LANDSLIDES, AN INDIAN CONTEXT

Landslides are the rapid movement of earth materials under the influence of gravity and belong to that family of short lived and suddenly occurring natural phenomenon that can cause extraordinary landscape changes and destruction of life and property. In the strict sense, landslides are relatively rapid down slope movement of soil and rock, which takes place characteristically on one or more discrete bounding slip surfaces which define the moving mass (Hutchinson, 1988). The causes of landslides are defined as external, which increases shear stress while shear strength remains constant, or as internal, which decreases the shear strength and leave the shear stress unchanged (Terzhaghi, 1950). Examples of external causes are steepening or heightening of slopes by erosion at the base, tectonic uplift or excavation, deposition of material on the upper part of the slope, and ground acceleration during earthquakes. Examples of internal causes are increase in soil water pressure, decrease in cohesion due to weathering or solution, and decrease in binding strength of roots caused by alteration or removal of vegetation. Such conditions or events seldom act alone to cause landslides (Terzhaghi, 1950; and Baker and Marshall, 1958). Factors such as slope steepening, progressive decrease in cohesion etc. operate so slowly that they are termed underlying causes. Other events or conditions may occur suddenly, such as rise of pore water pressure during a rainstorm or an earthquake. These are termed triggering causes. Landslides occur when a slope, already weakened by one or more underlying causes, is subjected to triggering event (Terzhaghi, 1950).

In India, it is estimated that the annual loss due to this phenomenon is about rupees 200 crores, emphasising the need for a concerted effort to mitigate its many fold miseries (Thampi et al., 1997). If one accepts the susceptible nature of the hilly regions in India by the very nature of the terrain with its steep slopes undergoing the natural diastrophism under tropical monsoon climate, the following regions need detailed assessment in terms of landslide hazards.
1. Western Himalayas
2. Eastern and North eastern Himalayas
3. Naga - Arakkan Mountain belt.
4. Western Ghats including Nilgiris
5. Plateau margins in the Peninsular India and Meghalaya in the North east India.

While the Himalayan region is known for its active tectonics which trigger the landslides, the Western Ghats and Nilgiris are geologically stable but has uplifted plateau margins influenced by neotectonic activity.

Being a heavy rainfall region, the hilly terrain of Kerala, falling in the Western Ghats belt, in general is prone to landslides. While the torrential, incessant rains or cloud bursts experienced in localised areas, are the immediate cause of most of the landslides in Kerala, the geological setting and the structural geological make up of the region can be attributed as the main cause of the landslides. The most prevalent, recurring and disastrous type of mass movements noted in Kerala are the “debris flows”. In local vernacular the event is called “Urul Pottal”. The characteristic pattern of this phenomenon is the swift and sudden downslope movement of highly saturated overburden containing varied assemblage of debris material ranging in size from soil particles to huge boulders destroying and carrying with it every thing that is lying in its path.

Davison (1889) was the first to use the term ‘creep’ after inventorying the landslides in polar areas. Varnes (1958) has classified landslides based on two main variables namely the type of materials and the type of movements and accordingly classified them as falls, slides (rotational and transitional), flows and complex slides. Hutchinson (1977) has indicated that the drainage is the principal measure used in the repair of landslides.

Choudhary (1980) categorised landslides into three types as (i) landslides due to exceptional causes (ii) ordinary landslides and (iii) landslides due to no apparent causes. Steepness of slope was demonstrated to be an important factor in both rainfall - triggered and earthquake - triggered landslides (Wieczorek et al., 1985).

Landslide events of some parts of Alps were attributed to the reactivation of Subric lineament caused due to post collision tectonics according to Schmidt et al., (1989). Bartarya and Valdiya (1989) observed that intense storms, even of short duration, generated widespread landslides along roads in many parts of Himalayas. Jagannathan (1991) in his geoenvironmental studies of landslide prone areas of Wayanad and Calicut districts of Kerala related the soil characteristics and weathering and lateritization to the occurrence of landslides.

Anbalagan (1993) related the landslide hazards of the Nainital area with the unplanned urbanization. Pradeep and Sinha (1995) have observed that structural discontinuity of lithological features and geotechnical parameters like shearing strength of soil, caused slope instability in Kaliaswar landslide of Garhwal Himalayas. Iverson (2000) studied the effects of rainfall infiltration on triggering landslides.

References

Anbalagan, R. and Tyagi, S.K., 1996. Landslide hazard mapping of a part of Kumaon Himalaya, U.P. India, Proc. Int. Conf. Disasters and Mitigation, Madras, 1, A 4.1 - A 4.11
Backer, R.F. and Marshal, H.C., 1958. Control and Correction, In : Landslides and Engineering Practice, Eckel, E.B., Editor, HRB Special Report 29, PP. 150 - 188.
Bartarya, S.K. and Valdiya, K.S., 1989. Landslides and erosion in the catchment of the Gaula river, Kumaun Lesser Himalaya, India, Mountain Research and Development, Vol. 9, No.4, PP. 405 - 419.
Chowdhary, R.N., 1980. Recent progress in evaluation and control of Landslides, Proceedings, International Symposium on Landslides (ISL 1980),April 7 - 11, New Delhi, Vol.1, PP. 313 - 318.
Davison, S., 1889. On the creeping of soil cap through the action of frost, Geology Magazine, New Ser ., Vol.3 No.6, PP. 255 - 261
Hutchinson, J.N. , 1977. Assessment of effectiveness of corrective measures in relation to geologic condition and types of slope movements, Bulletin of the International Association of Engineering Geology, No. 16, PP. 131 - 155.
iverson, R.M., 2000. Landslide triggering by rain infiltration, Water Resources Research, Vol.36, PP.1897 - 1910.
Jagannathan, V., 1991. Geoenvironmental studies of landslide prone areas in parts of Wayanad and Calicut districts, Kerala, Records of Geological Survey of India, Vol. 124, Part - 5, pp. 208 - 210.
Pradeep, K. and Sinha, U.N., 1995., Influence of structural discontinuity, petrographic features and geotechnical parameters on stability of Kalrasaur landslides, Garhwal Himalayas, Symposium on recent advances in Geological studies of NorthWest Himalyas and the fore deep, Abstracts, Lucknow, India, Feb. 21 - 23, PP. 318 - 319.
Schmidt, S.M., Aebli, H.R ., Heuer, R. and Zingg, A., 1989. The role of Pariadriatic line in the tectonic evolution of the Alps, In : M.P. , coward ,D. Dietrich, and R.G. Park, Editors, Alpine Tectonics, Geological Society, Special publication, Vol. 45, pp. 153 - 178.
Terzhaghi, K., 1950. Mechanism of landslides, In : Paige, S, Editor, Application of Geology to Engineering practice (Berkley volume), Geological Society of America, PP. 83 - 123.
Thampi, P.K., John Mathai., Sankar, G. and Sidharthan, S., 1997., Evaluation study in terms of mitigation in parts of Western Ghats of Kerala, Project report CESS.
Varnes, D.J., 1958. Landslides types and processes, In : E.B. Eckel, Landslides and engineering practice : Washington Highway research board, Special report 29, NAS - NRC Publication - 544, PP. 20 -47.
Wieczorek, G.F., Wilson, R.C. and Harp, E.L., 1985. Siesmic slope stability map of San Manteo County, California, U.S. Geological Suryey Miscellaneous Geologic Investigation map I - 12570, Scale 1: 62,500.