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Water Resources Research
水资源研究

RESEARCH ARTICLE
10.1029/2022WR032195
研究文章 10.1029/2022WR032195

Key Points: 要点:
  • Four field surveys on the mixing process downstream of a large river confluence were conducted
    在大河汇合处下游的混合过程进行了四次野外调查
  • Different mixing patterns were distinguished based on conductivity and hydro-acoustic measurements
    根据电导率和水声测量结果区分了不同的混合模式
  • Mixing dynamics around the confluence were controlled by the momentum flux ratio, secondary flow and the lock-exchange
    汇流区域的混合动力学受动量通量比、次生流和锁交换的控制

Correspondence to: 通讯地址:

S. Yuan, 袁山逸,

Citation: 引用:

Xu, L., Yuan, S., Tang, H., Qiu, J., Xiao, Y., Whittaker, C., & Gualtieri, C. (2022). Mixing dynamics at the large confluence between the Yangtze River and Poyang Lake. Water Resources Research, 58, e2022WR032195. https:// doi.org/10.1029/2022WR032195
徐磊,袁帅,唐辉,邱健,肖阳,惠特克,瓜尔蒂埃里(2022)。长江与鄱阳湖之间大型汇流处的混合动力学。水资源研究,58,e2022WR032195。https://doi.org/10.1029/2022WR032195
Received 14 FEB 2022
接收日期:2022 年 2 月 14 日
Accepted 17 OCT 2022
2022 年 10 月 17 日接受

Mixing Dynamics at the Large Confluence Between the Yangtze River and Poyang Lake
长江与鄱阳湖之间的大汇流处的混合动力学

Lei Xu (D), Saiyu Yuan (D), Hongwu Tang , Jiajian Qiu , Yang Xiao (D), Colin Whittaker (D), and
许磊 (D),袁赛宇 (D),唐宏武 ,邱佳健 ,肖扬 (D),科林·惠特克 (D),和
Carlo Gualtieri (i)
Carlo Gualtieri (i)
State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing, China,
河南省水文水资源与水利工程科学国家重点实验室,河海大学,中国南京,
Yangtze Institute for Conservation and Development, Nanjing, China, Department of Civil and Environmental Engineering,
长江保护与发展研究所,中国南京, 土木与环境工程学院,
The University of Auckland, Auckland, New Zealand, Department of Structures for Engineering and Architecture, University
奥克兰大学, 奥克兰, 新西兰, 工程与建筑结构系
of Napoli Federico II, Napoli, Italy
那不勒斯费德里科二世大学, 那不勒斯, 意大利

Abstract 摘要

Mixing processes downstream of river confluences impacts the ecology and the related environmental management of river networks. A clear understanding of such processes is challenging, especially for confluences having width-to-depth ratios larger than 100, due to the limited available field data. In this study, four field surveys based on hydro-acoustic and conductivity measurements were conducted near the confluence between the Yangtze River and the Poyang Lake, which are the largest river and freshwater lake in China, respectively. It was found that mixing dynamics at the confluence were controlled by a complex interaction among the momentum flux ratio, secondary flow and the lock-exchange flow associated to the density contrast between the two tributaries. Slow mixing was observed during high-flow conditions that generated dual counter-rotating secondary cells, with the downwelling flow acting as a barrier in the post-confluence channel. In contrast, more rapid mixing was observed during low-flow conditions when only a single channel-scale secondary flow was identified. The mixing processes were also affected by the lock-exchange flow associated to the density difference between the two confluent flows. Such lock-exchange enhanced mixing when the Yangtze River waters had higher temperature, that is, lower density than that of the Poyang Lake. In low flow condition, the penetration of the much larger momentum flux of Yangtze River created a "two-layers" structure with the contribution of the density difference, which further enhanced the curvature-induced helicity. The findings from the present study improve our current understanding of mixing dynamics in large river confluences.
河流汇合点下游的混合过程会影响河流网络的生态和相关环境管理。由于可用的野外数据有限,特别是对于宽深比大于100的汇合点的理解是具有挑战性的。本研究利用基于水声和电导率测量的四次野外调查,在长江和鄱阳湖的汇合处进行了研究,它们分别是中国最大的河流和淡水湖。研究发现,汇合点的混合动力学受动量通量比、次生流和由两个支流之间的密度差异引起的锁定交换流的复杂相互作用控制。高流量条件下观察到缓慢混合,生成了双对流旋转的次生细胞,下沉流充当了汇合后河道的障碍。相反,在低流量条件下观察到更快速的混合,只识别出一个通道尺度的次生流。 混合过程也受到两个汇合流之间密度差引起的锁交换流的影响。当长江水温较高、密度较低时,这种锁交换加强了混合作用。在低流量条件下,长江的动量通量远大于鄱阳湖,形成了"两层"结构,并通过密度差进一步增强了曲率诱导的旋度。本研究的发现提高了我们对大型河流汇合处混合动力学的理解。

Plain Language Summary The confluence between the Yangtze River and the Poyang Lake, which are the largest river and the largest freshwater lake in China, respectively, is one of the largest on the Earth. Understanding this mixing processes at such large-scale river confluence is significantly important for local flood control and aquatic ecology management, but even challenging due to the lack of detailed field data. Four field surveys were conducted to investigate hydrodynamics, water quality and mixing at the confluence of the Yangtze River and Poyang Lake. The effect of momentum and density difference between the two rivers and large-scale secondary flow on mixing processes was identified. The results of the present study indicated that such density difference can produce different patterns of vertical stratification which are related to the mixing between the Yangtze River and the Poyang Lake.
阐述语言摘要 中国最大的河流长江和最大的淡水湖鄱阳湖的汇合是地球上最大的之一。了解这种大规模河流汇合处的混合过程对于当地的防洪和水生态管理至关重要,但由于缺乏详细的现场数据而变得具有挑战性。进行了四次现场调查,以研究长江和鄱阳湖的汇合处的水动力学、水质和混合情况。确定了两条河流之间的动量和密度差异以及大尺度次生流对混合过程的影响。本研究结果表明,这种密度差异可以产生不同的垂直分层模式,与长江和鄱阳湖之间的混合有关。

