| The large radial sand ridges (RSR) in the southern Yellow Sea are famous for their unique subaqueous landscape with huge scale and complex topography. The RSR is the remarkable research area for a long time due to the complicated hydrodynamics, strong human activities here. This thesis sets up a hydrodynamic model using Delft3D with3kinds of grids to simulate the water level and current field. Data collected from40mooring stations in RSR is used for the model validation, and then the characteristics of tide and tide current are analyzed. Another model with high space resolution concentrated on the sediment transport and seabed topography changes. Finally, two ideal models are designed in order to reproduce the RSR evolution and revealthe factors on the formation of RSR. This research is of great significance for the exploitation of local resources.The result of model with a large domain, which includes the southern Yellow Sea and the north part of East China Sea, is almost in accordance with all the field observation data on velocity and water level. Difference between model result and field observation in couple of stations may caused by the utility of relative old data of topography, which was mainly collected in1979. The result of model shows a divergent and convergent flow field during the ebb and flood in RSR, respectively. The southern Yellow Sea and northern East China Sea are dominated by semidiurnal tides, i.e. the tidal constituent M2and S2, which, respectively, has maximum amplitude of2.5m and0.95m. High tidal amplitudes of M2and S2distribute in Hangzhou Bay, Jianggang and Haizhou Bay. The second dominating constituents are K1and O1. The progressive East Sea tidal wave and the rotating southern Yellow Sea tidal wave converge around the Jianggangsea, resulting in a large tide range and strong current along the Jianggang coast. Amphidromic points of M2and S2are found at the offshore area of the abandoned Yellow River mouth, which is same as the previous study. In the northern waters outside the coast of Jiangsu with a distance110-185km, there are east-southward Lagrangian residual currents, with magnitude of0.18-0.4m/s. The Eulerian residual currents are almost the same pattern with the Lagrangian residual currents in both direction and magnitude. The Stokes residual currents are distinctive in the shallow waters near the coast. The Stokes residual currents are about0.05m/s in the waters near shore, expect waters in Haizhou Bay, where the residual current is smaller than0.02m/s. Especially, the Stokes residual currents are landward, with magnitude bigger than0.05m/s, in Hangzhou Bay and Changjiang Estuary.Extracting the open boundary conditions from the big domain model, another high resolution model is established to simulate the suspended sediment transport and RSR evolution. The model result on the suspended sediment concentration (SSC) dovetails well with the field observation. Magnitude of SSC in RSR is in positive correlation with tide range. The area with high SSC (depth-average value>1.0kg/m3) in spring tides is much bigger than that in neap tides. There is distinct difference between the distribution of SSC in the northern and southern area. In the northern waters, especially Xiyang, Tiaozini and East Dongsha, the depth-averaged SSC can reach more than1.5kg/m3; while in the southern waters, SSC is always lower than0.8kg/m3.The result of model shows that, suspended sediment transport is east-southward in the northern tidal channels of RSR, with a net transport rate of0.4kg/(m·s). High SSC mainly occurs in5tidal channels:Xiyang, Chenjiawucao, Micaoshuyang, Huanghayang, nearest waters of Lengjiasha, among which Xiyang has a highest transport rate of2kg/(m·s). Net sediment transportation in Xiyang and ChenjiaWucao is seaward, while Huangshayang landward, MicaoshuYang landward at north side and seaward at south side. By comparing the sediment transportation among spring, middle and neap tides in the research waters, it is easy to find that the change of topography is dominated by in RSR is dominated by the transportation during spring tides, as net transportation rate of sediment during the spring tides is more than twice the net rate during middle tides, and4times the net rate during neap tides. Sediment transportation in spring, middle and neap tides are significantly different from each other. Take Xiyang for example, sediment transportation is seaward with magnitude of2kg/(m·s) in spring tides, landward with magnitude of0.5kg/(m-s) in neap tides, landward in the southern waters and sea-forward in the northern waters with magnitude of1.0kg/(m-s) in middles tides.Result of model based on the1979topography shows that the main tide channels are in erosion, among which Xiyang and Chenjiawucao are in most serious condition but Huangshayang and Lanshayang are relatively stable. Deep channels surround Tiaozini are also in erosion, being deeper and deeper. Dongsha, on the whole, is prograding, though the east part of Dongsha retreats westward due to the expansion of Chenjiawucao. As the result of extension of Chenjiawucao, the junction between Maozhusha and Waimaozhusha is being cut off. The southern channels are relative stable, except that some small channels developed on the tide flat between Jianggang and Lvsi.A model is designed to simulate the long-term evolution of the topography, assuming that the topography in RSR is in uniform slope. Its result shows that the radial currents in RSR do not depend on the radial topography. Sand ridges along the Jianggang coast, and then the northward sand ridges grow in the northern waters, eastward sand ridges grow in the southern waters. However, the farthest distance sand ridges can reach is smaller than90km from the coast and the topography change little in waters with depth bigger than20m in initial topography. Another ideal model is designed, assuming that there exists a huge sand ridge at the Jianggang offshore sea. Its result also shows that topography with depth deeper than20m in initial topography is still in little change. These two numerical experiments show that the radial currents in Jiangsu coastal zone exist independently and the radial currents are of great significance in the formation of RSR. These two experiments also show that the formation of RSR from null to stable condition needs about200years and that the existence of initial sand ridges will affect the formation time and distribution of sand ridges. For instance, assuming in an initial topography with uniform change but without sand ridges, the radical currents cannot mod out sand ridges in similar distribution of modern RSR. Especially, it is hard to change the offshore sand ridges90km away from the land... |
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