuities like bedding planes that dip steeply.
4. Rockfall is free falling of rock blocks over any steep slope keeping itself away from the slope. Rock falls occur from the superficial layers of the rock face, an occurrence that distinguishes it from rockslide which affects materials up to a substantial depth.
5. Landslides.

From the disaster management point of view, often all types of very rapid movements i.e. slump, debris slide, rockslide, rockfall and landslide, are called landslides since it is difficult for the general public to distinguish between these types.

Therefore, we shall take all of these events as the same in this section.

Landslides

Landslides are the rapid sliding of large masses of rock, debris, or earth down a slope. The shape and Size of the detached mass depend on the nature of discontinuities in the rock, the degree of weathering and the steepness of the slope.

Landslide Vulnerability Zones

In India, Landslide vulnerability zones are identified by the Geological Survey of India (GSI), M/o Mines. As per GSI, about 0.42 million km² covering nearly 12.6% of the land area of our country is prone to landslide hazards.

Based on past experiences, frequency and certain causal relationships with the controlling factors like geology, geomorphic agents, slope, land use, vegetation cover and human activities.

Our major Vulnerable zones are as follows:

1. Very High Vulnerability Zone: These include the following zones:
   • Himalayas – these areas are highly unstable and relatively young mountains and are tectonically active.
   • Those North-eastern Indian regions that experience frequent earthquakes and areas of intense human activities, particularly those related to the construction of roads, dams, etc. are included in this zone
   • Western Ghats and Nilgiris (Windward sides) are unstable due to High rainfall and steep slopes.
   • Andaman and Nicobar
     We have discussed the various causes of landslides in these zones in the next section.

2. High Vulnerability Zone: Areas that have almost similar conditions as the above areas, the only difference is the combination, intensity and frequency of the controlling factors. It includes:
   • Remaining region of the Himalayan states (Except trans-Himalayan sections)
   • North-eastern regions except the plains of Assam.
3. Moderate to Low Vulnerability Zone:
   • Areas that receive less precipitation such as the Trans-Himalayan areas of Ladakh and Spiti,
   • Undulated yet stable relief and low precipitation areas in the Aravalli Hills,
   • Rain shadow areas in the Western and Eastern Ghats and Deccan plateau.
   • Landslides due to mining and subsidence are most common in states like Jharkhand, Orissa, Chhattisgarh, Madhya Pradesh, Maharashtra, Andhra Pradesh, Karnataka, Tamil Nadu, Goa and Kerala.
4. Low Hazard & Very Low Hazard Areas: The remaining parts of India, particularly states like Rajasthan, Haryana, Uttar Pradesh, Bihar, West Bengal (except district Darjiling), Assam (except district Karbi Anglong) and Coastal regions of the southern States are safe as far as landslides are concerned.

Reasons of Landslides:

Controlling factors for landslides are highly localised.

Natural causes:

• Overloading due to heavy rainfall, saturation and lubrication of slope materials;
• Earthquakes can induce Landslides.
• Weather-Induced Landslide: It peaks in summer when cyclones, hurricanes and typhoons are more frequent and the monsoon season brings heavy rain to parts of Asia
• Excessive natural seepage: pressure between soil layers due to prolonged rainfall or seepage.
• Heavy drawdown of water from lakes, reservoirs and rivers leading to slow outflow of water from under the slopes or river banks;

Landslide in the Himalayas and Western Ghats:

Why do debris avalanches and landslides occur very frequently in the Himalayas?

There are many reasons for this.

1. The Himalayas are tectonically active.
2. They are mostly made up of sedimentary rocks and unconsolidated and semi-consolidated deposits.
3. The slopes are very steep.

Landslides in Western Ghats:

1. The Western Ghats along the west coast are relatively tectonically stable and are mostly made up of very hard rocks; but, still, debris avalanches and landslides occur though not as frequently as in the Himalayas, in these hills. Why?
2. Many slopes are steeper with almost vertical cliffs and escarpments in the Western Ghats and Nilgiri hills.
3. Mechanical weathering due to temperature changes and ranges is pronounced.
4. They receive heavy amounts of rainfall over short periods. So, there is almost direct rock fall quite frequently in these places along with landslides and debris avalanches.

