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...