Landslide, as the name suggest, is a kind of natural disaster where parts of a land slide down a slope. They result from displacements of rocks, debris and earth, breaking off the hill or mountain, and are driven by the gravity’s force. This usually happens when the rocks, soil or earth are unable to hold together as a result, these either move down slowly or fall disastrously causing destructions. Apart from the Landslides in the mountains or hills, there are even underwater Landslides known as Submarine Landslides and these cause high tidal waves which can damage coastal areas.

There are various ways in which the rocks, debris or earth fall off the hill slopes. These can also be called the Landslide movements and the basic Landslide movements are as follows:

• Fall: This movement is triggered by earthquake or erosion and it has an extremely fast rate. This movement mostly occurs at a rapid to extremely rapid rate and is characterised by a freefall of the rocks from a steep slope.

• Topple: Here the rocks tilt from the mountain break without falling or some of the rocks roll over towards the edge. These might subsequently lead to an abrupt fall of the rocks or the debris under natural circumstances. Even the topple movement takes place at a rapid rate..

• Flow: This form of Landslide is the most destructive and tumultuous one because of its water content. Therefore, it turns slurry as it loses cohesion and moves swiftly through the landscapes, making channels. Its main causes are rainfall, flooding or melting snow.

• Slide: It is the most common form of failure and can be categorised in to two types – rotational and translational slides. It can be called a rotational slide, when the rocks move in a circular pattern forming an arc-shaped rupture which is also known as a slump. In a translational slide, the breakdown happens in slabs of rocks and they slide down the slope.

• Spread: It is a gradual displacement of large volumes of rocks, silt or earth over a flat terrain. The movement is caused by a process known as liquefaction, where the loose sediments are transformed into a liquid-like state due to little or no cohesion at all, and this process is triggered by rapid ground motions mostly in the form of earthquakes.

• Creep: Here, the soil or sand on the top of a terrain flows slowly and smoothly slides down the slopes. It is mainly caused due to the changes in the moisture and temperature of the soil, which weakens its strength to get hold of the slope. This movement finally leads to tilting of trees and other upright objects on the slope.

Generally, the factors that result in the occurrence of Landslide typically consist of weather, climate, geology, vegetation, slope angle and stability, water content etc. It is important to understand how these factors are interlinked thus resulting in a Landslide and how humans impact these factors by modifying the processes of nature. Usually, there are a number of factors that contribute to a Landslide, but more often than not there is one which plays a pivotal role in the movement of material.

There are various causes of Landslides, both natural and human.

The natural causes of Landslides are as follows:

• Heavy rainfall

• Volcanic eruptions

• Earthquake

• Undercutting of cliffs and slopes by waves and river erosion.

• Degree of the slope and nature of the soil and rocks

Some of the human factors that contribute in Landslides are:

• Deforestation

• Leaking pipes, sewages and natural drainages

• Construction of roads, railways, buildings etc. cutting the slopes

• Excavation of rocks

• Overloading slopes.

• Mining

Though most of the Landslides are abrupt, there are some signs which can be noticed before the destruction occurs so that we can take precautions. Some of them are:

• New cracks and bulges in the ground, roads or pavements.

• Tilting poles, trees or fences.

• Displaced road surfaces

• A rapid increase in the water levels of bays with greater turbidity.

• Broken water channels and underground utilities.

There have been a number of major Landslides that caused massive destructions. Some of them are as under:

• Usoy Rock slide: This Landslide occurred in Tajikistan in 1911 and was a size of 2 billion cubic metres. It was triggered by the Usoy earthquake which had a magnitude of 7.4. It destroyed the Usoy village and killed 54 people. The rockslide was so powerful that it dammed the Murgab River and formed a 65-kilometre-long Lake Sarez.

• Rio Barrancas and Rio Colora do debris flow: This Landslide happened in the year 1914 in Argentina when a dam formed by another ancient Landslide was broken. It had a volume of 2 million cubic metres and flowed for nearly 300 kilometres devastating two small towns of Rio Barrancas and Rio Colorado.

• Cerro Condor-Sencca rockslide: With a volume of 5.5 million cubic metres, this Landslide was triggered by undercutting due to erosion. It was recorded in Peru in the year 1945 and formed a 100 metre high dam at Rio Montara River. The dam was destroyed after 73 days and the flood then caused destroyed many roads, bridges and farmlands.

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