Water Resources Research
水资源研究

Key Points: 要点：

S. Yuan, 袁山逸，

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

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。

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。

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。

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） 和 （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。

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）， 和 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

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 是 ，以及 ，其中 和 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。

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。 和 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 ，水动力学和混合过程会受到密度差异的影响。在本研究中， 在调查 1、3 和 4 中的范围为 2.2 到 2.9，而在调查 中为 （表 1）。因此，预计在我们所有的现场调查中，浮力作用都不可忽视，但在调查 2 中，它们的影响应该是有限的，并且不同，因为在调查 1、3 和 4 中，长江水域更密集，而调查 2 中则相反。

Figure 3 shows confluence bed elevations obtained by interpolating the values of water level minus the water depth collected with ADCP in the present surveys. Surveys 1 and 2 have maximum and minimum water depths,
图 3 显示了通过插值处理本次调查中 ADCP 收集的水位值减去水深值获得的汇合床高程。调查 1 和 2 的水深值最大和最小，

Figure 4. Map with depth-averaged velocities during the four surveys. Note that the shear layer was identified using the depth-averaged velocity, while the mixing interface data were gained from the surface water conductivity. Background images were obtained by using remote sensing (Sentinel-1/2 with low cloud cover conditions ) during each survey. The values of (with an arrow to indicate the direction of decreasing density gradient), and were consistent with Table 1.
图4. 在四次调查期间的深度平均流速图。请注意，剪切层是通过深度平均流速识别的，而混合界面数据是从水面电导率获取的。背景图像是通过遥感技术（Sentinel-1/2在低云层条件下）在每次调查期间获取的。 （箭头指示密度梯度减小的方向）、 和 的数值与表1中一致。

respectively, among the four surveys (Table 1). The thalweg of the two tributaries shows that the confluence, studied here, has relatively concordant beds and an asymmetric planform with an angle of . Thus, the bed discordant effect on secondary flow and mixing, mentioned in the previous literature studies (e.g., Biron, Best, et al., 1996; Biron, Roy, et al., 1996; Biron et al., 1999; Sukhodolov et al., 2017), was not relevant. In addition, because the discharge ratios were less than 1 , the thalwegs were located on the right bank of the post-confluence channel. The scour hole observed in Surveys 1,3, and 4 was stretched to form a deep channel in Survey 2, which is consistent with previous studies (Yuan et al., 2021). In addition, acceleration of flow through the deep channel may strengthen curvature-induced helicity during low flow condition (Lewis et al., 2020).
分别在四个调查中（表 1）显示两个支流的河槽，检查区域有相对一致的床铺和一个 角度的不对称平面。因此，先前的研究中提到的对次生流动和混合有床铺不一致的影响（例如，Biron、Best 等，1996 年；Biron、Roy 等，1996 年；Biron 等人，1999 年；Sukhodolov 等人，2017 年）是不相关的。另外，由于流量比小于 1，河槽位于汇合河道的右岸。在调查 1、3 和 4 中观察到的冲刷洞被拉伸形成调查 2 中的一个深槽，与先前的研究一致（Yuan 等，2021 年）。此外，深槽通过流动的加速可能在低流量条件下增强曲率引起的螺旋性（Lewis 等，2020 年）。

Figure 4 shows the depth-averaged velocity collected at each transect in the field surveys and remote sensing images during each survey. In medium and high flow conditions (Surveys 1,3, and 4), the floodplains on the right side of the Yangtze River and on the left side of the Poyang Lake were submerged. Near the junction apex between the Yangtze River and the Poyang Lake outflow channel, a stagnation zone whose size and location changed under the different flow conditions was observed. Previous studies indicated that the development of large stagnation zones is often associated with and close to 1 (Constantinescu et al., 2011). Although both parameters were close to 1 in Survey , the corresponding size of the stagnation zone was lower than that in Survey . This is probably related to the low-velocity flow in the submerged floodplain which reduced the lateral shear between the tributaries and affected the development of the stagnation zone. The location of the shear layer is defined by the strong velocity gradient between the two flows. As the Yangtze River and Poyang Lake entered the confluence, the channel width was almost , with the two waters merging about
图4显示了在每次调查中收集的横截面处的深度平均流速以及远程感测图像。在中等和高流量条件下（调查1、3和4），长江右侧和鄱阳湖左侧的洪泛平原被淹没。在长江和鄱阳湖出流河道交汇处的顶点附近观察到一个大小和位置在不同流量条件下变化的停滞区。先前的研究表明，大型停滞区的发展通常与 和 接近1有关（Constantinescu等，2011）。尽管在调查 中这两个参数接近1，但相应的停滞区大小低于调查 中的大小。这可能与淹没的洪泛平原中的低速流有关，这降低了支流之间的横向剪切并影响了停滞区的发展。剪切层的位置由两种流之间的强速度梯度定义。长江和鄱阳湖汇合时，河道宽度几乎为 ，两股水体交汇处形成了一个强烈且易于观察的剪切层。在第 1 和第 3 次调查中，由于长江一侧的动量通量较大，剪切层倾向于右岸。然而，在第 次调查中，鄱阳湖的流量将剪切层位置推向汇合后河道的中心。在河流汇合处通常会观察到流动偏转，这表现为汇合后河道中汇流流速矢量方向的变化。在第 1、3 和 4 次调查中，鄱阳湖流量完全重新对齐到长江汇合后的河道，大约在汇合点顶部下游 处发生，即 CYP7 处。
a strong and easily visible shear layer. In Surveys 1 and 3, the shear layer tended to the right bank due to the larger momentum flux on the Yangtze River side. However, in Survey , Poyang Lake flow pushed the shear layer position toward the center of the post-confluence channel. Flow deflection is commonly observed at river confluences, as indicated by the change in direction of the velocity vectors of the converging flows in the post-confluence-channel. During Surveys 1,3, and 4, complete re-alignment of the Poyang Lake flow to with the Yangtze River post-confluence channel occurred at about CYP7, which was located downstream of the junction apex.