	Himalayan Region	Western Ghats
Controlling factors of risk	Tectonic activity- The Himalayas mountain belt comprises tectonically unstable younger geological formations subjected to severe seismic activity. Weathering of rock: Due to various reasons like freezing and thawing and intense leaching, weathering of rock is pronounced. Human Construction: Such as road & rail projects and construction of Dams. Ex: Recent Chamoli disaster.	Neo-tectonic activity: Western Ghats and Nilgiris are geologically stable but have uplifted plateau margins influenced by neo-tectonic activity – such as due to reservoir-induced risks. Saturation-induced slides: Generally heavy rainfall is the major cause, which creates mudslides, drowning large areas of settlement. Nature of soil: Black soil and Red & Yellow soil change their form when saturated creating slides. Human activities: Improper land use practices such as heavy tilling, agricultural practices and settlement patterns have contributed to creep and withdrawal of toe support in many cases.
Impact on the landmass	Mass movement is huge:  The whole underlying lithology is displaced during sliding particularly due to the seismic factor. Great variation: a broad range of motions whereby falling, sliding and flowing under the influence of gravity dislodges earth material. Risk of Natural dam burst: At times, prolonged rainfall along with Landslides blocks the flow which can dislodge violently – bursting with heavy flash-flood. For example, the 1998 Mansarovar landslide killed 380 people.	Mass movement is seasonal: occurs during monsoon. Mud-slide, Earth flow and solifluction: are major types of sides that occur in the region. Rockslides are rare. Mainly impacts the top layer: Mass movements are confined to the overburden without affecting the underlying bedrock.

 

Manmade causes:

1. Removal of vegetation from the slopes,
2. Removal of support from below to materials above through natural or artificial means; For example, tunnel collapse.
3. Removal of material or load from over the original slope surfaces;
4. Overloading through the addition of materials naturally or by artificial filling; Construction of dams.
5. Interference with natural drainage,
6. Leaking water or sewer pipes,
7. Modification of slopes by construction of roads, railways, buildings, mining etc.
8. Explosions or machinery;
9. Reservoir-induced seepage and load

Consequences of Landslides:

Apart from the loss of life and property, it creates several other impacts.

• Blockades of transport channels: roadblocks, destruction of railway lines etc.
• Diversion of river courses due to landslides can also lead to flood and loss of life and property.
• Triggering Flash Flood: Landslides lead to the formation of artificial lakes, which can trigger flash floods in the region affected.
• Breaks Communication.

Mitigation of Landslide Risks

Prediction of Landslides

It is not difficult to gather information and monitor the possibility of landslides. Knowing the causes of landslides, those areas that are unstable and are susceptible to earthquakes, have poor drainage and are devoid of vegetation are prone to Landslides. We have already discussed these areas in the Himalayan and the Western Ghats region.

It is necessary to create awareness in these areas and have regular mock drills to mitigate the risks.

Regulatory Control:

It is always advisable to adopt area-specific measures to deal with landslides. Regulatory controls for construction and land usage are the best ways to mitigate its risks.

• Regulation of construction and other developmental activities such as roads and dams,
• Limiting agriculture in valleys and areas with moderate slopes, and
• Control of the development of large settlements in the high vulnerability zones should be enforced.

Policy actions:

• Promoting large-scale afforestation programmes
• Providing drainage channels
• Terrace farming should be encouraged in the northeastern hill states where Jhumming (Slash and Burn/Shifting Cultivation) is still prevalent.

National Landslide Risk Management Strategy (NLRMS)

National Landslide Risk Management Strategy (NLRMS) released by NDMA in 2019

• Generation of User-Friendly Landslide Hazard Maps: It recommends Landslide Hazard Zonation maps be prepared at the macroscale and meso level. It focuses on making use of advanced state-of-the-art tools such as Unmanned Aerial Vehicle (UAV), Terrestrial Laser Scanner, and very high-resolution Earth Observation (EO) data.
• Landslide Monitoring and Early Warning System: Technical recommendations for developing and implementing rainfall thresholds, Numerical Weather Prediction (NWP), Automatic Rain Gauges, etc. have been included.
• Stabilization of Slopes and Mitigation of Landslides
• Creation of Special Purpose Vehicle (SPV) for Landslide Management
• Capacity Building and Training of Stakeholders: Creation of the Centre for Landslide Research Studies and Management (CLRSM) to create a techno-scientific pool of expertise in the country.
• Preparation of Mountain Zone Regulations & Policies
• Awareness Programmes: A participatory approach has been defined so that each section of the community is involved in the awareness drive. Since the community is the first to confront the disaster before any aid reaches them, a mechanism of awareness is framed to involve and educate the community.

