## 2 Materials and Methods
### Data  
#### 2.2.1 Optical Satellite Data  
We used time series of optical satellite data, i.e., Landsat 5, Landsat 7 and Landsat 8 imagery (20 images overall) with 30 m spatial resolution, from 1984 to 2018 to semi-automatically map the evolution of the Butangbunasi landslide (Table 1). In particular, we selected the first cloud-free image of the area of interest available after a typhoon (including tropical storm) event that shows a noticeable change in the landslide area compared to pre-event images. This was done based on a visual inspection of the Landsat database and considering the historical hurricane/typhoon tracks of the National Oceanic and Atmospheric Administration (NOAA) Office for Coastal Management [60]. In addition, all major hurricanes crossing Taiwan within a radius of 100 km around the Butangbunasi landslide, which  
Figure 1: (a) Location of the study area in south-central Taiwan (R.O.C.; background data: © ESRI); and (b) the Butangbunasi landslide shown on a SPOT-5 satellite image from 9 September 2013.  
reached category H3 or above according to the Saffir-Simpson hurricane wind scale (SSHWS), were identified and the respective post-event Landsat imagery added to the database. The Landsat 5 scene from 1984 is the first available image for the study area and served as a starting point for the analysis. The Landsat 8 scene from 2018 does not follow a specific typhoon event but was used as the final image for the analysis to see if any changes in landslide area are also identified following a period without a heavy rainfall event. Moreover, this image temporally coincides with the field visit in November 2018 (cf. Figure 2c).  
All Landsat scenes were downloaded as Level-1 products from the EarthExplorer user interface of the United States Geological Survey (USGS). In order to increase the comparability across the Landsat imagery captured by different Landsat sensors and/or at different times, a top-of-atmosphere (TOA) calibration was performed on all satellite images [61]. To do so, the apparent reflectance function implemented in the ArcGIS Desktop version 10.7 was applied. The algorithm uses sun elevation, satellite position, acquisition date and sensor properties such as gain and bias settings for each spectral band [62]. In this way, the spectral differences between images from different dates and sensors were decreased. In addition to the Landsat images, the derived slope layer from the Advanced Land  
Figure 2: The Butangbunasi landslide area. (**a**) Unmanned aerial vehicle (UAV) image of the upper part of the Butangbunasi landslide. The image was taken on 13 June 2017. (**b**) UAV image of the temporary landslide-dammed lake at Butangbunasi and parts of the debris fan. The image was acquired on 29 April 2011. (**c**) Photograph of the Butangbunasi landslide from 15 November 2018. The photograph was taken from the Laonong river valley looking towards Butangbunasi; the actual landslide area is further up the tributary. The Butangbunasi debris fan, the former lake area, and the former landslide-dam area are indicated.  
Observing Satellite (ALOS) Pulsar digital elevation model (DEM; 12.5 m spatial resolution), acquired in 2008, was used as ancillary data to support the landslide mapping.