## 3 Results
### Spatio-Temporal Patterns of Coastline Change in Vietnam  
To provide an administrative spatial differentiation of coastline dynamics in Vietnam, Figure 8 shows the coastline change class proportions for each coastal province sorted by the erosion percentage. It is evident that Ca Mau in Southern Vietnam is the province most affected by erosion, with over 80% of its coastlines having retreated since 1986. Ca Mau is closely followed by Tien Giang with 70% and Bac Lieu with about 60% eroding coasts. Nam Dinh also shows more than 60% erosion, but is unlike the other provinces located in the north of Vietnam. Ninh Binh, also located in the north of the country, exclusively experienced accretion, followed by Hai Phong City (80% accretion) in the north and Soc Trang (70% accretion) in the south. The three provinces Binh Dinh (80% stable), Quang Ngai (70% stable), and Phu Yen (66% stable) with highest proportions of stable coastlines are located in central Vietnam.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Classification L1** & **Proportion [\%]** & **Classification L2 [m/Year]** & **Proportion [\%]** \\
**[m/Year]** & & & \\ \hline Stable (0.5 to \(-0.5\)) & 45.2 & & \\ \hline \multirow{4}{*}{Erosion (\(<\!-0.5\))} & \multirow{4}{*}{27.7} & Extreme erosion (\(<\!-5\)) & 39.2 \\  & & Severe erosion (\(-3\) to \(-5\)) & 9.6 \\ \cline{1-1}  & & Intense erosion (\(-3\) to \(-1\)) & 22.0 \\ \cline{1-1}  & & Moderate erosion (\(-1\) to \(-0.5\)) & 29.3 \\ \hline \multirow{4}{*}{Accretion (\(>\!0.5\))} & \multirow{4}{*}{27.1} & Extreme accretion (\(>\!5\)) & 49.5 \\ \cline{1-1}  & & Severe accretion (5 to 3) & 9.3 \\ \cline{1-1}  & & Intense accretion (3 to 1) & 23.7 \\ \cline{1-1}  & & Moderate accretion (1 to 0.5) & 17.5 \\ \hline \end{tabular}
\end{table}
Table 3: Proportion of coastline change classes in Vietnam.  
Figure 7: Classification of coastline change in Vietnam from 1986 to 2021, with zooms on two exemplary coastal segments showing the annual coastlines overlaid by the classification transects.  
The coastline change hotspot analysis is shown in Figure 9. The accretion and erosion rates as well as the affected coastal segment length for each of the selected hotspots are shown in Tables 4 and 5, respectively. The hotspot analysis shows small-scale extreme coastline dynamics in central Vietnam that have not been reflected in the provincial coastline change statistics. For example, erosion hotspot No. 10a is located close to the city Hoi An in Quang Nam province, which is considered a mostly stable province (see Figure 8). The erosion hotspot with the longest affected coastal segment of 102 km is located in the Mekong Delta, while the erosion hotspot with the highest erosion rate of \(-27.7\) m/year is found in the Red River Delta. The longest accretion segment of 39 km is close to Hai Phong City. The accretion hotspot with highest accretion rates is located south of the Red River Delta and stretches across the provinces of Thanh Hoa and Ninh Binh. It shows an average accretion rate of +47.3 m/year. In summary, it can be deduced that the majority of accretion hotspots length- and intensity-wise are located in the urbanized Red River Delta, while the majority of erosion hotspots lie in the Mekong Delta.