1,720,991 research outputs found
Hydrodynamic Responses of Fish Community Dynamics in Large‐Scale Morphologically Complex River Systems
Full text linkLarge rivers exhibit dynamic and complex geomorphic features, supporting some of the most biodiverse and ecologically productive ecosystems. However, their aquatic ecology is increasingly threatened by human modifications to natural channel morphology. The lack of systematic investigation and understanding of the interactions between hydrodynamics, water quality, and aquatic ecology within large-scale, morphologically complex rivers has led to fragmented ecological management. This study investigates the large-scale responses of fish communities to complex channel morphologies through two comprehensive field surveys of hydrodynamics, water quality, and fish distribution along a ∼30 km reach of the Yangtze River, encompassing channel narrowing, bifurcation, and the Yangtze River-Poyang Lake cascading confluence system. Both surveys observed elevated fish densities at the confluence and narrowing section, particularly with a distinct Confluence Hydro-Ecological Zone (CHEZ) within the post-confluence channel, marked by a pronounced increase in fish density and species diversity relative to both upstream and downstream sections. Mixing dynamics driven by the confluence momentum ratio (Mr) regulate the CHEZ: slow mixing in the equivalent momentum regime (Mr ≈ 1) amplified the increase in fish density within the CHEZ, while rapid mixing in the unequal momentum regime (Mr >> 1) weakened such effect but expanded the CHEZ. Density of large fish was primarily influenced by water depth and water-quality variability, whereas density of small fish was driven by seasonal population dynamics and hydrodynamic conditions. This study highlights that a comprehensive understanding of hydrodynamic configurations, water-quality contrasts, and seasonal biological dynamics at confluences can inform process-based strategies for sustainable river management
Hydrodynamic performance of swimming fish in the wake region of a semi-cylinder
No full textThe characteristics of environmental vortices significantly influence the behavioral strategies of fish. This study investigated how wake vortices from obstacles affected propulsion and stability disturbances in fish behavior by numerically quantifying hydrodynamic effects related to different scales of semi-cylinders. The strong reverse flow in the recirculation regions of semi-cylinders generated trailing edge vortices near the tail, which reduced thrust and induced instability upon wake vortex impingement. Decreasing the gap between the semi-cylinder and the fish increased the duration of disruption. Additionally, passing wake vortices induced drag on fish tails and destabilized the fish when the gap between the vortices was small. Notably, for fish behind small semi-cylinders, wake vortex impingement did not cause drag and instability; rather, the primary source of drag was the shed wake vortices. These factors extended the disruption region laterally from the edges of the semi-cylinder to twice its diameter outside the wake region. Only the far downstream area, beyond the recirculation flow, could provide a beneficial turbulence environment with low-momentum flow. The findings of this study may enhance the understanding of vortex-fish interactions and offer valuable insights for the design and path planning of bio-inspired underwater vehicles
Large Wood Transport and Accumulation Near the Separation Zone of a Channel Confluence
Full text linkFallen 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
Temporal variations of sediment and morphological characteristics at a large confluence accounting for the effects of floodplain submergence
No full textThe large confluence between the Yangtze River and the outflow channel of Poyang Lake is receiving attention due to its importance in flood control and ecological protection in the Yangtze River basin. There is a large floodplain along the outflow channel of Poyang Lake, which is submerged during high flow and dry during low flow. The effects of the submergence of this floodplain on sediment and morphological characteristics at this large confluence have not been known. Hence, a field investigation was done in March 2019 (relatively high flow, Survey 3) to complement the previous field studies done in August (high flow, Survey 1) and December 2018 (low flow, Survey 2) to identify the temporal variations of sediment and morphological characteristics considering the submergence of this large floodplain. The predominant sediment transport modes were wash load for Poyang Lake and confluence particles and mixed bedload/suspended load for the Yangtze River particles. The sediment transport processes were largely affected by both the secondary flows and the water density contrast between the tributaries with a lock-exchange sediment rich, denser flow moving across the inclined mixing interface in Surveys 1 and 2. The sediment flux across the mixing interface was weakened in Survey 3 when the density contrast was very small. The stagnation zone near the confluence apex had a low sediment concentration and played a role in preventing the sediment flux exchange between the two flows, and its size, and, thus, its importance as a barrier to sediment mixing were related to the submergence of the floodplain. The bed morphology with the local scour holes at the confluence was largely affected by the large-size helical cells, and this kind of effect was weakened as the secondary flows got restricted in Survey 3. The current results expand the database and knowledge on the sediment transport and morphological features of large river confluences
An optimized entropy-based model for estimating river confluence hydrodynamics: Accounting for the effects of velocity dip
No full textThe entropy model has been successfully used to calculate the flow velocity profile of rivers according to the
measured surface-flow velocity. However, it has limitations in the large river where the obvious secondary flow and flow dip (i.e., the location of maximum flow velocity moves down from the surface) occur. The confluence of rivers is usually characterized by strong secondary circulations, posing great challenges for the accurate flow velocity and discharge estimation. This paper aims to assess the effectiveness of the previous entropy-based approach in velocity estimation of a large confluence and then propose an optimized method. The flow velocity data of the large confluence between the Yangtze River and the Poyang Lake under different discharge ratios were used. The performance of the previous entropy model got worse for the stronger secondary circulation survey. To better estimate the confluence hydrodynamics, a moving average method was introduced with its advantage of smoothing data and integrated into the framework of the entropic method for the better assessment of dip parameter, and hence the better estimation of flow velocity profiles. Furthermore, the maximum velocity was found to locate at one third of the total depth below the water surface. The range of entropic function 0.48 ~ 0.68 could provide a basis for the determination under various flow conditions in rivers. The value of entropic function and the dip parameter got larger as the discharge ratio decreased. This research has expanded the application for estimating hydrodynamics in large rivers
