Understanding how to predict where the next landslide will happen in Sikkim - An analysis using the status report for initiating landslide studies by the Wadia Institute of Himalayan Geology

A status report that documents information regarding status of landslides in Sikkim and presents recommendations for preparing action plan for landslide studies.

This report presents information about the status of landslides in Sikkim. It was commissioned by the Department of Science and Technology and authored by the Wadia Institute of Geology. It draws upon several archives including that of the Directorate of Geology and Mining, Government of Sikkim. The report also presents recommendations for preparing an action plan for undertaking landslide studies.

Introduction
This status report was developed to aid the preparation of an action plan for undertaking landslide studies in Sikkim. It draws upon previous work carried out by the Geological Survey of India, The directorate of Geology and Mining (Government of Sikkim), Central Building Research Institute, Central Road Research Institute, Central Soil and Material Research, etc.

It speaks of the necessity for coordination and information exchange between the various institutions working on landslides, especially between central and state agencies.The authors also describe the necessity for large-scale mapping of landslides.

Geomorphology and climate
The geomorphology of the Teesta basin, besides other influencing factors, is strongly influenced by tectonics. This chapter describes the geomorphic features of Sikkim in some detail with the help of relevant maps. It divides the state into four distinct morphotectonic belts as follows:

  • Piedmont zone
  • Inner and foothill zone
  • Axial zone
  • Trans-axial zone

Sikkim's rugged topography, especially the altitudinal variations (from 200 to 7000 metres above sea level) has led to a large range of climate conditions. This chapter presents rainfall data for 18 stations in Sikkim. An isohytal map of the state is also presented. The authors observe that the maximum incidence of landslides occur in high-rainfall zones. not only that, but the incidence of landslides can be correlated to daily rainfall.

Regional geology and tectonic setting
Sikkim extends across a section of Eastern Himalaya covering the four  transverse zones from the sub- to the Tethys Himalaya. These and other geological features such as thrust surfaces and the occurrence of various rock groups are discussed in this chapter.Several maps and diagrams are included such as a geological map of the Sikkim and Darjeeling Himalaya, culminations in the north-eastern Himalayas, the tectonic map of Sikkim, and the tectonic trends of Sikkim.

The tectono-stratigraphic succession of rocks of Sikkim is presented in  a table which also details the districts in which the rocks are exposed.

This chapter also introduces the topic of natural hazards in Sikkim. It explains that regions with the presence of schists and phyllites are most susceptible to landslides due to their susceptibility to weathering, shear distortion and mineral structure. The authors discuss the periodicity, mode of generation, and history of earthquakes in the area. These earthquakes trigger landslides both directly and when they cause seismic deformations which become weak zones.

Inventory of landslides in Sikkim
This chapter presents, in tabular form, details about all the landslides in Sikkim that have been recorded in literature. While it does not claim to be exhaustive, it is nevertheless the most comprehensive list of landslides in Sikkim available so far. The parameters present in the table are as follows:

  • Slide name/location
  • Physiographic zone/tectonostratigraphic domains
  • Geomorphic characteristics
  • Lithology
  • Type of material/legolith
  • Nature of mass movement
  • Structural attributes
  • Causative factors
  • Activated since and current status (as of 2004)
  • Authors

In addition to the original document which you can download below in pdf format, this article also presents the above information in two additional formats. Certain parameters are presented in a spreadsheet which can be downloaded below, and the landslides for which latitude and longitude are available have been mapped below.


View Locations of some major landslides in Sikkim in a larger map

Discussion
This chapter takes a critical look at the information presented in the report so far. It points out the various lacunae in the data collection and suggests measures to remedy it. Further, the authors point out that rainfall, topography and geology have a combined influence on landslide incidence. They call for detailed mapping and collection of climatic, geological and landslide data for meaningful  landslide zonation.

Critical gaps and recommendations
This chapter discusses the critical gaps and makes the following recommendations to be undertaken urgently:

  • Preparation of topographic maps with scale 1:500 or 1:1000, with contour interval of 2-5 m
  • Preparation of geomorpholgy maps with scale 1:500 or 1:1000
  • Setting a network of automatic rainguages on slopes in areas with a known history of landslides
  • Preparation of overburden maps in areas with a known history of landslides
  • Detailed geotechnical investigations of known landslides

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