## 2 Material and Methods
As \"significant wave height H\({}_{\text{s}}\)\", we denote the height (constant) of a regular stationary sequence of waves (sinusoidal transverse profile), with height equal to the arithmetic average of the highest third of the waves that compose a specific marine event and cross a given marine area without interference between waves and seafloor. [PERSON] and [PERSON] [28] developed a set of curves showing the characteristic height H\({}_{\text{c}}\) and period Tc versus fetch and duration. These graphs are distinct from significant height (H\({}_{1/3}\)) and mean period (T\({}_{\text{z}}\)). There is uncertainty concerning the relationship between the visually and instrumentally derived quantities, but some bias (H\({}_{\text{c}}\) and T\({}_{\text{c}}\) both being slightly higher than H\({}_{1/3}\) and T\({}_{\text{z}}\), respectively) may be likely involved when using this type of graph. However, systematic errors are generally small compared with the random errors in individual observations and the differences are negligible for all practical purposes [29]. For the estimation of the characteristic wave height H\({}_{\text{c}}\) in relation to the wind force, known the duration of the anemometric event and the fetch, we used the Groen-Dorrestein nomogram [28] and cross-checked the values through Breugem-Holthuijsen nomogram [30]. UAS (Unmanned Aircraft System) photogrammetry combined with the Structure from Motion (SfM) technique [31; 32] has been used to analyze the seawall morphology along Via Partenope sections (Figure 1d). These methodologies represent a well-established method for mapping and creation of digital elevation models (DEMs) and ortho-images. In order to investigate the coastal section involved by the sea storm, a DJI Phantom 4 RTK aircraft with an on-board RTK (Real Time Kinematic) has been used to perform all the aerial imagery acquisitions. The DJI Phantom 4 RTK UAS mounted with a camera equipped with FC6310R lens (f = 8.8 mm), resolution of 5472 \(\times\) 3648 pixels. The GNSS RTK equipped UAS simplified the construction of highly accurate digital models through SfM photogrammetry, thus avoiding the need for ground control points (GCPs) [33] that can be difficult in a busy urban context. In this way, the high precision GNSS, allows the detection of the 3D position of the camera at the time of each capture, within a few centimeters. The position of the camera can be estimated automatically by the firmware and the coordinates are stored internally. Exif metadata have been referenced to the camera. The flight altitude was set to 50 m above the ground with a vertical image acquisition axis and Ground Sampling Distance (GSD) of 1.21 cm/pix. The GSD represents the distance between the centers of two consecutive pixels in the image on the ground [34]. For a given flying height H (m), the GSD is given by:  
\[\text{GSD}=\text{H}\times\text{P/f}, \tag{1}\]  
where P is the pixel size of the sensor (micro), and f is the focal length. Ninety-nine images were acquired following a single-grid plan, thus collecting pictures along a given direction only. In this case, the dataset consisted of radial imageries, and the entire coastal section was mapped. The SfM photogrammetric processing was performed by Agisoft Metashape(r) Professional software package [35]. The SfM-based photogrammetric processing resulted in a dense cloud point, 3D model, 2D orthomosaic, and DEMs data from the set of overlapping images for the area of interest. The obtained final value of root mean square error (RMSE), expressed in pixel units, was below 1 pixel RMSE for all the performed projections. The high resolution of the orthophotos and the accuracy of the entire post photogrammetric processing allowed for obtaining precise measurements within an accuracy range of \(\pm 2\) cm.