Vanes (1978) has classified landslides into five main groups; falls, topples slides, spreads and flows on the basis of the type of movement and material. Falls are masses of rock/soil that move down slope falling or bouncing through air.
These are abrupt movements of slope materials that become detached from steep cliffs, escarpments and road cuttings. Depending upon the type of material involved it may be rock fall, soil fall, debris fall.
Earth fall and boulder fall. Topples refer to those blocks of rocks that tilt or rotate forward or overturn on a pivot or hinge and then separate from the main mass falling on the slope and subsequently falling, sliding, bouncing and rolling down the slope. Slide is a mass movement in which a distinct surface of rupture or zone of weakness separates the slide material from the more stable, underlying material. Here movement is caused by a finite shear failure along one or several surfaces of rupture. Spreads are failures caused by liquefaction whereby water saturated, loose, cohesion less sediments are transformed into a liquefied state. Rapid ground motions such as earthquakes are responsible for this phenomenon.
Flows represent a rapid movement of a viscous fluid containing high proportion of water mixed with earth material, mostly coarse fragments and air. Flow can take place as one or more lobes that move at different rate depending upon the viscosity of material and the slope angle. Water is not essential for flows to occur, however, most flows form after periods of heavy rain fall (Singh, 2003, pp. 58-65).
Distribution of Landslides
In India there are three main areas which are vulnerable for landslides. These include Himalayas, Western Ghats and Southern Plateau. Uttaranchal, Jammu and Kashmir, West Bengal (mainly Darjeeling and Jalpaiguri districts), Himachal Pradesh and Arunachal Pradesh are the main states affected by the vagaries of landslides.
In the Himalayan region the zones of the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT) are the extremely vulnerable areas and are highly active. Along the MBT, the Indian plate along with its Siwalik front is slipping under the Lesser Himalayan crustal plate.
The shattered and highly deformed rocks have been tightly folded and repeatedly spit by thrust planes into multitudes of rock slabs, some of which are highly mylonitizedorarein a friable condition. Here Khairar, Sanarah, Gadela. Gautiya and Baur localities are extremely vulnerable for landslides (Joshi 1988 and Pande et al. 2002). The MCT has served as the plane along which the Lesser Himalayan domain has slid under the Great Himalayas causing tremendous uplift of the latter and wholesale splitting, shuttering and crushing of the Himalayan rocks. The tectonic movements along this disturbed zone assist slope failure. Here landslides occur frequently during the monsoon period. The Loharkhet and Sumgarh localities are prone for such landslides (Pande et al. 2002, p. 271).
The landslides of 15th August, 1977 on Indo-Tibet border (death of 44 persons and loss of property worth Rs. 2 million), of July 1975 in North Bengal (4,500 people rendered homeless due to landslides and consequent floods), of September 1970 in Uttaranchal (house collapse and toll of 223 persons), and of July 1970 in Alaknanda Valley (Chamoli worst affected; a caravan comprising 23 buses, 2 trucks and 6 taxies swept away by flood claiming toll of more than 100 persons and hundreds of cattle) and of July 23, 1983 in Karmi area of Bageswar district (causing flash floods in Saran river, and bringing untold miseries to local people in which 150 persons lost their life) are worthy of mention.
Landslides are very intimately associated with earthquakes. An earthquake above the magnitude of 4 on Richter scale is the threshold limit for the activation of landslides and faults. During the 1928 earthquake an area of 2,500 km between Bajang (Nepal) and Kapkot (Almora) was ravaged by landslides.
Western Ghats have precipitous slope along the Arabian Sea coast. Here landslide is a common phenomenon along the Konkan coast where crystalline basalt is interblended with volcanic ash. Here landslides are mainly activated by heavy rain fall and occasional occurrence of earthquakes (Koyna).
Landslides are also noticed along the slopes of Nilgiri and Anai Mudi Hills. The upper parts of these hills have a late rite capping and are devoid of vegetation. Hence heavy downpour of rain fall promotes rock mass to slide along the slopes.
Management of Landslides
There can be two-fold management of the landslides: (a) prevention and control measures, and (b) relief measures. Landslides cannot be entirely controlled only their severity can be reduced through timely and appropriate biological and engineering techniques.
These include a forestation of hi 11 slopes, controlling gully erosion by building small check dams, modification of steep slopes, draining of collected rain water by lined drainage, construction of buttress walls, retaining walls or crib walls, stabilization of the exposed toes of slopes, curbing mining, quarrying, road building and canal digging activities and promoting contour farming in landslide prone areas. Jalal (1985) has suggested three particular eco development programmes, comprising afforestation, masonry work and agricultural diversification in the MBT area to control the occurrence of landslides.
Equally important is to provide quick relief to the victims of landslides. There should be disaster mitigation center at state level with branches and control rooms at district and local level to provide quick relief to the victims. The landslide prone areas should have emergency shelter and relief camps with medical, transport and communication facilities. Peoples' awareness and training for disaster management may be imparted through schools.
The aim should be to put disaster management on the national agenda. International cooperation should be sought for technical assistance and technology transfer in the management of landslides. The NGOs and insurance companies may be involved to provide relief to the victims.