Fish diversity is often challenging due to its labor-intensive and time-consuming, particularly when it comes to detecting rare and endemic fishes in fast-flowing waters. Environmental DNA metabarcoding has emerged as a powerful tool that enables a comprehensive understanding of freshwater biodiversity. In this study, we applied eDNA metabarcoding to assess fish diversity and assembly patterns in a highly dynamic and complex freshwater ecosystem characterized by diverse rare and endemic fish species. Specifically, three extensive surveys were performed to characterize the structure and spatial variation of fish communities within the Yangtze River Upstream Nature Reserve of Rare and Endemic Fishes. A total of 166 fish species, spanning8 orders, 25 families and 99 genera, were identified. Seasonal variations in alpha diversity were found to be significant. Changes in species composition across different sample sites were primarily associated with the species turnover rather than nestedness across the three seasons. This finding suggested that the conservation efforts should be applied to the whole Nature Reserve rather than a specific area. Endemic fishes were found to be concentrated in the core areas of the reserve, which serve as critical habitats for rare and endemic fish species. Therefore, stronger protection measures should be implemented in these core areas. This study also demonstrated the feasibility of using eDNA technology to evaluate fish diversity in fast-flowing rivers, particularly when conducting large-scale, rapid monitoring over extensive spatial areas.

Water-level fluctuations caused by hydropower operations have the strongest effects in littoral zones, where they interfere with fish reproduction. Lithophilous species, which adhere eggs to gravel surfaces in these littoral zones, face heightened mortality as hydropeaking alternately inundates and exposes their spawning substrates. However, quantitative evidence connecting the intensity of hydropeaking with ecological impact of freshwater fish is still limited. This study addresses this knowledge gap by quantifying hatching success in fishes under daily hydropeaking conditions.Field experiments were conducted to determine the effect of dehydration duration on egg-hatching success, followed by the development of a water-level-fluctuation-based method to evaluate adhesive-egg spawning grounds and identify ecological thresholds for reservoir operation. Based on the effect of dewatering on fertilized egg survival, 6-h continuous exposure was identified as the ecological threshold for air-exposure-induced egg mortality. The spawning-ground deactivation rate was then proposed, defined as the ratio of the deactivated area (≥6 h exposure) to the total potential spawning area. Finally, it is recommended that discharge variations during the reproductive period be limited to ≤2600 m3 s-1 and water-level fluctuations be restricted to <1.5 m, to ensure that the majority of spawning grounds maintain intact ecological functions. The study elucidates the ecological mechanisms governing lithophilous fish egg hatching under daily hydropeaking and proposes ecological thresholds for reservoir operation. Furthermore, it provides a quantitative framework for assessing how hydropeaking affects lithophilous spawning habitats.

Abstract Bedload transport of coarse sediments involves complex particle‐flow interactions that govern sediment motion and bed surface evolution. In this study, a semi‐resolved CFD‐DEM method is developed to simulate bedload sediment dynamics, in which the background fluid information for each particle is calculated using a Gaussian kernel function. Both particle shape effects and free‐surface dynamics are explicitly considered. A bonded‐sphere model is employed to construct disk‐like non‐spherical particles, and a series of validation cases are performed to assess the reliability of the proposed method. Comparisons between spherical and non‐spherical particles are conducted under different flow conditions, including single‐particle settling, two‐particle interaction during settling, and bedload transport simulations. In addition, the proposed method enables the simulation of the evolution of bed surface self‐organization, including particle clustering patterns and imbricated structures. The results indicate that accounting for particle‐scale flow disturbances is essential for accurately reproducing bed surface self‐organized structures, which cannot be captured by conventional unresolved CFD‐DEM method.

To investigate the evolution of turbulent-structure scales in decelerating open-channel flow, this study uses a high-frequency particle image velocimetry system in combination with a 28 m high-precision variable-slope flume to conduct controlled flume experiments. The analysis includes cross-sectional specific energy, velocity profiles, turbulence intensity, Reynolds stress, cross-correlation, and power spectral density. The study examines the turbulent statistical characteristics of decelerating flow and the evolution of turbulent-structure scale distributions during streamwise development. The results show that the velocity profile within the decelerating-flow region generally follows a logarithmic distribution, whereas the outer-region velocity profile gradually deviates from the logarithmic law as water depth increases. Compared with uniform open-channel flow, decelerating flow exhibits significantly higher turbulence intensities and Reynolds-stress levels. During flow development, turbulent structures maintain stronger spatial coherence, with spatial correlation increasing as water depth increases. As the nonuniformity coefficient gamma increases, the turbulent-structure scale distribution shifts from bimodal to unimodal. Across the measured sections, the dominant turbulent-structure scales range approximately from lambda/H = 2.5 to 20, over the ranges Re tau = 596-849 and gamma = 1.2-2.8. During downstream development, turbulent kinetic energy increases progressively and is redistributed from large and small scales toward intermediate scales. These results provide new insight into turbulence-scale redistribution in decelerating open-channel flow.