1. Introduction 1. 引言

Confluences are locations where flows from different tributaries converge, resulting in mixing in the post-confluence channel. Complete transverse mixing between the two streams can occur over the mixing interface over a distance when two streams that have significantly different sediment loads, temperatures, or dissolved chemical and nutrient loads confluence (Gaudet & Roy, 1995; Lewis et al., 2020; Lewis & Rhoads, 2015). The mixing distance scales with the product of the post-confluence channel width by its aspect ratio (Rutherford, 1994). Thus, mixing at river confluences may occur over very long distances, especially for large rivers. This requirement is confirmed in many cases by aerial field measurements and satellite observations (Bouchez et al., 2010; Rathbun & Rostad, 2004; Stallard, 1987; Umar et al., 2018). However, in some other cases mixing could be strongly influenced by the complex flow structure within the confluence hydrodynamic zone (CHZ), and consequently occur over very different length scales (Kenworthy & Rhoads, 1995; Lane et al., 2008; Pouchoulin et al., 2020). This study focuses on the mixing dynamics within the CHZ of river confluences.
汇流是不同支流流量汇聚的地点,导致在汇流后的河道中混合。当具有显著不同的泥沙负荷、温度或溶解化学物质和营养负荷的两条河流汇合时,两条河流之间的完全横向混合可以在混合界面上的距离上发生(Gaudet & Roy, 1995; Lewis et al., 2020; Lewis & Rhoads, 2015)。混合距离与汇流后河道宽度乘以其纵横比的乘积成比例(Rutherford, 1994)。因此,在河流汇流处的混合可能发生在非常长的距离上,特别是对于大河而言。这一要求在许多情况下通过空中野外测量和卫星观测得到确认(Bouchez et al., 2010; Rathbun & Rostad, 2004; Stallard, 1987; Umar et al., 2018)。然而,在一些其他情况下,混合可能受到汇流水动力区域(CHZ)内复杂流动结构的强烈影响,因此可能发生在非常不同的长度尺度上(Kenworthy & Rhoads, 1995; Lane et al., 2008; Pouchoulin et al., 2020)。本研究重点研究了河流汇流处CHZ内的混合动力学。

