Chapter 2. Literature Review
2.8. Regional Scale Landslide Analysis
2.8.5. Indian Standard Code Provisions
Bureau of Indian Standards (BIS) has given guidelines for Macro level landslide hazard zonation in the scale of 1:50,000, IS 14496, Part 2 1998 (Reaffirmed 2002) in India.
BIS based Landslide Hazard Evaluation Factor (LHEF) rating scheme for landslide susceptibility zonation is a heuristic approach to landslide hazard assessment. Figure 2.48 shows the procedure for Landslide Hazard Zonation. BIS identified six landslide causative factors for hazard zonation viz. lithology, structure, slope morphometry, relative relief, land use-land cover and hydrological condition. In this method, the area under investigation is
divided into small mapping units to which numerical weights are assigned for each thematic data layer and finally TEHD (Total Estimated Hazard) is obtained by adding weights of all variables for each mapping unit and Landslide Hazard map is produced. The unit of study as recommended by BIS for Landslide Hazard Zonation is a slope facet. A slope facet is a part of hill slope showing consistent slope direction and inclination. The slope facets over a region are generally delineated from Topographical maps considering ridges, spurs, gullies and rivers. The slope facet map is considered as base map to prepare thematic data layers.
Depending on the THED values the facets are grouped in to Very Low Hazard (< 3.5), Low Hazard (3.5-5.0), Moderate Hazard (5.0-6.0), High Hazard (6.0-7.5) and Very High Hazard (>7.5) classes. BIS based LHEF rating scheme is a very simple and cost-effective method of landslide hazard mapping. However, subjectivity in weight assignment procedure exists in this method, which can affect the level of accuracy of hazard zonation map.
Figure 2.48 Landslide Hazard Zonation Procedure (IS 14496, Part 2 - 1998)
IS 14496 (Part 1) Mega-Regional (> 1:50,000), IS 14496 (Part 3) Micro-Regional (1:1,000 – 1:2,000) are yet to be framed and prescribed. No guidelines are available for Landslide Hazard Zonation on Meso-Scale (1:5000/10,000) and for further large scale (NDMA, 2004).
National Disaster Management Authority (NDMA) put forward an official document in the form of Management of Landslides and Snow Avalanches (2009). The document gives the definition of various terms associated with landslide studies. It defines Landslide Hazard
Zonation (LHZ) mapping is a tool to identify those areas that are, or could be, affected by landslides and assessing the probability of such landslides occurring within a specified period of time. The preparation of a LHZ map should include the study of the regional geology and geomorphic setting, slope conditions including existing and potential instability, and land use information.
The document highlights that scale is an important factor of LHZ mapping. Maps of 1:1,00,000 or 1:50,000 scales are inappropriate for regional studies since these are only indicative and do not provide adequate details. Larger scale maps on 1:10,000 or more should be taken up for detailed studies at the local level. There is also a lack of landslide inventory data, which lead to problems in validating landslide hazard maps. No organisation in our country has a sound database on landslide inventory. Taking into consideration the importance of developing a sound database on landslide inventory, the GSI has recently initiated a programme for the generation of landslide inventory maps and databases covering the landslide prone regions of our country.
The document reports that site-specific studies involving geotechnical investigations, including monitoring, have been carried out at some of the landslides. These include work on the Kaliasaur landslide along NH-58 near Srinagar, the Nainital landslide at Sher Ka Danda and 9.5 Mile, B2 and Lanta Khola landslides in Sikkim, the Powari landslide at km 367 on NH-21, Kinnaur district, Himachal Pradesh, and the Patalganga landslide on NH-58 near Pipalkoti, Uttarakhand. The DST has initiated various research projects such as the ones for NH-1A Sonapur in Meghalaya, the Tirumala Hills, eight specific sites in Uttarakhand, etc.
Standard Operating Procedure (SOP) – Landslide Investigation – put forward by the Geological Survey of India (GSI) states that at present there is no Guidelines available for LHZ on Meso Scale (1:5000/10,000). The main purpose of LHZ mapping on Meso Scale is to develop a detailed LHZ Map of a small area e.g. around a township, to assess the stability status of an existing localities incorporating more recognized important causative factors inducing slope instability, which cannot be considered in case of LHZ mapping on Macro Scale. GSI has also developed a methodology incorporating some more causative factors like slope erosion, shear strength of rock and overburden material etc. by modifying the BIS Code for LHZ on Macro Scale. GSI has proposed to develop a most acceptable Guideline both for Landslide Hazard Zonation/Landslide Risk Zonation on Macro (1:25,000/50,000) and Meso (1:5000/10,000) Scales as there are many shortcomings in the existing guidelines, however is
The SOP (GSI) states that an important aspect of landslide investigation called
“Stability Analysis” is seldom carried out. However, a landslide investigation cannot be completed without stability analysis, which is a direct measure of degree of instability of a slope. Proper geotechnical investigation of a landslide should be carried out to establish the causative factors of landslide. Detailed geotechnical mapping of landslide depicting geometry of the slide, crown and toe of the slide. Locations of longitudinal and transverse cracks, slide scars, old slide scar, plunge of striation lineation., Disposition of different litho-units, structure and state of weathering of rocks, type and composition of overburden. Location of severe toe erosion by streams or rivers, scouring pattern of slope along natural drainage, locations and attitude of steep, moderate and gentle slopes around the slide zone Zones of gully erosion, seepage/spring locations, dry or wet slope, elements at risk, tentative run out.
Such a study is essential for suggesting most appropriate remedial measures in a cost- effective manner to contain the slide.
Besides the above-mentioned codes and official documents, the IS 14680:1999 Landslide Control – provides some “Guidelines Landslide control methods for effective correction measures to avoid landslides in hill areas”.
The BIS codes for slope stability analysis, vis., IS 7894:1975 Code of Practice for Stability Analysis of Earth Dams and IS 8237:1985 Code of Practice for Protection of Slope for Reservoir Embankment are developed for, as the name suggest, for stability analysis of earth dams and reservoir embankment.