## 3 Results
### Start Bay
Figure 18a shows the SAR-derived shorelines on top of the DoD obtained from subtracting the elevations observed in 2016 from the elevations observed in 2018, so negative and positive elevation differences represent erosion and accretion, respectively. The elevation differences are well within \(\pm 1.5\) m, suggesting that sediment eroded from the northern end has been deposited in the southern end of the embayment (e.g., counterclockwise beach rotation). We notice that most of the elevation changes are in the upper swash zone, while the SAR-derived SLs are mostly located in the upper backshore and far from the main area of vertical change. The filtered shoreline points used to obtain the metrics of change from all the satellite observations available are shown in Figure 18b. More than 70% of the filtered shoreline locations are outside the area of appreciable vertical change except for points located on the northern end of the embayment. When using all the SAR data available to calculate the annual recession rate, we observe that the rotation is somehow captured as shown in Figure 18a,d, where eroding polygons and accreting polygons dominate different ends of the embayment. Polygons where annual change rate is within \(\pm 0.5\) m are not shown (e.g., transparent polygons), and as expected, the central part of the embayment is within these neutral polygons. We also notice that the direction of change observed in the DoD between 2016 and 2018 is opposite to the direction of change obtained when using all observations between 2015 and 2021: beach rotation is counterclockwise for the period 2016-2018 and clockwise for the period 2015 to 2021. Figure 19 shows the annual CRF for the entire Start Bay area obtained from all the SAR observations for the time between years 2015 and 2021, and as a visual reference, we also show the DoD between 2016 and 2018. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**SL Name** & **Count** & **Min (m)** & **Max (m)** & **Mean (m)** & **Std (m)** & **Median (m)** \\ \hline ASC 11 March 2016 \(\uparrow\) & 10324 & \(-2.7\) & 26.5 & 3.3 & 3.5 & 2.8 \\ ASC 10 September 2018 \(\uparrow\) & 10311 & \(-2.1\) & 50.0 & 4.2 & 6.3 & 3.1 \\ ASC 17 September 2020 \(\uparrow\downarrow\) & 8693 & \(-2.6\) & 29.5 & 2.3 & 3.2 & 2.4 \\ DESC 08 March 2016 \(\downarrow\) & 10826 & \(-2.5\) & 44.0 & 4.6 & 6.5 & 3.2 \\ \(\dagger\) DESC 12 September 2018 \(\uparrow\) & 10147 & \(-2.2\) & 40.6 & 4.6 & 5.5 & 3.8 \\ \(\dagger\) DESC 19 September 2020 \(\uparrow\) & 7791 & \(-2.7\) & 36.8 & 3.5 & 5.3 & 2.8 \\ \hline \multicolumn{7}{l}{\(\dagger\) Collection dates of DTMs and SLs within less than 24 h difference; \(\dagger\)ascending tide; \(\downarrow\) descending tide; \(\dagger\)\(\downarrow\) high tide.} \\ \end{tabular}
\end{table} 
Table 8: Main stats (count of non-nan values, minimum, maximum, mean, standard deviation, and median) of the terrain elevation values along the SLs extracted from the DTMs for Start Bay. Elevation values refer to OD Newlyn. The histograms show how the mean annual change rate is negative for both ASC and DESC SLs but of different values \(-0.6\) m/year and \(-0.2\) m/year, respectively. This is due to most of the metrics of change obtained from the DESC SLs resulting in neutral CRs (e.g., within \(\pm 0.5\) m/year), unlike for metrics of change obtained from the ASC SLs. The southern end of Start Bay is dominated by rocky outcrops that change very little in elevation over time. This stability seems to be better captured by the metrics of change derived from DESC SLs, which show neutral (\(\pm 0.5\) m/year) annual change rate at the southern end, while the metrics of change derived from ASC SLs show erosive annual CRs larger than \(-0.5\) m/year. Figure 18: Qualitative assessment of SAR shorelines detecting beach rotation at Blackpool Sands embayment in Start Bay. (**a**) Shorelines from ascending and descending orbits for the years 2016 and 2018 shown on top of the digital elevation model of difference.