# UAV-Based Evaluation of Rockfall Hazard in the Cultural Heritage Area of Kipinas Monastery, Greece
## 4 Rockfall Hazard and Risk Assessment  
### General  
The assessment of rockfall hazards and risks along roads and in other human-related locations is of great importance [24]. In order to determine the value of these parameters, a number of rating systems have been developed based on either qualitative or quantitative criteria. These approaches differ entirely in terms of input data, adopted analysis procedure and of course the final outputs. Qualitative-based methods define hazards, elements at risk and their vulnerabilities with the usage of qualitative descriptors, such as ranked attributes, weighted indices, rating systems, scoring schemes and classifications [74]. This type of evaluation can be based on either objective (statistical or mathematical) estimates, subjective (judgments or assumptions) estimates, or even a combination of both. Detailed ratings usually use several judging factors to provide an evaluation and/or score for the slope. The results of this method usually are expressed using relative terms, such as high, moderate and  
Figure 20: Location of main release points for rock blocks (seeders) and the trajectories of the potential blocks for: (**a**) sub-region U1 (**b**) sub-region U2 (**c**) sub-region U3 and (**d**) sub-region U4. low. Qualitative analyses provide useful information for hazard and risk management, for relative comparisons of different sites and for facilitating the prioritization of mitigation measures [75]. On the contrary, quantitative-based methods use numerical values or ranges of them instead of qualitative relative terms. Generally, quantitative hazard and risk analyses approach the evaluation of hazard in the form of a numerical probability method, which estimates the frequency of failures, including the intensity of a specific spatial distribution and attempts to quantify risk with the monetary expression of the potential damage. Even though the hazard evaluation, in qualitative terms, seems prevalent and the reliability of its results occur from the availability, quantity, quality and credibility of the derived data, the same do not apply for quantitative risk analysis. Several recent efforts have attempted to establish standard procedures for quantifying risk in terms of official national recommendations or guidelines [76]. In most quantitative risk analyses, in order to quantify the probability of potential losses that are related to the occurrence of a hazardous event, such as rockfalls, the parameters that are taken into consideration are the number of demolished structures, injured people and fatalities. However, this does not constitute a standard procedure for every risk analysis because it depends on the researcher's approach. Furthermore, the significance of the exported results varies as the acceptable and tolerable risk thresholds differ between countries and regulations. In general, qualitative analyses are more commonly used because they are easy to use and can be performed quickly [77]. For this reason, most rockfall hazard and risk assessments start with a qualitative approach to the investigated site as a whole and then adopt a detailed quantitative character, with thorough ratings to numerically differentiate the hazard and/or risk, respectively, at the predetermined specific region of the site. In other words, qualitative analysis represents an initial analysis step of the dominant hazards at a given site to evaluate the most dangerous areas further via quantitative methods. Despite this, nowadays, quantitative rockfall hazard and risk assessments are only possible when a detailed historical inventory of the rockfall events that have occurred in the study area is available [78]. However, the documentation of prior rockfall events is usually poorly made or even absent at most sites. Furthermore, complete rockfall inventories are rare in most parts of the world, due to the lack of reporting of small and medium events, which are usually not related to significant impacts [79].