[Objective] At present, the calculation methods for the discharge capacity of polygonal-line low-head practical weirs at sluices (such as the discrimination of the submergence limit and the calculation of the submergence coefficient) are still incomplete, which brings considerable inconvenience to related engineering design. This study aims to investigate the submergence limit discrimination and submergence coefficient calculation of polygonal-line low-head practical weir flow, and to propose corresponding discrimination criteria and calculation methods. [Methods] Hydraulic normal model tests on the discharge capacity of polygonal-line low-head practical weirs at sluices were conducted. The model conditions included a weir height T≤2.0 m (T=0, 0.5, 1.0, 1.5, and 2.0 m), a ratio of weir height to crest length T/δ≤0.4, and a submergence ratio h_s/H₀≤0.96, where h_s was the downstream water depth referenced to the weir crest, and H₀ was the total upstream head above the weir crest. The model scale was 35.5. [Results] (1) Under the experimental conditions of this study, the critical submergence limit (h_s/H₀) of the weir flow ranged from 0.748 to 0.787, which was lower than that of the broad-crested weir (0.8), indicating that the polygonal-line low-head practical weir at a sluice was more susceptible to submergence than a broad-crested weir. As the weir height T and T/δ increased, the critical submergence limit decreased correspondingly, and the difference between the critical submergence limit of the polygonal-line weir and that of the broad-crested weir increased accordingly. (2) Under the same submergence ratio h_s/H₀, the submergence coefficient σ of the polygonal-line low-head practical weir was smaller than that of the broad-crested weir σ₀. The absolute value of the relative error η between the two increased with increasing T, T/δ, and h_s/H₀. Compared with the geometric parameters of the polygonal-line low-head practical weir, namely the weir height T and the crest length δ, the variations in weir height T had a relatively greater influence on the relative error η. (3) When h_s/H₀ ranged from 0.8 to 0.96, the relative error η ranged from -1.5% to -7.6%. (4) Based on the weir height T and submergence ratio h_s/H₀, the relative error η can be obtained from the η-T-h_s/H₀ relationships shown in Table 4 and Figure 8. The corresponding submergence coefficient of the polygonal-line low-head practical weir could then be calculated using the formula and applied to discharge capacity calculation. (5) The results were validated by multiple hydraulic model test results of sluice projects. The discharge values calculated based on the present results were in good agreement with the measured values, with the absolute value of the relative error between the two being less than 1.5%. [Conclusion] The findings of this study can be applied to the calculation of discharge capacity of polygonal-line low-head practical weirs at sluices under the conditions of T≤2.0 m, T/δ≤0.4, and h_s/H₀≤0.96. Further in-depth studies are still required on the discharge characteristics of polygonal-line low-head practical weirs with T>2.0 m and larger ranges of T/δ. The proposed method has relatively high calculation accuracy and convenient application, and can provide a reference for the design and operation of similar projects.

[Objective] Ensuring safe and efficient downstream fish passage for hydraulic engineering structures remains a critical and less-solved challenge in eco-hydraulics. This review aims to systematically synthesize global research on the behavioral characteristics and hydrodynamic requirements of fish during downstream migration. The primary objectives are: (1) to consolidate the known hydrodynamic thresholds (e.g., velocity, turbulence, shear stress) for various fish species; (2) to evaluate the design and efficacy of different downstream passage facilities in relation to these hydrodynamic needs; and (3) to propose an integrated framework for future research and facility design that moves beyond single-parameter approaches to a holistic “fish-flow system” perspective.[Methods] Literature Synthesis and Categorization: literature was categorized along three primary themes: (1) empirical studies on fish behavior (e.g., active vs. passive descent) in response to hydrodynamic factors (velocity, turbulence, shear, acceleration, pressure); (2) technological assessments of different downstream passage facilities (bypasses, turbines, fish collection systems, specialized channels); and (3) advancements in research methodologies, encompassing in-situ field monitoring, laboratory flume experiments, and numerical modeling techniques like Computational Fluid Dynamics and individual-based models. Comparative Analysis and Critical Evaluation: quantitative data (e.g., preference velocity thresholds, injury limits) was synthesized into consolidated summaries (Table 1) and the strengths, weaknesses, and operational challenges of different passage technologies were assessed. Special attention was paid to inconsistencies, research gaps, and the applicability of findings across different fish species and geographies. [Results] 1) First, fish downstream behavior is systematically categorized into active descent (head-first, efficient) and passive descent (counter-current, inefficient), with transitions between these states triggered by specific hydrodynamic conditions. Preference and injury thresholds for critical hydrodynamic parameters have been quantified for several species. For instance, juvenile grass carp exhibit a preference velocity of 0.19-0.49 m/s, while Atlantic salmon smolts prefer 0.38-0.73 m/s. Turbulent kinetic energy (T_KE) below 0.03 m²/s² is generally preferred, with higher T_KE leading to disorientation and inefficient passage. Crucially, injury thresholds for shear strain rate vary significantly among species, from 500 s^-1 for trout to 2 179 s^-1 for juvenile Chinese carps. Similarly, pressure change gradients exceeding 50 kPa/s during descent or 15 kPa/s during ascent can cause barotrauma. 2) The critical evaluation of passage facilities reveals distinct performance profiles. Surface-oriented bypasses and specialized downstream channels (e.g., fish slides) generally offer a higher survival rate by leveraging fish surface-orientation and providing low-turbulence pathways. Despite improvements that can increase fish survival rates to over 97% in fish-friendly turbines, they still represent the highest-risk passage route compared to alternative passage routes. Collection and transportation systems are effective for high dams and complex terrain conditions, but require intensive operational maintenance and incur high costs. 3) One of the most significant result of this review is the proposal of an innovative, three-dimensional framework for defining and designing hydrodynamic conditions for downstream passage. This framework posits that effective passage requires the simultaneous satisfaction of three interconnected criteria: Necessity, Safety, and Timeliness. Necessity: Hydrodynamic conditions (e.g., specific “preference velocities” and “low-turbulence windows”) that actively attract fish and initiate downstream movement into the facility. Safety: Conditions that prevent injury or mortality, defined by rigid thresholds for damaging forces (e.g., shear strain rate, pressure gradient) throughout the entire passage route. Timeliness: Conditions that promote efficient and timely passage by minimizing behavioral delays (e.g., “non-direct descent” caused by adverse acceleration flows), ensuring fish are not energetically depleted or exposed to predators for extended periods. [Conclusion] Prevailing approach often focuses on isolated hydrodynamic parameters, which is insufficient for designing highly effective downstream passage facilities. The path forward requires a paradigm shift towards an integrated “fish-flow system” approach, as embodied by the proposed Necessity-Safety-Timeliness (N-S-T) framework. The ideal downstream passage is not merely a conduit with non-lethal hydraulic conditions; it is a system where the entrance flow (Necessity) seamlessly connects with a guiding, efficient internal flow (Timeliness), all while rigorously excluding hazardous forces (Safety). Future research can prioritize: (1) multi-factorial studies that explore the synergistic effects of coupled hydrodynamic parameters on fish behavior; (2) expanded research on adult fish and a wider range of non-salmond species, particularly those prevalent in Asian rivers; (3) the systematic development of a comprehensive, open-access database of hydrodynamic requirements; and (4) the integration of advanced monitoring technologies (e.g., AI-powered sonar, drones) and predictive numerical models for both design optimization and post-construction evaluation. By adopting this holistic perspective, the goal of harmonizing hydropower generation with sustainable fish conservation becomes increasingly attainable.

