The coastline of Vietnam - annual dynamics derived from 35 years of Earth Observation data

Kurzfassung

Currently, more than 40 % of the world’s population lives in coastal regions, a quarter of which situated less than 10 meters above sea level. Floods, saltwater intrusion, tsunamis, subsidence and erosion are among the natural risks to which coastal zones are exposed. The coastal region of Vietnam is particularly vulnerable to these risks because urbanization, agri- and aquaculture, tourism, infrastructure and industry are competing for the low lying, narrow and attractive areas close to the coast. Therefore, a profound understanding and monitoring of coastal processes is crucial to protect the environment, infrastructure and people. Coastline change analysis for Vietnam was conducted for a time series from 1984 to 2021 employing a cloud-based processing strategy on the Google Earth Engine. The analysis is based on Landsat-derived annual 75th percentile Modified Normalized Difference Water Index (MNDWI) composites, representing the mean high-water level and was executed for the entire shoreline of Vietnam. Contours were extracted on sub-pixel level. Linear regressions were calculated along shore-normal transects for quantifying coastline change rates. A hotspot analysis identified coastal segments with highest erosion and accretion rates. Coastal segments are considered hotspots when five neighboring transects are either erosional or accretional with a mean change rate of at least 5 m/yr. The transects were created with an interval of 200 m, which means that a hotspot includes at least 1,000 m of coastline. A validation of the automatic Landsat-based coastline detection yielded a sub-pixel accuracy of 8 m on a single Landsat acquisition. The results of the country-wide analysis show that the accumulated sum of accretion and erosion led to a slight increase of land area. Yet on local level, the erosion and accretion patterns are varying to a large extent. Erosion hotspots are located mainly at the coasts of the Mekong Delta (e.g. Cau Mau, Tien Giang, Bac Lieu) and the Nam Dinh province. The longest accretion segment of 39 km is located close to Hai Phong city. The central Vietnamese Thua Thien Hue province belongs not to the major hotspots of coastline dynamics. Yet, it is a good example why the understanding and monitoring of coastline dynamics is of high relevance. The province is home to the largest lagoon in Southeast Asia – the Tam Giang-Cau Hai lagoon. The lagoon is separated by an elongated and narrow sandy barrier from the sea, that serves as protection from waves, coastal flooding and storm surges for the lagoon and its hinterland. In addition, several settlements (communes) are located on the barrier, agri- and aquaculture are cultivated and recreational sites exist. The coastline change quantification for Hue province showed that more than half of the coast has been classified as predominantly stable with annual change rates of around 0.5 m/yr. However, 26 % of Hue‘s coast was found to be erosional with change rates higher than -0,5 m/yr on average, while 20 % mainly accreted with more than 0.5 m/yr. Hence, the total of Hue’s coasts slightly eroded by -0.13 m/yr on average. Yet, along the narrow barrier section south of the Thuan An inlet, intense average erosion rates were identified for the past 35 years. Five local hotspots with strong coastline change rates have been identified for Hue province with locally adapted hotspot identification parameters (transect interval of 100 m; minimum change rate of 3 m/yr). Three of which are erosional with maximum erosion rates of -15 m/yr and maximum accretion rates of + 18 m/yr and are located at the lagoon inlets, as well as two accretional. One of the erosion hotspots is located at the Thuan An inlet. It shows an average erosion rate of -4 m/yr and a length of 900 m. The erosional process has not been constant over time. While strong erosion started only in the 2000s, accretion predominated at the hotspot from 1988 to 1999. After 2014 the coastline dynamics stabilized. On the opposite side of the lagoon inlet, the reverse pattern could be observed. The headland has been classified as accretion hotspot with average accretion rates of +3 m/yr and a length of 1700 m. Phases of erosion and accretion also alternated here. Both processes are most likely linked through sediment redistribution. Severe erosion hotspots have further been identified north and south of the lagoon inlet in Tu Hien in the south of Hue province. The average erosion rate is between -3 and -4 m/yr and stretches over more than 5 km, only interrupted by the lagoon inlet. The coastal erosion has been rather constant and is not accompanied by equivalent accretion in the neighboring segment, indicating a deficit in the local sediment budget. A severe accretion hotspot is located north of Binh An with an average change rate of + 9 m/yr. According to visual interpretation, the accretion has been caused by the construction of a new harbor. Overall, even though most parts of Hue`s coastline remained stable over the past 35 years, strong coastline changes have been observed locally. The causes for coastline changes are multifold and interrelated. But it is reasonable to conclude that the erosion hotspots in Hue were caused by dynamic sediment redistribution as well as reduced coastal sediment availability. Accretion emerged both through natural sediment displacement and as a result of direct human intervention through stabilization, construction and land reclamation.