Categorisation Of Settlement Areas In Guwahati City: An Attempt To Identify Potentil Landslide Risk Zones Rajasmita Goswami Department of Geological Sciences Gauhati University Guwahati-781014 rajasmita@gmail.com S. Phukan Department of Geological Sciences Gauhati University Guwahati-781014 P. Phukon Department of Geological Sciences Gauhati University Guwahati-781014 Abstract Guwahati, the capital city of Assam, covers an area of about 231sq.km and has a population of more than 20 lakhs. The municipality area of Guwahati consists of plain areas, marshy lands interspersed with Precambrian residual hills. Because of the topographic factors the urbanization had been mostly in a ribbon growth pattern along the intermontanne narrow and low lying areas. The city has witnessed rapid growth in civil structures and population particularly during the last two decades. With the growth of the population there is now an acute shortage of suitable settlement areas. As a result the city has been witnessing expansion of the same in uphill direction, irrespective of its vulnerability. The settlement areas expanded to the hilly terrains are getting prone to landslide hazards, which has become a recurring and menacing problem as the thin weathered mantle overlying the massive crystalline rocks is highly susceptible to seepage pressure. Toe cutting of the natural slopes for construction of dwelling houses, loss of the vegetation cover further aggravate the situation. The present study is an attempt to classify the settlement areas of Guwahati city based on its slope to identify the potential landslide risk zones. The slope map of the city is prepared from a DEM, and the settlement areas are delineated from the landuse map prepared from supervised classification of a PAN sharpened IRS LISS3 image. The slope map is classified into four classes, viz., 0º-10º, 10º-20º, 20º-40º and greater than 40º. Both thematic maps are overlayed in GIS platform and the settlement areas are thus classified into four landslide risk zones based on slope, viz., no risk zone, moderately risk zone, high risk zone and very high risk zone. From the analysis the areas covered by each of the classes are identified and they are 125.15 sq.km, 8.81sq.km, 5.34 sq.km and 1.20 sq.km. respectively. Introduction: The Guwahati city (91º33'E-91º52'E and 26º08'N-26º14'N), the capital of Assam, has witnessed rapid growth in civil structures and population in last two decades. Being composed of residual hills of highly weathered Precambrian basement, low lying planes and marshy land the city is environmentally very vulnerable to unplanned expansion. With the growth of the population there is now an acute shortage of suitable settlement areas. As a result the city has been witnessing an uphill expansion of the same irrespective of its vulnerability. The settlement areas expanded to the hilly terrains are getting prone to landslide hazards, which has become a recurring and menacing problem as the thick weathered mantle overlying the massive crystalline rocks is highly susceptible to seepage pressure. Toe cutting of the natural slopes for construction of dwelling houses, loss of the vegetation cover further aggravate the situation. Landslide is a very common problem in the city during rainy season and every year many people die due to landslide after torrential or weeklong incessant rains. As the most of the Precambrian hills in the study area are highly weathered in nature and usually have moderately thick soil covers except in the areas occupied by granite plutons and depleted vegetal cover is, there is always a simple relation noticed in between slope and landslide, i.e. higher the slope greater is the chance of land slide. The present study tries to categorize proneness of settlement areas to landslide on the basis of slope. General Geological setup Guwahati city is situated in a part of the extensions of the Shillong plateau towards its north. The city is geologically made up of the Precambrian gneissic complex which is, directly overlain by Pleistocene-Holocene sediments. The hills are made up of the gneisses and granite bodies with quartzites,amphibolites and biotite schists, with the intermontane valleys are field with Pleistocene-Holocene sediments. The rocks are affected by two dominant sets of joints, intruded by quartz veins, aplite and pegmatite (Maswood and Goswami, 1974, Maswood, 1981, Shukla et al., 1989). The basement is overlain by a cover of Quaternary deposits of variable thickness composed