Landslides in India

Landslides, when catastrophic, can cause a lot of devastation. Landslides today are caused not just naturally but also because of unplanned human activity on the hills. In many places in India, people continue to carry out modifications on the hill slopes, without carrying our proper slope stability analysis, despite being aware of the consequences.

What are Landslides

The term ‘landslide’ means movement of rock, soil or debris down a slope under the influence of gravity; this movement usually when the slope gradient exceeds its threshold angle of stability (Crozier, 2002).

Landslides are caused due to a combination of various natural as well as anthropogenic factors. They cause substantial damage and claim around 5000 lives each year (Vasudevan and Ramanathan, 2016). In India, landslides commonly occur in the regions near the Himalayas in North and North-Eastern states, Western Ghats and Nilgiris (Das et al., 2014).

Landslides can be classified based on factors such as type of movement, type of material, rate of movement, activity, age of movement, morphological characteristics, size/scale etc. The most adopted classification is that of Varnes and Cruden of 1996, which classifies landslides based on two attributes, type of movement and type of material.

Landslide Movement

Landslide Movement can be described as falls, topples, translational slides, lateral spreads, and flows.

• In falls and topples, heavy blocks of material fall after separating from a very steep slope or cliff. Boulders tumbling down a slope would be a fall or topple.
• In translational slides, surface material is separated from the more stable underlying layer of a slope. An earthquake may shake the loosen top layer of soil from the harder earth beneath in this type of landslide.
• A lateral spread or flow is the movement of material sideways, or laterally. This happens when a powerful force, such as an earthquake, makes the ground move quickly, like a liquid.

Landslide Material

A landslide can involve rock, soil, vegetation, water, or some combination of all these. A landslide caused by a volcano can also contain hot volcanic ash and lava from the eruption. A landslide high in the mountains may have snow and snowmelt.

Why do Landslides Occur?

Factors that contribute towards landslides can be categorized as follows:

1. Geological factors such as weak and sheared material, the presence of fissures and joints, and contrasts in permeability or stiffness of the slope forming material
2. Morphological factors such as tectonic uplift, glacial rebound and erosion of the hill slope or toe
3. Physical factors such as heavy rainfall, rapid snow melt, and earthquakes. Also, anthropogenic (human caused) factors such as mining, deforestation and excavation of the hill slope or toe

Landslides in the North East

The North Eastern Region of India, also commonly referred to as the North-East comprises of the states of Assam, Arunachal Pradesh, Nagaland, Meghalaya, Manipur, Mizoram & Tripura (Mdoner, 2021). Most of the North-East is a hilly region and experiences landslide hazards, especially during the monsoons (Alam, 2001). In the North-East, these geo-environmental hazards have caused significant damage to life and property, damage to natural resources (e.g. vegetation, land and soil) and hamper developmental projects like roads, bridges and communication lines, etc.

The North East receives heavy rainfall each year, and torrential rains over the last few years have caused several flash floods and landslides in the region, causing a lot of devastation as people were rendered homeless, useful infrastructure such as roads were washed away.

Anthropogenic factors are human factors such as quarrying, indiscriminate excavation of the hill, the construction of large buildings on or near the hillslopes. These factors, along with other factors such as the presence of vertical joints, heavy rainfall, improper drainage, and water seepage through rock joints and bedding intersections can cause landslides to occur. Shale can also be a problematic material; water can cause weathering of the shale foundation, and cause sliding of the foundation resulting in a landslide (Vasudevan and Ramanathan, 2016).

While unplanned construction activities in the past few years have increase the instability of most hill slopes thus increase the chances of landslides, the North-East region itself is highly seismic and prone to earthquakes which is another reason why this region experiences frequent landslides.

• Landslide in Aizawl – Ramhlun Sports Complex Slide
• Laipuitlang Landslide, Aizawl City, Patkai hills

Analysing landslide hazards

For analysing landslide hazard of a particular region, various thematic maps based on different themes are used. Researchers commonly use thematic maps based on different themes such as drainage, geomorphology, geology, slope, Landuse/Landcover etc for analysing (Das et al., 2014).

