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LANDSLIDE HAZARD ZONATIONUSING THE RELATIVE EFFECTMETHOD IN SOUTH EASTERN PARTOF NILGIRIS, TAMILNADU, INDIA. Naveen Raj, T* Research scholar, Department of Geology, University of Madras, Maraimalai campus, Chennai - 600 025TAMILNADU INDIA. Ram Mohan.V Department of Geology, University of Madras, Maraimalai campus, Chennai - 600 025, INDIA Backiaraj. S Department of Geology, University of Madras, Maraimalai campus, Chennai - 600 025, INDIA Muthusamy.S Department of Applied Geology, University of Madras, Maraimalai campus, Chennai - 600 025, 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 NilgiriDistrict, Tamilnadu using the Relative Effect Method (REM) model, Geographic Information System (GIS) andremote sensing data have been evaluated. There are different methods of landslide hazard zonation with someadvantages and disadvantages. The authors suggest the Relative Effect Method (REM), which is statisticalmethod 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 bycalculating the ratio of the unit portion in coverage and landslide. The function that is used in this method islogarithmic. The advantages of the logarithmic function are in domain determination for output data andequality for plus and minus domains of calculated RE's. All the thematic layers are Display manipulate andanalysis has been carried out to evaluate layers such as geology, geomorphology, slope, soil, land use anddrainages. The computed index for each grid for each factor was summed and grouped into five classes. Thelandslide 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 downwardmovement 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 ahundred landslides within an area of 250 sq.kms in 1978 and in 1979 the incidence of landslides was on a muchlarger scale and nearly two hundred landslides were recorded in the Nilgiris district. Such severity of disastrouslandslides 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 mayhave a bearing on the landslide phenomena. Naveen Raj T et al. / International Journal of Engineering Science and Technology (IJEST)ISSN : 0975-5462Vol. 3 No. 4 April 20113260 Heavy rains in November, 1979 brought in large scale landslides in the Coonoor sector, eventuallyovertaking the landslide investigation which was in progress. The devastation due to landslides was even moresevere in 1979 than in 1978. 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, weretaken 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 12’30”N and11 o 35’00”N, and longitudes 76 o 35’30”E and 76 o 54’30”E, and covers an area of 526 km 2 approximately is givenin the (Fig.1). The area falls under survey of India Toposheet no 58 A/11 and 58 A/15.The minimum andmaximum altitudes are 550m and 2070m respectively above mean sea level. 2.1 Regional Geology The Nilgiri ranges comprise Archaean metamorphic rocks which include Charnockite. Charnockiterocks 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. 2.1.1 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 carriesenclaves of supra crustal rocks of divergent composition including metasedimentary sequences. The charnockitehas granulitic texture and carries quartz, feldspar, hypersthenes, garnet and hornblende. Biotite, apatite andzircon are present as accessory minerals. Variants of charnockite, especially the basic or ultrabasic types arefound in a few places. Some geologists have classified the charnockite as garnetiferous or non-garnetiferoustypes depending upon the presence or absence of garnet in the rock. Most of the peaks and high points in Nilgiridistrict are charnockite massifs. Naveen Raj T et al. / International Journal of Engineering Science and Technology (IJEST)ISSN : 0975-5462Vol. 3 No. 4 April 20113261 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 theMysore plateau towards north and merging gradually with the Western ghats in the north-west, west and south-west. The long axis of the plateau is in the direction of east-north-east. Over the years, phsiographers have beenmade a moot point that the Eastern Ghat abut into the Western Ghat in the Nilgiri ranges.The plateau has alength of 55km and a width of 32 km approximately, occupying an area of 1800 sq.km. It is bound by theBhavani river on the southern side and by the Moyar river in the north. The water divide in this part of thePeninsula 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 isquantitatively determined by introducing a ‘Relative Effect’ function (RE). Given an area of study that containsa 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 everyunit, 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/ASR= sld/SLDThe 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 meansthat 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. Thismeans 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 hasno 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 algebraicsummation of RE s, multiplied by alpha,Slide risk = ( Ʃ RE X α ) Naveen Raj T et al. / International Journal of Engineering Science and Technology (IJEST)ISSN : 0975-5462Vol. 3 No. 4 April 20113262 Where Alpha is zero if there is no risk of landslide (e.g., slopes less than 5 degrees), otherwise thevalue 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 RE s. Unitswith higher values of summation of absolute RE s, will be more effective and more important in landslidemanagement and hazard mitigation than those with lower values. 4. LANDSLIDE HAZARD MAPPING Interpretation of future landslide occurrence needs an understanding of conditions and processescontrolling 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 hydrologicfactor 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 areassociated with differing degrees of landslide hazards. The identification of the extension of these combinationsover 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 identifythe potential landslide hazard in the mountainous area. A landslide zonation map was prepared based on RE s 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 . Type % of coverage % of slide R.E. Charnockite 98.90912548 100 0.005Hornblende BiotiteGneiss 1.137262357 0 0 Table 2. Percentage of landuse and landcover units coverage and related slide. Type % of coverage % of slide R.E. Built-up land 3.65256806 4.229607251 0.064Crop Land 13.82908034 6.64652568 -0.318Decidious Forest 0.105765225 0 0Evergreen/Semi-Evergreen Forest4.047615423 1.510574018 -0.428Forest Blanks 0.745266358 0 0Forest Plantations 17.60607458 10.57401813 -0.221Tea Plantations 49.37337057 75.52870091 0.185Land with Scrub 4.289717083 1.510574018 -0.453Land without Scrub 0.259538887 0 0Reservoirs 0.227708383 0 0Scrub Forest 5.697075501 0 0Barren Rocky 0.037816761 0 0Tanks 0.061863516 0 0River 0.008637644 0 0Canal 0.111807122 0 0 Table 3 . Percentage of geomorphological units coverage and related slide. Type % of coverage % of slide R.E. Barren Valley 2.919067263 1.510574018 -0..286Barren Plateau 12.74673359 16.3141994 0.107Dissected Plateau 22.88946449 25.98187311 0.055Dissected Upland 50.56932372 41.3897281 -0.087Fracture Valley Fill 1.811756845 0.906344411 -0.301Reservoir 0.252361252 0 0Intermontane Valley 0.148134362 0 0Valley Fill 8.673132626 13.89728097 0.205 Naveen Raj T et al. / International Journal of Engineering Science and Technology (IJEST)ISSN : 0975-5462Vol. 3 No. 4 April 20113263