During Survey , complete re-alignment of the Poyang Lake flow to the Yangtze River post-confluence channel occurred at about CYP6, which was located about downstream of the junction apex, while a significant separation zone was not observed. However, near the junction apex low flow velocities toward upstream were observed upstream of the shear layer, due to the backwater effect into the Poyang Lake whose discharge was too small to affect the Yangtze flow direction. The confluent flows swerved from CYP4 to CYP6 with an anti-clockwise angle of nearly , mainly affected by the bend topography. Similarly, between CYP8 and CYP9, the direction of the flow velocity once again produced a clockwise change, which was consistent with the change of the thalweg in Figure 3b.
在 调查期间，鄱阳湖湖流向长江汇合通道的完全重新对齐发生在CYP6处，该处位于汇合角点下游约 ，但未观察到明显的分离区域。然而，在汇合角点附近，在剪切层上游观察到向上游的低流速，这是因为回水效应对鄱阳湖的影响很小，无法改变长江的流向。交汇流量在CYP4到CYP6之间发生逆时针方向约 的偏转，主要受到弯曲地貌的影响。同样，在CYP8和CYP9之间，流速的方向再次产生顺时针的变化，与图3b中河床最低点的变化一致。

Figures 5a-5d show the vectors of the secondary current velocity superimposed on the contours of the primary flow velocity using the Rozovskii definition during the medium and high flow conditions (Surveys 1,3, and 4). There were dual counter-rotating cells in the near-field cross-sections from CYP2 to CYP6 in all surveys. The left helical cell at the Yangtze side was generated by the Yangtze flow curvature, whose strength depended mainly on the flow curvature, and on (Constantinescu et al., 2011; Yuan et al., 2021). This helical cell got stronger in Survey 4 due to a larger force from the Poyang Lake, corresponding to the large momentum ratio . An anti-clockwise helical cell was observed on the right side near CYP2 for all the surveys. Since the secondary flow velocity has the same order of magnitude as the primary flow velocity, the helical cell cannot be attributed solely to the curvature of the Poyang Lake flow. Additional factors like the penetration of the near-bed Yangtze flow (as suggested by Yuan et al., 2021), which had a larger density and momentum flux into the Poyang flow cannot be discounted. Downstream of CYP2, the outflow of Poyang Lake was deflected clockwise owing to the squeezing of the flow by the Yangtze River (Figure 4). This, combined with the impact of the flow penetration, resulted in a considerable rise in the secondary flow velocity. It is worth noting that Survey 1 was dominated by a large-scale counter-clockwise helical cell after CYP6, but there were always dual counter-rotating cells in Surveys 3 and 4 till CYP8. This may be the result of the combined effect of relative density difference and momentum ratio. This will be discussed later.
图5a-5d显示了在中高流量条件下（调查1、3和4）使用Rozovskii定义，将次级流速矢量叠加在主要流速轮廓上的情况。在所有调查中，从CYP2到CYP6的近场横截面中存在双重逆时针旋转的细胞。长江侧的左螺旋细胞是由长江流曲率产生的，其强度主要取决于流曲率和 （Constantinescu等，2011年；Yuan等，2021年）。由于鄱阳湖的力量更大，这个螺旋细胞在第4次调查中变得更强，对应于较大的动量比 。在所有调查中，CYP2附近的右侧观察到了一个逆时针螺旋细胞。由于次级流速与主要流速具有相同数量级，螺旋细胞不能仅归因于鄱阳湖流曲率。像长江近床流的渗透（如Yuan等人，2021年所建议的）这样的额外因素，其具有更大的密度 和动量通量进入鄱阳湖流，不能被忽视。 CYP2下游，鄱阳湖的排水受到长江流域的挤压而顺时针偏转（图4）。结合流动穿透的影响，导致次生流速显著增加。值得注意的是，调查1在CYP6之后被大规模逆时针螺旋细胞所主导，但调查3和4一直到CYP8始终存在双向逆时针旋转的细胞。这可能是相对密度差异和动量比率的综合效应。这将在后面讨论。

Figure shows the vectors of the secondary velocity superimposed on the contours of the primary velocities under low flow condition (Survey 2). Compared with Surveys 1,3, and 4, the velocity in the Poyang Lake outflow channel was much smaller. Due to the minor density difference between the tributaries , helical cells caused by flow penetration were not observed at two sides of the shear layer at CYP2. While a single, channel-scale, clockwise secondary circulation occurred and developed from CYP4 to CYP8, whose size gradually decreased downstream, that is, the size of the secondary circulation had its maximum at CYP4. Interestingly. Yuan et al. (2021) reported that in December 2018 the size of the channel-scale helical cell had its maximum at CYP5 and CYP6, where the bend curvature was the largest. That difference may be related to the stratification observed during the present study in CYP4 caused by momentum flux and density contrast. The fast homogenization of velocity in Survey (Figure 4b) was likely related to such channel scale helical cell resulting a strengthened exchange of the lateral momentum (Lewis & Rhoads, 2015).
图 显示了在低流量条件下（第 2 次测量）在一次速度轮廓上叠加的次要速度矢量。与第 1、3 和 4 次测量相比，鄱阳湖出流河道的流速要小得多。由于支流之间的密度差异较小 ，在 CYP2 剪切层两侧未观察到由流动穿透引起的螺旋细胞。在 CYP4 到 CYP8 之间出现了单一的、通道尺度的、顺时针的次要环流，其大小逐渐在下游减小，也就是说，次要环流的大小在 CYP4 处达到最大。有趣的是，Yuan et al.（2021）报告称，2018 年 12 月，通道尺度螺旋细胞的大小在 CYP5 和 CYP6 处达到最大，其中的弯曲曲率最大。这种差异可能与 CYP4 中所观察到的由动量通量和密度对比引起的分层有关。第 2 次测量中速度的快速均质化（图 4b）很可能与这种尺度的通道螺旋细胞相关，导致了侧向动量的增强交换（Lewis & Rhoads, 2015）。

