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Landslide susceptibility mapping using the fuzzy gamma operation in a GIS, Kakan catchment area, Iran
Majid H. Tangestani
Dept. of Earth Sciences, Faculty of Sciences, Shiraz University, 71454 Shiraz, Iran
Tel: +98 711 2284572, Fax: +98 711 2280926, E-mail: tangestani@geology.susc.ac.ir
Introduction
Landslide hazard and risk zoning and mapping for urban and rural areas is widely performed around the world (Siddle et al 1991, Lee et al 1991, Hutchinson and Chandler 1991, Hutchinson et al 1991, Morgan et al 1992, Carrara et al 1991, and 1992, Moon et al 1992). A landslide zonation map divides the land surface into zones of varying degrees of stability, based on an estimated significance of casuative factors in inducing instability. Engineers, earth scientists, and planners are interested in assessment of landslide susceptibility and hazard because of two purposes: (1) The landslide hazard maps identify and delineate unstable hazard-prone areas, so that environmental regeneration programs can be initiated adopting suitable mitigation measures; (2) These maps help planners to choose favourable locations for siting development schemes, such as building and road construction. Even if the hazardous areas can not be avoided altogether, their recognition in the initial stages of planning may help to adopt suitabl
e precautionary measures.
The main factors which influence landsliding are discussed in Varnes (1984) and Hutchinson (1995). Normally the most important factors are bedrock geology (lithology, structure, degree of weathering), geomorphology (slope gradient, aspect, relative relief), soil (depth, structure, permeability, porosity), land use and land cover, and hydrologic conditions.
Soeters and van Westen (1996) and Leroi (1996) discuss the methods which can be used to assess probability of landsliding. Traditional methods of landslide hazard mapping have been based on extensive fieldwork by expert geologists in potentially dangerous areas. This is slow, expensive and very labor intensive operation, and as such can not be widely applied. With the increasing availability of high resolution spatial data sets, GIS, and computers with large and fast processing capacity, it is becoming possible to partially automate the landslide hazard and susceptibility mapping process and minimize fieldwork. Several studies have used GIS and statistics for landslide hazard and susceptibility mapping (Wadge 1988, Gupta and Joshi 1990, Wang Shu-Quiang and Unwin 1992, Pachauri and Pant 1992, Binaghi et al 1998, Guzzetti et al 1999, Skellariou and Ferentinou 2001, Gritzer et al 2001), but mapping studies using fuzzy approaches are limited (for example, Juang et al 1992, Davis and Keller 1997, Binaghi et al 1998, Ercanoglu and Gokceoglu 2002).
Northwestern Fars province, Iran, is affected by landslides. The Kakan area, 51° 42' 30" E to 51° 51' 30" E, and 30° 32' 30" N to 30° 43' 00" N, is mountainous and located in the geological Zagros Folded zone, and is subjected to heavy precipitation during fall and winter. A landslide susceptibility map was required as part of comprehensive detailed investigations for the development plans by local government. The objective of the present study is to generate the landslide susceptibility map of a landslide-prone area of 114 km2 in the Kakan catchment area, based on fuzzy approach. The study includes four main steps: (1) producing the causal factors maps by field studies and digital data processing; (2) evaluating the fuzzy membership functions for evidence maps using a modified method initially discussed by Anabalgan (1992); (3) the use of GIS to produce the index maps and generating the susceptibility map; (4) controlling the reliability of the susceptibility map.
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