Landslide Behaviour and Risk Reduction using SfM and 3D modelling techniques with Unmanned Aerial Systems (UAS). Chios island (Greece).

UAS have been increasingly utilized for research in Natural Hazards and Risk Management, especially when it comes to inaccessible study areas where the thorough examination of the existing geological-tectonic structures cannot be achieved only by field work. The study area is located on Chios island (North Aegean Sea, Greece) along the Chios-Kardamyla Road in the region from Mersinidi to Myliga, where the particular geodynamic and seismotectonic regime results in earthquakes which cause a great amount of natural disasters including many landslides. The largest part of the area was inaccessible. The use of SfM (Structure for Motion) techniques to obtain data from the UAV (Unmanned Aerial Vehicle-DJI Mavic Pro) flights above the study area led to detailed phototopographic, photomorphological, photogeological-tectonic and photogeotechnical mapping, detailed boundary and surface tectonic mapping and high-accuracy structural analysis in 3D environments. The combination of field work and UAS-based photogrammetry, provided complete and reliable results by following rapid and low-cost procedures by using Pix4D, ArcGIS, Rockware Rockworks 17, Rocscience Rocfall, Rocscience Slide and CAD software. The methodology was developed on the outline of the following workflow:

• Evaluation of existing geological, geotectonic, hydrogeological, seismotectonic and geotechnical data
• Flight project planning, according to: equipment specifications and capabilities, requirements of visual analysis, extent and morphology of the study area and expected weather conditions.
• Field mapping and UAS flight execution (imagery and footage capture).
• UAS imagery processing and interpretation: production of 3D models, Digital Surface Models (DSM), Digital Terrain Models (DTM) and Orthomosaics, formation boundaries recognition.
• Production of Geological-Tectonic maps for the study area.
• Research of the discontinuous tectonic deformation (SfM recognition and 3D mapping of tectonic lines and surfaces). Extraction of tectonic data (direction, dip, dip direction, aspect etc).
• Field and SfM tectonic data analysis and statistics (unification of tectonic data archive, weighting of the statistics, statistical processing and diagrams – density, rose, cyclographic projections etc).
• Research of the hydrogeological conditions of the area (determination of the role of groundwater in rock and soil movements according to hydrolithology and tectonic texture).
• Geotechnical mapping and hazard assessment.

Furthermore, this study includes the identification of the slope failures and the rock mass classification according to the internationally accepted stability calculation methodologies. Specific plans for rockfalls and rock slides, analysis of rockfall evolution and detailed simulation models of rockfalls were extracted. Appropriate measures and proposals for landslide risk reduction projects were also made. The evaluation of drilling results along the study area, the causes of landslides, the slope stability calculations and the proposed countermeasures are presented in the research. Especially regarding the carbonate rocks in the area, they have undergone tectonic strain that has led to their fragmentation into blocks and boulders. In combination with the water activity which reduces the shear strength of the discontinuities and the friction between a) the carbonate blocks and b) the carbonate mass and the clastic basement, these rock blocks are easily detached to overturn or slide on the downhill slopes, during intense precipitation or earthquake phenomena.