Inhibited swimming capacity of fish entrained in wake vortices behind a semi-cylinder
No full textThe loss of fish swimming capacity induced by those energetic vortical structures is detrimental, and limited
research on the mechanism of fish entrainment in the vortex has been reported. The present study investigated the entrainment of a fish in vortices generated in wake of a semi-cylinder. The swimming fish was modeled using an undulatory NACA0012 airfoil behind semi-cylinders with a diameter D from 0.1 to 2.4 times the fish body length (L). It was found that the fish couldn’t get higher propulsion from the low momentum flow for D > 0.8L. The wake of the fish was distorted by the shedding vortices and trailing-edge vortices (TEVs) arose at fishtail. The length- and time-scale of TEVs were positive correlated with these of oncoming co-directional vortices. The uncontrollable hydrodynamics induced by TEVs could elucidate the disruption of fish stability by surrounding vortices, manifested as a loss of propulsion, the generation of a unidirectional torque, and increased power consumption. The fish head gradually got energy from the turbulent flow of oncoming vortices, but fails to compensate adverse effects of the TEVs. The recovery of swimming capacity occurred only in the less disturbed flow between the wake vortices
Random-walk-path solution of unsteady flow equations for general channel networks
Full text linkThe random walk path method (RWP) is a stochastic model that transforms the solution of the unsteady open channel flow equations into a probability problem. In this paper, this method is introduced to solve the discretized Saint-Venant equations, with probabilistic representation of the water levels at junctions. Different boundary conditions, which specify either water levels or discharge hydrographs, can be implemented by specifying how walkers react when arriving at the boundaries. The pointwise solution at river junctions can be obtained via random walk simulations. The efficacy of the model was verified against the following test cases: (1) a real-world looped channel example documented in the manual of the software Hec-Ras and (2) a hypothetical channel system consisting of dendritic and divergent networks. The number of random simulations is an important aspect of the RWP method and needs to be chosen by considering the trade-off between precision and computational efficiency. The impact of the boundary water level or discharge on the water levels at internal nodes can be quantitatively evaluated by adopting the terminal weights, which present a distinct advantage of the RWP method. This assessment of water levels can serve as a guide in the operation of hydraulic controls, such as dams, sluices and pumps, to effectively regulate the flow in mitigating flood risks
Near-surface turbulent dissipation at a laboratory-scale confluence: implications on gas transfer
No full textRiver confluences contribute to the outflux of saturated dissolved gases in the water resulting from high dam discharges. This process is related to gas transfer across the water–air interface, which is primarily controlled by turbulent dissipation near the water surface. However, the near-surface turbulence dissipation is rarely reported in confluence hydrodynamics studies. This study conducted experiments with different discharge ratios to investigate near-surface turbulent motions at a laboratory-scale confluence. The higher dissipation
rate H∕U3 m of near-surface turbulence was mainly located inside the interfacial shear layer between the two incoming streams (~ 10–4) and the bank separation zone (10–4–10–3) where high shear was found in the mean flow. By contrast, the dissipation rates were much lower inside the incoming flows and outside the two regions of high shear (~ 10–5). The magnitudes of the dissipation rate inside the shear layer were comparable in experiments where the mixing interface was in the Kelvin–Helmholtz mode or in the wake mode. The dissipation rate was found to increase away from the free surface outside the shear layer, while it was more uniformly distributed over the depth inside the layer possibly due to the presence of strongly-coherent, vertically-orientated vortices. In the far field, the mean shear within the shear layer was largely weakened. Nonetheless, the effects of flow separation persisted and laterally expanded to occupy the entire cross section. The dissipation rate H∕U3 m of the confluent flow was more than 10–4 even at a distance of 10 times the channel width in the post-confluence channel
Mixing Dynamics at the Large Confluence Between the Yangtze River and Poyang Lake
Full text linkMixing processes downstream of river confluences impacts the ecology and the related environmental management of river networks. A clear understanding of such processes is challenging, especially for confluences having width-to-depth ratios larger than 100, due to the limited available field data.
In this study, four field surveys based on hydro-acoustic and conductivity measurements were conducted near
the confluence between the Yangtze River and the Poyang Lake, which are the largest river and freshwater
lake in China, respectively. It was found that mixing dynamics at the confluence were controlled by a complex interaction among the momentum flux ratio, secondary flow and the lock-exchange flow associated to the density contrast between the two tributaries. Slow mixing was observed during high-flow conditions that generated dual counter-rotating secondary cells, with the downwelling flow acting as a barrier in the post-confluence channel. In contrast, more rapid mixing was observed during low-flow conditions when only a single channel-scale secondary flow was identified. The mixing processes were also affected by the lock-exchange flow associated to the density difference between the two confluent flows. Such lock-exchange
enhanced mixing when the Yangtze River waters had higher temperature, that is, lower density than that of
the Poyang Lake. In low flow condition, the penetration of the much larger momentum flux of Yangtze River
created a “two-layers” structure with the contribution of the density difference, which further enhanced the
curvature-induced helicity. The findings from the present study improve our current understanding of mixing
dynamics in large river confluences
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