## 2 Materials and Methods
### Investigation
#### 2.3.1 Remote Detection Methods  
Remote sensing methods were employed during the investigation to promptly assess the aftermath of cyclone Tamara as part of the project. The Pleiades satellite image for Selanac from June 2014 was scrutinized, taken immediately after the landslide activation in May 2014. This satellite image served visualization purposes and facilitated the determination of the location and extent of the broader investigation area [4]. Subsequently, other remote sensing methods were utilized to primarily gather more precise data, such as generating a high-resolution Digital Terrain Model (DTM). In landslide modeling, we can consider two epochs crucial for analyzing the distribution of moved material due to instability:  
Epoch I: Referring to the stable terrain before the activation of the landslide;  
Epoch II: Referring to the terrain after the activation of the landslide. In order to obtain high-resolution orthophoto images and the DTM for the second epoch, after the activation of the flow, a DJI Phantom 4 Pro drone was used. The procedure for capturing and creating the DTM and orthophoto images was carried out in the following order: the study areas and the desired pixel resolution of the orthophoto images were defined; flight parameters (altitude, flight path, and waypoint locations) were calculated, and the necessary percentages of longitudinal and lateral image overlap were determined; orientation points on the terrain surface were defined, marked, and surveyed; control points on the terrain surface were defined, marked, and surveyed; drone flights were conducted; the collected images were processed photogrammetrically; point clouds were classified, terrain surfaces were derived, and DTM and orthophoto images were created. Comparing the DTMs of these epochs for both landslides served as a validation method for the final numerical models. The following figure (Figure 3) shows a high-resolution orthophoto of the large flow area where detailed field investigations and modeling were conducted. In addition to the defined elements, images of characteristic parts of the terrain are presented. To compare Digital Terrain Models from two epochs, the initial step involved resampling the model with a resolution of 30x30 cm to match the 5 m resolution used in the final modeling. To achieve a more precise comparison and obtain accurate data on mass differences before and after the activation of the debris flow, the scans from the two epochs overlapped (Figure 4). This overlap revealed the differences in terrain elevation within the debris flow zone. The image clearly shows that in the initial zone, the maximum depth reaches nearly 29 m, while in the deposition zone, differences of up to 31 m are recorded. Additionally, in the transportation zone, there is a noticeable area of significant erosion along the right gully (red zone), as well as material deposition in the left gully (green zone). It should be noted that, due to the nature of the processes involved, these differences are most representative in the initial zone, where material transport was the dominant process. In the deposition zone, the reported values indicate precise differences (with the deepest values corresponding to the Selanac river bed). However, since both material transport and deposition occurred in this area, the exact depth of the deposited material cannot be determined with certainty from these data. This is especially relevant for the transportation zone, where both significant erosion and continuous material deposition are dominant. The eroded and initiation zones were directly defined based on the results obtained from aerial photogrammetry (Figure 4). The red area in the initiation zone was identified as the initial block, while the red area in the transport zone was designated as the entrainment zone in both models. As noted, the two programs use different numerical approaches for calculating the eroded material.