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
本文是根据知识共享署名-非商业性-禁止演绎许可协议的开放获取文章,允许在任何媒介中使用和分发,前提是正确引用原始作品,使用非商业目的,且不得进行修改或改编。
According to the width-to-depth ratio, confluence scales could be classified as small , medium , and large scale , as suggested by Yuan et al. (2021). Research on confluence hydrodynamics at various scales is crucial for the scaling-up of river processes to the drainage network scale. The CHZ generally includes a zone of flow stagnation near the upstream junction corner, an area of flow deflection as the tributary enters the confluence, a shear layer and/or mixing interface between the two converging flows, a possible separation zone at the downstream junction corner, flow acceleration within the post-confluence channel, and flow recovery at the downstream end of the CHZ (Best, 1987; Bradbrook et al., 2000; Weber et al., 2001; Yuan, Xu, Tang, Xiao, & Gualtieri, 2022). Such flow structure, as well as sediment transport and morphodynamics, have been extensively studied (Best, 1987; Guillén-Ludeña et al., 2016, 2017; Herrero et al., 2016; Leite Ribeiro et al., 2012; Li et al., 2022; Rhoads & Kenworthy, 1995; Roy & Bergeron, 1990; Gualtieri et al., 2018; Ianniruberto et al., 2018; Sukhodolov & Sukhodolova, 2019; Yuan et al., 2016, 2018, 2021; Yuan, Zhu, et al., 2022). However, scale effects cannot be ignored when applying conceptual models developed for small-scale confluences to the larger-scale confluences that drain large basins. Compared to small-scale confluences, such large-scale confluences are likely to have a broader range of inflow conditions in terms of their water chemistry, sediment concentration, and densities (Gualtieri et al., 2019; Lane et al., 2008). Understanding the hydrodynamics and mixing processes at large-scale confluences is vital in efforts to control and regulate water with the broader goal of enhancing the ecological quality of the aquatic environment (Yuan, Xu, Tang, Xiao, & Whittaker, 2022). However, few field studies have examined the influence of the buoyancy and inertial effects on mixing dynamics at large-scale confluences (Lane et al., 2008 in the Paranà River; Gualtieri et al., 2019 in the Amazon River).
根据宽深比,汇流尺度可被分类为小尺度、中等尺度和大尺度,正如袁等人(2021年)所建议的。研究不同尺度下的汇流水动力学对于将河流过程扩展到排水网络尺度至关重要。CHZ通常包括靠近上游交汇角的流动停滞区、支流进入汇流时的流动偏转区、两个汇流流动之间的剪切层和/或混合界面、可能存在于下游交汇角的分离区、汇流后通道内的流动加速以及CHZ下游端的流动恢复(Best, 1987; Bradbrook et al., 2000; Weber et al., 2001; Yuan, Xu, Tang, Xiao, & Gualtieri, 2022)。这种流动结构以及泥沙输移和地貌动力学已被广泛研究(Best, 1987; Guillén-Ludeña et al., 2016, 2017; Herrero et al., 2016; Leite Ribeiro et al., 2012; Li et al., 2022; Rhoads & Kenworthy, 1995; Roy & Bergeron, 1990; Gualtieri et al., 2018; Ianniruberto et al., 2018; Sukhodolov & Sukhodolova, 2019; Yuan et al., 2016, 2018, 2021; Yuan, Zhu, et al., 2022)。 然而,在将为小尺度汇流开发的概念模型应用于排水大流域的大尺度汇流时,规模效应是不可忽视的。与小尺度汇流相比,这种大尺度汇流可能在水文化学、泥沙浓度和密度方面具有更广泛的入流条件范围。了解大尺度汇流的水动力学和混合过程对于控制和调节水资源,从而提高水生态环境质量的整体目标至关重要。然而,很少有现场研究探讨了浮力和惯性效应对大尺度汇流混合动力学的影响。
Mixing rates generally depends on the combined effects of molecular diffusion which is usually negligible, turbulent diffusion and lateral dispersion (Rutherford, 1994). Mixing is due first to the shear between the two incoming flows, characterized by turbulent eddies that develop at the mixing layer scale (Biron et al., 2019) and substantially enhance transverse mixing in the near-field region downstream of the confluence. In addition to shear dispersion, mixing may be enhanced by convective effects due to large-scale, persistent flow structures, often helical in shape. Helical motions generate lateral and vertical velocity components that can be observed in a meander bend (Best, 1988; Constantinescu et al., 2016; Mosley, 1976; Rhoads & Kenworthy, 1995, 1998; Rhoads & Sukhodolov, 2001); these have been shown to have a strong effect on mixing at a confluence (Lewis & Rhoads, 2015; Rhoads, 1996; Rhoads & Kenworthy, 1995; Rhoads & Sukhodolov, 2001). The dynamics of secondary circulation at river confluences have also received considerable attention. Secondary circulation cells are influenced by the momentum flux ratio between the two flows and the bed geometry (Best, 1988; Cheng & Constantinescu, 2020; Mosley, 1976; Sukhodolov & Sukhodolova, 2019). At large river confluences, empirical evidence and theoretical analysis suggest that the high width-to-depth ratio in wide rivers may impede the formation of coherent channel-size secondary flow cells (McLelland et al., 1996, 1999; Parsons et al., 2007). For example, at the large braid-bar confluences at Paranà River, the helical motion was restricted in the spatial extent to portions of the flow near the mixing interface (Szupiany et al., 2009), or even these channel-scale secondary circulation cells were absent (Parsons et al., 2007). They attributed this to the large channel width-to-depth ratio that allowed the effects of form roughness to become dominant, but the effects may be localized and not extend across the entire channel width. In the large river confluence between the Paranà and Paraguay Rivers, it was observed that the mixing length between the confluent flows was related to the presence or absence of a channel-scale circulation pattern (Lane et al., 2008). Such channel-scale circulation is generally controlled by the interaction between bed discordance, downstream topographic forcing, density effect and the momentum ratio between the confluent channels. Therefore, the factors that initiate and enhance (or inhibit) helical motions are still not fully understood at the large river confluences.
混合速率通常取决于分子扩散的综合效应,通常可以忽略不计,湍流扩散和横向扩散(Rutherford,1994)。混合首先是由两个流入流之间的剪切引起的,其特征是在混合层尺度上发展的湍流涡流(Biron等,2019),并且大大增强了汇合点下游近场区域的横向混合。除了剪切扩散外,混合可能会受到大尺度持续流结构的对流效应的增强,这些结构通常呈螺旋形。螺旋运动产生横向和垂直速度分量,可以在弯曲河道中观察到(Best,1988;Constantinescu等,2016;Mosley,1976;Rhoads&Kenworthy,1995,1998;Rhoads&Sukhodolov,2001);已经证明这对汇合点的混合有很强的影响(Lewis&Rhoads,2015;Rhoads,1996;Rhoads&Kenworthy,1995;Rhoads&Sukhodolov,2001)。河流汇合处的次生环流动力学也受到了相当多的关注。 次级环流受两个流体的动量通量比和河床几何形态的影响(Best, 1988; Cheng & Constantinescu, 2020; Mosley, 1976; Sukhodolov & Sukhodolova, 2019)。经验证据和理论分析表明,在大型河流汇合处,宽河的宽深比可能阻碍结构化的通道尺度次级环流的形成(McLelland et al., 1996, 1999; Parsons et al., 2007)。例如,在巴拉那河的大型纠缠砾石汇合处,螺旋运动仅限于靠近混合界面的部分流区(Szupiany et al., 2009),甚至这些通道尺度的次级环流可能不存在(Parsons et al., 2007)。他们将其归因于较大的河道宽深比导致形态粗糙度的影响占优势,但这种影响可能局部化并不延伸到整个河道宽度。在巴拉那河和巴拉圭河之间的大型河流汇合处,观察到混合流体之间的混合长度与通道尺度环流模式的存在与否相关(Lane et al., 2008)。 这种河道尺度的环流通常受床面不一致性、下游地形迫使、密度效应以及汇流河道之间的动量比相互作用的控制。因此,在大河汇流处,引发和增强(或抑制)螺旋运动的因素仍未完全理解。
Previous studies have demonstrated that density differences play an important role on confluence hydrodynamics and mixing if such differences produce buoyant forces comparable to inertial forces as indicated by the densimetric Froude number ) (Cheng & Constantinescu, 2018, 2020; Gualtieri et al., 2019; Herrero et al., 2018; Jiang et al., 2022; Laraque et al., 2009; Lewis & Rhoads, 2015; Lyubimova et al., 2014; Prats et al., 2010; Ramón et al., 2014, 2016). Underflows of denser water beneath less dense water have been documented at some large river confluences (Herrero et al., 2018; Lane et al., 2008; Laraque et al., 2009). Those buoyant forces might reinforce or weaken secondary motions associated with helical motion if the tributary waters have has a density lower or higher density than that of main channel waters, respectively (Horna-Munoz et al., 2020; Ramón et al., 2013). However, studies on how density differences affect confluence flows, such as the development of the mixing
先前的研究表明,密度差异在汇流水动力学和混合中起着重要作用,如果这种差异产生的浮力与惯性力相当,如密度弗洛德数所示(Cheng&Constantinescu,2018,2020; Gualtieri等,2019; Herrero等,2018; Jiang等,2022; Laraque等,2009; Lewis&Rhoads,2015; Lyubimova等,2014; Prats等,2010; Ramón等,2014,2016)。已经记录了在一些大河汇流处,密度较大的水在密度较小的水下面流动(Herrero等,2018; Lane等,2008; Laraque等,2009)。如果支流水的密度低于或高于主河道水的密度,这些浮力可能会加强或削弱与螺旋运动相关的次要运动(Horna-Munoz等,2020; Ramón等,2013)。然而,关于密度差异如何影响汇流流动的研究,比如混合的发展

