LANDSLIDE HAZARD ZONATION USING THE RELATIVE EFFECT METHOD IN SOUTH EASTERN PART OF NILGIRIS, TAMILNADU, INDIA.

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1 LANDSLIDE HAZARD ZONATION USING THE RELATIVE EFFECT METHOD IN SOUTH EASTERN PART OF NILGIRIS, TAMILNADU, INDIA. Naveen Raj, T* Research scholar, Department of Geology, University of Madras, Maraimalai campus, Chennai TAMILNADU INDIA. Ram Mohan.V Department of Geology, University of Madras, Maraimalai campus, Chennai , INDIA Backiaraj. S Department of Geology, University of Madras, Maraimalai campus, Chennai , INDIA Muthusamy.S Department of Applied Geology, University of Madras, Maraimalai campus, Chennai , INDIA ABSTRACT Landslides occur frequently due to climatologic and geologic conditions with high tectonic activities. In this paper, the landslide hazard and the effect of landslide-related factors at South Eastern part of Nilgiri District, Tamilnadu using the Relative Effect Method (REM) model, Geographic Information System (GIS) and remote sensing data have been evaluated. There are different methods of landslide hazard zonation with some advantages and disadvantages. The authors suggest the Relative Effect Method (REM), which is statistical method using GIS software for landslide hazard zonation. This method determines the relative effect (RE) of each unit, such as surface geology, slope morphometry, climatic conditions, land use and land cover by calculating the ratio of the unit portion in coverage and landslide. The function that is used in this method is logarithmic. The advantages of the logarithmic function are in domain determination for output data and equality for plus and minus domains of calculated RE's. All the thematic layers are Display manipulate and analysis has been carried out to evaluate layers such as geology, geomorphology, slope, soil, land use and drainages. The computed index for each grid for each factor was summed and grouped into five classes. The landslide susceptibility map can be used to reduce damage associated with landslides and to land cover planning. Keywords: Landslides, Relative Effective Method, GIS, Nilgiris, Hazard Zonation. 1. Introduction Landslides are frequent and annually recurring phenomena in the Nilgiri plateau. Outward and downward movement of mass, consisting of rocks and soils, due to natural or man-made process is termed as a landslide. When the landslides endanger humans and their installations, they are known as hazards and when they cause property damage and loss of life, they are known as disasters. The unprecedented rains caused more than a hundred landslides within an area of 250 sq.kms in 1978 and in 1979 the incidence of landslides was on a much larger scale and nearly two hundred landslides were recorded in the Nilgiris district. Such severity of disastrous landslides has not been felt in any part of the country so far. Detailed investigation of individual slides was later taken up. Data pertaining to the geological, geomorphological and hydrological features and other factors which, individually or in combination, trigger landslides were gathered in detail. These aspects were studied with an objective of suggesting comprehensive planning and designing of preventive structures and to outline precautionary measures. Available aerial photographs and landsat imagery were studied for identifying palaeoscars and regional structures which may have a bearing on the landslide phenomena. ISSN : Vol. 3 No. 4 April

2 Heavy rains in November, 1979 brought in large scale landslides in the Coonoor sector, eventually overtaking the landslide investigation which was in progress. The devastation due to landslides was even more severe in 1979 than in This new development entailed partial reorientation of work and modification of priorities. As the landslides of 1979 were more massive and of larger magnitude, detailed profiles of landslides, detailed mapping on larger scale and in a few instances, survey with terrestrial photogrammetric work, were taken up. 2. Research Area Fig 1: Base Map of the study area The area for which landslide susceptibility map (LSM) is prepared, lies between the latitudes 11 o N and 11 o N, and longitudes 76 o E and 76 o E, and covers an area of 526 km 2 approximately is given in the (Fig.1). The area falls under survey of India Toposheet no 58 A/11 and 58 A/15.The minimum and maximum altitudes are 550m and 2070m respectively above mean sea level. 2.1 Regional Geology The Nilgiri ranges comprise Archaean metamorphic rocks which include Charnockite. Charnockite rocks have been referred in earlier literature as Dharwar schists. They are at present included under the Sargur schists. A brief description of the individual rock formations is given below Charnockite Charnockite forms the bulk of the rock units in the Nilgiri district. This hypersthenes-bearing bluish grey rock forms the basement in high grade metamorphic terrain. It is interbanded with or carries enclaves of supra crustal rocks of divergent composition including metasedimentary sequences. The charnockite has granulitic texture and carries quartz, feldspar, hypersthenes, garnet and hornblende. Biotite, apatite and zircon are present as accessory minerals. Variants of charnockite, especially the basic or ultrabasic types are found in a few places. Some geologists have classified the charnockite as garnetiferous or non-garnetiferous types depending upon the presence or absence of garnet in the rock. Most of the peaks and high points in Nilgiri district are charnockite massifs. ISSN : Vol. 3 No. 4 April

