Past research on fluvial dynamics at the confluence of two alluvial rivers has mainly focused on downstream flow structure and bed scoring, often using laboratory experiments and numerical modeling. Little is investigated about yearly and episodic dynamics of confluence mouth bars that can affect downstream morphology using field measurements. In this study, we analyzed the migration of a confluence mouth bar of two free meandering alluvial rivers, the Amite and Comite Rivers in coastal Louisiana, USA from 2002 to 2017. Remote sensing images were utilized to investigate the decade‐long morphologic changes. To assess episodic dynamics, we employed terrestrial laser scanning measurements to acquire high‐accuracy digital elevation models at the confluence before and after three floods in 2017. Our study found that the Amite‐Comite confluence mouth bar migrated downstream 55 m in the past 15 years, and its angle reduced by 55° from 100° to 45°. The fast migration was a result of sediment deposition and channel deformation around the confluence mainly during the years when the tributary‐to‐main channel discharge was lower (<0.25). The study further reveals that a single moderate flood could strongly affect the mouth bar, as shown by an increase of the projected surface area by 114% and an increase of volume of the confluence mouth bar by 68%.

中文翻译：

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美国路易斯安那州两条冲积河流汇合条的形态和迁移的年代际变化和间歇变化

过去对两条冲积河流汇合处的河流动力学的研究主要集中在下游流量结构和河床计分上，通常使用实验室实验和数值模型。很少有关于汇合口条的年度和偶发动态的调查，这些动态影响可以通过现场测量来影响下游形态。在这项研究中，我们分析了2002年至2017年美国路易斯安那州沿海的两条自由蜿蜒冲积河-阿米特河和科米特河-的汇合口条的迁移情况。利用遥感图像调查了长达十年的形态变化。为了评估事件的动力学过程，我们采用了地面激光扫描测量方法来获取2017年三场洪水前后汇合处的高精度数字高程模型。我们的研究发现，过去15年中，Amite-Comite汇合口栏向下游迁移了55 m，其角度从100°减少到45°减少了55°。快速迁移主要是在支流至主河道流量较低（<0.25）的年份，汇流区周围沉积物沉积和河道变形所致。该研究进一步揭示，一次中等程度的洪水会严重影响口岸，如预计的表面积增加114％和汇合口岸的体积增加68％所示。