| The Bohai Sea is a shallow semi-enclosed inner shelf sea in China, which receives many terrestrial materials every year. Due to seasonal changes in wind strength, hydrodynamics are weak in the summer season and strong in the winter season, as characteristic for most of the Bohai Sea. The seabed surface sediment can be resuspended easily and transported by storm waves. So the winter season is a major period for sediment transport in the Bohai Sea. Hydrographic and suspended sediment data were collected in winter 2006 and summer 2007 along three survey transects at three time-series, ship-based stations off the Huanghe (Yellow River) delta, and at twenty-four grid survey stations in the adjacent Bohai Sea. Based on the verification of these data, using newer bathymetric and coastline data of 36 survey transects along the Huanghe delta in 2004 and the Bohai Bay in 2000, a 3-D high resolution (horizontal resolution of 1'×1') hydrodynamic and sediment transport numerical model, coupled with shallow sea wave model (SWAN), was used to study the seasonal variation characteristic of sediment transport in the whole Bohai Sea. Especially, the coastal areas were considered to be very important sediment sources in the winter season. The paleotopography of the Bohai Sea during the period of the maximum transgression (7 ka B.P.) was grossly recovered, and then the numerical simulation method was first applied to study the sedimentation of the ancient Bohai Sea and the ancient Huanghe delta.The major contents in this dissertation are shown as following:1. Based on the analysis of in-situ data, the results show that the sediment resuspension and its effects are the most important dynamic sediment processes in winter. Without considering the effect of sediment resuspension in winter, it would be hard to discern the sediment transport process. A zone of high SSC (>100 mg /l) with a width of about 35 km was formed along the delta coast. A much larger area of high SSC and higher SSC occurred in the bottom layer, indicating resuspension origins. The coincident locations of the three highest SSC centers around the abandoned Diaokou river mouth, present river mouth and the abandoned Qingshuigou river mouth, coupled with the distribution of three centers of high bottom shear stress zone caused by wave effects, offer convincing evidence that wave-induced resuspension caused the high SSC that we observed in the study area. The protuberant topography of all three areas and the strong NNE-NE-ENE wave direction in winter led to high bottom shear stress around the three river mouths, resulting in the creation of the three highest SSC centers, and they were the major sediment source areas in winter. The SSC in winter was much higher in the entire study area than that in summer except for the area around the present river mouth. The SSF at all the time-series stations were 2-122.5 times higher in winter than in summer except one station near to the present river mouth. This indicates that the intensity of sediment transport in winter is much stronger than in summer.2. The calculation of bottom shear stress under co-existing wave currents was modified in our model, and the bottom shear stress caused by currents was recovered after modification. According to the results of the seasonal variation characteristic of sediment transport, there were some obvious differences of the distribution of bottom shear stress and SSC in the Bohai Sea between winter and summer, especially in the coast area. As a result of strong waves in winter, a zone of high values was mainly alone the coast with water depth less than 15 meters and the highest centers lied in depths of approximately 4 to 10 meters in the winter season. The regions with highest values were mainly along the coast of the Huanghe delta and the Liaodong Bay. The values of bottom shear stress and SSC in those regions were about 10 times higher in winter than in summer. During the winter storm events, the values were further increased to more than 2 times higher than average conditions in winter. Most previous studies have no considering the main sources of sediment resuspension and no verification of measured data in winter, but in this paper the model can give a reasonable agreement between simulated results (velocity and suspended sediment concentration) and measured data.3. High resolution grids and newer topography were used in this study. According to the result of simulation, the shear front outside the abandoned Qingshuigou river mouth in the Laizhou Bay was found which is consistent with the measured data. The forming mechanism of this shear front was caused not only by a tidal phase gradient along the delta slope, but also by the topography of the protrusion of the abandoned Qingshuigou river mouth. This shear front played an important role in obstructing the sediment dispersion from Huanghe into the Laizhou Bay.4. Because of the greater water depth, the wider coastline and the weaker tidal waves from the Huanghai Sea, the tidal currents of the ancient Bohai Sea during the period of the maximum transgression (7 ka B.P.) were much weaker than the present Bohai Sea, especially around the west area of the ancient Bohai Sea. Therefore, under this weak tidal hydrodynamic environment, the sediment of the ancient Huanghe discharge into the sea deposited more easily and accumulated more rapidly around the river mouth, and it was beneficial to form the Huanghe delta. In addition, there was less sediment being transported from the Bohai Sea to other seas in China. The hydrodynamic in the Liaodong Bay is stronger than other places, which is the important reason for the surface sediment of the seabed with coarser particles than other places.
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