3 2.2 Geomorphology Fig 2: Geomorphology of the study area The Nilgiris hills, rising aloft from the uplands of Coimbatore is a plateau sloping steeply into the Mysore plateau towards north and merging gradually with the Western ghats in the north-west, west and southwest. The long axis of the plateau is in the direction of east-north-east. Over the years, phsiographers have been made a moot point that the Eastern Ghat abut into the Western Ghat in the Nilgiri ranges.the plateau has a length of 55km and a width of 32 km approximately, occupying an area of 1800 sq.km. It is bound by the Bhavani river on the southern side and by the Moyar river in the north. The water divide in this part of the Peninsula passes through the western edge of the plateau as shown in the (Fig 2). 3. METHODOLOGY In this study, the relative effect of a parameter as a determining factor of slope instability is quantitatively determined by introducing a Relative Effect function (RE). Given an area of study that contains a certain number of landslides, various thematic maps (geology, slope, soil thickness, soil texture, soil permeability, plant and forest) are prepared. Each map is covered individually by the landslide map. For every unit, the ratio of the unit area, a, to the total area of the study, A, and the ratio of the landslide area in the unit, sld, to the area of total landslide, SLD, are calculated; AR= a/a SR= sld/sld The relative effect function is then defined as: RE =Log (SR + ε), AR Where epsilon is a very positive value near zero. There are three cases for estimating a relative effect of each unit depending on it s RE. 1) RE is less than zero when the share of a unit in landsliding is less than its share in area Coverage. This means that it has an effect of decreasing landslide risk (negative effect). 2) RE is greater than zero when the share of a unit in landsliding is greater than its share in area coverage. This means that it has an effect of increasing landslide risk (positive effect). 3) RE is zero when the share of a unit in landsliding is equal to its share in area coverage. This means that it has no effect of decreasing or increasing landslide risk. The advantage of using logarithmic function is that the positive effect and negative effect are quantitively equal. Then using a GIS, all maps are integrated and an evaluation of landslide risk is determined by algebraic summation of REs, multiplied by alpha, Slide risk = (ƩRE X α) ISSN : Vol. 3 No. 4 April

4 Where Alpha is zero if there is no risk of landslide (e.g., slopes less than 5 degrees), otherwise the value of alpha is 1.The higher positive values of slide risk indicate a higher risk of landslide and the higher minus values of slide risk indicate a lower risk of landslide. We can also judge the effectiveness of a unit by simple summation of absolute values of REs. Units with higher values of summation of absolute REs, will be more effective and more important in landslide management and hazard mitigation than those with lower values. 4. LANDSLIDE HAZARD MAPPING Interpretation of future landslide occurrence needs an understanding of conditions and processes controlling landslides in the study area. Three physical factors such as past history, slope steepness, and bedrock are the minimum components necessary to assess landslide hazard zonation. It is also useful to add a hydrologic factor to reflect the important role which ground water often plays in the occurrence of landslides. An indication of this factor is usually obtained indirectly by looking at vegetation, slope orientation, or precipitation zones. All of these factors are capable of being mapped. Specific combinations of these factors are associated with differing degrees of landslide hazards. The identification of the extension of these combinations over the area being assessed results in a landslide hazard map. The scope of this study was to generate landslide hazard zonation maps that can be utilized to identify the potential landslide hazard in the mountainous area. A landslide zonation map was prepared based on REs of the geological units, soil type, landuse and landcover, geomorphology, drainage density, distance to drainage, lineament density, slope (Tables 1 to 8). Table 1. Percentage of geological units coverage and related slide. Charnockite Hornblende Biotite Gneiss Table 2. Percentage of landuse and landcover units coverage and related slide. Built-up land Crop Land Decidious Forest Evergreen/Semi Evergreen Forest Forest Blanks Forest Plantations Tea Plantations Land with Scrub Land without Scrub Reservoirs Scrub Forest Barren Rocky Tanks River Canal Table 3. Percentage of geomorphological units coverage and related slide. Barren Valley Barren Plateau Dissected Plateau Dissected Upland Fracture Valley Fill Reservoir Intermontane Valley Valley Fill ISSN : Vol. 3 No. 4 April

