## 5 Conclusions  
The coastal zone of Vietnam is particularly vulnerable to natural hazards, such as floods, saltwater intrusion, tsunamis, and erosion, since economic and population growth has been concentrated at the coast during the past decades. This study investigated the annual coastline dynamics of Vietnam over the past 35 years, exploiting the full potential of the Landsat satellite archive: (i) The development of a fully automated GEE-based approach allowed for the creation of annual MNDWI composites covering the entire coastal zone of Vietnam from 1986 to 2021. (ii) The mean-high water level could be represented in the annual composites through the creation of 75 th percentile images. Comparison with actual sea level data showed good agreement of the actual mean-high water level with the mean-high water level that was captured by Landsat throughout the year. The sub-pixel coastline extraction from MNDWI images yielded an average accuracy of 8.4 m on a single Landsat acquisition in central Vietnam. Comparison to previous studies on local coastline change rates in Vietnam shows that the automatic change quantification corresponds well to locally determined rates of change. (iii) The analysis of coastline change rates along shore-perpendicular transects revealed that almost half of the Vietnamese coast remained stable, while 27.7% experienced erosion and 27.1% experienced accretion over time. Within the erosional and accretional coastal segments, almost 50% of the transects show change rates above 5 m/year. (iv) The hotspot analysis identifies long stretches of coastline that show consistently high rates of erosion or accretion. Hotspots with highest erosion (\(-\)28 m/year) and accretion rates (\(+\)47 m/year) are concentrated in the two large river deltas of the Mekong River in the south and the Red River in the north. Both regions are additionally characterized by rapid economic and population growth. The highest erosion rates could presumably be related to natural redistribution of sediment while at the same time being accelerated through human-induced sediment deficits, such as river damming or sand mining. The highest observed accretion rates, in contrast, are most probably caused by direct human intervention in the form of mangrove plantation and land reclamation. However, to better understand the causes for coastline changes in Vietnam, further analysis including the type of the affected coast, prevailing natural processes, as well as coastal vulnerability, would be of interest. Furthermore, more extensive validation of the coastline extraction as well as the change quantification method is recommended in the future. Nevertheless, the presented coastline change analysis contributes an objective and valuable inventory, which may be used as a basis for future research. In addition, the approach offers the potential to be transferred to other regions, since only globally and publicly available datasets and tools were utilized. Conceptualization, R.L., F.B. and T.U. ; methodology, R.L., F.B. and T.U; software, R.L. ; validation, R.L. and F.B. ; writing--original draft preparation, R.L. ; writing--review and editing, R.L., F.B. and T.U. ; visualization, R.L. ; supervision, F.B. and T.U. All authors have read and agreed to the published version of the manuscript. This research received funding from the FloodAdaptVN project ([[https://www.floodadapt.eoc.dlr.de](https://www.floodadapt.eoc.dlr.de)]([https://www.floodadapt.eoc.dlr.de](https://www.floodadapt.eoc.dlr.de)), accessed on 18 May 2022), funded by the German Ministry of Education and Research (BMBF, funding no. 01 LE1905A1) in the framework of the FONA program ([[https://www.fona.de/en/](https://www.fona.de/en/)]([https://www.fona.de/en/](https://www.fona.de/en/)), accessed on 18 May 2022). The data that support the findings of this study are available from the corresponding author [[PERSON]], upon reasonable request.