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.
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These three phenomena are being discussed together because these are basically the hazards of mountain areas. Furthermore, rainfall (or snowfall) plays a crucial role although man-made causes increase these hazards to a very large extent.
The basic forecast in these events is that of rainfall (or snowfall) and strong winds but additional factors have to be kept in view. For landslides, the strength of the rock and any damage already occurred at ground have to be considered.
Heavy rains and heavy snowfalls initiate these phenomena. Further, these occur more frequently in areas, which are prone to these disasters due to the peculiar local geological features such as weakness of soil or rock.
Landslides and snow avalanches affect the remotely located often isolated, small communities in villages or hamlets in the mountain regions of the country where external assistance takes time to reach in times of emergency when the normally difficult terrain and tracks ,may become almost impossible to negotiate.
Webster’s Third New International Dictionary (1971) defines these phenomena as follows: Landslide: Rapid downward movement, under the influence of gravity, of a mass of rock, earth or artificial fill on a slope. Also, the mass that moves or has moved downwards.
As landslides involve movement of mass of rock, mud’s etc., down a slope, factors that promote such movement of mass provide the causes for landslides. Such causes can arise from a number of happenings.
In order to consider risk reduction measures, we should first have an idea of likely risks from landslides and snow avalanches. These are as follows:
In considering the effects and impacts of landslides and snow avalanches, the following special features of these hazards should be kept in mind.