The landslides are simply defined as the mass movement of rock, debris or earth down and have to include a broad range of motions whereby falling, sliding and flowing under the influence of gravity dislodges earth material. They often take place in conjunction with earthquakes, floods and volcanoes. The Himalayan Mountain, the north-east hill ranges and the Western Ghats and the Nilgiris experience considerable landslide activities of varying intensities.
Also called landslip; Downward mass movement of earth or rock on unstable slope including many forms resulting from differences in rock structure, coherence of material involved, degree of slope, amount of included water, extent of natural or artificial undercutting at the base of slope, relative rate of movement and relative quantity of material involved. Many terms cover these variations: creep, earth flow, mudflow, solifluction and debris avalanche are related forms in which mass movement is by flowage.
If shearing movement occurs on a surface on consolidated rock, the dislocated mass is a debris slide. Cliffs may become so steep through undercutting by rivers, glaciers or waves that masses of rocks will fall freely and constitute a rock-all type of landslide. Causes of Landslides: There are several factors which lead to the occurrence of landslides. Seismic activity, intensity of rainfall, steep slopes, rigidity of slopes, highly weathered rock layers, soil layers formed under gravity, poor drainage these all are natural factors that cause the landslides.
Not only this there are many man-made factors also which contribute to the occurrence of landslides. These are land use pattern, non-engineered construction, mining and quarrying, non-engineered excavation and deforestation leading to soil erosion. Protection Measures: Generally landslides happen where they have already occurred in the past, or in identifiable hazard locations. Following are the areas that are distinctly considered safe from landslides: I. Areas that have not moved in the past.
II. Relatively flat areas away from sudden changes in slope III. Areas at the top of or along ridges but set back from the edge of slopes. However, the homes built at the toe of steep slopes are frequently vulnerable to slides and debris flows that originate on property controlled by others. Adoption of slope stabilizing methods and professional site investigations by an engineering geologist and a technical Engineer has shown to reduce the landslide damage to over 95%.
But in many situations preventing landslides may be impractical Snow Avalanche: Large mass of snow or rock debris that moves rapidly down a mountain slope sweeping and grinding everything in its path. An avalanche begins when a mass of material overcomes frictional resistance of the sloping surface, often after its foundation is loosened by rains or is melted by a warm and dry wind. Variations caused by loud noises such as artillery fire, thunder or blasting can start the mass in motion.
Some snow avalanches develop during heavy snowstorms and slide while snow is still falling more often they occur after the snow has accumulated at the given site. The wet avalanche is perhaps the most dangerous of its large weight, heavy texture and the tendency to solidify as soon as it stops moving. The dry type is also very dangerous because its entraining of great amounts of air makes it act like a fluid; this kind of avalanche may flow up the opposite side of a narrow valley.
Avalanches carry a considerable amount of rock debris along with snow and therefore are significant geological agents; in addition to transporting unsorted materials to the bottoms of slopes, they may, if repeated, cause an important amount of erosion. From the above definitions and descriptions, it will be seen that landslides and snow avalanches are phenomena of mountain regions and both involve the swift and sudden movement of large masses of material falling or slipping down a hilly slope.
While landslide involves rock, soil and mud; snow avalanche primarily involves snow. While landslide may occur even in smaller hills or rocky terrains, snow. Landslides involve loosened or weakened rocks and mud whereas snow avalanche brings down accumulated or overhanging snow mass although it may collect rock and other debris on its way. Both the phenomena can be triggered by their own weights or by vibrations and also due to loud noise. Earthquakes or even minor tremors are known to have triggered landslides and snow avalanches.
Incidences of landslides are common in the various hilly regions of India but these are more in the Himalayas, in the Western Ghats (including Kerala), Nilagiris. There are occasional reports of landslides in the Vindhyachals and the Eastern Ghats as well. Landslides are more frequent during or after heavy rains. In India, snow avalanches occur in the Himalayan ranges and more so in the mountain regions of Kashmir, Himanchal Pradesh and the hills of West U. P. his is because the dense forest and vegetation cover in the eastern and northeastern Himalayas act as binding force and inhibit the slippage of snow mass The following are highly vulnerable to landslides and snow avalanches: (a) Deforested mountains especially in areas of heavy rainfall or snow fall; (b) Settlements (villages or hamlets) that are built on hill tops, steep slopes, softer soil; (c) Settlements built at the base of hills, steep slopes, mountain valleys; (d) Buildings with weak foundations; and (e) Roads and communication lines in mountain areas.
In considering the effects and impacts of landslides and snow avalanches, the following special features of these hazards should be kept in mind. (a) These disasters occur in remote mountain areas with difficult terrain and adverse weather conditions. (b) The communities (villages and hamlets) are small entities with weak housing, makeshift structures and poor resources. (c) Landslides and snow avalanches give almost no notice in most cases and enormous amount of rock, soil or snow come crashing with fantastic speed on the often-unprepared communities.
In the light of the above three considerations, the effects and impacts of landslides and snow avalanches may be divided into: i) Direct effects-physical damage Anything in the top of a landslide or in its path or at its bottom will suffer severe damage. The same is the case with a snow avalanche when anything in its path or falling areas will suffer severe damage. In case of a snow avalanche of “slab type” where massive slabs of hardened snow come hurtling down, the hit is very hard and devastating where as the “loose snow’ type of snow avalanche may engulf and cover larger area.
Blockages of roads, mountain passes and streams and damage to electric and communication lines are among the direct effects of landslides and snow avalanches apart from injuries and fatalities to human and cattle lives. Blockage of streams and later release of the impounded water create flash floods with disastrous effects. Falling of large volumes of debris from landslides or snow avalanches in mountain lakes can generate flash floods. Snow avalanches create additional suffering due to extremely low temperatures and the associated freezing effect.
Even if there are survivors among the victims, they may suffer hypothermia and frost-bite before help arrives. ii) Indirect Effects and long –term Impacts Apart from loss of houses, destruction of property and shattering of family life due to death or injury to kith and kin, the indirect effects and long-term impacts of landslides and snow avalanches lead to further loss of productivity (agriculture, poultry, small scale cottage industry, forest produce) in an already marginal productivity scenario