长江河口河槽沉积物捕集对河口大型工程的响应机制

For the past decades,significant changes in land boundary condition, water motion of channel occurred in response to large engineering projects as large area of land reclamation, deep waterway and freshwater reservior in the Yangtze estuary.They had led to the adjustments of sediment transport and entrapment, deposition and erosion of estuarine channels. And new change in morphology pattern composed of three bifurcations and four outlets to the sea by nature for the last 2000 years occurred. So, it is necessary to study the sediment transport and entrapment pattern of estuarine channels in response to the strong human interventions in the estuary. We propose to investigate a major problem occurring in the estuarine channels of Yangtze Estuary with the new change in development of erosion and deposition in waterways after engineering works. Specific objectives are: 1. to identify changes in hydrodynamics, suspended sediment concentration, grain size composition of the suspended sediment and bed sediment, and configurations of planar distribution, along-channel profiles and cross-channel profiles of the estuarine channels, with human interventions in the estuary; 2. to investigate the role of time-varying mixing conditions, wind, friction, nonlinear shallow water, spatial settling lag on along- and across-channel sediment entrapment with their subaqueous delta; 3. to identify and quantify the mechanisms responsible for the differences of the sediment trapping in the North Channel of the Yangtze estuary.

近年来，长江河口河槽动力沉积地貌发生显著变化，使得近两千年来自然作用形成的三级分汊四口入海格局出现了显著变异，迄今研究主要集中于其与流域大坝导致来沙减少的统计关联，但有关其与流域大坝及河口围海造地、深水航道、青草沙水库等大型工程的关联机制研究尚未见报道。本项目采用历史资料对比、现场观测、基于过程的解析模型与数值模型相结合的模拟分析等方法，分析河口大型工程导致的典型河槽横向与纵向水动力、悬沙和底沙分布、床面微地貌、纵剖面与横剖面及平面形态等变异特征，重点分析北港河槽径流、潮流、浅水分潮、风应力、密度梯度和沉降延迟对河槽纵横向沉积物输运与捕集的影响机制，探讨新的流域来沙条件下北港河槽纵横向沉积物捕集对青草沙水库和深水航道工程的叠加响应机制，以此增进对流域-河口-海洋系统内自然过程与人类活动相互作用机理的了解。

2015年1月至2017年8月，利用ADCP、多波束、浅地层和三维激光扫描仪等多模态传感器系统，对长江河口自口外-45m水深至北港、南槽、北槽、南港、南支河段乃至潮区界大通河段河槽动力、沉积、地貌进行了8个航次的定点和走航测量，开展了的同步走航测量和部分定点测量，并对重点岸段进行了三维激光扫描仪和RTK的准同步测量。而且，在项目执行期间，受项目负责人执行的其它项目野外测量启发，将原计划任务书中的长江入海河口拓展至长江中下游支流入干流河口，具体从松滋口以下的各个河口。.获得以下主要创新成果：(1) 构建潮汐河口侧向水沙输运解析模型，模拟分析河口地貌结构对水沙输运及捕集模式的影响，并应用到长江入海河口河槽横断面时发现河槽水动力受控于半日潮流，余流主要由陆相径流及本地非线性对流驱动，悬沙分布上北侧副槽远大于南侧主槽，水沙分布计算结果与实测结果在结构上基本一致。在各输沙因子的综合作用下，大量悬沙捕集于河槽北侧，使得河槽北部底层潮平均含沙量值达到最大值。(2) 三峡截流以来长江潮区界显著上移，洪季上移82km，枯季上移220km，潮区界变动河段冲刷地貌发育显著。(3) 长江三峡大坝至入海河口外-40m水深河槽侵蚀型沙波和巨型沙波的形态特征与发育机制。(4) 利用多波束测深技术评估长江河口桥墩冲刷状态。(5)长江大通以下干流河槽水下地形演变的人为驱动效应。(6)长江河口河槽冲淤对相对海平面变化的贡献。(7)弗劳德数在长江河口低流速区床面形态判别中的应用。共发表论文42篇，其中SCI论文10篇，专著1部，远超预期目标和任务。