## 3 Data and Methods  
The first out of two flight surveys was carried out before the medicine, more specifically on 18 July 2020 (Figure 7), using a commercial multi-rotary UAS (DJI Phantom 4 RTK) (Figure 7a), which carries a global navigation satellite system (GNSS) antenna with the ability of real-time kinematics (RTK) processing (Figure 7b). The same equipment was used also for the post-medicane survey, which was carried out on October 6 th, 2020 (Figure 7). The surveys were planned as part of the project \"LAERTIS--Innovative Risk Management System in the Region of Ionian Islands\" (MIS: 5010951) funded by the European Union under the National Strategic Reference Framework. To acquire comparable data, we repeated the survey at the same location (Figure 8). The aim of the multi-temporal UAS data collection and analysis was to create a series of digital surface models (DSMs) of the studied slope, along with orthophotomosaics, based on the structure-from-motion technique [33, 74]. By comparing these multi-temporal photogrammetric results it was possible to quantify the topographic changes of the studied surface. Although the same flight properties were used (path, elevation, and camera angle), the total number of acquired photographs was slightly different in the two surveys (Table 1), attributed to differences in satellite geometry during the flights, which in turn affects the UAS camera positioning, as well as the weather (wind direction and amplitude). Nevertheless, this is not considered to affect the overall methodology and the results. It took several flights to plan the flying strategy to satisfactorily cover the entire study area, due to the steep topography of the slopes surrounding the beach. A DJI Phantom 4 RTK UAS was operated at a maximum of 385 m above sea level (ASL) after it was launched from a suitable take-off area located at a beach view balcony located at 330 m ASL (Figure 7a). The covered area was about 1.2 km\({}^{2}\). Figure 7: Views of the equipment used for detecting the geomorphological changes triggered by the 2020 Medicare Ianos in the Myrtos coastal area: (**a**) a commercial multi-rotary UAS (DJI Phantom 4 RTK) with an on-board GNSS antenna with the ability of RTK processing was used as a rover antenna, (**b**) the DJI D-RTK-2 was used as the base station antenna. The onboard GNSS receiver, antenna, and inertial measurement unit (IMU) calculate the absolute camera positions and altitudes directly with high-accuracy measurements [75,76]. The preflight planning included the setup of the flight routes and elevation above the take-off area, the image acquisition angle, the front and side overlap between the successive aerial photographs, the UAS speed during the flight, as well as the camera function details. Additionally, setting up the connectivity with the GNSS base station network was also a crucial point for taking advantage of the RTK ability, and consequently increased the positioning accuracy during the data acquisition phase, even if the error would be largely minimized during the photogrammetric processing phase. The quality of the camera (FC6310R, 20 Mpx, 8.8 mm focal length) did not require the UAS to stop at waypoints to take oriented photographs to avoid motion blur in images due to possible short-term compensating UAS movements; therefore, it was not necessary to exclude many photographs at the quality control phase. The highest resolution obtained