APPLICATION OF GIS AND FACTOR OF SAFETY IN DETERMINING LANDSLIDE HAZARDOUS AREA IN TAPI WATERSHED, CHANGWAT NAKHIN SI THAMMARAT

Abstract

Factor of safety (Fs) equations proposed by Abe (1978), Gray and Megahan (1981), Gray and Leiser (1982), Wu et al (1979), and Coppin and Richards (1990) were employed to make landslide map of 2 sub-watershed in Tapi basin using the existing landslide map, interpreted from landsat taken in November 22, 1988 when severe landslided were occurred, together with the GIS techniques. Applicability and efficiency of each of those equations were verified based on the coincided value (CV) indicating percentage of overlapping both in size and location of actual and predicted landslide maps. Results showed that all equations gave the CV less than 20 %. When wind velocity of 40 km / hr, which was observed at the event day, was applied the Coppin and Richard's model yielded the highest CV (16.20 and 15.52 % for a given soil depth of 0.5 and 1.0 m respectively). The rest of model had lesser CV. Wu et al's model showed the highest CV of 16.07, 15.85 and 15.85 % respectively at a given soil depth of 2, 2.5 and 3 meters. Generally speaking, although CV is rather low, equation developed by Wu et al. (1979) seem to be most feasible for making landslide map in this area.