Hydrodynamic conditions play an essential role in algal growth and migration,but the mechanisms underlying algal responses to turbulence remain poorly understood.This paper systematically reviews the effects of turbulence on algal biomass accumulation and vertical migration,and further examines key factors affecting algal sensitivity to turbulent environments.First,turbulence regulates biomass by disrupting cellular processes,such as cell division and energy metabolism(photosynthesis and nutrient absorption).Second,turbulence alters vertical migration behavior by mediating algal buoyancy and mechanical stability.Finally,factors affecting algal turbulence sensitivity are analyzed in terms of cellular physiological structures and cell cycle phases.In view of current research gaps,future directions are proposed:deepening investigations into molecular regulatory mechanisms,establishing more comprehensive turbulence research frameworks,and improving coupled models linking algal physiology with turbulent physical structures to improve simulation accuracy.This review aims to provide theoretical support for understanding algal behavior under changing hydrodynamic conditions,developing bloom prevention and control strategies,and evaluating aquatic ecosystem services.

Fish egg drift is a hydrologically controlled long-distance transport process in which survival depends on whether eggs remain suspended during downstream transport. In highly engineered rivers, this process is governed jointly by unsteady hydrologic boundaries and local hydrodynamic conditions, yet their combined effects remain poorly resolved in existing fish egg drift studies. Here we present a comprehensive study of fish egg drifting under combined hydrological and hydrodynamic effects, using a dual-scale stochastic Lagrangian framework that integrates unsteady hydrologic forcing with local hydraulic modulation. Applied the new simulation scheme to a 123-km engineered reach of the upper Yangtze River, we found that settling dynamics are strongly linked to unsteady hydrologic boundaries: rising-flow conditions help maintain suspension and reduce settling-related loss during early development. In contrast, local hydraulics regulate transport pathways and concentration patterns, and intense hydraulic conditions can drive excessive shoreward dispersion and increase the potential for egg mortality. The modeled settling response is consistent with the tendency of many riverine fish to spawn during flooding or flow-rise periods, suggesting that spawning timing takes advantage of favorable hydrologic conditions. At the same time, the adverse effects of strong local hydraulics are consistent with the commonly observed upper flow-velocity preference for spawning. These results indicate that fish reproduction reflects a balance between beneficial hydrologic opportunity and harmful local hydraulic exposure, and provide a process-based basis for ecological-flow regulation and river management in engineered rivers.

Imbricated gravel beds have a distinctive surface morphology, which strongly affects riverbed stability and near-bed hydrodynamic processes. High-resolution digital elevation models combined with image processing techniques were used to quantitatively characterize imbricated surface features across three representative subregions of a gravel bar in the upper Yangtze River. The results indicate that (1) the long axes of imbricated pebbles are predominantly perpendicular to the flow direction; (2) greater bed-surface coarsening corresponds to steeper slope angles, more pronounced imbrication structures, and higher surface roughness; and (3) the self-similarity of the bed surface is anisotropic, being the strongest along the flow direction, and the Hurst index exhibits a quadratic relationship with the inclination index. A field-based method for identifying imbricated clusters was proposed. The results show that where imbrication is more pronounced, clusters tend to form more linear arrangements and increasingly align with the flow direction. These findings provide a new perspective for understanding the microtopographic features of gravel bed river surfaces.

Flow in a meandering river exhibits complex dynamics, particularly in meandering reaches within reservoir areas. This study focused on the Huanghuachang (HHC) Reach, a typical meandering reach in the Three Gorges Reservoir (TGR). Utilising a three-dimensional hydrodynamic model developed in OpenFOAM, this study compared the flow characteristics in the HHC Reach before and after the TGR impoundment. The results revealed that the impoundment of the TGR significantly decreased the flow velocities across the HHC Reach as a whole. At flow rates of 20 000 and 40 000 m3/s, the maximum depth-averaged flow velocities were reduced by about 59.4% and 31.4%, and the velocity reduction was particularly pronounced in areas identified as key zones for sediment deposition. Furthermore, both the secondary flow strength and turbulent kinetic energy were substantially weakened after the impoundment. While circulation structures at various cross-sections facilitated the downstream transport of sediment, their near absence in key deposition areas was likely a primary factor contributing to the uneven distribution of sediment deposition across the HHC Reach. These findings not only enhance the understanding of hydrodynamic characteristics in natural meandering rivers but also offer references for management of the TGR and other fluvial systems.

