Shallow Landslide Susceptibilty Mapping using SINMAP for Selected Areas in the Philippines

Among the deadliest calamities that plagued the Philippines were the landslide and flooding caused by consecutive tropical storms Mufia, Merbok, Winnie, and Nanmadol in Infanta, Quezon (2004), Typhoon Reming in Guinsaugon, Leyte (2006) and Typhoon Pepeng in Pangasinan (2009). In Quezon alone, the number of death and missing exceeded 1600, and the cost of damage was estimated at US$78.2M. Situated in the humid tropics, the Philippines will inevitably be a locus of climate-related disasters similar to those experienced recently. To aid the local government units in delineating areas located on hazardous zones in the country, the spatial distribution of rainfall-induced shallow landslide susceptibilities was modeled for the provinces of Quezon (906,960 ha), Leyte (651,505 ha), and Pangasinan (545,101 ha). The Stability INdex Mapping (SINMAP) model was applied based on the infinite slope stability model and a steady-state hydrology module. Using the hydrologic, soil and topographic parameters for each pixel on a 6-meter synthetic aperture radar-derived DEM (digital elevation model grid) or on a 1-meter LiDAR-derived DEM, Stability Index Maps were generated for the study areas. Soil properties were based on data available from Bureau of Soils and Water Management as of July 2013. These data were augmented by field data and constrained by values from direct shear strength testing, soil gradation, and hydraulic conductivity tests of soil. The model validation performed using the previously identified landslide inventory was found out to be about 90% accurate. The SINMAP model accuracy can be further improved by additional locations for field tests to better map out the spatial variation of soil properties.