of unconsolidated sand, silt and clay. Along many tracts occupied by paleo channels, the typical Brahmaputra sand with abundant biotite and mostly silt are encountered (Deka, 2001). Thin layers of residual clays, which are the weathered product of feldspar are found inter-wind with the alluvium at places. Geomorphology The geological setup is reflected in the landscaope of the study are with low lying Precambrian residual hills, doting al around interspersed with elongated low lying plains. Broadly three geomorphic units, viz. the denudo-structural hills (residual hills), the alluvial plains and the marshy lands including static water bodies can be identified from satellite images and topomaps. The major hills of the areas are Sarania Hills (193m), Nabagraha hills (217 m), Nilachal hill (193m) Chunchali hill (293m), Tetelia hills (221 m) Khanapara hills (303m) Narengi hills (168m). A unique picture of Guwahati is the presence of partly sited waterbodies locally known as beel, the largest is the Deepar Beel in the western fringe of city. Most if the earlier waterbodies, however, have been converted into built up land during last couple of years. Apart from Brahmaputra, the other major rivers are Bashista, Bahini, Bharalu and Amchang form the main drainage within the city. Methodology It is already mentioned that, in the Precambrian highly weathered residual hills of the city, with the rapid and unplanned growth of settlement and depletion of vegetal cover, slope is the causal factor in formation of landslides. We take the other parameters that may also play major role in triggering landslide, such as thickness and nature of the soil cover, vegetation etc. are similar in the entire settlement area of the city. Therefore, we have tried to categorize the proneness of settlement areas to landslide in the city based on slope. For the present study, only the settlement areas are taken into consideration. Other areas having higher slope are not included in the study. For this purpose, we have generated two thematic maps, viz., slope and landuse layers. The slope map is prepared from a 1arc second DEM of the area. The slope map is than reclassified to the desired classes (0º-10º, 10º-20º, 20º-40º and greater than 40º classes). The settlement areas are delineated from PAN sharpened IRS L3 image of November 2002 by supervised classification. First the image is classified into 8 classes, viz., water body, sandbar, marshy land, dense forest, thin forest, and settlement areas. The supervised classification is done in PCI Geomatica using maximum likelihood algorithm. After classification the resultant thematic map is smoothened using an average low pass filter. After that, all classes other than the ‘settlement area’ are merged to one class. Finally we get a thematic map, which has two classes – settlement area and non-settlement area. Now an overlay analysis is done in between the reclassified slope and landuse maps using simple Boolean operation (Table 1). The slope classes (Fig. 1) are identified as 1 (0-10º), 2 (10-20º), (20-40º), and 4 (>40º) and the landuse classes (Fig. 2) are identified as 1 (Settlement area) and 2 (Non settlement area) for the overlay operation. In the output layer (Fig. 3), every single pixel is unclassified if its value in the landuse map is 2, that means areas outside settlement area are not classified. All other pixels having value of 1 (settlement area) in the landuse map are classified into ‘no risk zone’, ‘moderate risk zone’, ‘high risk zone’, and ‘very high risk zone’ based on slope as shown in the table    Fig. 1  Fig. 2  Fig. 3  Fig. 4. Area occupied by different risk classes The entire 140.51 Sq km of the municipality area classified as settlement area in the supervised classification (Fig. 3) are classified into four classes, viz., very high risk zone (slope >40º), high risk zone (20- 40º), moderately high risk zone (10-20º) and no risk zone (0-10º). Areas occupied by different classes are shown in the Table 3 and their percentages are shown in the pie-diagram(Fig. 4). It is seen that most of the settlement area (125.15 Sq km) falls in the ‘no risk zone’. The ‘very high risk zone’ (slope > 40º) occupies only 1.20 Sq km, but ‘high risk zone’ (5.34 Sq km) and moderate risk zone (8.82 Sq km) occupy a sizeable portion of total settlement and along with ‘very high risk zone’ occupy 10.93% of total settlement area which can be said as very alarming situation. REFERENCES
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