interface (Rhoads & Sukhodolov, 2004), or helical motions induced by flow curvature (Ashmore et al., 1992; De Serres et al., 1999; Lewis & Rhoads, 2015; Paola, 1997; Rhoads, 1996; Rhoads & Sukhodolov, 2001), have received limited attention in published field studies (e.g., Lewis & Rhoads, 2015; Lewis et al., 2020; Duguay et al., 2022a; Rhoads & Johnson, 2018).
接口(Rhoads & Sukhodolov, 2004),或由流曲率引起的螺旋运动(Ashmore等,1992; De Serres等,1999; Lewis & Rhoads, 2015; Paola, 1997; Rhoads, 1996; Rhoads & Sukhodolov, 2001),在已发表的现场研究中受到了有限的关注(例如,Lewis & Rhoads, 2015; Lewis等,2020; Duguay等,2022a; Rhoads & Johnson, 2018)。
The present field study was undertaken to investigate hydrodynamics and mixing dynamics at a large confluence between the Yangtze River and the outflow channel of Poyang Lake. They are the largest river and the largest freshwater lake in China, respectively, and have significant density differences. Both have a dramatic influence on flood flows, water resources management, water chemistry as well as environmental and ecological protection within the Yangtze River basin. In the confluence reach, the Yangtze River has an average width of almost . The main goals of the present field study on such a large-scale, asymmetrical and concordant bed confluence are to investigate (a) the mixing patterns in the post-confluence channel; (b) how any change in the inflow conditions affects both hydrodynamics and mixing processes within the CHZ; and (c) how density differences between the incoming flows impact on confluence hydrodynamics, like the large-scale helical motions observed by Yuan et al. (2021). These results can improve our current understanding of and ability to predict watershed-scale pollutant transport and its ecological impacts.
本地野外研究旨在调查长江与鄱阳湖出流河道之间的水动力学和混合动力学。它们分别是中国最大的河流和最大的淡水湖,具有显著的密度差异。两者对长江流域内的洪水流量、水资源管理、水化学以及环境和生态保护都有显著影响。在汇流区域,长江的平均宽度几乎为 。对于这样一个大规模、不对称和一致的床体汇流的本地野外研究的主要目标是调查(a)汇流后河道中的混合模式;(b)流入条件的任何变化如何影响 CHZ 内的水动力学和混合过程;以及(c)流入流的密度差异如何影响汇流水动力学,如袁等人(2021 年)观察到的大规模螺旋运动。这些结果可以提高我们对流域尺度污染物输运及其生态影响的当前理解和预测能力。

2. Study Site, Field Procedures, and Methods
2. 研究地点、野外程序和方法

2.1. Study Site 2.1. 研究地点

The Yangtze River catchment is one of the largest drainage basins in the world; its annual water discharge and sediment load are ranked as the fifth and fourth largest in the world, respectively. The confluence of the Yangtze River and Poyang Lake is located about downstream the Three Gorges Dam (TGD) and upstream of the Yangtze River estuary (Figure 1a). The Poyang Lake watershed plays an important role in flood-mitigation storage and the protection of biodiversity. It is located at the junction of the south bank of the Yangtze River (Figure 1a), with an area of covering of the Yangtze River basin. The confluence is a key node for the exchange of water, sediment and pollutants between the Yangtze River and Poyang Lake. In summer, when the water level of the Yangtze River is higher than that of Poyang Lake, the backwater effect of Yangtze River on Poyang Lake affects lacustrine water chemistry. Moreover, previous research suggests that the TGD operation has affected the Yangtze River discharge and water level, ultimately altering the interaction between that river and the Poyang Lake. This in turn creates severe and extended dry seasons in the Poyang Lake (Guo et al., 2012).
长江流域是世界上最大的排水盆地之一;其年降水量 和泥沙负荷 位列全球第五和第四。长江与鄱阳湖交汇处位于三峡大坝(TGD)下游约 处,距长江河口上游 (图 1a)。鄱阳湖流域在防洪储存和保护生物多样性方面起着重要作用。它位于长江南岸交汇处(图 1a),面积 涵盖长江流域的 。这个交汇处是长江和鄱阳湖之间水、泥沙和污染物交换的关键节点。在夏季,当长江水位高于鄱阳湖水位时,长江对鄱阳湖的倒灌效应会影响湖泊水化学。此外,之前的研究表明,三峡大坝的运营已经改变了长江的排放和水位,最终改变了该河流与鄱阳湖之间的相互作用。 这反过来在鄱阳湖引发了严重而长期的干旱季节(郭等,2012年)。
For the 2020-2021 years (Figure 2), the average discharge in the Yangtze River and the Poyang Lake was 27227.9 and , respectively, which is close to the average annual flood of 30,146 and for the Yangtze River and the Poyang Lake, respectively. Near Jiujiang Station, Zhangjia Island divides the Yangtze River flow into two parts (each flow having total discharge) which meet about downstream from Jiujiang Station with an angle of convergence nearing (Figure 1b), that is, the branch at the left of this island also belongs to the Yangtze River. In this study, the Yangtze River mentioned hereafter means the branch at the right of the Zhangjia Island. The confluence of the Yangtze River and the Poyang Lake has an angle of about and its apex is located at the Meijia Island. The discharge of branch between the Unnamed Inland and Zhangjia Island is very small, accounted for less than of the discharge of the receive channel. In addition, the branch was located in the left bank of the receiving channel, and it is expected to have negligible effect on the mixing dynamics. Therefore, it was not considered in this study. Due to the large area covered and the shallow depth, the water temperature of Poyang Lake is higher in summer and lower in winter than that of the Yangtze River, with a temperature difference of about . In addition, at the confluence, water conductivity in the Yangtze River (average value among the four surveys) is generally larger than that in the Poyang Lake (average value among the four surveys), also contributing to a difference in water density between the tributaries.
对于2020-2021年(图2),长江和鄱阳湖的平均流量分别为27227.9和 ,接近长江和鄱阳湖的平均年洪水量30,146和 。在九江站附近,张家洲将长江流分为两部分(每部分具有 总流量),并在距九江站下游约 处相汇,相汇角接近 (图1b),也就是说,这个岛的左侧支流也属于长江。在本研究中,所称的长江是指张家洲右侧的支流。长江与鄱阳湖的汇合处角度约为 ,其顶点位于梅家岛。未命名内陆与张家洲之间的支流流量非常小,占接收渠道流量的不到 。此外,该支流位于接收渠道的左岸,并且预计对混合动力学造成微乎其微的影响。因此,本研究不考虑它。 由于覆盖面积大且水深较浅,鄱阳湖的水温比长江夏季高、冬季低,温差约为 。此外,在汇合处,长江的水电导率(四次调查的平均值 )通常比鄱阳湖的水电导率(四次调查的平均值 )要大,这也导致了支流之间水密度的差异。

