Peculiarities of the confluence of rivers in hydrodynamic backwater from the dam

Tang H., Qiu J., Yuan S., Xu L., Xiao Y., Liu M., Rennie C., Gualtieri C.

River confluences are nodes where the unique hydrological processes of two rivers meet, resulting in complex flow structure and water quality mixing processes. Thus, greater food availability and habitat complexity can occur at a confluence, making it a hotspot for fish productivity and diversity. Nonetheless, studies that relate fish community traits to specific habitat characteristics at large river confluences are limited. Two field surveys were conducted at the large confluence between the Yangtze River and the Poyang Lake outflow channel involving fish hydroacoustic detection, environmental DNA, and acoustic velocity profiling. The discharge ratios of this confluence in two surveys were similar, but the water quality conditions represented by turbidity differed greatly. The results demonstrated high spatial heterogeneity of fish density, size, and species near the confluence. In the Yangtze River with high flow velocity, the abundance of small-sized fish was substantially higher than that in Poyang Lake outflow channel with low velocity, while large-sized fish chose their habitats more freely between the two tributaries and post-confluence channel. The convergence of two tributaries created prominent spatial heterogeneity of habitat conditions in the post-confluence channel, thus highest fish abundance and species biodiversity occurred there. The species assemblage structure of the local fish community was greatly affected by the change of water quality, e.g., the intrusion of a turbid flow from Poyang Lake into the Yangtze River, and thus the exchange of fish communities between two tributaries was weakened. The present study highlights the ecological importance of river confluence in improving regional fish abundance and species diversity and provides the theoretical foundation for the conservation and management of the aquatic environment at the confluence and in the whole river ecosystem.

Shi J., Wang L., Yang Y., Huang T.

Thermal stratification has an important impact on the cycling of reservoir water quality. Uneven vertical distribution of water quality factors, such as dissolved oxygen and nutrients, occurs during seasonal stratification, which creates chemical stratification. Typically, thermocline and chemocline characteristics vary across different reservoirs. In this study, a field study was conducted in the Zhoucun Reservoir (ZCR), China, to investigate the variation in water quality during periods of thermal stratification. The results revealed the maximum water depth as 15 m at 122 mamsl, which is relatively shallow for stratified reservoirs. However, an extremely high thermocline strength index (TSI) was recorded, which indicates that the thermal stratification of ZCR water is very stable, contrary to our common understanding that stratification is more obvious at greater water depths. The chemical stratification strength index (IC-i) was compared with the TSI and the relative water column stability (RWCS). The TSI had a high correlation with IC-i, which explains the high stability of ZCR chemical stratification. Moreover, it highlights the suitability of TSI for characterising the chemical stability of reservoir stratification. Finally, this study also found that the formation and disappearance of thermal and chemical stratification in the ZCR were synchronous. As stratification directly influences a reservoir water quality regime, this study may be a helpful reference for optimising water quality management.

Li K., Tang H., Yuan S., Xiao Y., Xu L., Huang S., Rennie C.D., Gualtieri C.

• A field survey on flow at a large confluence of a compound tributary was conducted. • Kinetic energy and momentum correction coefficients at confluence were identified. • Floodplain flow decreased the size of stagnation zone and impacted its location. • Floodplain flow shifted the location of the mixing interface towards the centerline. • Floodplain flow was found to impact on the characteristics of the secondary flow. A field investigation was carried out in March 2019 at the large river confluence between the Yangtze River and the outflow channel with a large inner-side floodplain of Poyang Lake using acoustic Doppler current profiling (ADCP) and water quality sampling. The study was intended to complement previous field studies carried out under different flow conditions in August and December 2018 and to investigate the effects of the alternate flow mechanism (compound channel vs single channel) of the Poyang Lake outflow channel on confluence dynamics. The main hydrodynamics and morphodynamics features were identified and comparatively discussed with those from the previous field studies. The kinetic energy and momentum correction coefficients were found to peak near the confluence apex and decay farther downstream reflecting the gradual flow recovering process in the post-confluence channel. The alternate flow mechanism (compound channel vs single channel) of the Poyang Lake outflow channel and the change in momentum flux ratio M R were found to significantly impact on the characteristics of the stagnation zone, mixing interface and secondary flow at the confluence. The smallest and the largest stagnation zone were observed at the lowest M R and as the outflow channel acted as single channel, respectively, while also the location of the mixing interface was impacted. Furthermore, secondary flow consisting of dual counter-rotating helical cells and channel scale circulation were observed as the Poyang Lake outflow channel acted as compound channel and a single channel, respectively. These results provide novel information about confluence dynamics in large river systems where one tributary may have a floodplain flow in flooding conditions.

Lyubimova T.P., Lepikhin A.P., Parshakova Y.N., Bogomolov A.V.

