长江口南北槽分流口洪季水沙变化过程研究

Changes in water and sediment of the bifurcation mouth between the North and South Passage during the flood season,Yangtze Estuary

河口分流口的水沙变化过程是影响河口三角洲发育的核心环节,对下游河势的稳定起着关键性的作用。本文通过对北槽二、三期工程前后的南北槽分流口河段洪季大潮期间的同步水沙观测数据进行分析,以探讨长江口深水航道工程整治对分流口水沙过程的影响。结果表明:(1)北槽二期工程到三期工程后,分流口洪季以落潮优势流、优势沙为主的格局基本没有发生改变,但南槽优势流、优势沙出现略有变大,而北槽略有变小现象;(2)分流口洪季的水体输移主要受控于欧拉余流的变化,除北槽入口段水体净输移量一直较小外,其他河段在二、三期工程实施期间均有大幅提升,其中斯托克斯余流变化不大,拉格朗日余流与欧拉余流变化相一致,南槽呈波动状上升,北槽先增后减;(3)在二期到三期工程期间,北槽分流比明显减小,入口段落潮流速减小,含沙量较高,水体输移量降低,输沙强度减弱,由此导致二期工程以来北槽入口河段淤积强度加重。

The water and sediment change of the bifurcation mouth is one of the key parts to influence the estuarine delta formation,which is a determinative controlled factor on the stability of the downstream river situations.Here,based on the flood season water and sediment data observed synchronously in the bifurcation mouth between North and South Passage（BMNSP）during the operations of the second and third phases of the Deep Water Channel Project（DWCP）in the Yangtze Estuary,impacts of DWCP on hydrodynamics and sediment dynamics of the BMNSP were discussed.The results can be summarized as follows：（1）There had been almost no changes for the situation of both the ebb flow and ebb sediment dominance located at the BMNSP,even though the operations of DWCP had experienced time span from the second to the post-third phase.However,the coefficients of both the ebb flow and ebb sediment dominance of the upstream mouth in the South Passage showed an obvious change from low values in the second DWCP to high values in the third DWCP during the flood season.The corresponding coefficients of both the ebb flow and ebb sediment dominance in the North Passage showed inverse changes comparing with those in the South Passage.（2）The Euler residual flow in the flood season could directly determine the flow transport of the BMNSP.During the second and third phase of the DWCP,average transport discharge of the BMNSP was enhanced except the upstream of the North Passage where the average transport discharge still remained low.The change of the Lagrange residual flow of the BMNSP was similar to that of the Euler residual flow.However,the Stokes residual flow of the BMNSP has remained minor changes.（3）Due to the decreased ebb current velocities,increased suspended sediment concentrations,decreased flow transportation and weaken sediment transport rate of the upstream mouth of the North Passage from the second to the post-third DWCP,it could be the main reasons to induce the aggravation of accretions in the upstream mouth of the North Passage.