2.2. Instrumentation and Field Procedures
2.2. 仪器设备和野外操作

Four field surveys were carried out at this confluence during the years 2020 and 2021. According to the discharge sequences of Yangtze River in Figure 2, Survey 1 (August 2020) and Survey 4 (June 2021) were both carried out in high-flow conditions (defined as the discharge of Yangtze River ), after and before flood in
在 2020 年和 2021 年,该汇流处进行了四次野外调查。根据图 2 中长江的流量序列,调查 1(2020 年 8 月)和调查 4(2021 年 6 月)都是在高流量条件下进行的(定义为长江的流量 )。
Figure 1. (a) Map of the Yangtze River basin shows that the confluence of the Yangtze River and the Poyang Lake is in the middle and lower reach of the Yangtze River (near ); (b) locations of hydrological stations and islands; (c) map of measurements locations at the confluence during four surveys: The white lines mark cross sections for ADCP measurement, while the symbols indicate water chemistry sampling points.
图 1。 (a) 长江流域地图显示长江与鄱阳湖的汇合处位于长江中下游 (靠近 ); (b) 水文站和岛屿的位置; (c) 四次调查期间汇合处测量点的地图: 白线标记 ADCP 测量的横截面,符号表示水化学采样点。
Figure 2. Annual variation of water level and flow discharge at Jiujiang and Hukou stations (Figure 1b). The water level is based on the frozen base level whose zero datum is higher than that of 1,985 National Height Datum.
图 2。九江和湖口站 (图 1b) 的水位和流量的年变化。水位基于冻结基准面,其零点基准比 1985 年国家高程基准高

the Yangtze River, respectively. Survey 2 (December 2020) was conducted during a low flow condition (defined as the discharge of Yangtze River ), while Survey 3 was carried out in April 2021 in medium flow condition (defined as the discharge of Yangtze River was ) when the floodplain at the outflow of the Poyang Lake was just submerged.
长江。第 2 次调查 (2020 年 12 月) 在低流量条件下进行 (定义为长江的流量 ),而第 3 次调查于 2021 年 4 月在中等流量条件下进行 (定义为长江的流量为 ),此时鄱阳湖出口的洪泛原刚刚被淹没。
A total of 18 and 17 locations were considered in the two tributaries and the post confluence during Surveys 1 , 2, 4 and during Survey 3 (CYP8 and CYP8-A were merged into CYP8 which was in the middle of the two cross sections), respectively. These transects correspond to those previously considered by Yuan et al. (2021). In the present study, the transects in the post confluence channel were considered to analyze the mixing dynamics. Two acoustic Doppler current profiles (ADCPs), namely a Sontek River Surveyor M9 Instrument and a Rio Grande RDI Instrument, were used simultaneously. The two ADCPs were used at the same time to ensure that the flow discharge difference of a series of three repeat transects measured by the two instruments was within in each transect (Rhoads & Johnson, 2018). The 600-kHz Workhorse Rio Grande ADCP was configured for a sampling rate of , bin size of , blanking distance of , Water Mode 1, and Bottom Mode 5. These parameters were selected based on the default manufacture recommendations and previous studies for large rivers (Teledyne RD Instruments [TRDI], 2013; Jamieson et al., 2011; Rennie & Church, 2010). The ADCP M9 is a double frequency instrument, with four transducers at both 1 and . The frequency and thus cell sizes are changed automatically on-the-fly depending on the local water depth, if not configured otherwise (SonTek, 2012). Along the confluence cross-sections, the shallow margins were usually sampled at a frequency of , and the deeper channel at a frequency of , both in Narrowband mode. The vertical size of the measurement cells ranged for the and for the . The horizontal sampling size is determined as a function of the boat's movement in time, and the data acquisition frequency . The ADCPs provided three-dimensional flow velocities in each cross-section. In this study, the velocity data from the M9 were processed to analyze the flow structure. As the Sontek M9 firmware changes the acoustic operating frequency depending on the water depth and velocity (smart pulse) and only provide SNR (Signal to Noise Ratio), therefore the data from RDI were not only used to verify the discharge accuracy but also to provide the information about acoustic backscatter intensity which is related to suspended sediment concentration. As the surveys we being undertaken using a moving vessel, these raw velocities then must be corrected again for the boat velocity. There are two key methods for doing this. The first uses the bottom tracking to measure the boat velocity relative to the riverbed, under