Fallen trees enter the adjacent stream and are carried away downstream by the current. As the stream joins another one, the complex hydrodynamics near their confluence make the movement of wood hard ...to predict. These woods may accumulate near the confluence resulting in backwater and subsequent potential flooding. A laboratory study was conducted to investigate the movement and accumulation behavior of individual pieces of wood near the confluence. The characteristics of wood (i.e., the length, diameter, and density) and the hydraulic conditions (i.e., the discharge ratio and the release distance) were varied in this investigation. It was found that the wooden pieces released from the tributary got occasionally trapped in the flow separation zone of the confluence, whereupon they were mainly trapped by a clockwise vortex and continued to stay driven by a reverse cluster of currents within this zone. The accumulation probability of wood was mainly related to its length, the discharge ratio and the release distance. The effect of wood diameter and density within the tested parameters was negligible. The probability increased with an increase in the discharge ratio as well as a decrease in the release distance. The longer pieces had a higher probability of being trapped, whereas for those exceeding some critical value, the probability was nearly the same, or dropped sharply. A generalized model for wood accumulation near the confluence was developed for practical application. These findings carry significant implications for river management, particularly in preventing the risk of flooding caused by wood blockage.
Key Points
Conducting a laboratory study to investigate the transport, accumulation and trapping mechanism of wood near the confluence
Evaluating the wood accumulation probability depending on different wood characteristics and confluence hydrodynamic conditions
Wood released from the tributary may be trapped by the clockwise vortex and thus accumulate in the separation zone
Confluences act as critical nodes in a river network as they affect flow, sediment transport, water quality, and ecological patterns. A complete knowledge about hydro‐morpho‐sedimentary processes at ...river confluences is still incompleted and it has been usually accepted that secondary flows are weak because of the significant role of form roughness in large rivers. In this study, two field surveys were conducted on the flow structure, suspended sediment transport and morphology of the confluence between the Yangtze River (the largest river in China) and the Poyang Lake (the largest freshwater lake in China). Dual counter‐rotating cells were observed during high flow conditions and a single secondary cell appeared in low flow conditions. These helical cells restricted the core size of high sediment concentration and downwelling flows acted as a barrier hindering the exchange of sediment between the two rivers. Furthermore, the observed large scour hole was likely related to the downwelling and upwelling flows caused by helical motions. In low flow conditions, the scour hole looked like a deep channel, which was likely related to a long‐surviving helical cell. The scour hole disappeared further downstream, when either the helical motion got weak during low flow conditions, or when a reverse helical cell occurred during high flow conditions. Hydrodynamics, suspended sediment transport and morphological features observed at such a large confluence demonstrated that river planform geometry and discharge ratio affected the flow structure, especially the helical motion. This in turn affected sediment transport as well as the local bed morphology.
Key Points
Two field surveys on the river confluence of Yangtze River and the outflow channel of Poyang Lake were conducted
Flow structure, suspended sediment transport, and morphology of this large river confluence have been studied
Large‐size helical motions were observed that likely played important roles in suspended sediment transport and local morphology
•Discharge ratio and total flow flux significantly enhanced the release of PFASs.•The dissolved phase was the predominant loading form of released PFASs.•Short-chain PFASs dominated in the dissolved ...phase, while long-chain PFASs in SPM.•Distribution patterns of PFASs highly corresponded to the hydrodynamic zones.•Stream-wise and vertical flow velocities near the SWI were crucial to PFASs release.
The sediments in riverine environments contain notably high concentrations of perfluoroalkyl acids (PFAAs), which may be released into the water body under different hydrodynamic forces, such as those occurring at Y-shaped confluences. The release of PFAAs may pose a significant risk to the surrounding aquatic ecosystems. However, our understanding of the release and transport of PFAAs from sediments at Y-shaped confluences remains unclear. Thus, in this study, we performed a series of flume experiments to explore the effects of discharge ratio and total flow flux on the release and redistribution of PFAAs. The results indicated that these two parameters significantly affected the hydrodynamic features of confluences and the water physicochemical parameters. PFAA concentrations in the dissolved phase and suspended particulate matter (SPM) rose significantly as the discharge ratio and total flow flux increased. The dissolved phase was the predominant loading form of PFAAs, with short-chain PFAAs being the main kind, while long-chain PFAAs were dominant in the SPM. The spatial distribution pattern of PFAAs in sediments at the confluence exhibited a high degree of correspondence with hydrodynamic zones. The separation zone and maximum velocity zone were consistent with sediment regions with low and high capacities to release PFAAs, respectively. The patterns of variation in PFAA distribution were comparable to those observed in hydrodynamic zones as the discharge ratio and total flow flux varied. Furthermore, these two parameters altered the partitioning behaviors of PFAAs; specifically, the PFAAs in sediments tended to be released into the pore-water, while the liberated PFAAs tended to attach to SPM. Linear regression and correlation analyses suggested that the stream-wise and vertical flow velocity components near the sediment-water interface were the primary contributors to sediment suspension and PFAA exchange between the water column and pore-water. These findings will help us to understand the patterns of PFAA release in sediments at Y-shaped confluences and assist in the management of PFAA-contaminated sediments at these locations.