[Objective] This study aims to reveal the spatiotemporal distribution patterns and controlling mechanisms of bend circulation in a macrotidal estuarine environment. The specific objectives are as follows: (1) to conduct an in-depth analysis of the dynamic variations in longitudinal velocity, transverse water surface slope, and circulation intensity in the bend during flood and tidal periods; (2) to clarify the controlling role of strong tidal dynamics in the formation and evolution of bend circulation and to reveal its differences from those in conventional rivers. [Methods] Based on the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM) framework, a large-scale three-dimensional hydrodynamic model covering the Qiantang River Estuary from the hydropower station to Ganpu was established. The spatiotemporal distributions of longitudinal velocity, transverse water surface slope, and circulation intensity in the bend were simulated during both flood and tidal periods. The model employed an unstructured grid, with local refinement in the Wenyan bend to accurately resolve the channel topography and flow structures. Model driving conditions included astronomical tidal levels at the offshore open boundary and river discharge at the upstream boundary. Recent synchronous field measurements obtained in the Wenyan reach during spring tide and flood periods, including water level, flow velocity, and flow direction, were used for model calibration and validation. [Results] (1) The SCHISM model showed high applicability and remarkable computational efficiency for this complex application. Under the same parallel computing environment, its computational speed was nearly two orders of magnitude higher than that of conventional explicit Euler-type models (approximately 88 times faster). (2) The transverse water surface slope in the bend varied drastically during the flood tide acceleration stage, and a distinct bimodal pattern was identified for the first time. This behavior was markedly different from the unimodal or gradual variation patterns commonly observed in estuaries with moderate tidal ranges or in unidirectional rivers. In contrast, the variation in the transverse slope during ebb tide was much milder. Quantitative analysis showed that the maximum transverse slope during flood tide reached 5.3 times that during ebb tide, indicating pronounced tidal asymmetry. (3) During both flood and tidal periods, the bend circulation intensity exhibited a spatial pattern characterized by larger values near the bend apex and smaller values toward both ends. The core zone of maximum circulation intensity was consistently located at the cross-section on the downstream side of the bend apex. During the flood period, the maximum circulation intensity was approximately 0.08-0.12. Notably, the maximum circulation intensity during the flood tide of the autumn spring tide (0.09) was slightly greater than that during the 50-year return-period flood and was much greater than that during ebb tide. [Conclusion] (1) The SCHISM model is a powerful and efficient tool for simulating complex hydrodynamic processes in macrotidal estuaries. Its ultrafast computational capability (nearly 100 times faster than explicit models) represents a major technical advantage and has great potential for scientific and engineering applications requiring numerous simulation scenarios. (2) The hydrodynamic phenomena in bends of macrotidal estuaries are highly distinctive. The newly identified bimodal transverse slope pattern during flood tide, together with the pronounced tidal asymmetry, provides an important extension to conventional bend hydrodynamic theory and highlights the fundamental differences between macrotidal estuaries and ordinary rivers. (3) Tidal dynamics are an important factor controlling the generation and development of bend circulation, and their influence may even exceed that of extreme flood events.

[Objective] This study aims to solve the problem of insufficient energy dissipation and flow disorder in stilling basins with low Froude number. The V-shaped pier is proposed, and the forward displacement rate of hydraulic jump position P is introduced as an evaluation indicator to quantify the effect of hydraulic jump regulation. The main purpose is to systematically examine the effects of Froude number Fr, V-shaped pier angle θ, row spacing γ, first row pier position Γ, pier height ratio ξ, and contraction ratio β on energy dissipation rate η and P, to reveal the corresponding energy dissipation mechanisms and provide optimal design parameters for practical engineering applications. [Methods] Physical model tests and numerical simulation were adopted to analyze flow field structure, turbulent kinetic energy dissipation rate, and turbulent scale. The physical model consists an upstream water tank, a test section, a triangular water weir, and a backwater system. The experimental design based on L₁₈ (3⁷) orthogonal table was used to evaluate the effects of six factors at three levels on η and P. At the same time, a three-dimensional numerical model using the RNG k-ε turbulence model was established to simulate both the optimal V-shaped pier condition and the non-pier condition. [Results] The orthogonal test results showed that Fr had the most significant effect on the energy dissipation rate η, and η increased notably with the increase of Fr (2.77-4.91). Under the optimal scheme, the energy dissipation rate η reached 95.58%. For the forward displacement rate P of the hydraulic jump position, the first row pier position Γ and the pier height ratio ξ demonstrated the greatest effects, and P reached 49.42% under the optimal scheme. The variance analysis verified the significance of each factor and determined the globally optimal parameter combination at Fr=4.91. Numerical simulations indicated that compared with the non-pier condition, the V-shaped pier layout reduced the maximum velocity of the pre-jump section by 39.4%, significantly shortening the flow field homogenization distance. The peak turbulent kinetic energy dissipation rate ε near the first row of piers was 14.52% higher than that in the non-pier case, indicating enhanced energy dissipation through strong shear and vortex action. The turbulence length scale L_T near the pier decreased to about 0.02 m, effectively suppressing the development of large-scale vortices. Downstream of the second row of piers, the L_T was homogenized to about 0.10 m, and the vortex scale at the apron was reduced by 81.25% to 0.03 m, reflecting a multi-stage vortex synergy energy dissipation mechanism. [Conclusion] The V-shaped pier proves to be an efficient auxiliary energy dissipator for stilling basins with low Froude numbers. The optimal parameter combination was determined as θ=90°, γ=0.2L, Γ=0.2L, ξ=0.66, β=0.26, which achieved an η of 95.58% and P of 49.42%. Overall, the V-shaped pier fundamentally optimizes the flow field, significantly improves the flow stability and energy dissipation efficiency, and reduces the downstream scour risk through mechanisms of diversion, impingement, shear, and multi-stage vortex dissipation. Future studies should focus on establishing the optimal parameter combination relationships in a wider range of Froude numbers.