the assumption that the latter is stationary (i.e., there is no bedload transport). The second tracks the boat position using differential global positioning system (DGPS) (e.g., Zhao et al., 2014). In this study, boat position and velocity were determined using a DGPS receiver. The bottom tracking function of the ADCPs was not used due to the measurement errors that bed load transport can introduce into the results obtained (Rennie & Church, 2010; Szupiany et al., 2009). In addition, the data obtained by the ADCPs were corrected by adjusting the GC-BC (angle of average GPS course since start of transect minus ADP bottom-track course; value near 0 is desired) and (BT)/D(GPS) (Ratio of Bottom Track distance to GPS track distance) approximately 1 (SonTek, 2012). The DGPS-antenna was affixed to the port side mount directly above the ADCP. The DGPS-receiver provides time-stamped geographic coordinates at with up to sub-meter accuracy and was integrated with the ADCP to fully geo-reference velocity data at each ensemble. The compass was well calibrated, the boat velocity and track position of the survey lines were monitored online by the helmsman and were held constant as much as possible during the surveys. Due to the extremely harsh sampling conditions, for example, large-scale channel width and the large number of ships navigating in the area, a constant boat speed of approximately was used while collecting these transects where the flow velocities was quite high to ensure minimal lateral variations about the line, as well as to decrease the number of missing ensembles. The boat speed/ river velocity ratio was about 0.8 . To ensure that every measured velocity vectors are in the right direction, calibration tests were made for the inner compass of each device (heading, pitch, and roll).
在调查1、2、4期间,在两条支流和汇合后的区域分别考虑了18个和17个位置(CYP8和CYP8-A合并为CYP8,位于两个横截面的中间)。这些横截面对应于袁等人(2021年)先前考虑的那些。在本研究中,考虑了汇合后水道的横截面,并分析了混合动力学。同时使用了Sontek River Surveyor M9仪器和Rio Grande RDI仪器两款声学多普勒流速仪(ADCP)。两个ADCP同时使用,以确保由两种仪器测量的一系列三次重复横截面的流量差异在每个横截面上不超过1(Rhoads & Johnson, 2018)。600kHz Workhorse Rio Grande ADCP配置了采样率为2,单元大小为3,遮挡距离为4,水模式为1,底部模式为5。这些参数是基于默认制造建议和大河流的先前研究选择的(Teledyne RD Instruments [TRDI], 2013; Jamieson等,2011; Rennie & Church, 2010)。 ADCP M9是一款双频仪器,具有四个1和 的换能器。频率和细胞大小根据当地水深自动进行实时更改,除非另有配置(SonTek,2012)。在交汇断面上,浅部边缘通常以 的频率进行取样,而较深的河道以 的频率进行取样,都采用窄带模式。测量单元的垂直尺寸范围为 的范围为 。水平采样尺寸是根据船只在时间内的运动情况确定的,以及数据采集频率 。ADCP提供了每个断面的三维流速。在本研究中,通过处理M9的速度数据来分析流动结构。由于Sontek M9固件根据水深和水速改变声学工作频率(智能脉冲)并且仅提供SNR(信噪比),因此RDI的数据不仅用于验证流量的准确性,还用于提供与悬浮物浓度相关的声学回波强度的信息。 由于我们使用移动船只进行调查,这些原始速度必须再次校正以考虑船只速度。有两种关键方法可以做到这一点。第一种方法利用底部跟踪来测量船只速度相对于河床的速度,假设后者是静止的(即没有床载运输)。第二种方法使用差分全球定位系统(DGPS)来跟踪船只位置(例如,Zhao等人,2014年)。在这项研究中,船只位置和速度是使用DGPS接收器确定的。由于床载运输可能引入的测量误差,ADCP的底部跟踪功能未被使用(Rennie&Church,2010年;Szupiany等,2009年)。此外,通过调整GC-BC(自横截面开始以来的平均GPS航向角减去ADP底部跟踪航向角的角度;期望值接近0) (BT)/D(GPS)(底部跟踪距离与GPS跟踪距离的比率)约为1(SonTek,2012年),ADCP获取的数据进行了校正。DGPS天线直接安装在ADCP正上方的左舷支架上。 DGPS接收器可提供具有高达次米级精度的时间戳地理坐标,与ADCP集成后,可以完全将每个合奏的速度数据进行地理参考。指南针校准良好,船速和测量线路的航迹位置由舵手在线监控,并在测量过程中尽可能保持恒定。由于极端恶劣的取样条件,例如大型航道宽度和区域内大量船只航行,收集这些横截面时使用了大约16的恒定船速,其中流速非常高,以确保沿线的横向变化最小,并减少缺失的合奏数量。船速/河流速度比约为0.8。为确保每17个测量的速度矢量都是在正确的方向上,对每个设备的内置指南针进行了校准测试(航向、俯仰和横滚)。
Furthermore, during all the campaigns, local conductivity was measured over the water depth at 7 sampling points in Survey 1,6 sampling points in Surveys 2 and 3, and 10 sampling points in Survey 4 (Figure 1c) using an YSI EXO2 multi-parameter probe (EXO2 probe). Considering the density effects at this confluence, the sampling frequency of EXO2 probe was set as . It can generate time/depth series of water quality parameters in the vertical direction of each sampling point to continuously capture the water quality data. According to the manufacturer's specifications, the probes have a accuracy on the water depth measurements and accuracy on the electrical conductivity measurement. At the same time, the total suspended sediment (TSS) concentrations were measured using the water sampling (Figure 1c). Depending on the water depth, three different depth samples
此外,在所有的调查中,通过在调查1的7个采样点、调查2和3的6个采样点以及调查4的10个采样点上测量当地的电导率来测量水深(图1c),使用YSI EXO2多参数探头(EXO2探头)。考虑到在这个汇合处的密度效应,EXO2探头的采样频率设置为 。它可以在每个采样点的垂直方向生成水质参数的时间/深度序列,以连续捕获水质数据。根据制造商的规格,探头在水深测量上有 的精度,在电导率测量上有 的精度。同时,使用水采样测量总悬浮泥沙(TSS)浓度(图1c)。根据水深,有三种不同的深度样本。

