Sediment Dynamics And Evolution Of The Key Harbors In Guangxi, China

The total length of Guangxi Coast is 1595 km, which is near the south of Beibu bay. The coastline is twisting, and there are 50 little or big harbors, what's more, there are 6 harbors more than 60 km2.The Zhenzhu harbor, Fangcheng harbor, Qinzhou bay and Tieshan harbor were focused in the article, and the spatial and temporal characteristic of hydrology dynamic and sediment condition are studied, what's more, the sediment dynamic condition and the evolution pattern in the Qinzhou bay are analyzed. The data involved in the article includes the hydrology and sediment data continuously observed in 2009 in the four key harbors, the surface sediment sample collected in the qinzhou bay in 2008 and bathymetric surveyed in 1994,2004 and 2009.The characteristic hydrology and sediment in the key harbor is studied using the combine method of hydrology, sediment movement dynamics and the mathematical statistic. And the sediment characteristic, distribution pattern and its mechanism, and the transportation tendency are analyzed by the means of sedimentology, dynamic topography and the estuarine dynamic. What's more, the quantitative evaluation of the sea bed terrain evolution is carried on with method of digital elevation model that is DEM, and the deposition and erosion pattern of the key harbor is discussed.The main conclusions of this thesis are as follows:The tide and the tidal current in the Guangxi coast are controlled by the tidal wave from the Pacific Ocean, which belong to standing wave. The tide type in key harbor is the typical diurnal tide. The tide intensity is medium strong, but the tidal current power is relatively weak, the velocity surveyed ranges from 0.11m/s to 0.83m/s, and the ebb current is stronger than the flood current, additionally the current direction is along the deep trench or is parallel with the isobaths.The residual current outside of bay is stronger than in the bay, and the surface stronger than bottom, the velocity of residual current is below 10cm/s, and the fastest velocity is 26.9cm/s, additionally the residual current in the bottom of outside bay flows primarily toward the west. The suspended sediment content in the Guangxi key harbor is low, the mean value is about from 0.0163 kg/m3 to 0.0421 kg/m3, and its change along with the tide is not big, which belongs to the low sediment sea area. Generally speaking, the suspended sediment content in the outside sea of Qinzhou bay is biggest, and decreases toward east and west. The salinity is high in the key harbor, the mean value is 30.08%o in the surveyed sea, and it presents the tendency that the salinity in the west area is higher than in the east and the south higher than north. In a word; the hydrology and sediment is the characteristic of the strong tide and weak current and the high salinity and low sediment content.Based on the measured data of surface sediment samples in 2008, combined with sediment texture, topography and hydrodynamics, the study reveals that the sedimentary environments and their hydrodynamic processes. The results indicate that the transversal distribution of surface sediments in the outer Bay of Qinzhou presented from coarse to fine particle sizes, the degree of sorting from good to poor level. For the longitudinal distribution of surface sediments, there is a large percentage of the sand at the underwater slope, which is shallower than 5m, however, slit and clay predominate at the underwater slope, which is deeper than 5m. According to the differences in the hydrodynamics, morphologic features and sediment sources, the outer Bay of Qinzhou can be divided into three main sedimentary zones.The results show that it is a period of accumulation for the whole Qinzhou Bay from 2004 to 2009. Tidal channels are the scour center area, and others are depositing area during the past 5 years, beside it:depositing rate is about 0.16m/a, which is more and more heavery from coast to sea. Because sediment is not large in Qinzhou bay, silt content in the sea water is slight, and channel axial line is basically coinsident with the direction of flood current, the siltation after excavation of the channels will not be heavy. Furthermore, through the relationship between current velocity of the bay cervical and tidal Prism in inner bay, the critical condition of stability is presented. Inner bay at least maintained at 100km2, if less than this number, tidal channels will be shallow and instable, and this critical conditions is an important reference.