## 2 Materials and Methods
### SAR Imagery Datasets  
Our study makes use of S1 Level-1 Ground Range Detected (GRD) products, which are freely available at [[https://search.asf.alaska.edu/#/](https://search.asf.alaska.edu/#/)]([https://search.asf.alaska.edu/#/](https://search.asf.alaska.edu/#/)) (accessed on 6 December 2023) in high resolution or medium resolution (see Table 2). These data consist of SAR data that have been detected, multi-looked, and projected using the WGS84 Earth ellipsoid model. S1 consists of two satellites, which operate at C-band frequency (5.4 GHz), supporting both single (HH or VV) and dual polarization (HH + HV or VV + VH). SAR satellites typically follow two types of orbits: ascending (ASC), in which the satellite travels from the South Pole to the North Pole, and descending (DESC), in which it travels in the opposite direction. Each time the satellite orbits the Earth, it covers the same geographic area twice, once during the ASC pass and once during the DESC pass. As a result, an SAR application can acquire more frequent data over a specific area, which is particularly beneficial for monitoring dynamic processes or rapidly changing environmental conditions. The S1-SARinstrument acquires data in four exclusive modes [23] that differ in resolution, coverage, and revisit time:  
* Stripmap (SM). This is a standard SAR stripmap imaging mode where the ground swath is illuminated with a continuous sequence of pulses and with the antenna beam pointing to fixed azimuth and elevation angle.
* Interferometric wide swath (IW). Data are acquired in three swaths using the terrain observation with progressive scanning SAR (TOPSAR) imaging technique. In IW mode, bursts are synchronized from pass to pass to ensure the alignment of interferometric pairs.
* Extra wide swath (EW). Data are acquired in five swaths using the TOPSAR imaging technique. EW mode provides very large swath coverage at the expense of spatial resolution.
* Wave (WV). Data are acquired in small stripmap scenes called \"vignettes\", situated at regular intervals of a 100 km long track. The vignettes are acquired by alternating, acquiring one vignette at a near-range incidence angle and the next vignette at a far-range incidence angle (no WV images available for the study sites).  
Based on the number of available data and to ensure consistency and eliminate differences arising from varying data resolutions, among the options, for this study, we chose high-resolution products in IW mode with dual polarization (VV + VH). This choice offers the best resolution to the revisit time ratio, with a minimum revisit time of six days and 20 m \(\times\) 22 m spatial resolution, acceptable for coastal monitoring purposes. Figure 2(a) illustrates, as an example, the available datasets for the Start Bay study site (comparable numbers are also obtainable for the other two sites). The selected product for Start Bay provides a total of 1734 images for the period for which S1-SAR data are available, from April 2014 to July 2023 (e.g., 192 images per year for 9 years). Moreover, the authors decided to use VH polarization rather than VV polarization due to its lower sensitivity to sea conditions and improved contrast between land and sea, as depicted in Figure 2(b),c. This preference was further substantiated by a quantitative analysis presented in [24] to determine the best-suited polarization for these scenarios.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **HR (m \(\times\) m)** & **MR (m \(\times\) m)** \\ \hline SM & \(23\times 23\) & \(84\times 84\) \\ IW & \(20\times 22\) & \(88\times 87\) \\ EW & \(50\times 50\) & \(93\times 87\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: High- and medium-spatial-resolution Level-1 GRD.  
Figure 3: _Cont._