River confluences with a compound-channel tributary are common in large river networks, for example, the Yangtze River basin and the Amazon basin. They affect the confluence hydrodynamics, nutrient depletion and fish migration in the network, due to the seasonal exposure and submergence of the tributary floodplain. The corresponding influencing mechanisms are critical but poorly understood. This study investigated the hydrodynamic response of channel flow confluence to the tributary floodplain, that is, various widths and heights of the floodplain. Two kinds of secondary circulations were identified: One was due to the tributary streamline curvature, and the other produced by flow separation in the floodplain step. An increase in the floodplain size enhanced the strength of the secondary circulation in the compound tributary channel, but it disappeared near the junction because of the effects of the main channel. The decreased tributary cross-sectional area amplified the flow momentum, resulting in a larger separation zone near the tributary-side wall. The strength of floodplain-induced secondary circulation increased as the floodplain width increased, and it moved toward the tributary-side bank destroying the separation zone. Moreover, strong upwelling in the secondary circulation caused the rising water surface in the separation zone, which was supposed to be a region of falling water surface caused by energy loss and negative pressure. A conceptual model was proposed to summarize the hydrodynamics of confluence with different tributary floodplain topography. Our results provided a comprehensive knowledge of the confluence hydrodynamics, which have important implication for the flood management and ecological restoration of river basin.

中文翻译：

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河道水流汇合对支流漫滩地形的水动力响应


河流与复合河道支流的汇合在大型河流网络中很常见，例如长江流域和亚马逊流域。由于支流漫滩的季节性暴露和淹没，它们影响网络中的汇合水动力、养分消耗和鱼类洄游。相应的影响机制至关重要但人们却知之甚少。本研究研究了河道水流汇合到支流漫滩（即不同宽度和高度的漫滩）的水动力响应。确定了两种二次环流：一种是由于支流流线曲率引起的，另一种是由洪泛区台阶中的水流分离产生的。河漫滩面积的增大增强了复合支流河道内二次环流的强度，但在交汇处附近由于主河道的影响而消失。支流横截面积的减小放大了流动动量，导致支流侧壁附近有更大的分离区。漫滩引发的二次环流强度随着漫滩宽度的增加而增强，并向支流侧岸移动，破坏了分离带。此外，二次循环中的强烈上升流导致分离区水面上升，而该区域本应是由于能量损失和负压而导致水面下降的区域。提出了一个概念模型来总结不同支流漫滩地形汇流的水动力。我们的研究结果提供了对汇流水动力学的全面了解，这对于流域的洪水管理和生态恢复具有重要意义。