潮致余流及潮汐应变驱动的余流在河口水沙输运中的贡献机制

Over many tidal estuaries, the residual sediment transport could play an important role, although the mainly hydrodynamics is tidal current. With regard to the study of driven factors of residual flow on the estuarine sediment transport, traditional factors such as water density gradient, river discharge, wind shear stress were considered by many researchers. However, there are two components of residual flow, which have significant impact on sediment transport, still lack of attention. The first part of residual flow is induced by tidal rectification, i.e., the nonlinear advection terms of tidal momentum, the other component of residual flow is induced by tidal straining. The mainly aim of this study is to make clear the current structure of residual flow induced by tidal rectification and tidal straining and to examine their contribution on estuarine sediment transport. To achieve this, three transverse sections are chosen as our study area, which located in North Branch, North Channel and South Channel of Yangtze estuary, respectively. The Spatial and temporal variation of water vertical eddy viscosity will be computed according to the field observation and numerical modeling results. Following that, an analytical model will be developed to solve the current structure of residual flow induced by tidal rectification and tidal straining. At last, the contribution mechanism of these two residual flow components on estuarine sediment will be discussed according to the analytical model as well as observation data and numerical model.

尽管大多数潮汐河口的水动力以潮流为主，但从悬沙输运的角度，余流输沙不可忽视。在潮汐河口的余流结构及其驱动的悬沙输运的研究中，传统上主要考虑了密度梯度、径流、风应力等余流驱动因子，但针对非线性的潮动量对流项以及潮汐应变项驱动的余流结构及其输沙模式方面的研究仍然比较欠缺，值得进一步深入探讨。本项目拟选取长江口为研究区域，分别在北支、北港和南港上各选取一横断面作为研究对象进行对比研究。首先在现场观察、数值模型等手段的支持下计算各断面上垂向涡动粘滞系数的时、空变化特征，进而构建解析模型，从理论上求解潮动量非线性对流项和潮汐应变项驱动的余流在河口横断面上的结构特征，并计算各部分水动力因子的悬沙输运模式，最后结合实测资料和数值模拟结果讨论潮致余流和潮汐应变驱动的余流在潮汐河口悬沙输运及捕集系统中的贡献机制。

本项目中，通过构建潮汐河口横断面水沙输运解析模型，模拟分析河口横断面上的水沙及捕集模式，其中项目中重点考虑了潮动量非线性项驱动的潮致余流和潮汐应变项驱动的余流结构及其在输沙体系中的贡献。为探讨河床地貌结构对悬沙输运的影响，研究中参考其他研究成果选取了多种代表性河口形态，分别采用解析模型对其悬沙侧向输运模式进行模拟分析。研究结果表明，悬沙在各种河口断面上的分布模式有显著差异，主要体现在最大含沙量值及分布区域，以及悬沙空间结构均匀程度上，这种悬沙空间分布差异主要是潮致余流引起的。河口地貌结构差异会显著影响水平密度梯度及侧向环流结构，从而改变余流输沙结构，而河口水下地形坡度的陡峻程度与对流输沙有显著关系。另外，项目研究中还利用该解析模型讨论了长江口南港的水动力结构及悬沙侧向输运特征。南港悬沙分布上北侧副槽远大于南侧主槽，潮流输沙和余流输沙是南港侧向输沙函数的两个主要部分。南港中强劲径流削弱了涨潮流，增强了落潮流，使得向河槽南侧的涨潮流输沙小于向河槽北侧的落潮流输沙，导致潮流输沙指向河槽北侧。径流驱动的南港侧向余流在涨潮流方向上为一逆时针环流结构，余流输沙指向河槽北侧，因此南港悬沙捕集于河槽北侧。对比强径流的长江口南港和强水平密度梯度的James河口输沙模式发现，虽然两河口中潮差强度均属中等，但由于两者的余流主控因子不一样，导致悬沙分布模式有显著差异。潮汐应变驱动的余流结构取决于河口的垂向涡动粘滞系数与半日潮之间的相位差，大多数河口中（包括长江口），涨潮水体趋近于混合和落潮水体趋近于分潮，因此潮汐应变驱动的余流与重力环流类似，其强度与垂向涡动粘滞系数的波动振幅相关，一般说来与重力环流的强度相当，因此在输沙效应上，潮汐应变会加强重力环流输沙。在该基金项目的资助下，本研究共发表了学术论文15篇，其中SCI论文4篇，参加学术会议5次，培养硕士研究生4名。