## 5 Discussion  
This work explores the capabilities of UAS-aided surveys in the field of slope failures and shallow instabilities as a novel tool aiming to improve our understanding of identifying, delineating, and measuring these phenomena. The findings show that a UAS-aided post-event investigation is an effective and practical method able to extract the geometric properties of the slope and the mass movement processes with accuracy and identify failure surfaces, deposits, scarps, high-risk areas, and other morphological characteristics in a quantified way. The findings support and build on recent research in the field of detailed mapping of geomorphological features [86; 87] through a range of either manual or automated techniques [88], and demonstrate the strengths and weaknesses of the application. Concerning the advantages of the current approach, it has to be noted that in the course of a UAS-aided survey, the investigation follows a different sequence of steps compared to a more traditional approach, for example in a d-GNSS survey. In the latter, the local morphology can be described through pre-organized cross-sections or paths across or around the deposits, accompanied by photos from certain locations. In contrast, in the UAS-aided approach, the survey team collects data from a wider area, obtaining accurate morphometric information from a larger part of the slope. This allows not only for a more complete measurement coverage of the phenomena but also for a better understanding of the surrounding environment in which they occur (including its morphology, infrastructure, exposed assets, processes, etc. ), as photogrammetry allows the processing of multiple parts of the study site, allowing greater flexibility in site examination. In addition, with respect to data collection, a UAS-aided survey can acquire data that can be stored and re-examined after the field survey. The storage of terrain data allows the research team to revisit the study site virtually, even in the future, providing flexibility in scheduling data collection and data analysis. This can be particularly advantageous in the case of unsafe or remote sites or areas with restricted access, conditions which are not rare in locations with ongoing crises. It can be also useful to protect surveys from data loss from disruption of terrain elements (e.g., due to clean-up), preserving observations. Furthermore, the use of UAS permits capturing data from a site with minimum requirements regarding time and personnel as well as concerns regarding safety and access. In addition, during data collection, there is very limited disruption to visitors or clean-up personnel and authorities. Overall, these characteristics facilitate research and its applications in civil protection, without facing common practical limitations associated with post-disaster environments acknowledged in the literature [89]. Furthermore, the high resolution of the model allows qualitative observations on the role of various factors including geological formations and vegetation conditions and their spatial relationship with the observed phenomena. For example, it can be observed whether the sediment sources due to erosion occur across all lithologies found in the study site or only some of them. In addition, the highly detailed model allows observations on the condition of manmade objects and infrastructure and can assess their role or their position in relation to the hydro-geomorphic phenomena. Especially when the C2C method [90; 91] is applied effectively, i.e., the two point-clouds are successfully co-registered (by using common GCPs), a very high-resolution result is accomplished (Figure 11). Based on the latter approach, one can distinguish which parts of the road are subject to debris deposition or which buildings/structures are within the debris flow deposition areas.