in one vertical were collected. The EXO2 probe was also deployed at the edge of the vessel together with ADCP and provided the longitude and latitude information through the built-in GPS in the handheld device. The sensors of the EXO2 probe were below the water surface, and a section was set every in the study area to derive the distribution of surface water chemistry parameters. Before each measurement, the sensors were tested using the manufacturer's specifications for continuous water-chemistry monitoring.
在一个垂直方向收集了数据。EXO2 探头也与 ADCP 一起部署在容器边缘,并通过手持设备中的内置 GPS 提供经度和纬度信息。EXO2 探头的传感器位于水面以下 ,在研究区域的每 设置一个部分,以推导表面水化学参数的分布。在每次测量之前,根据制造商的规格对传感器进行测试,以进行连续的水化学监测。

2.3. Data Post-Processing
2.3. 数据后处理

RDI data and M9 data were exported as ASCII files and MATLAB files using Teledyne RDI WinRiverII software and RiverSurveyor Live software, respectively. The data for multiple transects were then analyzed using the velocity mapping tool (VMT), a suite of MATLAB routines with a graphical user interface (Parsons et al., 2013). VMT composites and averages ADCP velocity data from repeat transects along cross-sections, providing the capability to plot 3-D velocity vectors. In this study, the instantaneous velocity data from the M9 were averaged between a series of three repeated-transect lines and processed to analyze the flow structure, while the data from RDI were used mainly to verify their discharge accuracy during the field surveys and provide the information about acoustic backscatter intensity to analyze the mixing processes. The M9 and RDI data were smoothed using the VMT beforehand by adjusting the grid node spacing, horizontal/vertical vector spacing, and horizontal/vertical smoothing window.
RDI数据和M9数据分别以ASCII文件和MATLAB文件的形式通过Teledyne RDI WinRiverII软件和RiverSurveyor Live软件导出。然后使用速度映射工具(VMT)进行多次横截面数据分析,该工具是一套MATLAB例程,带有图形用户界面(Parsons等,2013)。VMT合成并平均了多次横截面沿横截面的ADCP速度数据,提供了绘制3D速度向量的能力。在本研究中,M9的瞬时速度数据被用来分析流动结构,RDI的数据主要用于验证野外调查期间的排水精度,并提供声学回波强度信息以分析混合过程。M9和RDI数据在通过调整网格节点间距、水平/垂直矢量间距和水平/垂直平滑窗口来预先使用VMT进行平滑处理。
In confluences field studies, it is commonly difficult to keep the transects orthogonal to the flow direction, especially at a high confluence angle. Some methods (e.g., Lane et al., 2000) can be used to minimize the secondary circulation in the transect but it is needed to know what the minimum secondary circulation transect path is, which is difficult to know a priori. Thus, in this study, the transects were chosen to be orthogonal to the stream path of the CHZ as much as possible to capture the mixing processes and the helical motions more accurately. The identification of helical motion at confluences based on patterns of secondary flow is sensitive to the frame of reference used to represent this flow (Lane et al., 2000; Rhoads & Kenworthy, 1998, 1999). The present study adopted the Rozovskii definition which had been used in the same site by Yuan et al. (2021) to calculate and plot the primary and secondary velocity. The Rozovskii reference frame rotated each vertical ensemble of velocity measurements such that primary and secondary velocity components are aligned parallel and perpendicular to the orientation of the depth-averaged velocity vector, respectively. This method was found useful to identify helical motion in strongly converging flows (Rhoads & Kenworthy, 1998; Rozovskii, 1954, 1957; Szupiany et al., 2009; Yuan et al., 2021).
在汇流领域研究中,通常很难保持横截面与流动方向正交,特别是在高汇流角处。一些方法(例如Lane等人,2000年)可以用来最小化横截面中的次生环流,但需要知道最小次生环流横截面路径,这是很难事先知道的。因此,在本研究中,选择尽可能使横截面与CHZ的流路正交,以更准确地捕捉混合过程和螺旋运动。基于次生流动模式识别汇流处的螺旋运动对所使用的参考框架敏感(Lane等人,2000年;Rhoads和Kenworthy,1998年,1999年)。当前研究采用了Rozovskii的定义,该定义已被Yuan等人(2021年)在同一地点使用,来计算和绘制主要和次要速度。Rozovskii参考框架使得每个垂直速度测量集合被旋转,以使主要和次要速度分量分别与深度平均速度矢量的方向平行和垂直对齐。 这种方法被发现对于识别强收敛流中的螺旋运动很有用(Rhoads & Kenworthy, 1998; Rozovskii, 1954, 1957; Szupiany 等,2009; Yuan 等,2021)。
Confluence bathymetry and distribution of water chemistry parameters on the surface in each survey were prepared using a continuous recording of depth values using the ADCP and water chemistry parameters along the planned trajectory. These depth values and water chemistry parameters were then interpolated on a grid covering the area of interest, using a kriging procedure which is widely used in geostatistics (Gualtieri et al., 2017; Herrero et al., 2018; Yuan et al., 2021).
在每次调查中,通过使用 ADCP 连续记录深度值和沿着计划轨迹的水化学参数,准备了汇合地形和水化学参数在表面上的分布。然后,利用克里金插值程序在覆盖感兴趣区域的网格上插值这些深度值和水化学参数,克里金程序在地统计学中被广泛使用(Gualtieri 等,2017; Herrero 等,2018; Yuan 等,2021)。
Water density was calculated from the measured water temperature and then adjusted to consider the contribution from the TSS concentration and specific conductivity (Ford & Johnson, 1983; Gualtieri et al., 2019; Ramòn et al., 2013; Moreira et al., 2016). The following equations were applied:
水密度是根据测得的水温计算的,然后根据 TSS 浓度和特定电导率的贡献进行调整(Ford & Johnson, 1983; Gualtieri 等,2019; Ramòn 等,2013; Moreira 等,2016)。以下方程式被应用:
where is the density of pure water, which can be accurately calculated (Kell, 1975). is specific conductivity ( is conductivity at ), and are coefficients correlated with temperature and , which can be obtained by the RHO_LAMBDA approach (Moreira et al., 2016). represents the density difference caused by different suspended sediment concentrations. SG is the specific gravity of suspended solids, assumed equal to 2.65 .
其中 是纯水的密度,可以通过准确计算得出(Kell,1975)。 是特定电导率( 是在 处的电导率), 是与温度和 相关的系数,可以通过 RHO_LAMBDA 方法获得(Moreira 等人,2016)。 代表不同悬浮泥沙浓度引起的密度差异。SG 是悬浮固体的比重,假定等于 2.65。
Table 1 表 1
Main Flow Properties and Water Chemistry Characteristics of Yangtze River and Poyang Lake
长江和鄱阳湖的主要流动特性和水化学特征
Date of the survey
调查日期
23 August 2020: Survey
2020 年 8 月 23 日:调查
1
21 December
Survey 2
16 April 2021: Survey 3
2021 年 4 月 16 日:第三次调查
01 June 2021: Survey 4
2021 年 6 月 1 日:调查 4
Yangtze River (Main 长江(主要支流)
channel, measured in Y2)
(在 Y2 处测量的河道)
Temperature 28.3 11.5 16.7 22.3
59.3 32.4 45.3 28.6
Density 996.4 999.8 999.1 997.9
Wetted area 29,721 8,266 15,414 26,273
1.03 0.92 0.99 0.83
Poyang Lake (Tributary; 鄱阳湖(支流;
measured in P3) 在 P3 测量)
Temperature 30.6 8.6 18.0 24.9
Conductivity 84.1 220.8 150.5 109.4
7.8 25.5 29.1 21.1
Density 995.5 998.7 997.1
Confluence (CYP1) 汇流 (CYP1) 0.84 0.62 0.80 0.99
Width 2488 2041 2170 2422
Mean depth 17.35 6.32 12.75 16.27
Water level (m) 水位 (m) 18.94 9.09 12.52 18.69
Momentum flux ratio
动量通量比
0.17 0.06 0.23 1.19
Discharge ratio  放水比率 0.37 0.19 0.37 0.83
Velocity ratio  速度比率 0.45 0.31 0.63 1.43
8.4 -1.7 3.6 7.4
Note. TSS total suspended sediments; average cross-sectional velocity (measured using the ADCP); water density (calculated using Equations ) and is , and , where and are defined as average velocity and cross-sectional depth at the transect CYP1, respectively. The water level values were provided by the Hukou hydrologic station.
注:TSS 总悬浮泥沙; 平均横截面流速(使用 ADCP 测量); 水密度(使用方程式 计算)和 ,以及 ,其中 被定义为横截面 CYP1 处的平均流速和横截面深度。水位值由壶口水文站提供。