[Objective] Under changing environmental factors such as sediment deposition, riverbed incision, and backwater effects induced by cascade reservoir impoundments, low-head hydraulic hubs in plain river basins often face the challenge that the design discharge curves of sluice structures do not match actual scheduling operation. To address the issues of low accuracy in the design discharge curves of low-head hydraulic hubs, this study, based on the analysis of the causes of discharge curve discrepancies, proposes a calibration method for discharge curves of sluice gates and demonstrates its performance through an engineering case study. [Methods] Taking a navigation and hydropower hub in the middle reaches of the Ganjiang River as an example, field measurements of discharge were conducted for sluice gates using a moving-vessel ADCP (Acoustic Doppler Current Profiler) measurement technique. The prototype observation results were used to calibrate empirical coefficients of hydraulic formulas and to validate the accuracy of a three-dimensional CFD (Computational Fluid Dynamics) numerical model. Under free-flow operation conditions of the sluice gates, the validated three-dimensional numerical model was employed to calculate discharge capacities of the sluice gates. The simulation results were further used to calibrate empirical coefficients in hydraulic formulas, enabling the calculation of sluice-gate discharge under different combinations of upstream and downstream water levels. [Results] The N-H-Q curve interpolation method provided high accuracy in calculating power-generation reference discharge and could be used to analyze the respective proportions of power-generation discharge and spilled discharge in the total outbound flow of the hub. When the sluice gate was partially opened to 0.5 m, the flow pattern downstream of the gate was submerged. When the gate was partially opened to 1.5 m, the flow transitioned to a free-flow pattern. Under the same conditions, the flow patterns through the sluice gate openings obtained from the three-dimensional CFD numerical simulation were consistent with field observations. Discharge values calculated using hydraulic formulas and three-dimensional CFD calculations agreed well with measured data, with a maximum deviation of less than 5% and an average deviation of approximately 1%. Considering variations in upstream and downstream water levels of the hub and the operational range of sluice gate openings, a family of calibrated discharge curves for partially opened gates was calculated. When the gate was fully open for free discharge, different submergence coefficient calculation methods were required to determine the discharge depending on the degree of downstream submergence. [Conclusion] By integrating hydraulic calculation, prototype observation,and three-dimensional numerical simulation,a family of discharge curves under both controlled and free-flow operations of sluice gates is developed,adaptable to changing field boundary conditions and unaffected by time-varying environmental factors such as inflow magnitude,downstream backwater effects,or unsteady water level-discharge relationships.The calibrated discharge curves deviate from the measured data by less than 5% and can be used for practical application in hub scheduling and operation.

[Objective] Shared approach channels were formed during the reconstruction and expansion of ship locks due to limited spatial conditions. This study focuses on the complex unsteady flow induced by water discharge from multi-line ship locks and its impact on navigation safety. Taking the double-line ship locks of the Sanjiang approach channel at the Gezhouba Project as the research object, this study systematically investigates the superposition patterns of flow fluctuations and flow velocity variations within the shared approach channel under different water discharge combinations using a mathematical model. The objective is to reveal the hydrodynamic response characteristics of the shared approach channel during coordinated operation of multi-line ship locks, thereby providing a theoretical basis for optimizing the operation scheduling of multi-line ship locks, improving navigation flow conditions, and enhancing navigation safety. [Methods] Based on the N-S equations for two-dimensional incompressible flow, a two-dimensional hydrodynamic mathematical model covering the downstream area of the Gezhouba Project and the Sanjiang approach channel was established. The model was validated using measured hydrological data and showed good accuracy and reliability. By setting different water discharge combination scenarios, various operation modes were simulated, including simultaneous discharge and staggered discharge of the double-line ship locks. The water level fluctuation process, flow velocity distribution, and their variation patterns within the shared approach channel were analyzed. Special attention was given to the staggered discharge conditions, under which the spatiotemporal interactions and response characteristics of the fluctuations generated by successive discharges were investigated. The superposition characteristics of wave crests and troughs and their effects on the flow regime within the shared approach channel were further examined. [Results] Under staggered water discharge conditions, the forward flow generated by the later-discharging ship lock met the reverse flow produced by the earlier-discharging ship lock within the approach channel, which significantly reduced the reverse flow velocity in the shared approach channel and was beneficial to ship navigation control. Compared with single-line ship lock discharge, during double-line ship lock discharge, the increment of water level wave crests in the approach channel first increased and then decreased with increasing staggered time, and finally tended to remain unchanged at zero. The difference in wave troughs was positive at first and then became negative, and ultimately tended to remain stable. When the double-line ship locks discharged with a staggered time of 12-22 minutes, smaller wave troughs occurred in the approach channel, which were favorable for ship navigation. In terms of wave superposition, the wave crests in the first cycle exhibited a pronounced linear superposition characteristic. Under specific staggered discharge conditions, the superposition effect of wave troughs showed only minor differences from the results of linear superposition, whereas nonlinear characteristics were observed at other staggered times. In addition, different discharge combinations produced distinct effects on the longitudinal flow velocity distribution along the shared approach channel. Significant variations in velocity gradients and flow directions were observed, particularly in the lock gate area and the middle section of the approach channel. [Conclusion] Staggered water discharge can regulate the reverse flow velocity within the approach channel and can further reduce wave troughs in the shared approach channel within specific staggered time intervals. The findings provide guidance for optimizing the layout of shared approach channels for existing and reconstructed multi-line ship locks, formulating reasonable discharge timing schemes, and mitigating the adverse effects of complex flow conditions on navigation safety. The results also offer scientific reference for improving navigation flow conditions in the Gezhouba navigation capacity expansion project and similar hydraulic hubs.

[Objective] The unsteady motion of secondary flow, namely Dean vortices, in bent pipes can induce thermal fatigue and mechanical fatigue, posing a serious threat to the safe operation of industrial piping systems. In recent years, the swirl switching phenomenon, as a fundamental fluid mechanics issue in turbulent flow through 90° bends, has gradually attracted widespread academic attention. However, most existing studies focus mainly on time-averaged hydraulic characteristics such as pressure distribution and mean velocity distribution in bends, while research on the unsteady dynamic characteristics of secondary flow in pipes remains relatively limited. [Methods] Through the extensive literature review, this study collected and organized existing research results on the swirl switching phenomenon in turbulent flow through 90° bends. From the perspectives of upstream inflow conditions, flow separation in bends, flow structures, oscillation characteristic frequencies, the influence range of secondary flow, and flow control, the research progress on the swirl switching phenomenon is systematically summarized. [Results] The correlation between the swirl switching phenomenon and upstream inflow conditions in spatially developing turbulent flow through 90° bends still requires further verification. Flow separation on the inner side of the bend is not a necessary condition for triggering Dean vortex oscillation. Significant discrepancies remain regarding the dominant proper orthogonal decomposition modes representing Dean vortex oscillation. The characteristic frequencies of the swirl switching phenomenon extracted using POD analysis are lower than those obtained by other analysis methods, and extracting characteristic frequencies based on variations of single local physical quantities exhibits considerable randomness. The spatial scale of Dean vortex oscillation, its interaction with the streamwise main flow, and its spatiotemporal evolution mechanisms still require further investigation. The triggering mechanism of the swirl switching phenomenon remains unclear, and how to effectively control it also requires more in-depth research on the underlying flow mechanisms. [Conclusion] Current domestic and international understanding of the swirl switching phenomenon remains incomplete. Its dynamic characteristics and spatio-temporal evolution mechanisms are still unclear, and high-precision three-dimensional flow field data are lacking. It is proposed that further exploration and investigation of the triggering mechanisms, intrinsic modes, characteristic oscillation frequencies, spatial structures, and flow control methods of the swirl switching phenomenon should be conducted using high-fidelity direct numerical simulations and advanced three-dimensional time-resolved flow field measurement techniques.