National Landslide Susceptibility Mapping (NLSM)

It was initiated by the Geological Survey of India (GSI) in 2014 to offer seamless landslide susceptibility maps and landslide inventory maps of the entire landslide-prone areas of India.

Avalanche

An avalanche is a rapid flow of snow down a slope, such as a hill or mountain. It is primarily composed of flowing snow and air. However, large avalanches can capture and move ice, rocks, and trees too.

They occur in two general forms, or some combination:

1. Slab avalanchesmade of tightly packed snow, triggered by a collapse of an underlying weak snow layer. These further can be composed of various types of snow slabs.
   • Fresh and damp Snow
   • Old and Brittle Snow 
   • Glacier Avalanche – It is composed of extremely dense glacier slabs.
2. Loose snow avalanchesmade of looser snow. After being set off, avalanches usually accelerate rapidly and grow in mass and volume as they capture more snow. If an avalanche moves fast enough, some of the snow may mix with the air, forming a powder snow avalanche.

Source: media.prothomalo.com

Major Causes of Avalanche

Natural factors:

1. Earthquakes: They disrupt the stability of the latent snow causing it to slide.
2. Snowstorm and Wind: Wind normally blows from one side of the slope of the mountain to another side. This may cause a snowstorm.
3. Heavy snowfall: it deposits snow in unstable areas and puts pressure on the snow-pack. Precipitation during the summer months is the leading cause of wet snow avalanches.
4. Layers of Snow: There are conditions where snow is already on the mountains and has turned into ice. Then, fresh snow falls on top which can easily slide down.
5. Steep Slopes: Layers of snow build up become heavy and slide down the mountain.
6. Warm Temperature: Warm temperatures that can last several hours a day can weaken some of the upper layers of snow and cause it to slide down.
7. Movement of Animal: This too can destabilise the snow.

Human Activity:

Humans have contributed to the start of many avalanches in recent years.

1. Winter sports: Winter sports that require steep slopes often put pressure on the snowpack which it cannot deal with.
2. Vibration or Movement: The use of All Terrain Vehicles and Snowmobiles creates vibrations within the snow that it cannot withstand. Coupled with the gravitational pull, it is one of the quickest ways to cause an avalanche.
3. Construction activities: can cause vibrations which can trigger avalanches. For example the recent Chamoli disaster.
4. Mining: It hollows the relief making the slope unstable.

Strategies to mitigate avalanche risks:

1. Stabilization of slopes: slopes with pitches greater than 25 degrees are dangerous.
2. Plantation:Treeless slopes and gullies are particularly susceptible to avalanches. The absence of trees may reflect that previous avalanches have occurred in the area.
3. Avoiding Hiking after the storm: Avoid hiking immediately after a storm. Most avalanches occur at the time of or shortly after a heavy snowfall.
4. Snow retention and retardation structures (snow racks, avalanche snow bridges, snow nets), are used in the upper path of probable avalanche paths. Snow guarddevices (used to increase snow retention on roofs). For example avalanche nets and avalanche dams.
5. Snow redistribution structures (wind baffles, snow fences): Snow deflection structures are used to deflect and confine the moving snow within the avalanche track. They should not deflect the avalanche sharply, because in the latter case, they may be easily overrun by snow.
6. Monitoring natural movement: For example, HIMANSH is a project in the Spiti district of Himachal Pradesh using Terrestrial Laser Scanners (TLS) mounted on UAVs that would digitize the glacier motion and snow cover variations with exceptional precision.
7. Regulation of construction and mining activities: This significantly enhances the risk of disaster.
8. Local community participation: For planning and risk assessment in the area.

Avalanches are one of the most severe disasters that India faces. In the past couple of decades, several hundred of our soldiers have died in Siachin, and disasters like Chamoli are common in the Himalayan region. India needs a comprehensive policy in this regard with regulatory updates and the use of technology for the prevention and mitigation of the disaster.

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