The transport characteristics of semi-buoyant fish eggs significantly influence their drift, dispersal, and recruitment in rivers. Due to physical differences between eggs and flows, we hypothesize their motion is asynchronous with flow dynamics. Spherical surrogate particles mimicking real fish eggs in hydraulic properties were employed to investigate the three-dimensional movements in decelerating flows. Results indicated that surrogate eggs' longitudinal velocity lagged approximately 22% behind flow velocities, while their vertical velocity was strongly flow-dependent. Eggs dispersed evenly laterally along the main flow axis. Furthermore, increased travel distance led to higher egg loss rates, attributed to gravitational settling and trapping effect. These outcomes provide valuable insights into evaluating the distribution patterns of semi-buoyant fish eggs during long-distance river drifting.

Critical swimming speed (Ucrit) is a key ecological indicator of fish swimming performance. However, most existing predictive models rely on empirical fitting, which limits their physical interpretability and dimensional consistency. In this study, we developed a physics-based prediction framework for Ucrit by integrating thrust–resistance balance theory with dimensional analysis. Using five representative cyprinid species, we conducted controlled swimming and morphometric measurements, demonstrating that body dimensions follow geometric scaling with body length. The resulting model is formulated within a unified framework and involves a single dimensionless coefficient, which serves as an effective parameter that can be calibrated to account for species-specific morphological and physiological differences. Cross-validation revealed that the proposed model achieves high predictive accuracy across species and body sizes, maintaining consistent predictive performance across the observed juvenile and adult size ranges. Compared with existing empirical models, our model reduces parameter complexity while improving interpretability and providing more bounded predictions across the observed juvenile and adult size ranges. This study advances the theoretical understanding of fish swimming biomechanics and provides a rigorous methodological foundation for the ecological application of critical swimming speed, including in fishway design, habitat assessment, and aquatic biodiversity conservation.

Objectives As one of the “four major Chinese carps” in the Yangtze River, unlike general mechanical movement, grass carp (Ctenopharyngodon idellus) is a kind of organism with locomotor vigor, which can control its movement process through its own muscle nerves, making it difficult to study the swimming dynamics of grass carp. In addition to guiding fishery production, the study of the swimming behavior of grass carp is also of great importance in the construction of ecological waterways.Methods Numerous studies have proved that the shallow ridge-deep pool riverbed topography created by damming and other engineering projects forms a slow-flowing area, which has been proven to be a suitable habitat for fish.The flow augmentation method was used to study the swimming locomotor characteristics of juvenile grass carp in a deep pool with variable velocity currents. The hydrodynamic environment of a continuous shallow deep pool was created by topographic design modifications.The flume test was carried out by Particle Image Velocimetry (PIV) to study the variation of flow structure in deep pools and to analyze the flow characteristics under different flow conditions in terms of two dimensions: flow velocity characteristics and flow structure.Behavioral characteristics of grass carp, including the active area and tail swing frequency of grass carp, were observed in the experiments to reveal the relationship between the movement process of grass carp and the response of the hydrodynamic environment under variable-speed currents in deep pools.The swimming movement of grass carp is essentially a process in which the state of movement of an object changes under the action of an external force. The grass carp will be subjected to the force exerted on it by the surrounding water body in the process of swimming, which will make the grass carp undergo a stress reaction and produce thrust through muscle contraction to counteract the water.Based on Newton’s second law, the concept of current-induced force (Fw) was proposed, and the force on the juvenile grass carp in the variable-speed current of a deep pool was analyzed by treating the grass carp as a general object that can generate its own motion thrust. The total energy dissipation coefficient (Cps) during the movement of the juvenile grass carp was calculated to evaluate the energy consumption of the juvenile grass carp during swimming movement.Results and Discussions 1) The flow velocity of the water in the deep pool is significantly smaller than the shallow flow velocity, and due to the hindering effect of the dam, the water flowing from upstream to downstream undergoes reflux, forming flat-axis vortex structures of different sizes. As the dam body is inundated by the water flow, the area of reflux is getting smaller and smaller, gradually moving in the downstream direction of the deep pool. When the flow rate is greater than 1.5 L/s, the vortex region starts to appear in the deep pool, and the area of the vortex region increases with the flow rate and extends downstream from the upstream of the deep pool gradually. 2) As the flow velocity increased, the grass carp showed an obvious clustering effect. The complex flow environment of the deep pool provided a good habitat and activity site for grass carp, which preferred to move in an area with a large water depth, low flow velocity, and turbulent flow pattern.There is a correlation between the area of grass carp larval activity and deep pools.The percentage of the activity area of grass carp decreased from 80.99% to 28.54% in the experiment, which not only reflected the clustering effect among fish groups but also reflected the suitable environmental preference of grass carp, and the suitable habitat area of grass carp in the deep pool gradually decreased with the increase of flow. 3) The turbulent water flow in the deep pool always caused the grass carp to exhibit intermittent and sudden tail wagging phenomena, and the overall fluctuation of tail wagging frequency was large. However, with the increase in flow velocity at the inlet, the frequency of tail wagging gradually increased, but the range of frequency fluctuations in the frequency of the grass carp tail swing gradually decreased. In the low-flow environment, the test fish were subjected to less flow coercion, and the movement of the fish was influenced by other subjective factors (e.g., hunger, fatigue, etc.), which resulted in different wagging frequencies and different degrees of activity for different test fish, and therefore the wagging frequency of grass carp fluctuated more at this time. When the flow rate increases, the fish usually increase the frequency of the tail swing to obtain power to maintain their own stability or upstream.4) As long as it is in motion, it is inevitable to produce energy dissipation. The main movement of fish organisms is swimming, and tail swinging is the main energy dissipation behavior during swimming. Based on Newton’s second law and force analysis on grass carp, the total energy dissipation coefficient of grass carp larvae in variable-speed deep pool currents was calculated.The total energy dissipation factor for grass carp movement was lowest when water flowed over the dam and at a flow rate of 1LB/s. When the flow rate was less than 1LB/s, the total energy dissipation coefficient of grass carp decreased with the increase in flow rate; when the flow rate was more than 1LB/s, the total energy dissipation coefficient of grass carp increased with the increase in flow rate.Conclusions The above study quantifies the movement process of grass carp larvae, fills a gap in the research on the movement characteristics of grass carp under variable-speed water flow, and provides guiding suggestions for fish habitat creation projects as well as the design of fishways.