[Objective] Lateral inflow of pump station can easily cause flow separation and backflow as the water body in the forebay changes direction, which deteriorates the flow pattern in the forebay and seriously threatens the efficient and stable operation of pump station units. [Methods] In this study, the computational fluid dynamics (CFD) method was used to calculate the flow field of the inflow building of Dazhaihe pump station, and the effectiveness of the numerical calculation method was verified by physical model tests. Quantitative and qualitative comparative analyses were conducted to evaluate the flow field characteristics of the forebay and the open inflow basin under five flow field regulation measures, including flow-guiding grille, flow-straightening sills, and different types of guide walls. [Results] (1) Based on the RNG k-ε turbulence model, the inflow conditions of lateral inflow forebay and inflow basin of the pump station in the initial design scheme were analyzed, and the effectiveness of numerical calculation method for the flow field in the pump station forebay was validated through hydraulic physical model tests. (2) The combination of arc and linear guide walls achieved optimal flow regulation effect. This measure facilitated the redistribution of flow velocity, reduced the non-uniformity of flow distribution across inflow basins, and stabilized lateral inflow. The area proportion of high-velocity zones at characteristic cross-sections was relatively small, and the average velocity along the horizontal centerline of this cross-section was slightly higher than theoretical cross-sectional average velocity. The velocity distribution within the inflow basin became more reasonable with reduced variability, significantly improving the overall flow field. (3) By comprehensively comparing five evaluation indicators—axial velocity distribution uniformity at the inlet of bell-mouth pipes in submersible pumps, the range of axial velocity distribution uniformity, velocity-weighted average angle, characteristic values of vorticity, and head loss coefficients between characteristic cross-sections—the combined application of arc and linear guide walls achieved optimal inflow conditions in the inflow basins of all units. Compared with the initial design scheme, the axial velocity distribution uniformity at the inlet surfaces of bell-mouth pipes in submersible pumps increased by 14.8% on average, and the velocity-weighted average angle increased by an average of 9.2°. Additionally, the range of axial velocity distribution uniformity between units was the smallest, and no vortex ropes were observed at the inlets of bell-mouth pipes in any unit. [Conclusions] The combined regulation measure using curved and linear guide walls is proven effective in mitigating the impact of adverse flow patterns on pump station units, providing reliable guidance for improving flow conditions in similar lateral inflow pump stations. For practical engineering applications, factors such as project timelines and construction complexity should be considered. The findings of this study offer feasible technical support and recommendations for the design and operation of future similar pump stations and hydraulic structures. Current research primarily focuses on flow field optimization of the lateral inflow forebay at the preliminary design stage, and further on-site testing of pump stations is required to evaluate the actual regulatory effects of the optimized schemes.

[Objective] Topographic and shoreline changes caused by human activities such as tidal flat reclamation, channel deepening by dredging, and seabed sand mining further interfere with tidal wave propagation in estuarine and coastal areas. These changes have significant impacts on key environmental and engineering issues, including channel maintenance and management, the safety and stability of coastal hydraulic structures, the transport patterns of sediment and pollutants, and saltwater intrusion. Previous studies on tidal asymmetry in the Pearl River Estuary have mostly focused on tidal asymmetry itself; however, tidal current asymmetry induced by topographic and shoreline changes exerts a greater influence on material transport. To systematically evaluate the disturbance mechanisms of such human activities on the tidal dynamic system, this study investigates the tidal current asymmetry in the Lingding Bay area caused by topographic and shoreline changes using the skewness method. [Methods] A high-resolution hydrodynamic numerical model covering the river network, estuary, and adjacent sea areas of the Pearl River Delta was established based on Delft3D-Flow Flexible Mesh. The model was rigorously calibrated and validated using observed water levels and current velocity data from multiple stations. The simulated results agreed well with the observations, indicating that the model had reliable predictive capability. On this basis, sensitivity experiments with different shoreline and topographic configurations were conducted. Using the T-Tide harmonic analysis tool, the variations in amplitude and phase of the main astronomical tidal constituents O1, K1, M2, and S2 and the shallow-water constituents M4 and MS4 between the 1970s and the 2010s were analyzed and summarized. The skewness method was adopted to calculate the flow velocity asymmetry (FVA) and flow duration asymmetry (FDA), to reveal the spatial distribution patterns of FVA and FDA in different decades, and to explore the contributions of major tidal constituent combinations to FVA and FDA. [Results and Conclusion] (1) The tidal dynamics in the Lingding Bay area exhibited a distinct tidal current asymmetry. Shoreline extension caused by reclamation and topographic incision resulting from channel dredging led to a general increase in the amplitudes of major shallow-water constituents and a northward shift of tidal wave phases, while the phase difference between the bay mouth and bay head decreased. These changes collectively weakened the ebb dominance and shortened the ebb duration, thereby enhancing the overall tidal current asymmetry in this region. (2) For different types of tidal current asymmetry, FVA was mainly controlled by topographic changes, with the combinations of K1/O1/M2 tidal constituents and the residual current term playing a dominant role. The FVA generally exhibited ebb dominance, favoring seaward sediment transport. The skewness values of FVA within Lingding Bay still showed a spatially decreasing trend along the bay. (3) In contrast, FDA was more sensitive to shoreline changes. Its variation was mainly controlled by the combinations of tidal constituents such as M2/M4 and M2/S2/MS4. Topographic and shoreline changes induced by human activities further enhanced the contribution of these tidal constituent combinations to FDA. Overall, the FDA exhibited a shorter ebb duration, with its asymmetry increasing from the bay mouth toward the bay head.