Water conductivity was a suitable tracer to illustrate the mixing processes at the confluence of the Yangtze River and the Poyang Lake, which naturally have a large difference in water conductivity. Moreover, water conductivity is a conservative parameter (Gaspar, 1987) and can be continuously recorded in situ using multiparameter water chemistry meter, as suggested by Moody (1995) and Pouchoulin et al. (2020).
水电导率是描述长江与鄱阳湖汇流处混合过程的合适示踪指标，由于长江与鄱阳湖在水电导率上存在较大的差异。此外，水电导率是一个保守参数（Gaspar，1987），可以使用多参数水化学仪在原位连续记录，正如 Moody（1995）和 Pouchoulin 等人所建议的。

Figure 6 shows the contours of conductivity near water surface at the confluence during the four field surveys. For the medium and high flow conditions (Figures 3a-3d), the maps of conductivity highlighted slow mixing processes with clear mixing interfaces near the water surface, maintained for a long distance. It is
图 6 显示了四次野外调查期间汇流处水表面附近的电导率等值线图，对于中高流量条件（图 3a-3d），电导率图表明缓慢的混合过程，并在水表面附近形成清晰的混合界面，并保持了很长距离。

Figure 5. Secondary flows in representative cross sections for (a) Survey 1; (b) Survey 2 2020; (c) Survey 3; (d) Survey 4 looking downstream (Vector: secondary velocity; contour: primary velocity; WL: water level with units of ). The values of (with an arrow to indicate the direction of decreasing density gradient) and were consistent with Table 1 .
图 5 显示了代表性横截面中的二次流动情况，包括（a）第 1 次调查；（b）2020 年第 2 次调查；（c）第 3 次调查；（d）第 4 次调查，向下看（矢量：二次速度；等高线：一次速度；WL：水位，单位为 ）。 的值（箭头指示密度梯度下降方向）和 与表 1 中的值一致。

noteworthy that, in Survey 1, the flow originating from the Poyang Lake, with lower water conductivity, seemed to cross over the flow originating from the Yangtze River with higher water conductivity. However, this interesting phenomenon was not obvious in Surveys 3 and 4, where clear mixing interfaces persisted till the end of the marked study area. Comparatively, in Survey 2 under the low flow condition the mixing interface near the water surface disappeared near CYP5 (Figure 3b), suggesting a rapid mixing process. These results were also consistent with the remote sensing images in Figure 4. In Surveys 3 and 4, another clear mixing interface of different conductivity was observed in the tributary (namely Poyang outflow channel) when the large floodplain on the left of the Poyang outflow channel was submerged (Figures and ). This interface persisted for a long distance, and the development of this interface could be due to the erosion of sediment on the floodplain. As the water depth increased (Figures 3c to 3d), the conductivity difference in the Poyang outflow decreased from 22 to . The mixing interface in the tributary was no longer evident in Survey 1, probably because the large water depth and low flow velocity caused by the backwater effect of Yangtze River on Poyang Lake.
值得注意的是，在调查1中，来自鄱阳湖的水流，水电导率较低，似乎穿过了来自长江的水电导率较高的水流。然而，这一有趣的现象在调查3和4中并不明显，清晰的混合界面一直持续到标记研究区域的末端。相比之下，在低流量条件下的调查2中，水面附近的混合界面在CYP5附近消失（图3b），表明了一个快速的混合过程。这些结果也与图4中的遥感图像一致。在调查3和4中，当鄱阳湖出流河道左侧的大洪泛平原被淹没时，观察到了另一个不同电导率的清晰混合界面（即鄱阳湖出流河道）。这个界面持续了很长一段距离，这个界面的发展可能是由于洪泛平原上的泥沙侵蚀。随着水深的增加（图3c至3d），鄱阳湖出流的电导率差从22减少到23。 在调查 1 中，支流中的混合界面不再明显，可能是因为长江对鄱阳湖的返流效应导致水深较大、流速较低。

Figure 7 presents the vertical profile of water conductivity at sampling points, in . In all the surveys, upstream of CYP2 the distribution of water conductivity in both Yangtze River and Poyang Lake sides was almost uniform at each sampling point without any clear vertical stratification. While on the Yangtze River side in Survey 1, from CYP2 to CYP5, the vertical distribution of conductivity also remained uniform; on the
图 7 展示了取样点处水电导率的垂直剖面，在 。在所有调查中，在 CYP2 上游，长江和鄱阳湖两侧的水电导率分布在每个取样点几乎是均匀的，没有明显的垂直分层。而在调查 1 中，从 CYP2 到 CYP5，长江一侧的电导率垂直分布也保持均匀；在

Figure 6. Contours of water conductivity near the water surface during four field surveys. The values of (with an arrow to indicate the direction of decreasing density gradient) and were consistent with Table 1 . The symbols indicate water chemistry sampling points in each survey. The values of average conductivity in floodplain (ACF) and average conductivity in main channel of Poyang Lake outflow (ACM) were shown with units of . Background images were obtained by using remote sensing (Sentinel-1/2 with low cloud cover conditions ) during each survey.
图6. 在四次现场调查期间，水面附近的水电导率等高线。 和 的数值与表1一致。符号表示每次调查中的水化学采样点。洪泛平原的平均电导率（ACF）和鄱阳湖出流主河道的平均电导率（ACM）的数值以 为单位显示。背景图像是在每次调查期间利用遥感（Sentinel-1/2在低云层条件下 ）获得的。

