# UAV-Based Evaluation of Rockfall Hazard in the Cultural Heritage Area of Kipinas Monastery, Greece
## 4 Rockfall Hazard and Risk Assessment
### Background  
The first rockfall hazard assessment methodology was proposed for railway networks by [81] in 1976. This method was based on a qualitative approach to evaluate the hazard level and prioritize the remediation need with alphabetical descriptors from \"A\" (the most hazardous regions which require immediate intervention) up to \"F\" (the most stable regions which do not require any intervention). This first attempt at a rating system was initially applied along the Canadian Pacific Railroad [81] and was then used as the cornerstone for any rating system methodology, related to rockfall hazards and risks, ever developed afterwards.  
One of the first and most well-known rockfall hazard rating methods is called the Rockfall Hazard Rating System (RHRS) and was developed by [82] in Oregon (USA), in order to provide the local Department of Transportation with a simple, repeatable and easy-to-use standard rockfall hazard system along railways [83]. This system is considered as the basis for most rating systems, which further developed it in order to satisfy the requirements of variable transportation departments. Just to mention a few, the most widely used modified RHRS methods are: the Rock Slope Rating Procedure for New York (USA) [84; 85; 86; 87], the mRHIRS for Italy [24], the MORFR IS for Missouri (USA) [88; 89; 90], the TRHRS for Tennessee (USA) [91; 92; 93; 94], the CRHRS for Colorado (USA) [95; 96; 97] and the RHRON for Ontario (Canada) [98; 99; 100].  
Other important rating systems include the Rock Slope Hazard Index (RSHI) system, which was developed for rock slope inspection in highways in Scotland from [101]. The authors in [10] applied a quantitative hazard and risk methodology for the highways and railways in British Columbia, which was based on the magnitude and frequency distribution of rockfall events. The authors in [102] also proposed a new rockfall hazard rating system for use along a railway, named Quantitative Risk Assessment (QRA). This system was developed to quantify the probability of potential losses that are related to a hazardous event, with absolute results that can be used to compare different locations, thus providing a benchmark for prioritizing mitigation measures [102]. To this day, QRA constitutes one of the most popular quantitative methodologies.  
A common ground between all the systems that were mentioned above is that all of them are orientated to anthropogenic cut slopes for highways and railways and not to natural ones. Some researchers have proposed systems that are applicable to natural slopes, but most of them are constructed for specific locations only. The authors in [103] proposed a hazard estimation methodology, specifically for a part of the transportation network across Central Italy, with 3D rockfall trajectories and their spatial distribution in order to estimate the risk. In a related way, [104] presented another quantitative approach with 3D rockfall trajectories for risk evaluation at the Sola d'Andorra slope (Andorra Principality) before and after the implementation of risk mitigation works. Lastly, [105] proposed a qualitative rockfall risk rating system, applicable for the estimation of risk for both natural and anthropogenic slopes, which was developed for a case study at the historical site of Monemvasia, Greece.