LANDSLIDE HAZARD EVALUATION AND A STUDY ON INFLUENCE OF NEOTECTONIC LINEAMENTS

Transcription

1 Chapter - 6 LANDSLIDE HAZARD EVALUATION AND A STUDY ON INFLUENCE OF NEOTECTONIC LINEAMENTS 6.1 GENERAL The study area is bounded by hills like Shevaroys, Chitteri and Kalrayan in the north and Kolli and Pachchai malai hills in the south, many hillocks like Kanjamalai, Godumalai, Bodamalai, Nainar malai and Paittur hills are at the centre part of the study area. Signatures of Neotectonic activity made the study on landslide an inevitable one. Since the landslide is a phenomenon happening in high relief region, hill boundary map was prepared and all the thematic maps like geology, geomorphology, landuse, structure and lineaments and slope were generated for hilly part of the study area. Though more than 60 percent of the region is covered with hills and high relief topography, landslide vulnerability study was never been done earlier nor the records of landslide occurrences are available. Moreover, the hills are highly dissected with complex geomorphology and varied landuse pattern. So, neither the information value method nor weight of evidence method nor GIS based slope method become suitable for the present study. Knowledge based index overlay method can be a tailor made fit for the present study area. In this Method, the user assigns weightages to various terrain systems and scores to the sub variables of the each of the terrain systems (Bonham-Carter 1994). In this method, the user assigns such weightages and scores on the basis of his knowledge about the terrain and the visibly appreciated contribution of different terrain variables over landslides. Hence, most of the scientists and technocrats claim this method to be very useful in Landslide Vulnerability Zonation Mapping (Aleotti and Chowdhury 1999; Lulseged Ayalew et al., 2004; LEE et al., 2004; Sati et al., 1998; Ramakrishnan et al., 2002). So, accordingly this method was carried out

2 for the study area and Landslide Vulnerability Zonation Mapping was done. The systematic study on terrain parameters has revealed the neotectonic activity in the study area. The influence of neo active tectonism as triggering mechanism on landslide was studied. 6.2 LANDSLIDE HAZARD EVALUATION BY INDEX OVERLAY METHOD Methodology In this method all the raster GIS layers of the 7 terrain systems (Fig. 6.2 to 6.8) were used and for all these 7 terrains systems weightages (Wi) were assigned on the basis of their possible contribution to landslides. Then for the each sub variable of the 7 terrain systems, scores were assigned (Sij). Then these scores (Sij) of the sub variables were multiplied with the corresponding weightage (Wi) of the terrain systems and landslide vulnerability weightages (W x Sij) were worked out for each sub variable. For example, Wi for lithology layer was 7 and Sij for the Fissile hornblende gneiss was 8. So the finally accrued Landslide weightages (Wi x Sij) for the Fissile hornblende gneiss was 56 (7 x 8). Similarly, for each sub variable of the 7 terrain systems landslide vulnerability weightages were worked out and the corresponding weightages were assigned to pixels of such sub variables. Such 7 weighted GIS raster terrain system layers having 7,012,505 pixels each were integrated using GIS and integrated GIS layer was generated. As in such integration, each pixel of each layer was added with corresponding pixels of the 7 terrain parameter thematic layers, such integrated layer obviously had same number of 7,012,505 pixels, but having the total weightages of all 7 themes and its sub variables. The finally accrued weightages of these 7,012,505 pixels of the integrated layer were divided by the sum of the weightages ( Wi) of all the 7 terrain parameters and final weightages were worked out for integrated final GIS layer. Then the dynamic range of the final weightages of the 7,012,505 pixels were rescaled from 0 to 100 and again grouped into 5 zones of landslide vulnerability zones (Fig. 6.9). 340

3 Thus Index Overlay based Landslide Vulnerability Zonation map having five grades of landslide vulnerability was prepared. The Figure 6.1 explains the general methodology adapted in the present study for Index Overlay method based Landslide Vulnerability Zonation Mapping. Satellite Data SRTM Data Topographic Sheet from SOI (1:50000 scale) Soil Map from NBSS& LUP Slope District Resource Map from GSI Soil Map Geomorphology Lithology Landuse / Landcover Lineament Lineament Density Drainage Drainage Density Assigning Weightage (Wi) and Scores (Sij) Raster Integration Sum of Weightages / ( Wi) Rescaling and Classification Landslide Vulnerability Zonation Map Neotectonics Vs Landslides Field Validation Fig. 6.1 Methodology Flow Chart- Index Overlay method based LVZ mapping 341

