## 2 Study Area and Materials
### Environmental Factors  
Intensive rainfall-triggered debris flows are controlled by the interaction of various factors including topography, hydrology, soil, and forests [39]. Topography and hydrology influence debris flow initiation through the effect of gradient on slope stability with rainfall. These factors also determine the concentration and dispersion of the material and the material balance on the slope associated with the slope stability. In addition, soil and timber factors on the slope affect the spatial distribution of debris flows. These factors are significant controls, and can be represented as spatial distribution from digital elevation models (DEM) and soil and forest maps. Geology and faults were not considered as environmental factors in this study because shallow soil failure was mainly related to positive pore water pressure in saturated soils by intensive rainfall [39; 40; 41]. In this study, 18 environmental factors were considered for landslide susceptibility modeling based on ANN and BT (Table 1, Figure 4). Figure 3: Digital aerial photographs of (**a**) pre-; (**b**,**c**) post-landslide occurrences; and (**d**) covered blue plastic at landslide scar after landslide occurrence in 2011. Figure 4: _Cont._  
Topographic and hydrologic factors were extracted from the DEM for determining the relationship between these factors and debris flow using SAGA GIS modules [42]. A DEM with a 5 \(\times\) 5 m grid format was generated from a triangulated irregular network (TIN) derived from a digital elevation contour with 5 m interval lines in ArcGIS 10.2. Soil and forest factors were also extracted from soil and forest maps with a scale of 1:5000. The extracted topographic factors were slope, aspect, plan curvature, convexity, topographic position index (TPI), terrain ruggedness index (TRI), min-slope position (MSP), and landforms (Figure 4a-h). The considered hydrologic factors were slope length (SL), stream power index (SPI), and topographic wetness index (TWI) (Figure 4i-k). Slope indicated the steepness of a hill, and aspect  
Figure 4: Spatial database of the landslide causative factors. was the steepest downhill direction. Plan curvature was perpendicular to the slope and affects the divergence and convergence of flow across the surface. Terrain surface convexity was described as positive surface curvature and represented the percentage of convex-upward cells [43]. TPI was the difference between the elevation of each cell and the mean elevation for a neighborhood of cells [44]. Negative values represented lower features than surrounding features, values near zero were flat areas, and positive values represented features typically higher. TRI were absolute values obtained by squaring the difference between the value of a cell and neighbor cells, and convex and concave areas could have similar values. MSP were assigned a 0 value, while maximum vertical distances to the mid-slope in crest or valley directions were assigned a 1 value. Landform classification (cl 1: deeply incised streams, cl 2: shallow valleys, cl 3: upland drainages, cl 4: U-shape valleys, cl 5: plains, cl 6: open slopes, cl 7: upper slopes, cl8: local ridges, cl 9: mid-slope ridges, and cl 10: high ridges) was derived by ranges of TPI values [45]. SL was based on specific catchment areas and slope, with the former used as a substitute for slope length. SPI represented the erosive power of a water flow [46]. TWI indicated the effect of topography on the location of the saturated area size of runoff generation [46]. In general, higher SL and SPI, and lower TPI, represented a higher landslide susceptibility. The attribute columns in the digital soil map (Table 1) included land-use, material, thickness, and topographical values (Figure 4l-o). Land-use was classified into natural grasses, forests, paddy fields, and farm orchard areas. Soil material included three classes: gneiss, acidic residuum, and granite residuum.