## 5 Discussion
For instance, in the present case, we identified important areas of erosion upslope of the beach and showed that parts of the access road, the parking area, and the local convenience stores are situated within the erosion and deposition areas. Figure 12 illustrates these observations in detail, delineating zones throughout the study sites. Examination of the vertical changes shows clearly that the road leading to the site was covered in debris from upstream in many locations, while large mudflows, rock avalanches, and debris cones are revealed, as the results indicate connections between the source and the final deposition locations of the sediments. The C2C method led to a quite detailed temporal change (before and after the lanos extreme event) based on the two point-cloud elevation differences (Figure 13). Given the very high intensity of the event, the unaffected areas, which can be distinguished using the current application (see Figure 12), can be considered as the parts of the site that have a lower risk of mass movement phenomena, whereas the affected areas can be delineated as risk-prone. This distinction can be further fine-tuned with continuous monitoring of a site and can become the basis for sustainable development, in the sense that it allows undisrupted usage through an in-depth understanding of risk. This in turn allows the adaptation of the existing uses of the site taking into consideration risk level zonation. One other advantage of the use of a UAS is the availability of on-demand imagery, in comparison for example with satellites that, although very effective, have limitations due to cloud canopy or image unavailability due to satellite period, etc. The resolution of UAS-derived imagery can be significantly higher than satellite images even using consumer-grade equipment. Within the frame of co-registration, it is a scientific principle that in order to succeed accurate intercomparison, an accurate georeferencing is needed. The most prevalent georeferencing method, the use of ground control points (GCPs), is a very reliable method; however, the necessity to be evenly distributed in addition to the morphological challenges of the terrain can make the GCP method time-consuming and unreliable concerning the position uncertainty over time. Considering the fact that the morphological changes must be monitored due to extreme weather events, it is sadly difficult to control the locations of the GCPs contingent on the weather impact and the surface deformations. Studies have shown that the results of photogrammetry applications compete in practicality and accuracy with the more common and costly terrestrial laser scanning approaches [92; 93]. Figure 12: A perspective pseudo-3D view of the affected area, with a color scale showing vertical changes (erosion and deposition), and the distinctly affected elements (including high-visitation areas, infrastructure, and buildings), parts of the access road that suffered deposition from debris flows and rock avalanches, indicating the higher risk parts of the site, as well as the unaffected areas of Myrtos. Figure 13: Graph showing the percentage of the points from the point cloud that have either increased their elevation (red bars) showing deposition or decreased their elevation (blue bars) showing erosion. With regard to limitations, in general, the altitude is inversely proportional to the distinctive shooting ability of the camera; therefore, the altitude should be adapted concerning the required resolution and the extent of the site under study. In this sense, the user should always select the technique by keeping in mind that there is a tradeoff between study area size and resolution, calculating available time by considering safety and equipment characteristics. In addition, the actual aircrafts used to capture the data are sensitive to strong winds and rain, which are both not uncommon between or after storm incidents, especially in winter and autumn months. Bad weather may prohibit aerial surveys.