Poyang Lake side (i.e., CYP4) the conductivity near the bed was larger than that near the surface. This reveals that entrainment was taking place in the lower part of the water column of Yangtze River with high conductivity. Further downstream such distribution was still observed with a gradual increasing of conductivity over depth that confirms the ongoing development of mixing process (Figure 7a). Such a helical mixing structure was also observed in Figure 6a, clearly speeding up the mixing between the two confluent flows. The counter helix mixing structure seemed to occur in Survey 2 so that at CYP4 in the Poyang Lake side the water conductivity near the water surface in the Poyang Lake side was larger than that near the bed. Rapid mixing was found and the distribution was much more uniform downstream of CYP5 in Survey 2 (Figure 7b) than in the other surveys. The abrupt drop in the conductivity profile in Survey 2 may be caused by small momentum ratio and density difference (as discussed later). In Surveys 3 and 4, vertically uniform profiles were observed for long distance on both sides of the mixing layer. In Survey 3, although the vertical distribution of water conductivity was almost uniform at all sampling points, conductivity values on the Poyang Lake side gradually increased, indicating a slow development of the mixing process between two confluent flows (Figure 7c). In Survey 4 the uniformity conductivity near the bank was seen till the end of the study area in agreement with the contour of conductivity near the water surface (Figure 6d). This revealed a very weak transverse mixing. While the conductivity of the sampling points located near the mixing layer (CYP0/3/6-M) indicated that the mixing layer in Survey 4 also had a significant lateral tilt with a tilt amplitude increasing downstream. This suggests that the mixing process may developed greatly under the surface water. Therefore, the conductivity at the surface water and in a limited number of sampling points may not clearly describe the mixing process in the post-confluence channel and further data analysis is needed.
鄱阳湖边（即CYP4）底部附近的电导率大于表面附近的电导率。这表明，在电导率较高的长江水域的较低水层发生了混合作用。进一步向下游观察到这种分布，电导率逐渐增加，深度证实了混合过程的持续发展（图7a）。在图6a中也观察到了这样的螺旋混合结构，明显加快了两个汇流流体之间的混合。第二次调查中似乎出现了反方向的螺旋混合结构，使得在鄱阳湖一侧的CYP4处，与床面附近相比，水面附近的电导率更大。在第二次调查中，下游的CYP5处的混合更为快速，分布更加均匀（图7b），相比其他调查，电导率曲线的突然下降可能由于动量比和密度差异小（稍后将讨论）。在第3和第4次调查中，在混合层的两侧长距离上观察到垂直均匀的剖面。 在第三次调查中，尽管水电导率的垂直分布在所有取样点几乎是均匀的，但鄱阳湖一侧的电导率值逐渐增加，表明两股汇流流动之间的混合过程发展缓慢（图7c）。在第四次调查中，沿岸附近的电导率均匀性一直保持到研究区域末端，与水面附近电导率等高线一致（图6d）。这表明横向混合非常弱。位于混合层附近的取样点的电导率（CYP0/3/6-M）表明，第四次调查中的混合层也具有明显的横向倾斜，倾斜幅度随下游增加。这表明混合过程可能在地表水下得到很大发展。因此，表面水的电导率和有限数量的取样点可能无法清楚描述汇合后水道中的混合过程，需要进一步的数据分析。

To gain insights into this difference, the cross-sectional distribution of backscatter (BS) gained by the ADCP measurements was analyzed. BS value could be related to TSS concentration after a proper calibration (Szupiany et al., 2009) and used to identify the location of the mixing interface between two flows of different TSS concentration (Gualtieri et al., 2019). Figure 8 presents the contours of the BS intensity distribution directly measured by RDI at CYP2, CYP4, CYP6, and CYP8 during Surveys 1, 3, and 4. The red and yellow zones indicate the
为了深入了解这种差异，分析了由 ADCP 测量获得的回波散射（BS）的横截面分布。BS 值经过适当校准后可以与 TSS 浓度相关联（Szupiany 等，2009 年），并可用于识别两种不同 TSS 浓度流之间混合界面的位置（Gualtieri 等，2019 年）。图 8 展示了在调查 1、3 和 4 期间由 RDI 直接测量的 CYP2、CYP4、CYP6 和 CYP8 处 BS 强度分布的等高线。红色和黄色区域表示

Figure 7. Vertical profiles of conductivity in the field surveys. The locations of the sampling points are marked in Figures 1c and 6. The values of (with an arrow to indicate the direction of decreasing density gradient) and were consistent with Table 1.
图 7. 田野调查中的电导率垂直剖面。采样点位置在图 1c 和 6 中标出。 的数值（带有箭头表示密度梯度递减方向）和 与表 1 一致。

sediment rich waters of the Yangtze River and the blue indicates Poyang Lake waters with low TSS concentration. The dotted lines demarcate the location of the mixing interfaces. The mixing patterns derived from the analysis of the BS data were consistent with the distribution of water conductivity (Figures 6 and 7). In Survey , at CYP2 a significant difference in BS between the tributaries was observed across the mixing interface which was tilted obviously. Farther downstream, the mixing interface was subjected to a complex development due to the entrainment of sediment-laden flow at the bed moving from the Yangtze River side into the Poyang Lake pool, which was a kind of "double helix structure" of mixing pattern as aforementioned. At CYP8, the low sediment-laden Poyang Lake waters even moved to the center of river downstream of CYP6, while the sediment-rich Yangtze River waters moved toward the right bank. The same process also occurred in Survey 4 . Although the existence of dual-counter rotating cells restricted the mixing process of two flows, the entrainment of sediment-laden flow at the bed moved from the Yangtze River side into the Poyang Lake pool due to the density different. Because the and TSS of the two tributaries in Survey 4 were close (Table 1), the low sediment-laden Poyang Lake waters moved to the center of river till CYP8-A, and the double helix structure was not as obvious as that in Survey 1. In Survey , although the mixing layer was also tilted, the sediment-rich waters on the Yangtze River side and the low sediment-laden waters on the Poyang Lake side were constrained on both the sides of the mixing layer by the dual-counter rotating cells, resulting in a slow mixing. The reason for the difference results between Survey 3 and Survey 1,4 will be discussed later.
长江的富含沉积物的水域和蓝色表示鄱阳湖水的悬浮物浓度较低。虚线标示混合界面的位置。通过对数据进行分析，得出的混合模式与水的电导率分布（图6和7）一致。在调查 中，在CYP2的支流之间的混合界面上观察到明显的BS差异。在更下游，由于由长江一侧的含沉积物流动的床移动到鄱阳湖池中，混合界面经历了复杂的发展，形成了前述的“双螺旋结构”的混合模式。在CYP8，鄱阳湖水的低含沉积物区甚至移动到CYP6以后的河道中心，而富含沉积物的长江水则向右岸移动。在第4次调查中也发生了同样的过程 。 尽管双逆时针旋转细胞的存在限制了两股流体的混合过程，但由于密度差异，Poyang湖床部的含沙流体夹带从长江一侧流入长江池。由于调查4中的两条支流的 和TSS接近（见表1），低含沙量的鄱阳湖水流动到了河流的中心位置，直到CYP8-A，双螺旋结构并不像调查1中那样明显。在调查 中，虽然混合层也倾斜了，但长江一侧富含沙的水和鄱阳湖一侧含沙量低的水被双逆时针旋转细胞约束在混合层的两侧，导致混合速度较慢。调查3和调查1、4之间的差异结果将在稍后讨论。