The paper presents the results of field measurements and numerical modeling of the influence of various factors on the formation of coherent structures in the confluence zone of the Sylva and Chusovaya rivers, which are dammed by the Kamskaya Hydroelectric Power Station (HPS). A characteristic feature of the measured parameters in the zone under study is that they experience both seasonal fluctuations and fluctuations of much higher frequency associated with intraday regulation of the HPS operation. These intraday fluctuations give rise to coherent structures with periodicity T~2–10 min, which manifest themselves in the fluctuations of the specific electrical conductivity of water. The flow velocity also experiences significant fluctuations with a sufficiently wide frequency spectrum, although the characteristic period of its fluctuations is less than the period of electrical conductivity fluctuations and is equal to ~1 min. In order to study the features of the formation of such structures, numerical simulation was carried out within the framework of the three-dimensional approach. Calculations were performed for a 300-meter-long stretch of the Chusovaya River, which is located downstream of the confluence of Chusovaya and Sylva rivers and is the site of the Chusovskoy water intake of Perm city. It was found that the intraday irregularity of HPS operation gives rise to the occurrence of vortex structures in this layer, leading to the temporal variation of concentration at a given point of space and the formation of the wave structure of the concentration field at different moments of time. Time period and spatial scale of such vortex structures depend on the ratio of velocities of water masses and difference in their mineralization and, accordingly, in densities. Moreover, the period of fluctuations is proportional to the ratio of flow velocities. These estimations are of fundamental importance for the implementation of stable selective intake of water with required consumer properties under conditions of intraday irregularity of hydroelectric power station operation.

Wen L., Wang C., Li Z., Zhao L., Lyu S., Leppäranta M., Kirillin G., Chen S.

There are thousands of lakes in the Tibetan Plateau (TP), and most are saline. However, little is known about the responses of TP lakes to climate change, especially saline ones. We investigated the thermal responses of the largest freshwater lake (Ngoring Lake) in the TP and its nearby small saline lake (Hajiang Salt Pond) to climate change using the improved lake scheme in the Community Land model (CLM4-LISSS), in which we primarily developed the salinity parameterizations previously evaluated in the Great Salt Lake in USA and further considered the effect of salinity on the temperature of the maximum density of saline water in the present study. The improved lake model with salinity parameterizations was first applied to a saline lake in the TP, where saline lakes make up the majority of water bodies. The CLM4-LISSS model could effectively simulate lake surface water temperature (LSWT), lake water temperature (LT) and ice thickness in Ngoring Lake. Additionally, the model including our salinity parameterizations significantly improved simulations of LSWT and LT in Hajiang Salt Pond, especially in winter. The LSWT of the two completely opposite lakes were warming in the simulations at a rate above 0.6 °C/decade. Meteorological forces were the main driving factor, with increasing downward longwave radiation, air temperature and air humidity, as well as weakening winds contributing to LSWT increase. Compared to a hypothetical shallow freshwater lake, the greater depth of Ngoring Lake made its surface warm faster, and salinity slightly accelerated the warming of Hajiang Salt Pond. Monthly mean LSWT differences between the two lakes were induced by salinity effects in cold periods and lake depth in the unfrozen period. In response to a warming climate, the LSWT in the ice-free Hajiang Salt Pond rapidly increased from January to April due to the warming climate, whereas the LSWT of Ngoring Lake increased faster in the first and last month of the ice-cover period due to later ice-on and earlier ice-off. This study will provide a useful tool for saline lakes in the TP and help deepen our knowledge about the responses of TP lakes, especially the saline lakes, to climate change, as well as response differences between freshwater and saline lakes and the reasons for these differences.

Lyubimova T., Lepikhin A., Parshakova Y., Bogomolov A., Lyakhin Y.

The creation of reservoirs in water streams leads to significant changes in the hydrological regime of water bodies: it allows smoothing the peaks of maximum water discharge during a flood period and regulating low-water flow. The creation of reservoirs with significant storage capacity makes it possible to solve a wide range of water-management problems, including the use of falling water energy for hydropower purposes, and maintenance of the uninterrupted water supply and navigation. Since constructed dams are usually operated by hydropower companies, the regulatory regime for the discharge of water into the lower pool is often determined by the daily electricity consumption regime. Intra-day variations in the volume of water discharges through hydroelectric power stations generate multidirectional streams in the upper pool, which can affect the operation of other water withdrawal systems. This paper considers the effect of intraday variations in water discharges into the lower pool on the dynamic and physical properties of the water mass in the region of the location of drinking water-intake heads of Perm city and the quality of the withdrawn water.

Horna‐Munoz D., Constantinescu G., Rhoads B., Lewis Q., Sukhodolov A.