4 6.2.2 Assignment of Weightages (Wi) and Scores (Sij) to Terrain Systems As discussed in the methodology, first the weightages (Wi) were assigned to the 7 terrain parameters as shown in Table 6-1. While assigning so, the maximum weightage of 10 was assigned to the slopes Similarly, the weightage 10 was assigned to all the other three terrain parameters related to slope ridges and the joints as many earlier workers have observed that these parameters significantly control landslides. Ramasamy et al. (2008) also inferred that the slope ridge lines and joints and its intersections play a vital role. They have done so on the basis their detailed GIS based studies. Various derivatives of lineaments and geomorphology were assigned weightages 9 and 8 respectively. Thus, varied weightages ranging from 4 to 10 were assigned to 7 different terrain system parameters (Table 6-1). Subsequent to the assignment of weightages to 7 terrain system parameters, scores (Sij) were assigned to different sub variables of the each of the 7 terrain system parameters. For example, the geomorphology layer had 10 sub variables namely - cliff/ escarpments, denudational slope, intermontane valley, hilltop weathered, dissected plateau, fracture valley fill, undissected plateau, fracture valley barren, residual hills, linear ridge/dyke and they were regrouped into six. They were assigned the scores of 10, 8, 7, 6, 5 and 4 as the cliff/ escarpments, denudational slope are more susceptible to landslides, followed by intermontane valley, hilltop weathered, dissected plateau and trailed by the fracture valley fill, fracture valley barren and residual hills (Table 6.4). Subsequently, the scores of these sub variables were multiplied with the corresponding weightage 8 of the geomorphology layer and thus these 6 groups have respectively accrued the landslide vulnerability weightage of 80, 64, 56, 48, 40 and 32 as shown in Table 6-4. Then, these values were assigned to the corresponding pixels numbering over of cliff/ escarpments and denudational slope, pixels of intermontane valley and hilltop weathered, pixels of dissected plateau,

5 pixels of fracture valley fill, pixels of undissected plateau and pixels of, fracture valley barren, residual hills and linear ridge/dyke in the raster GIS layer of geomorphology (Fig. 6.4) and such Index Overly based weighted GIS layer on geomorphology was prepared. Similarly, for the remaining 6 terrain system parameters, the landslide vulnerability weightages were worked out by assigning suitable scores to various sub variables of each of the parameters as shown in Table 6.1 to 6.8. Thus, finally the landslide vulnerability weightages were assigned to the corresponding pixels of various sub variables using the raster GIS layers Fig. 6.2, 6.3, 6.4, 6.5, 6.6, 6.7 and 6.8 related to various terrain system parameters. Table 6-1 Weightages assigned to 7 Terrain systems Sl.No Themes Weightage (Wi) 1. Slope Lineament density 9 3. Geomorphology 8 4. Lithology 7 5. Landuse land cover 6 6. Drainage density 5 7. Soil 4 Total ( Wi) = 49 Table 6-2 Weightages and Scores Slope Sl.No Slope Name Weightage (Wi) Score (Sij) (Wi x Sij) 1. Steep Slope Moderate Slope Shallow Slope Rolling surface/plain

6 Fig 6.2 Landslide -Weighted Slope Map Table 6-3 Weightages and Scores Lineament density Sl.No Lineament Density Weightage (Wi) Score (Sij) (Wi x Sij) 1. Very High > High Moderate Low Very Low

7 Fig. 6.3 Landslide -Weighted Lineament Density Map Table 6-4 Weightages and Scores Geomorphology Sl.No 1. Geomorphology Class Cliff/ Escarpments, Denudational Slope 2. Intermontane Valley, Hilltop Weathered 3. Dissected Plateau 4. Fracture Valley Fill 5. Undissected Plateau Weightage (Wi) Score (Sij) (Wi x Sij) Fracture Valley Barren, Residual Hills, Linear Ridge/Dyke

8 Fig. 6.4 Landslide -Weighted Geomorphology Map Table 6-5 Weightages and Scores Lithology Sl. No. Lithology Class Weigh tage (Wi) Score (Sij) 1. Laterite Fissile Hornblende Gneiss, Amphibolite, (Wi x Sij) 3. Charnockite, Siderite-Ankerite Gneiss, Epidote Hornblende Gneiss, Hornblende Biotite Gneiss Basic Dyke, Granite, Pink Migmatite Carbonatite, Calc granulite and Limestone Magnetite- Quartzite, Fuchsite Kyanite- Magnetite-Silimanite Quartzite, Garnetiferous Gabbro, Pyroxene Granulite, Fluvial (Sand, Silt, Gravel, Clay), Ultramafic rocks

