# UAV-Based Evaluation of Rockfall Hazard in the Cultural Heritage Area of Kipinas Monastery, Greece  
[PERSON]  
1 Laboratory of Engineering Geology & Hydrogeology, Aristotle University of Thessaloniki (AUTH), 54124 Thessaloniki, Greece 1  
[PERSON]  
2 Geotechnical Division, School of Civil Engineering, National Technical University of Athens (NTUA), 2  
predefined time period [1,8,9]. Hence, the concepts of magnitude (volume of the blocks), severity (amount of developed energy), location (spatial probability of occurrence) and frequency (temporal recurrence of the event) are included. At last, rockfall risk is a more comprehensive term which involves all of the above fused with the human factor. It can be defined as a measure of the probability and severity of an adverse effect to health, property or the environment [10,11,12]. In other words, risk is given by three basic components: the hazard, the exposure of the elements at risk and their vulnerability. These terms are characterized by both spatial and non-spatial attributes. As rockfalls represent a special type of landslide hazard due to their rapid movements with various trajectories, the accurate prediction of their potential hazard constitutes a very difficult task [13,14]. Thus, rockfall hazard and risk analyses are inherently some of the most elaborate and challenging assessments in geoengineering. Their complexity as a natural phenomenon has resulted in the proposition of several different methodologies regarding stability assessments for rock cliffs [15]. These approaches rely mainly on (i) the accurate recognition of the failure mechanism and (ii) on the prediction of potential displacements of rock masses as a whole [16,17,18], or, in some cases, the displacement of individual prone rock blocks [19]. Hence, the proposed methodologies for rockfall hazard assessments vary from the probabilistic modeling of potentially unstable rock block trajectories [20,21,22,23], to qualitative approaches via the implementation of a rating system [24]. A common area between all of the proposed methodologies of rockfall hazard assessments, regardless of their approach to the subject (i.e., quantitative or qualitative), is their substantial need for data concerning the critical slope profiles, the material properties, the discontinuity surfaces that can potentially contribute to slope instabilities and the size of possibly unstable rock blocks [25]. In the past decades, due to the confined available means, all of these important data for rockfall hazard and risk assessments were extracted completely by traditional engineering geological field obtained measurements and geoengineering assumptions. Therefore, the difficulties and the complexity of these assessments derive mainly from the lack of accurate data (spatial- and temporal-based), the site-specific rockfall properties, the limited capabilities in the quantification and the visual interpretation of rockfall spatial distribution and the heterogeneity of vulnerability of various elements at risk [26,27,28,29]. Most of the above deterring factors have been eased in recent decades and, in many cases, surpassed by the introduction of innovative remote sensing technologies as a common practice in hazard and risk assessments. Two of the most well-known technologies, which can be classified based on their sensors, are the active, such as Terrestrial Laser Scanners (TLS), and the passive, such as photogrammetric surveys via Remotely Piloted Aircraft Systems (RPAS), also known as Unmanned Aerial Vehicles (UAV). The latter uses flexible multicopters equipped with digital cameras and other various sensors [30,31,32,33] and is considered a flexible and powerful tool with many applications in geosciences, and especially in rockfall investigations [34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,47,49,48,49,50]. This occurred mainly due its low cost and ease of transportability in harsh environments, in conjunction of course with the reduction in the human risk factor during rockfall-related surveys.