Confluences are locations of complex hydrodynamic conditions within river systems. The effects on hydrodynamics and mixing of temperature-induced density differences between incoming flows are investigated at a small-size, concordant bed confluence. To evaluate density effects, results of eddy-resolving simulations for a densimetric Froude number Fr = 4.9 (weak-density-effects cases) and Fr = 1.6 (strong-density-effects cases) are compared to results of simulations in which the densities of the incoming flows do not differ (no-density-effects cases). Flow patterns predicted for both weak- and strong-density-effects cases show that secondary flow develops with increasing distance from the confluence apex. The pattern of secondary flow is characterized by denser fluid on one side of the confluence moving near the bed toward the side of the downstream channel corresponding to the less dense fluid and the less dense fluid moving near the free surface in the opposite direction. This pattern of fluid motion is similar to a spatially evolving lock-exchange cross flow. In the strong-density-effects simulations, a cross-stream cell of secondary flow develops at the density interface between the flows, similar to interfacial billows generated in classical lock-exchange flows. Density effects increase global mixing with respect to corresponding no-density-effects cases regardless of whether the high-momentum stream contains the higher-density fluid or the lower-density fluid. When density effects are weak, the lock-exchange mechanism either reinforces the pattern of mixing associated with secondary flow induced by inertial forces, particularly helical motion, or opposes this pattern of mixing, depending on which tributary contains the denser fluid. When density effects are strong, flow from the upstream channel with the denser fluid moves under the flow from the upstream channel with the less dense fluid.

Cheng Z., Constantinescu G.

Stratification effects induced by density differences between the incoming flows are investigated at a medium-size stream confluence with a highly discordant bed. The relatively large temperature difference ΔT = 4.7 °C between the main, high-discharge tributary and the low-discharge tributary together with the relatively small velocity of the flow in the main channel translate in a very high Richardson number, Ri = 1.89, which suggests that stratification effects have an important effect on flow hydrodynamics and thermal mixing. Simulation results confirm this and show that for the case when the velocity of the low-discharge, lower-temperature tributary is much smaller than that of the high-discharge tributary, the denser fluid from the low-discharge tributary plunges rapidly toward the bottom and the confluence apex. It then moves as a near-bed current of denser fluid across the main-tributary side of the main channel until the opposing bank where it reaches the free surface. It then continues to move parallel to the bank line. Meanwhile, the less-dense fluid from the main tributary moves over the near-bed current of denser fluid into the central part of the main channel. This induces a two-layer structure of the flow inside the upstream part of the confluence, which explains the very different mean flow patterns near the bed and near the free surface. Flow hydrodynamics and mixing in the Ri = 1.89 simulation are found to be very different when compared to those observed in the no-density-effects (Ri = 0) case where there is no coupling between the temperature and the momentum equations via the Boussinesq approximation. In this case, the fluid from the low-discharge tributary remains on the corresponding side of the main channel. Away from the confluence apex, the volume of mixed fluid is several times larger in the Ri = 1.89 simulation, which demonstrates that under certain conditions two streams of unequal densities can mix much faster compared to the case when the two streams have the same density. The large-scale flow patterns, eddy dynamics and volume of mixed fluid in the Ri = 1.89 simulation were also found to present major differences with a case where the main, high-discharge tributary contained the lower-temperature (denser) fluid.

Gualtieri C., Ianniruberto M., Filizola N.

Confluences are common components of all riverine systems, and characterized by converging streamlines and potential mixing of separate flows. The fluid dynamics of confluences possess a highly complex structure with several common types of flow features observed. An investigation was carried out in both low and relatively high flow conditions at the confluence of the Negro and Solimoes Rivers, Brazil, which ranks among the largest river junctions on Earth. During this field research, acoustic Doppler velocity profiling (ADCP) and water quality sampling were applied to investigate hydrodynamics, sediment transport and mixing characteristics at this confluence. It was found that the location and the width of the mixing interface were closely related to changes of the discharge ratio between the tributaries due to both seasonal and annual variations. Second, a timescales analysis was applied to comparatively identify the contribution to mixing at the Negro/Solimoes confluence of four processes: difference in (1) velocity and (2) density between the rivers, (3) bed friction, including form drag, and (4) change in channel width. The analysis demonstrated that adjustments of confluence hydrodynamics and morphodynamics can modify the relative importance of each contribution. Immediately downstream of the junction the effects of differences in velocity and density were comparable, while farther downstream the latter was predominant and the role of bed friction was significant. At the end, this study suggests that mixing at the Negro/Solimoes confluence can be explained as a combination of the four hydrologic and morphologic processes.

Cheng Z., Constantinescu G.

Umar M., Rhoads B.L., Greenberg J.A.