9 Fig. 6.5 Landslide -Weighted Geology Map 347

10 Table 6-6 Weightages and Scores Landuse / Land cover Sl.No. Landuse and Land Cover Class Weightag e (Wi) Score (Sij) (Wi x Sij) 1. Scrub Forest, Forest Blank Wastelands/Land Without Scrub Built-up, Crop Land in Forest, Crop Land, Gullied/ Ravenous Land, Salt Affected Land, Wastelands/Land With Scrub 5. Deciduous Open, Plantations Water bodies 6. Barren Rocky/ Stony Waste Deciduous Forest, Forest Plantations 8. Abandoned Quarries/ Mining Area Fig. 6.6 Landslide- Weighted Landuse / Land cover Map 348

11 Table 6-7 Weightages and Scores Drainage density Sl.No Drainage Density Weightage (Wi) Score (Sij) (Wi x Sij) 1. Very high > High Moderate Low Very low < Fig. 6.7 Landslide -Weighted Drainage Density Map 349

12 Table 6-8 Weightages and Scores Soil* Sl.No Soil Type Wi Sij Wi xsij 1. Loam Soil -On Slope Summit Clay, Gravelly Loam On Steep Slope Gravelly Clay On High 3. Gravelly Loam, Gravelly Clay, Clay, Calcareous Loam, Loam Soil, Calcareous Clay- On Moderate Slope 4. Gravelly Loam, Gravelly Clay, Clay, Loam Soil On gentle Slope Calcareous Loam, Calcareous Clay On Level Fig. 6.8 Landslide-Weighted Soil Map 350

13 6.2.3 GIS Integration and Index Overlay Method Based Landslide Vulnerability Zonation Mapping Finally, all these 7 weighted GIS raster layers of the different terrain parameters were added using raster calculator menu in spatial analyst extension in ArcGIS. In this process, each terrain parameter having 7,012,505 pixels were added with corresponding pixels of the other 6 parameters and the resultant integrated GIS output obviously again had 7,012,505 number of pixels. The summed up values ( Wi x Sij) of each pixels ranges from 414 to 111. Then the summed up values of the entire 7 weighted GIS layer ( Wi x Sij) were divided by the sum of the weightage ( Wi) i.e. 49 (Table.6.1). That means each of the finally accrued weightages ( Wi x Sij) of the 7,012,505 pixels were divided by ( Wi) i.e., ( Wi x Sij)/ ( Wi) and these values were assigned to the each of the pixels in the Integrated layer. The same has varied from to Such dynamic range was rescaled from 0 to 100 which were further grouped in to 5 classes. That is pixels with 70 weightage Very High Vulnerable Zones 69 to 60 weightage High Vulnerable Zones 59 to 50 weightage Moderate Vulnerable Zones 49 to 40 weightage Low Vulnerable Zones 39 weightage Very Low Vulnerable Zones Thus, the Index Overlay based Landslide Vulnerability Zonation Map was prepared (Fig. 6.9). 6.3 A STUDY ON INFLUENCE OF NEOTECTONIC LINEAMENTS Landslide is phenomenon related to high relief region and leaving behind the anthropogenic causes, rainfall and seismicity are the main external causative factors. As the name indicates they cannot be studied on a hill slope, they usually affect a larger area. These factors at many times are responsible for triggering landslides. 351

14 Fig. 6.9 Landslide Vulnerability Zonation Map Rainfall and Landslides An analysis of rainfall pattern for the study area was done for over the period of 10 years. The average rainfall level of the region exceeds at some places like Shevaroy and northern part of Kalrayan hills. So, the rainfall is considered as as one of the prime triggering mechanisms. Mostly the landslides that are happening in this region are triggered by rainfall so as in any landslide in Tamilnadu. The figure 6.10 gives an idea of the rainfall pattern in Salem and adjoining area. For an instance 1 st June 2009 heavy 352

