## 3 Results  
### Bull Island  
Figure 9 illustrates an overview of the SAR-SLs extracted for Bull Island. They were generated by processing the available SAR images between June 2016 and March 2022.  
As RL, the high-water-mark shoreline produced by Ordnance Survey Ireland ([[https://www.geohive.ie/pages/data](https://www.geohive.ie/pages/data)]([https://www.geohive.ie/pages/data](https://www.geohive.ie/pages/data)) (accessed on 6 December 2023)) was used. Along the RL, 93 polygons were created, and for each of them, SAR CRs were produced in order to validate it \"quantitatively\". The speckle effect is minimal along the front beach (Figure 9a). The majority of the SLs appear to be well concentrated and unaffected by sea conditions, suggesting a relatively calm environment during data acquisition. Figure 9b illustrates the points selected for the annual SL CR after applying a minimal filter (heatmap >0.175%) and the GMD technique. Considering the flat beach geometry, the authors note that there is good overall agreement between the results obtained from SAR ASC and DESC tracks. The geometry distortions (shadows/layovers) do not appear to have a noticeable effect on the SLs. This results in the SLs tending to overlap with one another. The slight differences observed are attributed to variations in the number of SAR images available. Bull Island is not covered equally from ASC (759) and DESC (386) tracks, which may lead to minimal differences in certain polygons.  
The Bull Island shoreline consists of 5 km of low-lying sand dunes. The shoreline change shows a gradual transition from advancing shoreline in the southern part to receding in the northern end. The central part lies within the stability range. The overall trend is accretion, covering 2/3 of the shoreline. However, the average shoreline change rate is negative (receding), \(-\)0.54 m/year, as the northern part shows high erosion values up to \(-\)9.95 m/year (Table 4).  
Overall, there is a high degree of alignment between the CRs derived from DSA and SAR shorelines. This is confirmed in the visual inspection of the profiles (Figure 10) and the strong correlation coefficients (\(R^{2}>0.9\)) between the two datasets (Table 5). In general, SAR annual SL CRs are lower than the DSA counterparts, in both average values and coverage. Regarding the mean rates, the SAR CRs are lower than the DSA rates (\(\sim\)0.5 m/year) and display larger receding rates. The receding shoreline extent is larger in both SAR passes (ASC/DESC) when compared with DSA rates.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**SL Annual CRs** & **N** & **Mean** & **Max** & **Min** & **Advancing** & **Receding** \\ \hline SAR ASC & 93 & \(-\)0.95 & \(-\)11.72 & 1.36 & 47/93 & 46/93 \\ SAR DESC & 93 & \(-\)1.18 & \(-\)10.71 & 1.46 & 40/93 & 53/93 \\ VL DSAS & 93 & \(-\)0.54 & \(-\)9.95 & 3.51 & 62/93 & 31/93 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Shoreline annual CR stats for SAR ASC, SAR DESC, and DSA vegetation line (VL DSAS).  
Figure 9: SLs produced for Bull Island including both ASC and DESC tracks (**a**). Filtering applied to the scene and points selected to produce time series and change rate (**b**). Source of aerial imagery: Google, ©2023 Maxar Technologies.