Landslide Hazard Zonation Mapping Using GIS Technique - Case Study From Nilgiri District Tamil Nadu - India

C. Mohana Doss and K. Sunitha
Department Of Geology,
Anna University, Chennai - 25.

Landslide is a common natural hazard that usually occurs in mountainous areas. They are the most devastating and frequently occurring environmental hazards landslide is a common term generally used for all forms of rapid mass environment.

In general landslide activity is related to the following factors. Slope, geology, structure, lineament, geomorphology, climate, rainfall and landuse. The principal factors that initiate landslides are heavy and prolonged rainfall, cutting and deep excavations on slopes for buildings, earthquake shocks and tremors widespread deforestation and population pressure.

In this paper an attempt has been made to prepare a land slide hazard zonation mass of Nilgiri districts covering far of Conoor and Kotagiri, Ooty taluks using GIS systems. A study has been made on the extent to which some important factors like slope land use lineament density rainfall and geomorphology exert this control over triggering of land slopes various thematic maps were prepared and these were ranked bases on frequently of occurrence of past landslides for the case of certain factor such as slope and lineament density the ranking does not appear to follow conventional geology, relatively low slope percentage were given like ranking, origin high lineament density regions were given low ranking become of frequency of past landslides. Therefore a second landslide zonation map was prepared by modifying the rank. High slopes were given a high rank. The same procedure was also followed into the case of lineament density map and drainage density map.

GIS is a very fast and accurate methods by which large volume of the integrating of various landslide triggering factors has resulted in a map which in a certain extent a is ale to predict the susceptibility of an area of landslides and a regional scale. The software IDRISI was used for the work.

Description Of The Study Area
The study area is located in Nilgiri district a mountainous terrain in the NW part of Tamil Nadu state the study area covers part of Ooty Coonoor and Kotagiri taluks. The area is between 11 15' to 11 30' N latitude and 76 45' to 76 55' East. Longitude. The study area covers about 500 sq. kilometers. Physiographically the Nilgiri hills rise abruptly form the plains to an average elevation of 1370 m above MSL. Many of the hill peaks have steep rocky escarpment with or without soil cover around which radial drainage pattern are seen.

The Nilgiri ranges comprise archaen metamorphic rocks which include quartzite, biotite gneiss, magnetite quartzite, hornblende granulite, pegmatite, dolerite and quartz veins. Charnockite forms the bulk of the rock units in the study area.

The Nilgiri hills is a plateau sloping steeply into the Mysore plateau towards north and merging gradually with the western ghats in the NW west and SW. The plateau is divided into a highly dissected plateau and a less dissected undulating plateau.

Nilgiri plateau has been formed by three systems of faults along the peripherals namely a E-NE fault, a North-West fault and a third fault along the northern boundary of the platea. The regional foliation is towards E-NE. Most of the lineament may represent fractures along which there may or may not have been movement. Many of the lineaments represent faults.

The major area in the Nilgiri is either under vegetation or is used for plantation. The reserves forest area have over the years has reduced and more and more thick woods have turned into vegetated scrub lands.

Methodology
Various thematic maps like geology geomorphology, landuse, rainfall. Lineament density, drainage, soil and slope were prepared in the scale of 1:50, 000. Toposheets, satellite images and published maps were used to compile the themes. The maps were scanned and input into the system. Each map was geometrically corrected for latitude and longitude. The thematic maps were digitized and polygons and each feature in as there was given an identity. A sample of 25 landslides from the inventory map was taken and the number of landslides occurring in each feature of a thmatic map was counted.

The map layers were considered as separate entities and each feature in a map layer was ranked according to the frequency of occurrence of landslides. High the number of landslide occurs higher the rank. Thus, rank I indicates low susceptibility and rank 4 indicates a high susceptibility analysis was done by overlay. The eight thematic maps were overlaid on one another successively. The eight thematic maps were taken as 4 pairs and each pairs was overlaid were item successively overlaid.

Landslide Susceptibility ranking

Landslide susceptibility value	Susceptibility rank
29-32	Very high
23-28	Moderate to high
17-22	Low to Moderate
8-16	Very low

The maps thus devices the area in to form zones of varying slope stability. The map was used for analyses the control of slope land used geomorphology and lineament density in triggering slides. The slope map, lineament density map and drainage density maps were also ranked according to expected occurrence of landslides to landslide susceptibility increases with increasing slope, lineament density and drainage density. Thus higher slope percentages were given a higher rank as they are usually more susceptible to sliding. Overlay of all the terms were done and a landslide hazard zonation map was prepared. This was compared with the landslide hazard zonation map based on past slides.

Analysis And Information
The landslide hazard zonation map for the study area covering 500 square kilometers clearly indicates that there are few zones of low susceptibility. Landslides can occur through out major portion of the study area. Nearly 18% of the total study area falls in the very high susceptibility zone. Fifteen landslides from the landslide inventory map were overlaid on the map. Nine landslides fell in the highly susceptible zone, three in the moderately susceptible and two in the low to moderate zones are also susceptible areas and sliding may be triggered.

Recommendations
The following recommendations are given for landslides hazard mitigation based on the present work for the regions, where slope in the main triggering factor the upper parts of the slope can be unloaded or the lower slopes can be loaded. A general flattening of the slope can be achieved. Boulders on the upper slops can be removed to arrest their movement.
Rainfall is an important cause for landslides surface water should be deviated and not allowed to infiltrate in to the ground. Horizontal drains and drainage galleries can be made for the purpose. The run-off can be intercepted by cut-of drains or through the nearest culvert. The conduit of the culvert slopes should not be allowed to get choked up and proper care should be taken to remove the block periodically.
Retaining and strengthening structures like retaining walls and cribs can be used to protect unstable slopes. Hill slopes are cut at the toe and provide space for construction of building and rocks stability. Reinforced earth retaining walls are capable of strengthening slopes.
Afforestation ploys a key role in increasing the stability of the slopes. The barrier slops can be planked with tree a number of grasses like lemon grass have been known to protect slopes from sliding.

Refrences
Gupta R. P, Joshi B. C. 1990. Landslide Hazard Zoning using GIS approach - a case study from the Ramganga catchment - Himalayas, Engineering Geology 28 119 - 131.
Hydro geological conditions in Nilgiri district 1984, Central Ground Water Board, Dept. of Irrigation Tamil Nadu India
Nilgiri landslide. 1982. Geological Survey of India Miscellaneous publications No 57.
Pachouri A. K , and Pant. M. 1992 Landslide hazard mapping base on geological attributes. Engineering Geology. 32, 81 - 100.
Report on the study of landslides of no Vol 993 in Nilgiri District 1993, The Geotechnical cell Coonoor.