15 rainfall in Salem causes rockfall in Kariaperumal Karadu Hills near Nethimedu (Annexure-IG) which actually falls in very low vulnerable zone (PE1 and PE2 in Table 6.10 and Fig. 6.11) Fig 6.10 Rainfall trend for the past 12 years in mm Neotectonic Lineaments Vs Landslides Klaudia Ratzinger et al. (2006) have studied the Printopa landslide in Greece by combined analysis of optical (LANDSAT ETM) and SAR intensity (ERS-1/2) remote sensing data with results from differential SAR interferometry in a GIS environment to assess the potential for landslide hazard on a regional basis. Patwary et al. (2009) have done landslide hazard potential zone in one of the structurally disturbed and active zones in western Himalaya around Rishikesh. The neotectonic activities in the southern part of Indian continent is considered as pulsatory tectonics with E-W trending arching, N-S trending extension NE-SW sinistral and NW-SE dextral strike slip faults (Ramasamy SM., 2006a). 353

16 The high hills with steep slopes are controlled by newer evolution of the plateaus probably tectonic plateaus, bounded by NE-SW, NNE-SSW, NNW-SSE and NW-SE, lineaments. These differently oriented lineaments make the slope of the plateaus very much vulnerable to landslides. Moreover the plateaus are highly dissected, may be as a result of cumulative effects of all the tectonic events in this region. Higher degree of deformation and recrystallization makes the rocks break easily and ready to slide. The continuously drifting and subducting Indian plate causes the accretion, arching and grabening of this region. The E-W trend penetrating the normal NE-SW trend in the Attur valley and further northern zone may be corresponds to the tear faulting transverse to the northerly arching landmass. The arching and the probable tear faulting could cause the already dissected plateaus to widen up or loosen up across N-S trending lineaments. The slump scars were observed in the Kalrayan hills along N-S trending lineaments NL18, NL24, NL46, L58 and in Shevaroys along NL24. The alignment of 12 neotectonic lineaments along 0 to 10 degrees north, 10 neotectonic lineaments along 30 to 40 degrees north and 18 neotectonic lineaments along 80 to 90 degrees north suggests the possibility of landslides along these lineaments. Apparently, there exists a possible correlation of landslides with Neotectonic activity along these lineaments (Fig.6.11). 6.4 FIELD VALIDATION The shaping of the present topography, the relative relief and landforms are greatly influenced by the neo tectonics which keeps the hilly environment prone for landslide apart from rainfall. Field visit was made to Shevaroys, Kalrayan, Kolli, Godumalai, Perumamalai, Kanjamalai, and Kariyaperumal karadu hills in the study area and the slide or rockfall or slump scar were located with the help of GPS (Plate-X&XI). The GPS locations were correlated with landslide vulnerability zones (Fig.6.11) and their relations with the neotectonic and seismogenic lineaments were tabulated (Table 6.9). 354

17 There were incidences directly recorded from local people in Motaiyanur (location near K15 in table 6.9, Plate-XIE) village in Kalrayan where the (GPS location : E 78 44' 23.99", N 11 51' 35.99") huge slide was happened during an earth quake Ambur earthquake whose magnitude is 3.8 on Richter s scale and in Kariaperumal Karadu near Salem (GPS location : PE1- E 78 7' 47.99", N 11 39' 0") rocks are rolled down and destroyed the house and injured a women during heavy rainfall in 1 st June of 2009 (Annexure- IG). Similar happenings were common in Salem-Yercaud ghat road (Annexure-IH) and created havoc in 1992, since the Yercaud and other villages were cut off from the nearest main town for about a month. The paleo-tectonism and neotectonism have had the direct influence in weakening the rocks by shearing and shattering and induced higher degree of weathering of rocks. These can be observed in the field that thick weathered zone at highly dissected plateaus like Kalrayan and Chitteri that supports good cultivation at the hill top near the places of major lineaments. The lineaments with N-S, NNE-SSW, E-W, NE-SW trend has higher influence in the observed locations, as these lineaments cut the rocks intensively and make them vulnerable to slide. Especially in Shevaroys, along the road from Salem to Yercaud via Kondappanyakanpatti, slide scars and landslide were observed which are all debris slide in nature whereas the other road from Salem via Kuppanur to Yercaud, that is south eastern flank, many rock falls are observed than debris slides. Slump scars were observed along N-S lineaments NL24 in Shevaroys and along N-S trending NL18, NL46 and L58 in Kalrayan. Anbazhagan et al. (2005) observed land subsidence in southeast of Kolli hills and attributed that to fracture developed causes. This incidence was published in the daily Dhina Thanthi in 19 th December 2005 and The Hindu 25 th December 2005 which might probably be of similar nature to that of subsidence at Shevaroys and Kalrayan along Neotectonic lineaments. 355