(c)

Figure 8. Distribution of the backscatter intensity and secondary flow for transects CYP2, CYP4, CYP6, and CYP8 during Survey 1 (a), Survey 3 (b), and Survey 4 (c; CYP8-A) Looking downstream (Vector: secondary velocity; contour: backscatter intensity). Note the use of different scales for different surveys (Top color bar for Survey 1, and 4; Bottom color bar for Survey 3). The values of (with an arrow to indicate the direction of decreasing density gradient) and were consistent with Table 1.
图 8。调查 1（a）、调查 3（b）和调查 4（c；CYP8-A）期间横贯 CYP2、CYP4、CYP6 和 CYP8 的回波强度和二次流分布。向下游观察（矢量：二次速度；等高线：回波强度）。请注意不同调查使用不同比例尺（调查 1 和 4 的顶部色标；调查 3 的底部色标）。 的值（带箭头指示密度梯度减小方向）和 与表 1 一致。

Mixing at confluences is commonly characterized based on the streamwise variation of the standard deviation of a tracer (Gaudet & Roy, 1995; Horna-Munoz et al., 2020; Lewis et al., 2020; Lewis & Rhoads, 2015). In the present study, the streamwise development of mixing is estimated using a normalized mixing metric developed by Lewis and Rhoads (2015) and Lewis et al. (2020). The metric is based upon the calculation of the standard deviation of the conductivity at each cross-section in the as well as at an upstream composite cross-section consisting of conductivity for the two incoming flows . The standard deviation for the composited cross section was calculated using more than 500 values of the conductivity which were measured by the EXO2 probe during the investigation of bathymetry and distribution of water chemistry parameters near the water surface. The proportion of the 500 values assigned to each tributary flow depends upon the ratio of cross-sectional areas of these flows. The surface mixing rate between the confluence apex and a downstream cross-section is computed as a percentage based on the change in the normalized standard deviation from the upstream section to the current section: . If the cross section of interest has the same tracer standard deviation as the hypothetical upstream cross section , no mixing occurs, and equals 0 . If the nondimensional conductivity field at a cross section is completely uniform , mixing is complete and equals 100 .
在汇流处的混合通常是基于示踪剂标准偏差的沿流变化而表征的（Gaudet & Roy, 1995; Horna-Munoz等，2020; Lewis等，2020; Lewis & Rhoads, 2015）。在本研究中，混合的沿流发展是使用Lewis和Rhoads（2015）以及Lewis等人（2020）开发的标准化混合度量来估计的。该度量基于在每个横截面 处以及由两个流入流的电导率组成的上游复合横截面的电导率的标准偏差的计算。复合横截面的标准偏差是使用EXO2探头测量的500多个电导率值计算的，这些值是在调查水底地形和水化学参数分布时测量的。分配给每个支流流量的500个值的比例取决于这些流量的横截面面积比。 集合顶点和下游横截面之间的表面混合率按照标准差的标准化变化计算为百分比 ： 。如果感兴趣的横截面与假设的上游横截面具有相同的示踪剂标准差 ，则不会发生混合， 等于 0 。如果横截面上的无量纲导电率场完全均匀 ，则混合完全， 等于 100 。

Figure 9. Longitudinal distribution of the surface mixing rate given as a percentage of mixed conductivity at the surface of cross section, . The distance is measured along the centerline of the mixing interface near the water surface with distance at the confluence apex (CYP1)
图 9. 集合顶点处（CYP1）水面附近的混合界面中心线上，表面混合率按照混合导电率的百分比给出， 距离以 测量。

Figure 9 presents the longitudinal distribution of for the four surveys, and the spatial change of with distance can reflect the mixing rate near the water surface (Rhoads & Johnson, 2018). The plot confirms that among the four surveys, Survey 2 had the most rapid mixing as it was almost completed ( approximately 70%) at CPY5. For the other three surveys, upstream of CYP6, the mixing rate was about the same. However, farther downstream it increases significantly in Survey 1 , which was related to the double helix structure of mixing pattern. At the downstream end of the study area that percentage was of approximately in Survey 2, while for Survey 1, Survey 3 and Survey 4 it was in the order of and 30, respectively.
图 9 展示了四次调查中 的纵向分布，以及 随距离的空间变化可以反映水面附近的混合速率（Rhoads＆Johnson，2018）。图表证实，在四次调查中，调查 2 的混合速率最快，因为在 CPY5 处几乎完成（ 约为 70％）。对于其他三次调查，在 CYP6 的上游，混合速率大致相同。然而，在下游更远的地方，调查 1 中的混合速率显着增加，这与混合模式的双螺旋结构有关。在研究区域的下游末端，调查 2 的百分比约为 ，而对于调查 1、调查 3 和调查 4，百分比分别为 和 30。