Although past work has noted that contrasts in turbidity often are detectable on remotely sensed images of rivers downstream from confluences, no systematic methodology has been developed for assessing mixing over distance of confluent flows with differing surficial suspended sediment concentrations (SSSC). In contrast to field measurements of mixing below confluences, satellite remote-sensing can provide detailed information on spatial distributions of SSSC over long distances. This paper presents a methodology that uses remote-sensing data to estimate spatial patterns of SSSC downstream of confluences along large rivers and to determine changes in the amount of mixing over distance from confluences. The method develops a calibrated Random Forest (RF) model by relating training SSSC data from river gaging stations to derived spectral indices for the pixels corresponding to gaging-station locations. The calibrated model is then used to predict SSSC values for every river pixel in a remotely sensed image, which provides the basis for mapping of spatial variability in SSSCs along the river. The pixel data are used to estimate average surficial values of SSSC at cross sections spaced uniformly along the river. Based on the cross-section data, a mixing metric is computed for each cross section. The spatial pattern of change in this metric over distance can be used to define rates and length scales of surficial mixing of suspended sediment downstream of a confluence. This type of information is useful for exploring the potential influence of various controlling factors on mixing downstream of confluences, for evaluating how mixing in a river system varies over time and space, and for determining how these variations influence water quality and ecological conditions along the river.

Taira D.E., Schettini E.B., Silvestrini J.H.

The purpose of this numerical work is focused on the dynamics of a stably stratified inclined mixing layer. Both effects, stratifi- cation and slope, are considered through relevant flow parameters. Chebyshev's approximations and Direct Numerical Simulation (DNS) are used in the context of linear stability analysis for different Richardson numbers and slopes. Two-dimensional temporal and spatial simulations are employed to examine baroclinic layer and the evolution of primary and secondary Kelvin-Helmholtz instabilities. In three-dimensional configuration, only stratification effects are considered. The numerical results show persistence of the translative instability with formation of intense longitudinal vortices highly influenced by the Richardson number.

Lyubimova T., Lepikhin A., Konovalov V., Parshakova Y., Tiunov A.

Summary The peculiarities of the formation of density currents in the zone of confluence of two rivers with strongly different hydrochemical regimes are studied numerically and experimentally. The three-dimensional numerical simulation shows that the water of the river of higher mineralization and density flows under the water of the river of lower mineralization and density and vice versa. And besides, such overlapping of the water streams is observed both upstream and downstream of the confluence of two rivers. The results of numerical simulation are supported by the data of expedition observations and in situ measurements. A similar phenomenon, namely, a flow of two overlapped oppositely directed water streams was previously discovered in the mouth zone of the rivers flowing in the sea. Our study reveals the existence of a new type of the hydrological systems, in which such a phenomenon occurs.

Lyubimova T.P., Roux B., Luo S., Parshakova Y.N., Shumilova N.S.

Abstract. The present study concerns the 3-D distribution of pollutants emitted from a coastal outfall in the presence of strong sea currents. The problem is solved using the nonlinear Reynolds-averaged Navier–Stokes equations in the framework of the k-ε model. The constants of the logarithmic law for the vertical velocity profile in the bottom boundary layer are obtained by processing experimental data from acoustic Doppler current profilers (ADCPs). The near-field distribution of pollutants at different distances from the diffuser is obtained in terms of the ambient flow velocity (steady or with tidal effect) and outfall discharge characteristics. It is shown that even in the case where the effluent density is substantially lower than the ambient sea water density the plume can impact the seabed, creating a risk of pollution of removable bottom sediments.

Bouchez J., Lajeunesse E., Gaillardet J., France-Lanord C., Dutra-Maia P., Maurice L.

Rivers continuously discharge dissolved material to the oceans. Dissolved compounds partially result from water–rock interactions, which produce a large range of water chemical and isotopic compositions. These waters are collected by rivers, that are commonly assumed to be well-mixed with regard to their different tributaries, as a result of turbulent dispersion. In this paper, we test this hypothesis on the Solimoes River (at Manacapuru), the largest tributary of the Amazon River, by analyzing the sodium concentration and strontium isotopic composition of river water on a transverse section at different depths. High-precision measurements reveal lateral heterogeneities. This reflects poor mixing between two main river masses, that have distinct chemical and isotopic signatures, a hundred kilometers downstream from their confluence: the Solimoes mainstream and the Purus River. Using sodium concentration data, the transverse dispersion coefficient is estimated for the studied Solimoes reach (the Earth's largest river on which such an estimate now exists), and is found to be 1.8 ± 0.2 m2/s. Comparison with previously reported data highlights the potential role of bed morphology and islands in the efficiency of lateral mixing in large rivers. We finally demonstrate that the characteristic length of lateral mixing downstream from confluences in large rivers is at least of several tens of kilometers.