[Objective] Freak wave is a marine disaster characterized by extremely large wave height, strong nonlinearity, and high destructiveness. The results of wave superposition method for simulating freak waves are influenced by multiple parameters, and the sensitivity and interaction mechanisms of these factors require systematic investigation. [Methods] Based on a self-developed viscous-flow numerical wave tank, we conducted a numerical simulation on the generation of freak waves and their influencing factors. First, the reliability of the numerical model was verified against physical experimental data. Subsequently, the harmonic separation method was employed to examine the influence of wave group nonlinearity on wave surface deformation, focusing characteristics, and frequency spectrum structure. Through numerical experiments, the effects of key parameters—including spectral type, number of constituent waves, spectral bandwidth, spectral peak frequency, and water depth—were investigated. [Results] 1) During the generation of a freak wave, wave-wave nonlinear interactions caused energy to transfer from the primary frequency to both high and low frequencies, resulting in significant spectral broadening. Low-frequency free pseudo-harmonics propagated faster, leading to an actual wave height slightly larger than the theoretical value. High-frequency bound harmonics formed a tail wave, which had a minor influence on the shape of the main peak. 2) The spectral type significantly influenced the wave profile characteristics: the JONSWAP and P-M spectra, with concentrated energy, tended to generate freak waves with steep crests. The CWA spectrum produced gentle wave profiles; the CWS spectrum yielded the smallest focused amplitude. 3) The number of constituent waves affected the focusing recurrence period. An insufficient number could generate secondary focused waves. It was recommended to use 29 constituent waves to balance computational accuracy and efficiency. 4) Under finite water depth conditions, the focused amplitude reached its maximum when the spectral bandwidth was 0.7 Hz, indicating that the amplitude was co-modulated by the spectral bandwidth, water depth, and spectral peak frequency. 5) An increase in the spectral peak frequency enhanced nonlinearity, resulting in wave profile steepening. However, an excessively high frequency led to wave breaking, thereby reducing the amplitude. 6) Water depth influenced the wave profile by altering the dispersion characteristics. A greater water depth resulted in faster wave speed and a higher amplitude, whereas an excessively small water depth readily induced wave breaking. [Conclusion] The main innovations of this research include: establishing a high-precision viscous-flow numerical model capable of accurately simulating the evolution of nonlinear waves including breaking effects; employing the harmonic separation method to reveal the influence mechanism of wave group nonlinearity on wave surface structure and energy distribution; and clarifying the coupling effects of various factors under finite water depth conditions through multi-parameter sensitivity experiments. The findings of this study deepen the understanding of freak wave generation mechanisms, provide an important theoretical basis and parameter selection guidance for laboratory simulation of freak waves.

[Objective] The centrifugal and inertial forces of discharge flow in curved stilling basins lead to the impact of the flow on the concave bank and water surface rise, and result in the formation of a significant transverse water surface gradient, thereby inducing hazards threatening the structural safety, including scouring and deepening of the concave bank and sediment deposition on the convex bank. To address these challenges, this study investigates the energy dissipation and diversion characteristics of a combined energy dissipator comprising rough strips and trapezoidal piers arranged within the stilling basin. This study aims to provide a systematically validated optimized design scheme and theoretical prediction tool for engineering applications addressing complex hydraulic problems. [Methods] Taking the TGZBL Reservoir in Xinjiang as the engineering background, a 1∶60 scale physical model was constructed following the gravity similarity criterion. The model consisted of four components: a straight approach channel, a diffusion section composed of an Ogee curve and a reverse curve segment, a main curved stilling basin section incorporating an arc segment, and a discharge channel. Water depths at 35 cross-sections were measured using 0.1 mm precision point gauges, while flow velocities at two-thirds the water depth beneath the surface were recorded at left (A), middle (C), and right (E) measurement points in key cross-sections. Evaluation metrics included the energy dissipation rate η based on the Bernoulli's equation and the coefficient of variation of the transverse water surface gradient C_v to quantify the dispersion degree and assess energy dissipation and flow diversion performance. An orthogonal experimental design was employed at the core of the study, with seven influencing factors selected, including the relative width of the rough strips (b), the angle between the rough strips and the cross-section (θ), the relative height of the rough strips (h₁), the height ratio of the rough strips (λ), the relative spacing of the trapezoidal piers (Δζ), the length of the trapezoidal piers (ϕ), and the longitudinal section dimensions of the trapezoidal piers (γ). Each factor was set at three levels, and 18 test conditions were arranged using the L₁₈ (3⁷) orthogonal array. [Results] Main effects analysis of variance (ANOVA) revealed that the most significant factors influencing the energy dissipation rate η were the trapezoidal pier spacing Δζ (P=0.005) and the relative height of the rough strips h₁ (P=0.045), with the degree of influence ranked as: Δζ > h₁> θ > λ > ϕ > b > γ. This was because Δζ directly altered the number and water-facing area of the trapezoidal piers, enhancing counterflow resistance and turbulent dissipation, while variations in h₁ effectively redirected high-kinetic-energy flow from the concave bank toward the convex bank, maximizing the energy dissipation capacity of the latter. For the flow diversion effect C_v, the most influential factors were the relative width of the rough strips b (P=0.005), the length of the trapezoidal piers ϕ (P=0.011), the height ratio of the rough strips λ (P=0.015), and the spacing of the trapezoidal piers Δζ (P=0.023), ranked as: b > ϕ > λ > Δζ > h₁ > γ > θ. Among these, b, h₁, and λ collectively determined the obstructive and frictional effects of the rough strips on concave-bank flow, forcing redirection toward the convex bank and thereby achieving more uniform water depth distribution within the basin. Through post hoc multiple comparisons and comprehensive analysis of factor-level trends, the optimal parameter combination balancing both energy dissipation and flow diversion effects was determined as A2B3C3D3E3F3G1. Validation tests for this optimal configuration demonstrated significant improvements compared to the initial condition without energy dissipators: the energy dissipation rate η increased from 72.47% to 82.22% (a net gain of 9.75%). The coefficient of variation of the transverse water surface gradient C_v decreased from 0.981 9 to 0.161 2 (an 83.58% reduction). The vortex flow within the basin was mitigated, with average flow velocities at the concave and convex banks declining from 7.72 m/s and 12.42 m/s to 3.03 m/s and 3.43 m/s, respectively (reductions of 60.75% and 72.38%), and the inlet velocity of the discharge channel was substantially lowered. To translate the research findings into practical tools, a multi-factor evaluation model incorporating 11 dimensionless parameters was established based on dimensional analysis principles. Through multiple regression analysis, logarithmic function equations (adjusted R²=0.946) for predicting energy dissipation rate η and asymptotic function equations (adjusted R²=0.804) for forecasting the coefficient of variation of the transverse water surface gradient C_v were respectively developed. To ensure model reliability, six independent working conditions (including the optimal combination), which had not been involved in the fitting process, were selected for validation. The results demonstrated that the relative errors between predicted and measured values for both η and C_v remained below 10%, confirming the established semi-theoretical and semi-empirical formulas achieved excellent precision and applicability. [Conclusion] Verifications through anti-sliding and anti-overturning stability calculations confirm that the combined energy dissipator meets safety requirements under design hydraulic loads, ensuring its safety and effectiveness in practical engineering applications.