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•Enhanced nitrogen removal processes widely exist in river confluences.•Microbial community in confluences can sustain in downstream area.•Contributions of microorganism in confluences to lower ...reaches decrease with distance.•Confluence characteristics determine influence scope of microorganism in confluences.
River confluences are crucial parts of the river networks, which connect different rivers and promote material exchange between them. Joining of two rivers would cause sharp shifts of flow dynamics and form confluence hydrodynamic zone (CHZ) with specific flow structure and biogeochemical processes. However, CHZs are small areas compared with river networks, whether and how the biogeochemical processes in CHZs would influence the lower reaches and even the whole river networks remain unclear. To address this gap, the present study focused on the nitrogen dynamics in a river network consisting a series of confluences, and combined molecular biological tools, gene-centric modelling approach and SourceTracker analysis to reveal the microorganisms induced nitrogen transformation processes in CHZs and other normal hydrodynamic zones (NHZs). Results illustrated that the enriched microbial species in CHZs were associated with nitrogen transformation, such as Terrimonas and Sphingobacterium, which were reported to be vital participants in nitrate and nitrite reduction processes. The reactions rates of different nitrogen transformation processes, calculated based on functional genes abundances, further revealed that N2 production rates were significantly higher in CHZs than NHZs (p < 0.05), demonstrating CHZs were the hotspots for nitrogen removal in the researched river network. Besides, the specific microbial communities and stimulated nitrogen removal processes in CHZ would sustain in lower reaches. SourceTracker analysis illustrated that microbial communities would exert significant influences on lower reaches, and the influence scope was controlled by confluences characteristics, such as flow ratio between tributary and main stream. Higher discharge ratio would increase the influence scope of microbial community in CHZ. These results indicate that reasonable design of confluence characteristics can be applied as a critical engineering measure for the improvement of ecological health of river networks.
Vegetation greatly affects the flow characteristics and contaminant transport in river confluences. In this study, the flow characteristics and contaminant transport in the non‐vegetated/vegetated ...Y‐shaped confluence were explored systematically through a series of experiments. A total of 10 scenarios were designed to answer the three main research questions: what is the difference between the flow characteristics and contaminant transport in (1) asymmetrical and Y‐shaped confluences; (2) non‐vegetated/vegetated Y‐shaped confluences; (3) vegetated Y‐shaped confluences with different confluence ratios? The experimental results revealed that vegetation remarkably changes the internal flow structure in Y‐shaped confluences. Briefly, the velocity profile can be divided into three vertical layers within the vegetated system, but it remains nearly constant in the non‐vegetated channel. Vegetation changes the circulation location and reduces the intensity of the secondary current, weakening the strength of contaminant mixing. However, the turbulent kinetic energy within the vegetated system is larger than that in the non‐vegetated case, and it peaks at the top of the vegetation canopy. Under different confluence ratio cases, the overall fluctuation of the longitudinal dispersion coefficients along the cross‐sections in the mainstream was similar but increasing the confluence ratio causes the circulation to appear to advance and enhances its intensity. In addition, the vegetation density (200 item/m2) in this study render the manning roughness coefficient at 0.068, which is larger than that under lower vegetation density cases. The outcomes from this study are helpful for both environmental and river management applications.
The interaction of vegetation and channel confluence renders flow characteristics more complex. Vegetation can redistribute the flow velocity zone, change the internal flow structure, and weaken the strength of contaminant mixing. Increasing the confluence ratio causes the circulation to appear to advance and enhances its intensity.
River confluences are sites that operate with complex interactions between material and energy, provided by the combination of different flow patterns that present continuous changes in flow ...structure and sediment transport. In addition, river confluences have intrinsic ecological value, where biophysical processes and ecosystem services may be concentrated. This study aimed to analyze the morphology, flow structure, and hydrosedimentary interaction along tributary-trunk river confluences in a bedrock-alluvial plateau basin. The study included a field investigation into the mixed bedrock-alluvial basin of morphology, bed sediments, and hydrologic condition (including the flow structure) of 29 confluences to identify and analyze the similarities and differences between these confluences at the basin scale. For example, we considered the Ivaí River Basin, a medium-sized dam-free river that exhibits a mixed bedrock-alluvial bed and heterogeneity in both geology and longitudinal profile. Our findings show that the flow structure and morphology are conditioned by geological aspects, and the composition of the flow is extremely affected by the Ivaí River. In general, our results presented distinct differences from field-based data described in the literature for alluvial river confluences.
