# UAV-Based Evaluation of Rockfall Hazard in the Cultural Heritage Area of Kipinas Monastery, Greece
## 2 Methods and Data
### Engineering Geological Field Survey  
The first and most important method used was an in-depth engineering geological survey in the area of interest. The necessity of detailed and intense field work is derived from the urgent need to gain a primary understanding of the engineering geological model of the site. This enables the identification of failure modes and possible volumes and of their possible impact. With the engineering geological model established, the evaluation of all the computer-based measurements from the UAV-derived data can be achieved and potential errors can be revised. Furthermore, via the field survey, many important parameters for the hazard assessment, which could not be obtained remotely, were estimated.  
From an engineering geological point of view, the limestone rock mass is moderately jointed, intersected by numerous major vertical fractures, which ultimately form the local face of the cliff. As mentioned before, the limestone is karstified and voids of large dimensions are formed, undermining the rock slope.  
According to the findings of the engineering geological field survey, two (2) main orientations were detected for the slope's face, due to its constant dip direction alternations, alongside the investigated area. Additionally, mainly three (3) major discontinuity sets and the bedding were identified, as presented in the Schmidt stereographic projection in Figure 2. The three major discontinuity sets are characterized as very steep in general (dip angle \(\geq\) 80\({}^{\circ}\)) and two of them are transversal to the slope plane; thus, they form the rock slope face in most places. The bedding is presented as very mild up to horizontal in some (dip angle \(\leq\) 15\({}^{\circ}\)) sub-regions of the slope.  
It is worth noting that due to the difficult terrain of the study area and the large scale of the hazardous regions, it was decided that the capabilities of the UAV should be taken advantage of, thus the estimation of both the spacing and the distance of the discontinuities were taken from the 3D point cloud model in accordance of course with the field-based observations. The spacing of the discontinuities has relatively high values (more than 5 m only in specific locations), hence the sizes of the rock blocks vary from moderate to very large (1.5 m\({}^{3}\) up to larger than 5.5 m\({}^{3}\)). In places, mainly due to stress relief, the size of the rock blocks is significantly smaller (0.5 m\({}^{3}\) up to 2.5 m\({}^{3}\)), especially on the eastern part of the slope. The distance of the discontinuity planes varies significantly depending on the degree of fracturing of the rock. In general, it varies in a range between 2 m and 5 m, although locally the distance can be lower than 2 m.  
Figure 1: Conceptual scheme of the proposed methodology.  
The joint wall compressive strength is high (estimated up to 24 MPa), while the roughness of the discontinuity planes can be characterized as rough (JRC ranges between 8 and 10). In general, there is no infilling material in the discontinuities' surfaces and the basic angle of friction along the discontinuities was adopted to be equal to \(\varphi=30^{\circ}\).  
Studying the engineering geological conditions that control the investigated slope, the face of the slope is generally characterized by relaxation of the rock mass, due to its high inclination. In conjunction with the spacing and the distance of discontinuities, moderate-to-large prone blocks are formatted. The main reason that these instabilities are not widespread across the slope is the \"nature\" of the rock mass itself and its superior mechanical properties, which restrain the prone-to-failure blocks, causing a temporal stability on the slope.