Momentum flux ratio, channel-scale secondary cells, and density difference are common factors that affect the mixing processes between two confluent flows (Cheng & Constantinescu, 2021; Constantinescu et al., 2012, 2014, 2016; Horna-Munoz et al., 2020; Lane et al., 2008; Lewis et al., 2020; Lewis & Rhoads, 2015; Rhoads & Sukhodolov, 2008). In this section, attempts have made to determine the possible causes that result in the different mixing patterns of these four surveys.
动量通量比、通道尺度次生细胞和密度差异是影响两个汇合流之间混合过程的常见因素（Cheng & Constantinescu, 2021; Constantinescu 等，2012, 2014, 2016; Horna-Munoz 等，2020; Lane 等，2008; Lewis 等，2020; Lewis & Rhoads, 2015; Rhoads & Sukhodolov, 2008）。在本节中，尝试确定导致这四项调查不同混合模式的可能原因。

Laboratory (Best, 1987, 1988), field (Gualtieri et al., 2018; Lane et al., 2008; Lewis & Rhoads, 2015; Rhoads, 1996; Rhoads & Kenworthy, 1998; Rhoads & Sukhodolov, 2001; Yuan et al., 2021), and numerical (Bradbrook et al., 2000, 2001; Constantinescu et al., 2016) studies of river confluences have shown that the has a critical effect on the interaction between two confluent streams. The shear layer presents high levels of turbulence and large-scale coherent structures, which enhanced lateral mixing. For all the surveys, a distinct velocity contrast exists between the two confluent flows. The depth-averaged velocity patterns show that the shear layer is relatively aligned with the upstream mixing interface, especially in Survey 2. The location of the shear layer and mixing interface shifted toward the right bank for low (Surveys ) and toward the center of the post-confluence channel for high (Survey 4), Therefore, we can infer that dominates the transversal location of the mixing interface. Furthermore, in Surveys 1,3,4 complete re-alignment of the Poyang Lake flow with the Yangtze River post-confluence channel occurred near CYP7 and the shear layers disappears, even though the mixing interface extended downstream till the end of the study reach. That is, lateral transport associated with these coherent structures within the shear layer is insufficient to produce conspicuous mixing between
实验室（Best, 1987, 1988），野外（Gualtieri等，2018；Lane等，2008；Lewis＆Rhoads，2015；Rhoads，1996；Rhoads＆Kenworthy，1998；Rhoads＆Sukhodolov，2001；Yuan等，2021），和数值（Bradbrook等，2000，2001；Constantinescu等，2016）研究表明，河流汇合处对两条汇合流之间的相互作用有关键影响。剪切层呈现出高水平湍流和大尺度的一致结构，增强了横向混合。在所有调查中，两条汇合流之间存在明显的速度对比。深度平均速度模式显示，剪切层与上游混合界面相对较为对齐，尤其是在第二次调查中。剪切层和混合界面的位置在低 （第二次调查）时向右岸移动，在高 （第四次调查）时向汇合后水道中心移动，因此，我们可以推断 主导混合界面的横向位置。 此外，在 1、3、4 次调查中，鄱阳湖流域与长江汇流河道的流动发生了完全重新对齐，发生在 CYP7 附近，剪切层消失了，尽管混合界面一直扩展到研究范围的末端。也就是说，在剪切层内与这些相干结构相关的横向输运不足以产生明显的混合。
two flows, suggested by Lane et al. (2008). Therefore, other driving factors which cause different mixing patterns and rates in the four surveys need further discussion.
Lane 等人（2008 年）的研究建议，这两股流体之间存在着不同的混合模式和速率，因此需要进一步讨论导致这种差异的驱动因素。

The results for all four surveys indicated that the development of secondary flow can play an important role in mixing dynamics. The presence of single channel-scale single secondary cell (Survey 2), or dual secondary cells (Surveys 1, 3, and 4) contribute to the corresponding mixing patterns between the two confluent flows. The helical motions on the left side restricted the size of the core of high sediment concentration. The downwelling flows between the dual counter-rotating helical cells acted as a barrier preventing mixing between the two rivers (Yuan et al., 2021). These results are in agreement with recent studies on mixing at other confluences, which have indicated that little to no mixing takes place within the CHZ when dual helical cells develop (Chen et al., 2017; Konsoer & Rhoads, 2014; Rhoads & Johnson, 2018; Riley et al., 2015). However, as the helical cell from the Poyang Lake became dominant and almost occupied the whole cross section (after CYP6 in Survey 1 and after CYP8 in Survey 4), mixing was enhanced. The flow from the Yangtze River was advected near the bed into the flow from the lateral tributary, thereby contributing to the "double helix" structure observed in Surveys 1 and 4 (Figures and ). On the other hand, there were always dual counter-rotating cells, extending till the end of study area in Survey 3, which resulting in a relatively slow mixing rate (Figures 6 and 8). That is, dual counter-rotating helical cells limit the mixing processes at the post-confluence channel (Surveys 1, 3 and 4), especially when is small (Survey 3). At confluences, subsurface thermal mixing is greatly enhanced by secondary currents associated with helical motion, especially when a single dominant cell develops (Lewis & Rhoads, 2015; Rhoads & Johnson, 2018). Although the main cause of rapid mixing process in Survey 2 were the small momentum ratio ( ) and density effect, the development of a curvature-driven large-scale helical motion also played an important role. This aspect has been supported by recent field studies on mixing at confluences (Herrero et al., 2018; Lane et al., 2008; Sukhodolov & Sukhodolova, 2019).
所有四次调查的结果表明，次级流的发展在混合动态中起着重要作用。单一通道尺度的单一次级细胞（第 2 次调查）或双次级细胞（第 1、3 和 4 次调查）有助于两个汇流流之间的相应混合模式。左侧的螺旋运动限制了高沉积物浓度的核心尺寸。双对旋转螺旋细胞之间的下沉流作为一道障碍，阻止了两个河流之间的混合（Yuan 等，2021 年）。这些结果与最近在其他河汇交汇处进行的混合研究结果一致，表明当双螺旋细胞发展时，在 CHZ 内几乎没有或很少发生混合（Chen 等，2017 年；Konsoer＆Rhoads，2014 年；Rhoads＆Johnson，2018 年；Riley 等，2015 年）。然而，当鄱阳湖的螺旋细胞变得主导并几乎占据整个横截面（第 1 次调查后的 CYP6 和第 4 次调查后的 CYP8），混合被增强。 长江的水流被输送到床附近的水流中，从而导致了在调查 1 和 4 中观察到的“双螺旋”结构（图 和 ）。另一方面，在调查 3 中一直存在着延伸至研究区域末端的双逆时针旋转的细胞，导致相对较慢的混合速率（图 6 和 8）。也就是说，双逆时针旋转的螺旋细胞限制了汇合后河道的混合过程（调查 1、3 和 4），特别是当 较小时（调查 3）。在汇合处，与螺旋运动相关的次生流体运动极大地增强了地下热混合，特别是当单一主导细胞发展时（Lewis＆Rhoads，2015；Rhoads＆Johnson，2018）。尽管调查 2 中混合过程迅速的主要原因是小的动量比（ ）和密度效应，但曲率驱动的大尺度螺旋运动的发展也起到了重要作用。这一方面得到了最近关于汇合处混合的现场研究的支持（Herrero 等，2018；Lane 等，2008；Sukhodolov＆Sukhodolova，2019）。