5 Table 4. Percentage of slope units coverage and related slide > Table 5. Percentage of lineament density units coverage and related slide Table 6. Percentage of drainage density units coverage and related slide Table 7. Percentage of distance to drainage units coverage and related slide Table 8. Percentage of soil units coverage and related slide. Clayey Soils Loamy Soils Table 9. Percentage area of risk classes Class Area (m 2 ) Area percentage from Basin Very Low Low Moderate High Very High Sum Based on the RE s value, Landslide Zonation Map (LZM) was prepared. It is divided into five classes namely very low, low, moderate, high and very high is shown in the (Fig 3) ISSN : Vol. 3 No. 4 April

6 Fig 3 Landslide Hazard Zonation Map 5. Conclusions In lulc units, built-up land and tea plantations shows the positive values indicates that high possibility for landslides and also shows the rapid development of urban in the study area whereas in geomorphology units, barren plateau, dissected plateau and valley shows the positive values.through this study, it is evinced again that the geomatics technology is a proven tool for landslide studies in order to properly understand, identify and suggest remedial measures. References [1] Aleotti, P. and Chowdury, R.: 1999, Landslide hazard assessment: summary review and new perspectives, Bull. Eng. Geol. Envir., 58(1), [2] Brabb, E. E.: 1984, Innovative approaches to landslide hazard mapping, In: Landslides-Glissements de Terrain, IV International Symposium on Landslides, Vol. 1, Toronto, Canada, pp [3] Carrara, A., Cardinali, M., Detti, R., Guzzetti, F., Psqui, V., and Reichenbach, P.: 1991, GIS techniques and statistical models in evaluating landslide hazard, Earth Surface Processes and Landforms 16, [4] Carrara, A., Cardinali, M., Guzzetti, F., and Reichenbach, P.: 1995, GIS technology in mapping landslide hazard, In: A. Carrara and F. Guzzetti (eds.), Geographical Information Systems in Assessing Natural Hazards, Kluwer Academic Publisher, The Netherlands, pp [5] Chung, C. F. and Fabbri, A. G.: 1999, Probabilistic prediction models for landslide hazard mapping, Photogram metric Engineering & Remote Sensing 65(12), [6] Cruden, DM (1991) A simple definition of a landslide. Bull Int Assoc Eng Geol V. 43, pp [7] Cruden DM, Varnes DJ: 1996, Landslide types and processes. In: Turner AK, Schuster RL (eds) Landslides investigation and mitigation. Special Report 247. Transportation Re-search Board, Washington, pp , scale 1:24,000, 8 pp. [8] Ganapathi, G.P., Mahendran, K. and Sekar. S.K. (2010) Need and Urgency of Landslide Risk Planning for Nilgiri District, Tamil Nadu State, India, Int. Jour. of Geomatics and Geosciences. V. 1, No 1, pp [9] Jaiswal. P et al.: 2010 Quantitative assessment of landslide risk in India, Natural Hazards Earth System Sciences., 10, [10] Jaiswal, P. and van Westen, C. J.:2009, Estimating temporal probability for landslide initiation along transportation routes based on rainfall thresholds, Geomorphology, 112, ISSN : Vol. 3 No. 4 April

7 [11] Seshagiri, D. N., Upendran, R. and Lakshmikantham, C.B. (1980) Nilgiris Landslides: Report on the First Phase of geological Investigation A collaborative Study. Unpublished Report of State Geology Branch, Industries Department. 48 p. [12] Van Westen, C. J.: 1993, Application of Geographic Information Systems to Landslide Hazard Zonation, ITC Publication 15, 245 pp. [13] Van Westen C.J.: GIS in landslide hazard zonation: A review with examples from the Colombian Andes. In: Price M.F.& Heywood D.I. (eds.), Taylor and Francis, London, 1994, [14] Varnes, D. J.: 1978, Slope movement: types and processes, In: R. L. Schuster and R. J. Krizek (eds), Landslides: Analysis and trol, Transportation Research Board, National Academy of Sciences, Washington D.C. Special Report 176, Chapter 2: [15] Varnes, D.J.:1984, Landslide hazard zonation: a review of principles and practice, Natural Hazards 3, UNESCO: 63p. [16] Venugopal, D.:2004, Development-Conservation Dilemma in the Nilgiri Mountains of South India, J. Mt. Sci., 1(1), ISSN : Vol. 3 No. 4 April