## 2 Material and Methods
### DEM Acquisition  
#### 2.3.1 Data Sources  
The airborne LiDAR DEM data were provided by the Satellite Survey Center, Depart of Land Administration, Ministry of the Interior, and Central Geological Survey, Ministry of Economic Affairs. Airborne LiDAR technology produces effective data for obtaining large-area and high-resolution digital terrain features with the characteristics of rapid measurement, and lower unit cost [40]. Therefore, airborne LiDAR technology has been widely applied on slope hazard mapping, disaster prevention, interpretation of large-scale landslide areas, and changes in sediment transport in river channels [40,41] and survey of agriculture and forestry resources [42,43]. With the assistance of GPS, and IMU (Inertial Measurement Unit), each reach LiDAR return have positioning data. LiDAR data are composed of dense point clouds. The higher point cloud density can be used to build a high-density DEM.  
The LiDAR DEM acquired in 2010 was used in this study. The grid spacing size was 1 m. Overlapping between swaths was greater than 40%. The point density of the areas where the elevation below 800 m is higher than 2 points/m\({}^{2}\), whereas the elevation above 800 m is higher than 1.5 points/m\({}^{2}\). The TWD97 coordinate system is used. The external error evaluation of airborne LiDAR is to select a certain proportion of ground control points and LiDAR data for comparative analysis to evaluate the overall accuracy. Ground control areas were selected from bare land, dwarf vegetation (short grass, dwarf trees, tea gardens, etc.), sheltered forests, densely covered forest, metropolitan areas, etc. Each category selects about 30 points for the precise measurement of RTK-GPS or total station. The elevation difference between ground point and LiDAR DEM is mostly within 10 cm [44].  
The images before Typhoon Morakot were provided by the Aerial Survey Office, Forestry Bureau, Council of Agriculture, Executive Yuan, and were obtained from an airborne platform with an Airborne Digital Sensor ADS40 airborne camera or Z/I Imaging DMC. The Airborne Digital Sensor ADS40 is a commercially available airborne camera developed by Leica. It can provide five spectral bands (Panchromatic, R, G, B, NIR), three panchromatic CCD lines, four multispectral CCD lines, a 12-bit dynamic range CCD chain, 14-bit Resolution A/D converter, 16-bit data channel, and get true color 3D data in photogrammetry. The DMC system developed by Intergraph that provides four multispectral (R, G, B, NIR), 3k \(\times\) 2k camera heads, CCD frame sensor technology delivers the best geometric accuracy, and 12-bit per pixel radiometric resolution ensures exceptional image clarity. Data acquisition of images after Typhoon Morakot was performed using the Skywalker X8 carry with Nikon D800E with a fixed lens of 50 mm focal length. In addition, the average rainfall intensity of the 1 st survey period is 120.04 mm/rainy days and the rainfall intensity of 2 nd and 3 rd survey period are 45.50 and 41.97 mm/rainy days, respectively. The rainfall intensity caused by typhoon Morakot is 3 times more than the regular periods and this observed difference may cause the measurable difference of topographic process. The detailed information of the image source is shown in Table 1.