Past literature suggested that the hydrodynamics and mixing processes at confluences are affected by density difference (Gualtieri et al., 2019; Jiang et al., 2022; Lewis & Rhoads, 2015; Lyubimova et al., 2014; Ramón et al., 2013, 2014). The lock-exchange mechanism generated by the density difference, may work with/against the curvature-induced structure of helical cells in the Yangtze River (Horna-Munoz et al., 2020). The Yangtze River flow had large density effect in Surveys 1 and , and the advective mixing by helical motion was impeded as the lock-exchange acted against this pattern (Figure 10a). Density differences promoted outward movement of fluid near the bed and inward movement of fluid near the surface from the Poyang Lake having a lower density, in opposition to the influence of helical motion on near-surface and near-bed flow. As the helical motion decayed downstream, the warm, less dense water of Poyang Lake gradually moved to the middle of the channel near the surface while cool, dense water of Yangtze River crossed the bed and moved to the right bank, and a single channel-scale secondary flow formed downstream (after CYP6 in Survey 1; after CYP8 in Survey 4) to enhance the water mixing (Figures 8a and 8c). Although the backscatter profiles (Figure 8) also suggest that the core of lighter fluid was confined in the center of the channel in Figure 10a, this may be only until the denser flow upwells. Eventually, the lighter fluid will spread out laterally to form a thin layer near the surface if there is no force to keep it there against the pull of gravity. It is worth noting that the surface mixing rate of Survey 4 was the slowest, which may be related to the largest momentum flux ratio increasing the mixing distance. Therefore, it is improper to indicate the mixing rate using remote sensing images or surface water quality data at large-scale river confluences, especially those with density differences. Even though value in the Survey was close to that in Surveys 1 and 4, it also mixed slowly without a clear stratification (Figures and . On the one hand, the density effects might have been weakened by the larger relative bed roughness associated with the lower water depth in Survey 3 (Best & Roy, 1991; Constantinescu et al., 2016). On the other hand, the density effect of Survey 3 may not be as large as the density Froude number indicated (Duguay et al., 2022b; Pouchoulin et al., 2020).
过去的文献表明，汇合处的水动力学和混合过程受密度差异影响（Gualtieri等，2019年；江等，2022年；Lewis & Rhoads，2015年；Lyubimova等，2014年；Ramon等，2013年，2014年）。由密度差异产生的锁定交换机制可能会与/反对扬子河螺旋细胞的曲率诱导结构相互作用（Horna-Munoz等，2020年）。在调查1和0中，扬子河流在密度上产生了很大的影响，而由于锁定交换作用于该模式，螺旋运动的迁移混合受到了阻碍（图10a）。密度差异促使抚仙湖床近处的流体向外移动，使密度较低的抚仙湖表面附近的流体向内移动，与螺旋运动对近表面和近床流动的影响相矛盾。 随着螺旋运动在下游的减弱，鄱阳湖温暖、密度较低的水逐渐移动到河道中间的表面附近，而长江的凉爽、密度较高的水则穿过河床向右岸移动，形成了单一的河道尺度的次要流动（在第一次调查后CYP6; 在第4次调查后CYP8），以增强水体的混合（图8a和8c）。虽然回波剖面（图8）也在图10a中表明较轻的流体核心在河道中心受限，但这可能只是在较密集的流动上升之前。最终，如果没有力量阻止它受到重力拉扯，较轻的流体将横向扩散形成靠近表面的薄层。值得注意的是，第4次调查的表面混合速率最慢，这可能与动量通量比 增加了混合距离有关。因此，在大尺度的河流交汇处特别是密度差异较大的地方，用遥感影像或表面水质数据来指示混合速率是不妥的。 尽管调查 中的 值接近于调查 1 和 4 中的值，但它也在没有明显分层的情况下缓慢混合（图 和 ）。一方面，密度效应可能已经被相对较低的水深所关联的较大相对床面粗糙度削弱（Best＆Roy，1991; Constantinescu 等，2016）。另一方面，第 3 次调查的密度影响可能并不像密度弗洛德数量所显示的那样大（Duguay 等，2022b; Pouchoulin 等，2020）。

In Survey , the rapid mixing can be related to the large lateral expansion of the Yangtze River into the Poyang Lake portion of the confluence due to the large momentum and density difference, resulting in a vertical stratification at CYP4 (Figure 7b). The abrupt drop in the conductivity profile in Survey 2 might not be
在 调查中，Yangtze 河向鄱阳湖汇合部分的侧向扩张较大可能与大动量和密度差异有关，导致在 CYP4 处的垂直分层（图 7b）。调查 2 中电导率剖面的急剧下降可能不
(a)