3. Results and Discussion
3. 结果和讨论

3.1. Hydraulic and Density Conditions
3.1. 水力和密度条件

Table 1 lists the hydraulic parameters of the Yangtze River and Poyang Lake calculated from the ADCP measurements during the four surveys. Surveys 1 and 3 have similar discharge ratios ; the parameters , , etc. are introduced at Table 1.) and momentum flux ratios ( . Despite the discharge of Poyang Lake in Survey 1 being nearly twice that in Survey 3, the average velocity in the outflow channel was lower in Survey 1 than in Survey 3. This may be due to the Yangtze River's increased backwater effects in August leading to a more than doubled wetted area in the Poyang Lake and to a lower velocity. It is worth noting that in Survey 4 momentum flux ratio ( ) was close to 1 , suggesting that the wake mode within the mixing interface may play an important role in CHZ. The largest differences in discharge and flow velocities were observed in Survey 2 , leading to a substantially lower momentum flux ratio than in the other surveys.
表 1 列出了从四次调查中通过 ADCP 测量计算出的长江和鄱阳湖的水力参数。调查 1 和 3 具有相似的流量比 ;参数 等在表 1 中介绍。)和动量通量比( 。尽管调查 1 中鄱阳湖的流量几乎是调查 3 的两倍,但出流河道中的平均流速在调查 1 中低于调查 3。这可能是由于长江在 8 月增加的返水效应导致鄱阳湖的润湿面积增加了一倍以上,流速降低。值得注意的是,在调查 4 中,动量通量比( )接近 1,表明混合界面内的尾流模式可能在 CHZ 中发挥重要作用。调查 2 中观察到的流量和流速的最大差异,导致动量通量比 明显低于其他调查。
Table 1 also lists the water chemistry characteristics, and the relative density difference observed during the field surveys. A notation is adopted to convey both the magnitude and direction of (Duguay et al., 2022b). In Survey 1, the Yangtze River was estimated to be denser than the Poyang Lake and because the Yangtze River is the left tributary, density decreased across the mixing interface from left to right. We denoted this as with the arrow pointing in the lateral direction of decreasing density. Temperature, water conductivity, and TSS
表 1 还列出了水化学特征以及在现场调查中观测到的相对密度差 。采用一种符号来传达 的大小和方向(Duguay 等,2022 年 b)。在调查 1 中,估计长江比鄱阳湖密度更大,因为长江是左支流,密度在混合界面从左到右逐渐减小。我们用 表示,箭头指向横向密度减小的方向。温度、水电导率和 TSS

Figure 3. Bed elevations of the confluence during the field surveys. Bed elevations were calculated by the minus of the water level at Hukou Station and the water depth of the whole study site measured by ADCP. and are the discharge in the Yangtze River and Poyang Lake outflow, respectively. The values of (with an arrow to indicate the direction of decreasing density gradient), and were consistent with Table 1 . Background images were obtained by using remote sensing (Sentinel-1/2 with low cloud cover conditions ) during each survey.
图 3. 在现场调查期间的汇合处床面高程。通过计算壶口站水位减去整个研究区测得的水深(ADCP 测量)来计算床面高程。 分别是长江和鄱阳湖的出流 的值(带有箭头来指示密度梯度减小的方向),以及 与表 1 的结果一致。背景图像是在每次调查期间利用遥感获得的(使用 Sentinel-1/2 和低云层条件 )。
showed significant differences between the two incoming flows; hence they were applied to quantify the density difference. The water temperature in the Yangtze River was about lower than that in Poyang Lake in Survey 1,3 and 4, while it was the opposite in Survey 2. This could be explained considering that the temperature of Poyang Lake with a large surface area could be significantly affected by the solar irradiance and ambient air temperature. A large difference in water conductivity was found between the two tributaries. In addition, as water supply in the Poyang Lake region is primarily derived from groundwater during the dry season (Bing, 2018; Xu et al., 2021), the conductivity of Poyang Lake in Survey 2 was nearly three times that in Survey 1 (Table 1). TSS concentration in the Yangtze River showed a positive correlation with its average velocity ). Interestingly, TSS concentration of Poyang Lake was only in Survey 1, a much lower value than in other surveys (in average ). This may be due to the stronger backwater effect of the Yangtze River on the outflow of Poyang Lake (Fang et al., 2012)
两个入流之间显示了显著差异;因此,它们被用来量化密度差异。在调查1、3和4中,长江水温比鄱阳湖低约 ,而在调查2中则相反。这可以解释为鄱阳湖的温度受到太阳辐射和环境空气温度的显著影响,因为鄱阳湖具有较大的表面积。两条支流之间发现了水电导率的巨大差异。此外,由于鄱阳湖地区的水源在旱季主要来自地下水(Bing,2018;Xu等,2021),因此在调查2中,鄱阳湖的电导率几乎是调查1的三倍(表1)。长江中的TSS浓度与其平均流速呈正相关 )。有趣的是,鄱阳湖的TSS浓度在调查1中仅为 ,远低于其他调查的平均值(平均 )。这可能是由于长江对鄱阳湖出流的强翻水效应(方等,2012)所致。
The observed temperature, conductivity and TSS data were then used to calculate the water density in each tributary according to Equations 1-3. Previous literature (Gualtieri et al., 2019; Herrero et al., 2018; Horna-Munoz et al., 2020; Lewis & Rhoads, 2015; Lyubimova et al., 2014; Pouchoulin et al., 2020; Ramón et al., 2013, 2014) suggested that hydrodynamics and mixing processes are affected by density difference if . In the present study, was in the range from 2.2 to 2.9 in Surveys 1,3 and 4, while it was in Survey (Table 1). Therefore, it is expected that the buoyant forces cannot be ignored in all our field surveys, but in Survey 2 their effect should be limited and different because in Surveys 1,3 and 4, Yangtze River waters were denser, while the opposite was in Survey 2 .
然后使用观测到的温度、电导率和 TSS 数据根据方程式 1-3 计算每个支流中的水密度。先前的文献(Gualtieri 等,2019 年;Herrero 等,2018 年;Horna-Munoz 等,2020 年;Lewis 和 Rhoads,2015 年;Lyubimova 等,2014 年;Pouchoulin 等,2020 年;Ramón 等,2013 年,2014 年)指出,如果