【作者】 杜家笔；

【导师】 汪亚平；

【作者基本信息】 南京大学 ， 海洋地质学， 2012， 硕士

【摘要】 南黄海辐射沙脊群以其形态特殊、地形复杂、沙脊规模巨大而著称,因其地貌演变迅速,水动力条件复杂,且位于陆、海和人类活动相互作用最强烈的海岸带,辐射沙脊群一直是海洋学家重点关注的研究对象。本文基于Delft3D模型,采用三种空间尺度的网格,建立了辐射沙脊群海域的水动力数值模型,并利用40个测站数据进行验证,进而分析了研究区的潮汐、潮流特征。在此基础上,建立了沉积物输运和长周期地貌演化模型,模拟辐射沙脊群海域冲淤变化；设计了两个理想实验,试图探讨辐射沙脊的演变过程及其影响因素。这些研究成果对该地区的自然资源开发利用有指导意义和参考价值。研究表明,包括南黄海、局部东海海域的大区域模型的计算结果与2005-2011年实测水位、流速结果吻合良好。少数测站的流向与模拟结果有一定误差,可能与模型使用的地形数据(地形数据的获取时间为1979年,相对偏旧)有关。模拟结果显示,在涨潮和落潮时刻,辐射沙脊群海域流场分别呈辐聚和辐散状；南黄海、东海北部主要受半日分潮M2、S2分潮控制,这两个分潮的最大振幅分别可达2.50m和0.95m,振幅高值主要分布在杭州湾、弶港和海州湾,其次为全日分潮K1、01。东海前进潮波和南黄海旋转潮波共同作用,在弶港汇合,造成弶港海域潮差大、潮流强的特征。M2、S2分潮在废黄河口外海出现无潮点,无潮点位置与前人的研究结果基本一致。在江苏岸外110-185km处、辐射沙脊群东部外缘,出现一股大小为0.18-0.4m/s(东南向)的拉格朗日余流。欧拉余流场的空间分布趋势及数值大小与拉格朗日余流场基本一致。斯托克斯效应在近岸海域较为显著,除海州湾区域小于0.02m/s外,其他近岸海域平均为0.05m/s,杭州湾和长江口海域斯托克斯漂流方向均向陆,数值一般大于0.50rn/s。以大区域模型的结果为边界条件,使用高空间分辨率网格的小尺度模型模拟了辐射沙脊群海域的悬沙分布、悬沙输运及地形演化过程。其中,悬沙浓度模拟结果与实测数据大致吻合；辐射沙脊群海域悬沙浓度数值大小与潮差成正比,大潮期间垂线平均悬沙浓度高值(垂线平均>1kg/m3)分布范围远大于小潮期间的高值分布范围。整个辐射沙脊群海域以弶港为分界线,南、北两侧悬沙浓度分布格局差异显著。北部海域,尤其是西洋、条子泥和东沙东部海域,垂线平均悬沙浓度可达1.5kg/m3以上；南部海域,悬沙浓度整体较低,一般在0.8kg/m3以下。模拟结果表明,辐射沙脊群北部海域悬沙向东南方向输运,单宽净输运率约为0.4kg/(m·s),至辐射沙脊群北部边缘处转为东南方向。悬沙输运率高值区分布在西洋、陈家坞槽、草米树洋、黄沙洋、冷家沙外围等五个海域,其中以西洋最为显著,悬沙净输运率高达2kg/(m·s)。陈家坞槽和西洋水道内的悬沙均向海输运,黄沙洋水道则向陆输运,草米树洋水道北侧向岸、南侧向海。对比大、中、小潮辐射沙脊群海域的悬沙输运格局,发现研究区地形演变主要受控于大潮期间的悬沙输运格局,大潮期间的悬沙净输运率数值是中潮期间的2倍以上,小潮的四倍以上。大中小潮的悬沙输运格局有显著差异,以西洋水道为例：西洋水道大潮期间悬沙向海输运,净输运率平均可达2kg/(m·s),小潮期间,悬沙向陆输运,净输运率一般小于0.5kg/(m·s),中潮期间北部向海,南部向陆,净输运率平均为1.0kg/(m·s)。基于1979年海底地形下的数值模拟结果显示,辐射沙脊群海域主要水道以侵蚀过程为主,其中以西洋和陈家坞槽最为显著,黄沙洋、烂沙洋等南部海域的水道相对稳定。条子泥周围水道不断被加深,沙脊不断淤高。东沙东部受陈家坞槽扩张影响向西后退,整体仍处于淤长状态。毛竹沙与竹根沙的连接处因为陈家坞槽的延伸而逐渐被切断,毛竹沙、外毛竹沙与竹根沙有分离的趋势。南部海域相对较稳定,在弶港——吕泗岸段的潮滩出现规模较小的水道,并逐渐扩张加深。假定海底地形由陆向海均匀变化,建立了一个长周期地貌演化模型。模拟结果表明,江苏近岸海域以弶港为顶点的辐射状潮流场不依赖于原始海底地形。潮流脊首先在弶港海域发育,弶港以北发育正北、北偏东方向的潮流脊,南部发育正东、东偏南方向的潮流脊,整体呈辐射状。但潮流脊向海延伸不超过90km,其演化基本局限于原20m等深线以浅海域。假定海底海底地形为均匀斜坡、并在弶港东部海域由陆向海有一较大沙脊存在,建立了另外一个长周期地貌演化模型。结果表明,水动力对20m以深海域海底地形的影响依然十分微弱,中央沙脊20m以下的砂体基本维持原状。上述两个理想实验的结果都显示,江苏近岸辐射状沙脊的形成与海底原始地形无关,潮流在研究区辐聚、辐散的空间分布格局是辐射状沙脊群形成的最主要的动力因素。辐射状沙脊体系从无到有,并维持稳定状态,大约需要200年时间。此外,理想实验还证实了,原始海底有、无沙脊,对辐射状沙脊形成的时间尺度和分布格局有显著影响。例如,均匀斜坡地形、海底无沙脊的条件下,辐射状的潮流场也无法塑造出目前的沙脊分布形态,尤其是无法形成现今离岸约90km以外沙脊的分布形态。更多还原

【Abstract】 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更多还原