Sea-Level Variation Along The Coast Of Vietnam

Ocean is of great significance to the nature and human life. It plays a very important role, such as providing tidal energy, heat energy, wave energy, etc.. Ocean, especially continental strata is rich in resources, such as metal, petroleum, etc., which are important resources for industrial development. Marine fishery, aquaculture, aquatic products exporting & processing industry, the development of the shipbuilding industry, and marine development machinery are tightly linked with the ocean. Human benefits from the sea, however, at the same time also faces many challenges from the sea, such as abnormal sea level rise, typhoon, storm surge, the intensified salinity, coastal erosion phenomenon such as instability in coastal land-form. These natural disasters derived from ocean have caused serious threat and damage to the life and property safety of coastal residents and coastal infrastructure. Under the background of global climate change, the situation has become more serious. Therefore the study of ocean, grasping the law of sea level change, are issues of global attention at present. With the achievements of ocean research, we will be likely to put forward early warning and forecasting on all kinds of marine disasters, and to support and guarantee ocean development and the sustainable development of marine economy based on the law of nature.The observation record data from tide stations is the basic data for the study of eustatic sea level change, which has the characteristics of long-term and stability. However, since tide stations are located on islands or mainland coastal areas, the observed values from tide stations are affected by the vertical plate movement that the station locates. Satellite altimetry, as a spatial measurement technology, is adopting satellite as the carrier to measure the global sea surface relying on space, electronic and microwave isometry technology etc.. In order to determine the water level fluctuation characteristics and the trends of mean sea level of south China sea and Vietnam coastal water, this paper has utilized a lot of Vietnam coastal tide station data records and 16 years' satellite altimetry data on the research. The specific achievements of the research include:Study the change characteristics and trends of sea level, sea tide, residual water level (no tidal level) of Vietnam's coastal sea area and extreme water level statistics forecast The number of tide stations used for the research is up to a total of 33, distributed in six representative coasts from north to south, a minority on offshore islands. The time span of the data is between 20-48 years (1961-2009) with the sampling frequency of generally 1h. Chapter VI of the paper states a brief calculation of global sea level change and that of South China sea by taking the T/P and Jason-1 data from 1993 to 2008.(1) Studied the sea level change of each sea area by using the long-term observation data of 10 tide stations at six representative coasts from north to south. A mean rate of rise 1.9mm/a is obtained. The 6 sea areas, except for north-middle, which presents as a mean dropping of -3.2mm/a, are presenting as rising (north-north, north-middle, south-middle, east-south, west-south) (1961-2010). The mean rise rate of 1990 to 2010 is 2.4mm/a. The average difference between the highest and the lowest sea level is 20-50 cm for many years.(2) Studied coastal tidal characteristics and trends by using the data of the 6 representative sea areas. The tidal range (the difference between the highest water level and lowest one) variation of the 6 sea areas is with an overall average rise of 2.4mm/a, among which the north-middle area has an average dropping of-2.2mm/a; by using the hourly of tidal observation records during the long-term observation of representative tide stations at Vietnam's coastal areas, the tidal harmonic constants of each year were obtained through analysis. And then according to the time series of the tidal harmonic constants, the partial tidal harmonic constants and variation trends were analyzed by adopting the linear regression model; according to the tidal properties classification criteria of the V index of the main diurnal tides and semi-diurnal tides, including four classes as regular and irregular diurnal tides, regular and irregular semi-diurnal tides. The different distribution of tidal types, the order of magnitude of amplitude as well as the 18-year periodic variation rate were provided, combining with ENSO (El Nino Southern Oscillation), the results were explained. To research the tidal structure and variation at Vietnam coastal areas is the important content of the paper.(3) Studied and calculated the extreme water levels and predictions of each sea area based on three probability distribution modes (Pearson-?, Gumbel and Weibull) by using the 50 years' observation data from the 10 tide stations of the 6 sea areas. The tidal analysis software package developed by the author has solved 114 partial tidal harmonic constants of the reference data and calculated astronomical tide level time series at different time intervals based on this, estimated the maximum water level for many years at different recurrence intervals (10 years-500 years), compared the three kinds of calculation results with a difference less than 5cm, suggesting to take the average value of the three results preferably, and discussed the the security and economic of using maximum water level for many years on engineering design.(4) Conducted statistic analysis on the residual water level (increase or decrease of water level) change of the 6 representative sea areas and has found that the monthly average water level time series presenting as rising of most of the stations and that is basically consistent with the long-term change trends of monthly mean sea level. Respectively counted the number of occurrences of interlacing of average water level rising and dropping and corresponding duration. Provided the different duration and cumulative frequency of water level increasing and decreasing and has obtained that the frequency less than 1 day is>90%, and<1% when longer than 2 days (typhoon emergence period) and that the average cumulative frequency is>96% when the water level increasing is>50cm, and the average cumulative frequency is>97% when the water level decreasing is<-50cm.(5) Analyzed the global sea level change and that of South China sea by taking the T/P and Jason-1 data from 1993 to 2008 (16 years). They are generally presenting as rising. The rising velocities of each year are different. The sea level change of different sea areas of South Sea has large fluctuations. The average velocity is 2.72±0.01mm/a, a little bit higher that the global average rising velocity 2.14±0.06mm/a.