## 3 Results
### Validation  
The displacement between the reference shoreline (Rapid Eye) and the extracted shoreline (Landsat 8) was calculated along 2167 shore-perpendicular transects. The total length of the reference shoreline amounts to 183 km. As shown in Figure 10, the median displacement between the RapidEye and the Landsat shorelines equals to 8.4 m with a standard deviation of 10.4 m. Taking the displacement direction into account, the median shoreline displacement yields \(-7.5\) m, which implies that the Landsat shoreline tends to be detected slightly seawards of the reference shoreline. The 75 th percentile MNDWI images have been created to capture the mean high-water level of each year. Figure 11 shows measured sea levels at Qui Nhon station in central Vietnam for three exemplary years. The number of acquisitions varies among the examples (1997: few, 2008: average, and 2015: maximum). Hourly sea levels are represented in blue  
Figure 8: Coastline change per province, showing the proportion of transects with erosion, accretion and stable conditions from 1986 to 2021.  
in the left column of Figure 11, superimposed by the Landsat acquisition timestamps in orange. The right column shows the resulting sea levels only at Landsat acquisition time stamps. The black line represents the 75 th percentile of the sea levels at Landsat acquisition, while the grey line shows the 75 th percentile of all sea levels. It can be deduced that Landsat captures different sea levels over the course of the year, which generally allows for the representation of a mean high-water level by annual 75 th percentile images. However, with less acquisitions, the Landsat percentile image tends to slightly underestimate the actual mean high-water level. Figure 9: Coastline change hotspots in Vietnam from 1986 to 2021. Only the hotspots with extreme average erosion and accretion rates (-5 m/year) were selected. The size of the circle differs by the length of the affected coastal segment. The two zoom images additionally show the annual coastlines overlaid by the filtered hotspot transects and circles. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No.** & **Length [m]** & \
\begin{tabular}{c} **Mean Change** \\ **Rate [m/year]** \\ \end{tabular} &
\begin{tabular}{c} **Mean Standard** \\ **Error [m]** \\ \end{tabular} & **Province** \\ \hline
0b & 39,400 & 34.4 & 4.9 & Thai Binh/Hai Phong City \\
1b & 32,000 & 22.6 & 3.3 & Soc Trang \\
2b & 22,600 & 39.0 & 3.3 & Ca Mau \\
3b & 20,400 & 23.5 & 1.9 & Soc Trang/Tra Vinh \\
4b & 17,200 & 8.6 & 2.5 & Bac Lieu \\
5b & 16,200 & 47.3 & 6.9 & Thanh Ho/Ninh Binh \\
6b & 15,800 & 36.8 & 3.2 & Quang,Ninh \\
7b & 14,600 & 11.4 & 1.6 & Kien Giang \\
8b & 14,200 & 14.8 & 1.0 & Kien Giang \\
9b & 12,600 & 25.3 & 2.5 & Quang Ninh \\
10b & 12,000 & 15.2 & 1.7 & Quang Ninh \\
11b & 11,800 & 10.2 & 1.8 & Tv Vinh \\
12b & 11,400 & 21.1 & 2.5 & Quang Ninh \\
13b & 9400 & 15.3 & 2.4 & Da Nang City \\
14b & 9400 & 13.0 & 5.8 & Quang Ninh \\
15b & 9400 & 5.1 & 0.5 & Kien Giang \\
16b & 9400 & 11.5 & 3.7 & Nam Dinh \\
17b & 8800 & 15.6 & 3.7 & Nam Dinh \\
18b & 7600 & 7.9 & 2.2 & Quang Ninh \\
19b & 7200 & 6.7 & 1.3 & Kien Gang \\
20b & 6800 & 24.9 & 3.6 & Ben Tre \\
21b & 6400 & 28.9 & 4.1 & Ben Tre \\
22b & 6200 & 20.1 & 5.2 & Thai Binh \\
23b & 6200 & 6.1 & 0.3 & Kien Giang \\
24b & 6000 & 12.3 & 3.2 & Quang Ninh \\
25b & 6000 & 51.5 & 13.9 & Hai Phong City/Quang Ninh \\
26b & 6000 & 6.8 & 0.8 & Tra Vinh \\
27b & 6000 & 11.0 & 4.8 & Quang Ninh \\
28b & 6000 & 21.4 & 8.2 & Nam Dinh \\
29b & 5800 & 6.5 & 0.9 & Quang Ninh \\
30b & 5000 & 21.8 & 3.9 & Ha Tinh \\
31b & 4800 & 8.8 & 1.2 & Hai Phong City \\
32b & 4400 & 9.5 & 1.7 & Khanh Hoa \\
33b & 4400 & 10.9 & 1.7 & Hai Phong City \\
34b & 4400 & 25.1 & 2.8 & Kien Giang \\
35b & 4200 & 19.2 & 2.0 & Tien Giang \\
36b & 4200 & 6.1 & 1.0 & Binh Thuan \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Accretion hotspots in Vietnam from 1986 to 2021.