## 2 Materials and Methods
### Validation and Interpretation of SAR Shorelines and Change Rates
#### 2.5.2 Salinas Beach  
At the western boundary of Salinas beach (profiles No. 57 to No. 61 in Figure 7), changes in the dry beach area are limited due to the existing seawall on the backshore. Therefore, all analyses in this study site are associated with changes observed at the eastern boundary of the beach (profiles No. 1 to No. 9 in Figure 7), next to the inlet gro \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Date Acquisition** & **Source** & **Provider** & **Resolution (m \(\times\) m)** \\ \hline
2 June 2016 & Google Earth Pro & SPOT 6 & 6 \(\times\) 6 \\
7 May 2017 & Google Earth Pro & SPOT 7 & 6 \(\times\) 6 \\
24 June 2018 & Google Earth Pro & WV2 Maxar technologies & 2 \(\times\) 2 \\
1 June 2020 & Google Earth Pro & WV2 Maxar technologies & 2 \(\times\) 2 \\
26 April 2021 & Google Earth Pro & WV2 Maxar technologies & 2 \(\times\) 2 \\
28 March 2022 & Google Earth Pro & WV2 Maxar technologies & 2 \(\times\) 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: List of satellite images in the DSA analysis for Bull Island (Ireland) and their spatial resolution. The analyses are made based on the distances of SAR-SLs from the reference line located landward of the dune (green dots in Figure 7) (\(d_{M}\); see Section 2.4.2). The ability of SAR-SLs to detect morphological changes in the beach profile is assessed at two different timescales:  
* Inter-annual changes based on a qualitative assessment of the erosional/accretional processes observed according to the SAR-SL distance from the baseline in comparison to those observed in topographic field surveys. The beach subaerial topography is monitored on a regular basis by means of in situ surveys provided by the Ministry of the Ecological Transition and the Demographic Challenge of the Government of Spain with a variable precision ranging from 10 to 0.1 m. Four surveys are available since 2015: May 2016, September 2019, September 2021, and August 2022. The topographic surveys cover the beach foreshore and backshore, including the inter-tidal area, berm, and dry beach, up to the toe of the dune or seawall. The survey dated September 2021 also covers the dune seaward slope and crest. From these topographic surveys, the shape of the beach profile is extracted along transects placed in the center of polygons (Figure 7). The vertical reference level for these topographic surveys (chart datum of Aviles Port) is located 274 cm below the local mean sea level. Other vertical references, such as the highest astronomical tide, mean high water, mean low water, and the lowest astronomical tide, are shown in Figure 8 with results of the topographic surveys for beach profile No. 6 as an example, along with the window that indicates where SAR-SLs are identified. * Seasonal changes, based on the correlation between the time series of SAR-SL position (distance from the baseline) and local hydrodynamics. Time series of wave parameters, tides, and surge levels are obtained from the databases Global Ocean Waves [39], Global Ocean Tides [40], and Global Ocean Surges [41], respectively (see time series in Figure A1). Regarding waves, the Global Ocean Waves dataset consists of hindcast data generated with the WaveWatch III numerical model. In the study area, the dataset spans from 1979 to 2021 with a temporal resolution of one hour and a spatial resolution of 0.25 degrees. Regarding the astronomical tide, the Global Ocean Tides provides hourly time series of astronomical tides with global coverage and a spatial resolution of 0.25 degrees. Astronomical tides are generated from the harmonic constants derived from the TPXO global tide model in its different versions developed by Oregon State  
Figure 7: SAR polygons and beach profiles, indicated by numbers, for Salinas beach. Source of aerial imagery: Esri, World Imagery Metadata. University.