•Study of flow structure, morphology, and bed sediments in river confluences based on field measurements.•Influence of physiographic properties on hydro-sedimentary aspects in river confluences.•The characteristics of river confluences in mixed bedrock-alluvial channels.
•Multi-trophic microbiota distribution in channel confluence was analyzed.•Flow velocity governed multi-trophic microbiota composition.•Predation among multi-trophic microbiota was enhanced in low ...velocity region.•Flow conditions affected nitrogen cycle by changing the food web's composition.•Low velocity region in channel confluence promoted nitrogen removal.
Identifying the distribution of multi-trophic microbiota under the complicated hydrodynamic characteristics of channel confluences and evaluating the microbial contributions to biogeochemical processes are vital for river regulation and ecological function protection. However, relevant studies mainly focus on bacterial community distribution in confluence, neglecting the essential role of multi-trophic microbiota in the aquatic ecosystems and biogeochemical processes. To address this knowledge gap, this study investigated the distribution of multi-trophic microbiota and the underlying assembly process under the hydraulic characteristics in the confluence and described the direct and indirect effects of multi-trophic microbiota on the nitrogen dynamics. Results revealed that, in a river confluence, eukaryotic communities were governed by deterministic processes (52.4%) and bacterial communities were determined by stochastic processes (74.3%). The response of higher trophic levels to environmental factors was intensively higher than that of lower trophic microbiota, resulting in higher trophic microbiota were significantly different between regions with varied environmental conditions (P < 0.05). Flow velocity was the driving force controlling the assembly and composition of multi-trophic microbiota and interactions among multi-trophic levels, and further made a significant difference to nitrogen dynamics. In regions with lower flow velocity, interactions among multi-trophic levels were more complex. There were intense nitrate and nitrite reduction and anammox reactions via direct impacts of protozoan and metazoan and the top-down control (protozoan and metazoan prey on heterotrophic bacteria) among multi-trophic microbiota. Results and findings reveal the ecological effect on river nitrogen removal in a river confluence under complex hydraulic conditions and provide useful information for river management.
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Initially these were Franklin's main collaborators: her colleague at King's College London, biophysicist Maurice Wilkins, and molecular biologists Francis Crick and James Watson at the University of ...Cambridge, UK. X-ray diffraction data from Franklin, Wilkins and Franklin's student Raymond Gosling. Besides this confluence of theory and experiment - which Watson and Crick did not acknowledge in their original paper - management support was essential to the project's success. D. Bernal observed in his obituary of Franklin that she was an enthusiastic collaborator and mentor, and happier at Birkbeck College in London than King's, leading a team that worked on the tobacco mosaic virus.
Turbulent air–sea heat fluxes were computed from in situ high‐frequency micrometeorological data during two research cruises performed in the southwestern Atlantic Ocean (SWAO) occurring in June 2012 ...and October 2014. Two different and dynamical areas were covered by the cruises: the Brazil‐Malvinas Confluence (BMC) and the Southern Brazilian Continental Shelf (SBCS). The eddy covariance (EC) method was used to estimate the air–sea sensible‐ and latent‐heat fluxes. This article compares these novel high‐frequency estimates of heat fluxes with bulk parametrizations made at the same location and time from independent measurements taken on board the ships. When comparing the EC and bulk‐estimated time series of sensible‐heat fluxes, we found a good agreement both in their magnitude and variability, with small bias (generally <20 W·m−2) between the datasets from the two study areas in the SWAO. However, the EC and bulk latent‐heat flux comparisons show large biases ranging from 75 W·m−2 to 100 W·m−2 in the SBCS and BMC, respectively. These biases were always associated with short‐term, high‐frequency environmental perturbations occurring either in the atmosphere or in the ocean with the majority related to strong wind burst events and large air–sea temperature gradients. The short period changes in atmospheric conditions were mostly related to the passage of transient synoptic systems over the two study areas. The large air–sea temperature gradients were mostly linked to the surface characteristics of the BMC and SBCS regions, where sharp oceanographic fronts are located. Our results are able to contribute to improving weather and climate simulations of the mid‐ to high latitudes of South America, a region largely influenced by the sea‐surface temperature patterns of the SWAO in combination with the frequent propagation of transient atmospheric systems.
Novel data from two research cruises performed in the southwestern Atlantic Ocean were used to describe the ocean–atmosphere latent‐ and sensible‐heat fluxes. We found that these heat fluxes were modulated by high‐frequency environmental changes in the atmosphere or in the ocean such as episodes of strong wind bursts, transient systems passage, or large differences between the SST and the air temperature at sea level.
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