[Objective] Hydrological stations play a crucial role in monitoring hydrological regime changes. Achieving online flow calculation at hydrological stations and improving flow calculation accuracy are of great research significance for hydrological monitoring, flood and drought disaster prevention, and water resource management. [Methods] By monitoring the upstream and downstream water levels of hydrological stations as boundary conditions, a one-dimensional hydrodynamic model of the river reach near the hydrological station was constructed. The Kalman filter technique was employed to automatically calibrate the model’s roughness parameters based on measured flow data from the station. Using the upstream and downstream water levels as inputs and the automatically calibrated roughness data as outputs, a BP neural network was constructed to fit the complex relationship between water levels and model roughness. During online flow calculation, the roughness values were corrected using the real-time upstream and downstream water levels and the water level-roughness neural network relationship to improve flow calculation accuracy. By correcting the roughness based on real-time upstream and downstream water levels and using the constructed one-dimensional hydrodynamic model for simulation calculation, online flow calculation at hydrological stations was achieved. [Results] Taking Lanxi Hydrological Station as an example, the accuracy of online peak flow calculation at Lanxi Station using the proposed method was higher than that of the currently used index velocity method. For three major flood events selected, the flood flow calculation accuracy at Lanxi Station using the proposed method was higher than that of the index velocity method currently used at Lanxi Station. The reason was that the index velocity method, when establishing the relationship between index velocity and cross-sectional average velocity, used only boat-measured flow data to calibrate the relationship, which could lead to significant errors and consequently larger errors in peak flow simulation. In contrast, this study constructed a one-dimensional hydrodynamic model, used measured flow data to automatically calibrate the model roughness parameters, corrected roughness based on real-time upstream and downstream water levels to perform online flow calculation with the model, and utilized more real-time water level information than the index velocity method for model calibration and assimilation, thus achieving higher peak flow calculation accuracy. [Conclusion] This study achieves online flow calculation at hydrological stations and improves flow calculation accuracy by utilizing upstream and downstream water levels. The applicability of the method is verified using Lanxi Hydrological Station as an example, demonstrating significantly improved flow calculation accuracy compared to the index velocity method, particularly during major floods with high water levels. The method proposed in this paper is suitable for online flow calculation at hydrological stations located in upstream or midstream river reaches with relatively stable riverbed cross-sections where sediment erosion and deposition effects can be neglected. Considering the relatively low cost of water level gauges, the method demonstrates good application prospects and promotion value.

[Objective] In hydraulic projects, flood-discharge structures operating under compound water inlet conditions often exhibit complex vertical-axis vortices at the inlet in which the approach flow direction differs from the mainstream river direction. Traditional vortex elimination measures perform poorly under such conditions, and severe air-entraining vortices may threaten structural safety and operational efficiency. Based on a pumped-storage power station, this study aims to clarify vortex mechanisms through physical modelling and to develop an efficient and economical vortex elimination measure that completely suppresses air-entraining vortices, thereby providing design guidance for similar projects. [Methods] A 1∶50 undistorted clear-water physical model was built under Froude similarity to ensure geometric, kinematic, and dynamic similitude. Several operating conditions (check, design, energy dissipation, and scour protection) were simulated to reproduce actual operation. Vortex characteristics were observed (air-entraining vortices up to 7.5 cm diameter) without any suppression measures. Five vortex elimination schemes were then tested: Scheme 1—conventional vertical vortex elimination beams with an optimized inlet; Schemes 2-4 —addition of 30° triangular vortex elimination plates to the beams, adjusting beam spacing and width, and reducing the inlet angle α; Scheme 5—a comprehensive optimization scheme using monolithic concrete to simplify the structure. Digital imaging and thin-walled triangular weirs were used to quantitatively analyze vortex elimination effect and flow improvement of each scheme. [Results] (1) Cause of vortices: superposition of lateral and longitudinal inflows under compound conditions markedly increased initial circulation, generating strong vortices within 30° of the inlet (100% type-F vortices at stage 3).(2) Comparison of vortex elimination performance: conventional beams (Scheme 1) merely downgraded type-F to type-D vortices without eliminating air entrainment. Adding 30° vortex elimination plates and optimizing beam spacing (Schemes 2-4) reduced vortex diameter from 7.5 cm to 0.1-0.3 cm, leaving only minor concave vortices (type B) at stage 3. Scheme 5 (preferred) completely eliminated air-entraining vortices while simplifying construction and maintaining smooth flow under all operating conditions. (3) Innovations: 30° triangular vortex elimination plates conformed to the inlet geometry, reducing the inflow angle α and suppressing initial circulation. Widening the central vertical beam enhanced vortex interception and disrupted vortex structure. Scheme 5 replaced complex components with a monolithic concrete pour, balancing effectiveness and constructability. [Conclusions] Vortex intensity under compound water inlet conditions is directly linked to the inlet angle α and initial circulation. Lateral flow amplifies complexity and hazards. Combining vortex elimination beams with 30° triangular vortex elimination plates completely eliminates air-entraining vortices, reducing vortex size by over 98% and remaining effective under all operating conditions. The optimized Scheme 5 balances vortex elimination performance with economy and offers a transferable design paradigm. The findings overcome the limitations of traditional vortex elimination measures and provide valuable guidance for high-head, multi-directional hydraulic projects.

[Objective] As an important part of the side inlet/outlet of a pumped storage power station, the length of the adjustment section directly affects the hydraulic characteristics of the inlet/outlet under bidirectional flow conditions as well as the project cost. This paper aims to investigate the effect of different adjustment section lengths on the hydraulic characteristics of side inlet/outlet, focusing on changes in head loss, velocity distribution, and discharge allocation, and to recommend an appropriate range of adjustment section lengths that meets design codes. [Methods] A three-dimensional mathematical model of the side inlet/outlet of the Lianghekou pumped storage power station was established. The Reynolds Stress Model (RSM) was adopted and validated against physical model test results. The velocity curves obtained from numerical simulation showed good agreement with the experimental measurements, confirming the applicability of the selected turbulence model to the study of inlet/outlet hydraulic characteristics. [Results] For the power generation condition (outflow), increasing the adjustment section length reduced inlet/outlet head loss, homogenized the velocity distribution at the trash rack section, and kept the discharge non-uniformity across openings within 5%. For the pumping condition (inflow), increasing the adjustment section length also reduced inlet/outlet head loss and maintained discharge non-uniformity within 10%. When the adjustment section length L relative to the diffusion section length T satisfied L≥0.3T, the bidirectional hydraulic characteristics of the inlet/outlet were favorable; when 0.1T≤L<0.3T, the bidirectional hydraulic characteristics were slightly poorer but still met code design requirements. [Conclusion] The research results provide support for the design and optimization of adjustment sections.

[Objective] Appropriately reducing the diameter of branch pipes is an effective way to lower the superimposed water hammer pressure in water conveyance systems. However, there is limited research on how replacing a single large-diameter branch pipe with multiple smaller parallel branch pipes affects the hydraulic transient process in such systems. Moreover, there is a lack of research on how to design the diameters of these parallel branches to minimize hydraulic transients while maintaining flow requirements and ensuring economic efficiency. Based on the Qitai high-head gravity-flow water conveyance project, we investigated the impact of branch pipe arrangement on hydraulic transients and proposes principles for determining branch pipe diameters. [Methods] Using numerical simulation, we compared the hydraulic transient behavior in complex pipelines with a single branch pipe versus multiple small-diameter parallel branches, and examined the impact of parallel arrangements on water hammer pressure and pressure fluctuation duration, summarized the design method for branch pipe diameters in parallel schemes, and verified the engineering applicability of the conclusions using a real-world case. [Results] When the number of parallel branch pipes increased from 2 to 5, the maximum positive pressure head in both main and branch pipes significantly decreased under various scenarios. Under the most critical condition, the maximum water hammer pressure was reduced by 10.32% in the main pipe and 48.75% in the branch pipes, respectively, compared to the single large-diameter branch layout. Simultaneous valve closures in the branches did not result in additional pressure increases in the main pipe, and water hammer waves between branches did not interfere with one another. Moreover, replacing a single large branch with multiple smaller branches effectively eliminated negative pressure heads in the branches, thus preventing negative-pressure-induced water hammer, and shortened the duration of pressure fluctuations. The maximum reductions in pressure fluctuation duration at key points in the main and branch pipes were 63.16% and 46.15%, respectively. For stable hydraulic transitions and economic feasibility, the ratio of branch to main pipe diameter at the connection point should be between 0.12<β<0.28. In the Qitai project, using two parallel branch pipes (β=0.226) increased construction costs by 33.33% compared to the single large branch solution, but reduced investment in water hammer protection by 39.10%. [Conclusions] Under the premise of safe water delivery, a water supply scheme that replaces a single large-diameter branch pipe with multiple smaller parallel branches can effectively reduce valve-closing water hammer pressure and shorten the duration of pipeline pressure fluctuations.