(b)

Figure 10. Conceptual model showing patterns of lateral mixing between two flows with different densities at the confluence of Yangtze River and Poyang Lake. (a) Yangtze River contains fluid denser than the Poyang Lake, and (b) Poyang Lake contains fluid denser than the Yangtze River. Red arrows correspond to flow from the Yangtze River and blue arrows correspond to flow in the Poyang Lake, with solid arrows representing flow near the surface and dashed arrows representing flow near the bed. Curved arrows in cross-section denote strong, curvature-induced cells of helical flow, while the dashed arrows show the development of the near-bed and free-surface intrusions associated with the lock-exchange like flow. Dark blue corresponds to unmixed fluid with cold temperature and higher density, Red corresponds to unmixed fluid with warm temperature and lower density, and light blue corresponds to mixed fluid between the two unmixed fluid mentioned above.
图10. 概念模型显示在长江与鄱阳湖交汇处两股不同密度流体间的横向混合模式。(a) 长江含有比鄱阳湖更密的流体，(b) 鄱阳湖含有比长江更密的流体。红色箭头表示长江的流动，蓝色箭头表示鄱阳湖中的流动，实线箭头代表近水表面处的流动，虚线箭头代表近床层的流动。横截面中的曲线箭头表示强烈的曲率诱导的螺旋流细胞，而虚线箭头展示了与类似于锁定交换的流动相关的近底层和自由水面侵入的发展。深蓝色代表未混合流体，具有低温和较高密度，红色代表未混合流体，具有高温和较低密度，浅蓝色代表两种上述未混合流体之间的混合流体。

only caused by momentum ratio effects that could cause a much less abrupt gradient due to turbulent momentum exchange along the mixing interface. The rapid drop is more consistent with the interface between stratified layers of a lighter and a denser fluid due to the density difference. Despite the density effect was weaker than that in higher flow conditions, density difference and curvature-induced cells of helical flow were working together (Figure 10b). Both the lock exchange and the helical cell moved from Poyang Lake to Yangtze River at the bed and from Yangtze River to Poyang Lake at the free surface. In addition, the was the lowest and Yangtze River pushed strongly against Poyang Lake side to form a two-layer structure. Hence, the penetration of the Yangtze River flow at the free surface was enhanced (Lane et al., 2008). This explains the fastest mixing found in Survey 2.
由于动量比例效应引起的，可能会在混合界面沿湍流动量交换引起更平缓的梯度下降。快速下降更符合较轻和较密流体之间分层界面的特征，这是由密度差异引起的。尽管密度效应 弱于较高流量条件下的效应，但密度差异和曲率引起的螺旋流细胞是共同起作用的（图10b）。无论是锁定交换还是螺旋细胞都从鄱阳湖移动到长江底部，从长江移动到鄱阳湖的自由水面。此外， 最低，长江强烈推动鄱阳湖一侧形成双层结构。因此，长江流量在自由水面的侵蚀得到加强（Lane等人，2008）。这解释了在第二次调查中发现的最快混合现象。

From the above results and discussion, it is possible to assess the contribution of the momentum flux ratio, large-scale secondary cells, and the density effects to river mixing patterns at the large-scale river confluence of Yangtze River and Poyang Lake. Table 2 lists the causes of different mixing patterns in these four surveys. A summary of the main causes of mixing patterns when mixing is slow or rapid is given in Table 2.
根据上述结果和讨论，可以评估动量通量比、大尺度次生环流以及密度效应对长江和鄱阳湖大尺度河流汇合处的混合模式的贡献。表 2 列出了这四次调查中不同混合模式的原因。当混合速度较慢或较快时，表 2 总结了混合模式的主要原因。

Four field surveys were conducted to investigate the lateral mixing dynamics of a large river confluence (combined width ) between the Yangtze River (the largest river in China) and the outflow channel of Poyang Lake (the largest freshwater lake in China). Those surveys were carried out during medium and high (Survey 1,3, and 4), as well as low flow condition (Survey 2).
进行了四次现场调查，研究了长江（中国最大的河流）与鄱阳湖（中国最大的淡水湖）出流河道之间的大型河流汇合（综合宽度 ）的横向混合动力学。这些调查分别在中高（调查 1、3 和 4）和低流量条件（调查 2）下进行。

The main important conclusions drawn from this study are:
该研究得出的主要重要结论是：

Table 2 表 2

Causes of Different Mixing Patterns in These Four Surveys
这四个调查中不同混合模式的原因

The data are available for download: https://doi.org/10.5281/zenodo. 6866986
这些数据可供下载：https://doi.org/10.5281/zenodo.6866986

This research was funded by the National Key R&D Program of China (Grant 2022YFC3200032), National Natural Science Foundation of China (51779080; U2040205; 52079044), the Fundamental Research Funds for the Central Universities (20195025712), the 111 Project (B17015), the Fok Ying Tung Education Foundation (520013312), and the Jiangsu Province Project (2021001). The authors would like to thank Professor Bidya Sagar Pani of the Indian Institute of Technology-Bombay for help in revising this work. Thanks are also extended to Yunqiang Zhu, Yuchen Zheng, Yihong Chen, Chenhui Wang, Hao Wang, Chun Li, and Jiaming Yang of Hohai University for their support during the field surveys.
本研究得到了中国国家重点研发计划资助（2022YFC3200032 号），中国国家自然科学基金资助（51779080 号；U2040205 号；52079044 号），中央高校基本科研业务费资助（20195025712 号），111 计划资助（B17015 号），霍英东教育基金资助（520013312 号），以及江苏省项目资助（2021001 号）。作者要感谢印度理工学院孟买分校的 Bidya Sagar Pani 教授在修订本文时提供的帮助。还要感谢河海大学的朱云强、郑宇辰、陈奕宏、王晨晖、王浩、李春和杨佳明在野外调查期间的支持。

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