• Base Map: A map on which geographical and topographical data (boundaries, transportation, river, landmarks etc) is plotted and provides general information of the area.
• Drainage Map: It shows the pattern of drainage network available in the area. Drainage network is vital in the identification of landslide prone areas as it reflects the slope formation of the region and its erosional aspects.
• Geomorphology Map: It shows the spatial distribution of different landforms and near surface deposits, and the processes and time that act on the formation of these landforms.
• Geology Maps: It shows the composition and relationship among rock and sediment near the earth’s upper surface. It helps to understand the rock composition and structure within a study area and their role in the causes of landslide in the region.
• Landuse/Landcover Map: This map helps identify the different classes of land and helps understand changes in landuse pattern of the study area over a different period.
• Contour Map: A contour line is an imaginary line that joins points of equal elevation on the earth surface. It is one of the most important ways of showing the relief feature of earth on the map.
• Slope Aspect Map: It shows the direction and steepness of hilly terrain slope. The direction of the slope in respect to sun has huge influence on its structural and chemical composition of the soil slope.
• Digital Elevation Model (DEM): This is a three-dimensional representation of the earth’s topography through remote sensing technique. It is used to generate contour, slope, aspect map.

Conducting Stability Analysis of Slopes

There are various ways to conduct stability of slopes and to derive the associated Factor of Safety (FoS). Method of Slices, such as the Ordinary or Normal Method of Slices, Bishop’s method, are used to calculate the stability of slopes (Bishop, 1967).

Prediction of Landslides

Simple forms of landslides could also be predicted to some extent using deterministic and probabilistic analysis. Most of these predictive attempts try to associate landslide activity with rainfall records or occurrence of earthquake or with slope surface movement measurements. Researchers however have not had much success with their prediction attempts in complex situations (Bhandari & Jeyatharan 1994).

Recommendations and Remedial Measures

Here are protective measures that the authorities as well as the community should undertake in the North East states of India:

• Slopes that are identified as vulnerable to landslides should be bolstered using technical methods. Crated masonry wall should be provided at various positions on the slope.
• The drainage system of the cities should be fixed to allow water to flow freely through the channel without affecting slope.
• Mining and any quarrying activities on in and around the hills must be strictly banned.
• No new construction work, or human settlement, should be allowed in areas identified as high-risk zones. People already living in the most risk prone areas should be rehabilitated to safer areas through government development schemes such as JNNURM (Jawaharlal Nehru National Urban Renewal Mission) (Mohua, 2021).
• In general, the excavated soil should ideally be taken away from the site and not be dumped on the side of the excavation as it puts more load on the slope. Jute geogrid can be placed on the slope for growth of vegetations (Sarkar and Venkataraman, 1989).
• Trees should be retained as far as possible. Even if logging (cutting of trees) is to be undertaken, it should be done in a controlled manner. People should plant more deep-rooted trees along the slope as the roots of the tree would hold the soil more tightly and make the soil less saturated, thus reducing the chances of landslides.

It’s extremely important for the city authorities to prepare standard guidelines for landslide management and mitigation for their respective cities. These guidelines should be shared with the administrative department, with all the stakeholders who are associated with disaster management, and also with the community.

It is also important to create an awareness programme among the community in the North-Eastern region to help them better understand the causes and impacts of landslides, the perils of houses created on steep slopes, and to teach them of natural ways to mitigate landslides.

Conclusion

Landslides may be caused either by natural process or human activity or combination of both. Landslides can be caused by earthquake, rains, logging, erosion and even development activities. Erosion is also a cause of landslides; when soil gets extremely saturated or loose, it can result in a landslide if there’s nothing holding the soil together. Landslides are also caused due to development activities that causse destruction of trees and result in modifications of hills. Unplanned construction activities, constructing buildings on a steep hill side can cause landslides and but lives in danger.

The stability of a hilly terrain is highly influenced by the angle of its slope, which is defined as the degree of inclination of a feature relative to the horizontal plane. Most hills have moderate slope but without much vegetation cover. Development work carried out in parts of North-East has resulted into instability of slopes in the region; construction of large number of settlements over the hills without any appropriate measure has also played a crucial role in the slope failure in the region (Das et al., 2014).

In most places in India, including the North East, it is common to see modifications carried out in hill slopes without proper slope stability analysis, even though the consequences are well known. In other cases, stable slopes are converted into unstable slopes by the people residing in the area by artificial modification/alterations of the hill slope without comprehending its consequences. In the past, such cases have caused loss of many lives in the North East even though the size of such landslides were very small.

This study presented an account of landslides in India,, including the North Eastern states of India. It was noted that physical factors such as heavy rainfall and earthquakes, along with anthropogenic (human caused) factors such as excavation and deforestation on the hill slopes, and improper drainage systems cause landslides in the North East.

Several recommendations and remedial measures were presented that outlined how landslides could be minimized using modern scientific and technological advancements, through community awareness programmes, and by having standard guidelines for landslide management and mitigation.

References

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