[Objective] The integrated operation of water diversion and power generation hubs faces challenges in maintaining hydraulic stability during emergencies, particularly under sudden load rejection conditions. This study aims to develop an adaptive emergency regulation strategy to coordinate the operation between diversion sluices and power generation units, with the objectives of minimizing flow and water level fluctuations in the main canal, reducing decision-making response time, and ensuring the operational safety and efficiency of the water conveyance system. This study addresses the critical gap in existing systems where the lack of an adaptive flow regulation mechanism between sluices and turbines leads to prolonged hydraulic transients and potential safety hazards during emergencies. [Methods] A comprehensive methodology combining theoretical analysis, numerical modeling, and prototype validation was adopted. The emergency operation principle was established based on the goal of rapidly restoring flow into the main canal. An adaptive emergency regulation strategy was then developed by integrating the hydraulic relationship between power units and diversion sluices, along with operational constraints such as unit vibration range and gate opening limitations. A one-dimensional hydrodynamic model was established for the canal system extending from the hub to the downstream check gate, simulating unsteady flow under different disturbance scenarios. The model was calibrated and validated using operational data from the prototype. A typical case involving full load rejection from both power generation units was simulated to compare the proposed adaptive strategy with conventional manual operation. Key performance indicators included water level deviation, flow variation, and response time. [Results] The simulation results demonstrated a significant improvement in hydraulic stability under the proposed adaptive emergency strategy. Compared to conventional manual operation, the adaptive approach reduced the maximum water level fluctuation in typical canal cross-sections by over 40%. The maximum flow fluctuation was reduced by 58%, indicating a markedly smoother transition during emergency load rejection. Furthermore, the adaptive strategy enabled an immediate response to emergency conditions, reducing decision-making and execution time to near real-time. The coordination mechanism between the sluice and power generation units effectively balanced flow discontinuities caused by sudden turbine shutdown, thereby limiting the propagation of disturbances along the canal. The hydraulic response under adaptive control showed faster convergence to stable conditions, significantly reducing the risks of overtopping or drying in the canal. [Conclusions] This study successfully develops and validates an adaptive emergency regulation strategy for water diversion and power generation hubs, significantly enhancing operational safety and efficiency during unexpected events. The proposed strategy introduces an innovative real-time coordination mechanism between sluice gates and power generation units, representing a substantial improvement over traditional human-operated systems that often result in delayed and suboptimal responses. The findings provide a scientific and technical foundation for automated emergency management in multi-objective hydraulic hubs. The adaptive method not only ensures rapid flow recovery and minimizes hydraulic transients but also supports the maximization of power generation benefits without compromising water supply security. This approach has broad applicability in similar large-scale water diversion projects worldwide, highlighting its significance in advancing smart water management and emergency response technologies.

[Objective] Designing the shape of bifurcated pipes has long been a focus in water diversion projects. Current research on the erosion of pipes by sediment-laden flows remains limited, and studies specifically on the erosion and wear of Y-shaped bifurcated pipes are even rarer. This study aims to investigate the erosion and wear characteristics of Y-shaped bifurcated pipes and optimize their shape. [Methods] We employed numerical simulation and physical model experiments to investigate the solid-liquid two-phase flow within Y-shaped bifurcated pipes during the standalone operation of the main pipe. The influence of an elliptical arc chamfer scheme on the hydraulic and wear characteristics of the bifurcated pipe was explored. The numerical simulation results demonstrated strong consistency with the experimental data, validating the reliability of the numerical simulation method. [Results] (1) During standalone operation of the main pipe, compared with the bifurcated pipe with a circular arc chamfer, the bifurcated pipe with an elliptical arc chamfer had a more open space and gradual variations in the flow cross-section, resulting in smoother internal flow patterns. Its head loss coefficient was smaller, reduced by 6.9% compared to the circular arc chamfer pipe. The local low-pressure distribution at the bifurcation angle significantly improved, and the minimum pressure increased by 1.47 m water head compared with the circular arc chamfer bifurcated pipe, indicating that the elliptical arc chamfer bifurcated pipe could effectively improve the low-pressure distribution and enhance the bifurcated pipe’s cavitation resistance. (2) When the main pipe operated independently, the wear on the bifurcated pipe was most severe near the inlet and outlet of the main pipe, followed by the crotch area, while the branch pipe area experienced the least wear. The particle mass flow rate had the greatest impact on the wear distribution of the bifurcated pipe, followed by the particle impact velocity. By adopting an elliptical arc chamfer, the water flow could move smoothly, and the mass flow rate of particles acting on the pipe walls was reduced, thereby mitigating wear at the main pipe outlet and the crotch area of the bifurcated pipe. The average wear at the corresponding positions was reduced by 22.26% and 21.03%, respectively, compared to the circular arc chamfer bifurcated pipe. [Conclusions] During standalone operation of the main pipe, compared with bifurcated pipes with a circular arc chamfer, bifurcated pipes with an elliptical arc chamfer demonstrate a 1.47 m water head increase in minimum pressure and significant reduction in abrasion at the main pipe outlet and the crotch area. These findings indicate that elliptical arc chamfer bifurcated pipes effectively enhance both cavitation resistance and abrasion resistance of bifurcated pipes, providing valuable references for bifurcated pipe design.