18 Fig.6.11 Locations of Landslides and Neotectonic Lineaments 356

19 6.5 SYNTHESIS Index overlay method is an effective method in mapping a large area where previous records of landslides are not available or the terrain is inaccessible. Seven system parameters pertaining to landslides were selected after thorough literature survey and weightages and scores were assigned on the basis of knowledge about the terrain and the visibly appreciated contribution of different terrain variables over landslides. The geoinformatics based analysis has led to the identification of landslide prone zones which were further validated with field study. The lineament characterization was restricted to the locations where landslides occurrence were verified. Though the rainfall causes landslide the cumulative effects of tectonism and weathering has made the rocks ready to slide. Undoubtedly, the lineaments with N-S orientation have cut the general trend of NE-SW foliation and NE-SW trending lineaments in acute angle and make the rocks unstable to hang on the slope. The alignment of slump scar along N-S lineaments suggests sinking of land mass or probably widening along east west direction corresponding to the arching. The slopes of the plateaus are very steep and are bounded by many cliffs and fault scarps with triangular facets and cutting across by many neotectonic and seismotectonic lineaments suggests that this region is still prone for many more landslides in the years to come. 357

20 Table 6.9 GPS Locations of Landslide Occurrences GPS location Longitude (In decimal degrees) Latitude (In decimal degrees) Height in 'm' Zone Type of landslide Proximal lineaments Remarks Ga V Rock fall/ rock slide L54 Go V Rock fall/ rock slide NL10,NL11 Go V Rock fall/ rock slide NL11, NL28 K V Mud/ debris slide NL12,NL20 K V Slump scar NL29 K V Mud/ debris slide L58 K V Mud/ debris slide L58 Along N-S K MV Slump scar NL18 Along N-S K MV Slump scar NL18 Along N-S K MV Slump scar NL18 Along N-S K MV Slump scar NL18 Along N-S K V Mud/ debris slide L25, NL44 K V Mud/ debris slide L25, NL44 K V Mud/ debris slide L25, NL44 K V Mud/ debris slide L25, NL44 K V Mud/ debris slide NL44 K V Mud/ debris slide NL44 K V Mud/ debris slide NL46 *Ambur Ka MV Rock fall/ debris slide NL 25 Ka MV Rock fall/ debris slide NL 25 Ka HV Rock fall/ debris slide NL50 Ka V Rock fall/ debris slide NL50 Ka V Rock fall/ debris Slide NL50 Kan V Rock fall/ rock slide NL35, NL41 Ko V Mud/ debris slide NL34 Ko V Mud/ debris slide NL34 Ko V Mud/ debris slide NL34 Ko HV Mud/ debris slide NL34, NL42 Ko V Mud/ debris slide - Na V Mud/ debris slide L65,NL48 PE VLV Rock fall/ rock slide NL35, NL37 #Rainfall PE VLV Rock fall/ rock slide NL35, NL37 #Rainfall S MV Mud/ debris slide NL38 S MV Mud/ debris slide NL38 S MV Mud/ debris slide NL38 S MV Mud/ debris slide NL38 358

21 S MV Soil creep NL38 S MV Soil creep NL38 S MV Mud/ debris slide NL38 S MV Mud/ debris slide NL38 S MV Mud/ debris slide NL38 S MV Mud/ debris slide NL38 S MV Mud/ debris slide NL38 S MV Mud/ debris slide NL38 S HV Mud/ debris slide NL38 S V Mud/ debris slide NL38 S V Mud/ debris slide NL38 S HV Slump scar NL37, NL44 S V Slump scar NL24, NL37, NL44 S V Slump scar NL24, NL37, NL44 S V Slump scar NL24, NL37, NL44 S V Slump scar NL24, NL37, NL44 S HV Mud/ debris slide NL24, NL9 S HV Rock fall NL24, NL9 S HV Rock fall NL24,NL9 S HV Rock fall NL24,NL9 S HV Rock fall NL24,NL9 S HV Rock fall NL24,NL9 S HV Rock fall NL24,NL9, NL8 Along N-S Along N-S Along N-S Along N-S Refer HV-HIGHLY VULNERABLE; V-VULNERALE; MV-MODERATELY VULNERABLE; VLV- VERY LOW VULNERABLE ZONE NL-NEOTECTONIC LINEAMENT; L-SIGNIFICANT LINEAMENT * DUE TO 2008 VELLORE (AMBUR) EARTHQUAKE (78.80º, 12.80º), MAGNITUDE 3.8, # HEAVY RAINFALL IN SALEM DISTRICT ON 1 st JUNE

22 360

23 361