Sea Level Variation and Trend Analysis by Comparing Mann-Kendall Test and Innovative Trend Analysis in Front of the Red River Delta, Vietnam (1961-2020)

In this study, we analyze sea surface height referenced against the WGS84 ellipsoid at the Hon Dau tidal gauge station (Hai Phong, Vietnam), in front of the Red River Delta, between 1961 and 2020. The annual sea level varied from 165.23 cm to 206.06 cm in this period (+20.28 cm over 60 years). The average water level was 190.87 cm for 60 years, with higher annual values in recent years, especially from 2016 to the present (above 201.5 cm). The Mann-Kendall (MK) test with Sen's slope estimator and Sen's innovative trend analysis (ITA) were applied and compared to estimate the sea level rise. These methods showed complete agreement among tests with significant rising trends of about 3.38 mm/year with the MK test and 3.08 mm/year with the ITA method for 1961-2020. During the last 20 years (2001-2020), the mean sea level increased about 7.16 mm/year (MK test and Sen's slope), 7.38 mm/year (ITA method), and around twice higher than the rate of the region and globally. The MK test and ITA method provided similar results for periods: 1961-2020, 1961-1980, and 2001-2020, with relatively stable monotonic related trend conditions. For the period 1981-2000, with a more nonmonotonic trend, the MK test and ITA method provided different trends and allowed to illustrate the specificity of each method.