Foundation scour is one of the primary causes of hydraulic failures in river-crossing bridges. By integrating flume experiments, prototype observations, numerical simulations, and artificial intelligence methods, this study reviews research on foundation scour of river-crossing bridges over the past six decades, summarizes progress in three aspects of general scour, contraction scour, and local scour, analyzes the limitations in existing research, and proposes future research directions. In terms of physical mechanisms, most existing studies focus on bridge foundation scour under simplified boundary conditions such as straight channels, non-cohesive riverbeds, and cylindrical structures. However, cohesive sediments prevalent in natural rivers exhibit complex force interactions and high randomness, resulting in scour processes for bridge foundations that differ significantly from those in non-cohesive riverbeds. Moreover, in common natural channels such as braided, branching, confluence, and alternating wide-narrow channels, water-sediment dynamics and riverbed evolution involve numerous factors with strong uncertainties, making scour mechanisms for bridge foundations more complex than those in straight channels. Therefore, future research must focus on scour mechanisms under more boundary conditions commonly found in natural rivers to improve the theoretical framework for foundation scour of river-crossing bridges. Regarding scour prediction methods, existing research primarily relies on flume experiments and prototype observations of specific bridges. The former’s prediction accuracy is severely affected by scale effects, while the latter has limited applicability. To date, there is a lack of predictive formulas or analytical models that quantitatively consider the scale effects on bridge foundation scour. Data-driven models such as artificial neural networks and deep learning can effectively compensate for the inability of conventional prediction methods for bridge foundation scour to account for complex boundary conditions. In particular, multi-module multilayer perceptrons (multi-module MLPs) can construct hybrid neural networks incorporating physical scour mechanisms, showing great potential in addressing the challenges of predicting scour under complex boundary conditions. In numerical modeling, existing methods are often applicable to low Reynolds number conditions, with insufficient accuracy in capturing turbulence at high Reynolds numbers and absence of standardized grid size criteria. Sediment transport is frequently computed using empirical formulas, and dynamic grid technologies often suffer from low precision. Existing numerical methods exhibit inadequate coupling between turbulence models and sediment transport models. Moreover, current numerical simulations are limited to non-cohesive riverbeds, with few models applicable to cohesive riverbeds and virtually no reported models suitable for stratified riverbeds. Therefore, numerical models for bridge foundation scour require in-depth investigation to address these issues in the future, improving their applicability and reliability under complex boundary conditions. In addition, intensified human interventions—including sand mining, channel regulation, and dam construction—have triggered rapid riverbed degradation in many rivers. These degradation events often occur at scales, rates, and complexity far beyond conventional understanding of general riverbed degradation, resulting in highly destructive and abrupt changes. Future research should systematically investigate riverbed evolution under human disturbances. To build a more comprehensive understanding of foundation scour of river-crossing bridges, future studies should better integrate flume experiments, prototype monitoring, numerical modeling, theoretical analysis, artificial neural networks, and deep learning methods. This will enable systematic investigation of bridge scour under human disturbance and complex boundary conditions, thereby improving the theoretical system and developing more widely applicable and reliable scour design methods.

[Objectives] Traditional culvert designs often result in excessively high flow velocities within the channel, impeding the upstream movement of weak-swimming fish species. Installing small triangular baffles inside culverts has the potential to provide upstream passage for small fish while maintaining discharge capacity. This study aims to clarify the hydraulic effects of triangular baffles by arranging multiple small baffles along one side of a flume to simulate internal culvert structures and verify the hydraulic effects through flume experiments. [Methods] Three-dimensional velocity data were collected using an Acoustic Doppler Velocimeter (ADV) to analyze the distribution patterns of turbulent kinetic energy and Reynolds stress. The quadrant analysis method was employed to quantitatively assess the impact of the baffle system on flow velocity distribution, turbulence characteristics, and momentum transport modes. [Results] The results showed that the triangular baffles created stable low velocity zones (LVZs) along the sidewall, with longitudinal velocities ranging from -4 to 15 cm/s, and velocities at the outer edge of the baffles around 25 cm/s, below the critical swimming speed of small fish such as Rhinogobius giurinus. In the mainstream zone, the lateral profiles of longitudinal velocity were nearly identical, ranging from 25 to 30 cm/s, indicating that the small triangular baffles had minimal impact on mainstream flow and thus preserved discharge capacity, achieving synergistic optimization of hydraulic efficiency and ecological function. The proportion of the low velocity zone area remained relatively consistent along the flow path, accounting for 14.80%-18.07% of the total cross-sectional area, demonstrating the feasibility of using triangular baffles to stably expand LVZs. The baffles significantly enhanced turbulence intensity in the region near the baffle-side sidewall, generating clockwise vortices and positive horizontal Reynolds stress that play an important role in maintaining swimming stability. Although the turbulent kinetic energy and Reynolds stress in downstream LVZs were higher than those in high-speed regions without baffles, they remained below the threshold of fish swimming preferences. This moderate turbulence enhancement not only provided energy for swimming but also avoided excessive turbulence that could impair the sense of direction or balance. Momentum exchange was dominated by jetting (Q₂) and sweeping (Q₄) events, whose dominance increased with higher threshold parameter H₀ (with a contribution rate of about 60% at H₀=4). The transient vortices formed had planes parallel to the fish’s spine and body axis, reducing energy loss during upstream movement and improving swimming efficiency through vortex energy transfer. This provided a more favorable flow environment for weak-swimming fish species. [Conclusions] This study identifies the distribution patterns of mean flow and turbulence characteristics and introduces quadrant analysis into the study of culvert turbulence-fish behavior interactions. It reveals the promoting effect of small baffle structures in fish upstream migration and addresses the lack of detailed flow field and turbulence structure analyses in previous research. The findings offer a feasible hydraulic optimization paradigm and model reference for the design of eco-friendly culverts.

[Objective] To address the issue that conventional river regulation structures struggle to dynamically adapt to the highly variable characteristics of natural rivers, this study develops an innovative active flow-regulating vane system. [Methods] The system combined a vertically adjustable and rotatable vane structure with a remote intelligent control module. It allowed real-time monitoring and dynamic adjustment of flow parameters, thereby overcoming the limitations of traditional fixed structures such as spur dikes and deflector vanes. To investigate its applicability in curved river channels, the flow-regulating vanes were arranged in a 180°U-shaped bend model. The verified RNG k-ε turbulence model and VOF method were used to conduct numerical simulations of the bend’s flow field characteristics before and after the vane installation. The impact of the flow-regulating vanes on the hydrodynamic structure of the bend was analyzed. [Results] 1) Numerical results showed that when the top of the flow-regulating vanes was flush with the free water surface (at a flow rate of 7.9 L/s), the longitudinal velocity near the convex bank region increased by 21.67% compared to the original bend, while the maximum transverse velocity in the central region decreased by 70.33%, effectively weakening the transverse circulation. When the vanes were submerged to 0.3 times the water depth (at a flow rate of 15.8 L/s), the longitudinal velocity still increased by 13.64%, and the transverse velocity decreased by 37.63%. 2) Analysis of the flow field structure revealed that the vanes could split the original single clockwise vortex circulation structure within the bend into two vortices rotating in the same direction, which reduced the flow’s kinetic energy, lowered the circulation velocity, and decreased transverse sediment transport. 3) The distribution of bed shear stress showed that, after the installation of the flow-regulating vanes, the bed shear stress within the bend was uniformly distributed along the convex bank side, which helped alleviate sedimentation on the convex bank while avoiding concentrated scouring. Moreover, the suspended design of the vanes reduced flow obstruction at the bend bottom, solving the sedimentation problem caused by decreased flow velocities around traditional structures fixed to the riverbed, making it a viable option for flow regulation in hardened bend channels.