Saltwater intrusion in artificial canals is commonly caused by gravity currents; however, the influence of ship motion on gravity currents remains unclear. This study investigates the behavior of gravity currents influenced by ship motion through laboratory experiments, focusing on fluid dynamics and the mixing process between dense and light fluids under a single-ship passage disturbance. A lock-exchange setup with a ship-model control system was used, where ship velocity was linked to propeller rotation via the apparent advance coefficient. Particle image velocimetry and planar laser-induced fluorescence imaging were used to capture the synchronized data of the velocity and density fields. Three cases with different ship speeds were conducted: 2×, 3×, and 6× the current frontal velocities (uf). Three main stages of flow behavior were observed (approach, compression, and mixing), with the mixing stage further divided into three sub-phases: mixing enhancement, decay, and stratification. The findings revealed that higher ship speeds amplified density oscillations and sustained fluctuation periods, with the ship's wake generating significant turbulence and fluid mixing, particularly in the mixing stage. An exponential power-law decay model was applied to the turbulence intensity, which highlighted an increased stratification over time, ultimately reducing turbulent kinetic energy production. During mixing enhancement, the density change rate and turbulence intensity exhibited a linear relationship, which transitioned to a quadratic function in the decay phase, highlighting the dynamics between mixing and turbulence within the fluid. This study enhances our understanding of the effects of a single disturbance induced by ship motion on gravity currents.

Restoration of complex river ecosystems requires an understanding of the availability of habitat for populations and multi-scale choices. Habitat unit (HU) mapping classifies river habitats, aiding in establishing the connection between the physical and biological conditions of rivers. Mid-scale HU classification can effectively predict fish habitat utilization patterns and has been developed and applied in numerous small to medium-sized river classifications. However, the delimitation of habitat units (HUs) for large rivers remains in its preliminary stages. This study used a two-dimensional (2D) hydrodynamic numerical model to calculate hydrodynamic data and forms ecological expert opinions based on the swimming abilities of typical fish species and geomorphic characteristics. A HU mapping procedure was constructed to describe the mesohabitat heterogeneity of HUs in the fluctuating backwater areas of the upper Yangtze River. Results indicated: (1) Rich diversity of habitats in the investigated river segment. HUs exhibit evident patterns in their planar, longitudinal, and lateral distribution. (2) Discharge magnitude influences the stability of HUs, leading to transitions between HU types. During the storage and falling periods, no dominant HU was observed, whereas the fast channel unit dominates during flood period, resulting in a reduction of the preferred habitat of fish. (3) Field monitoring of fish communities validates the potential of HU mapping in describing habitat utilization, which HUs such as pools and riffles were favoured by fish, while fast channel and slackwater unit were actively avoided, confirming the efficacy of the procedure. This research holds significant implications for habitat restoration in river management.

Fish often swim in structured group formations (fish schooling). Hydrodynamics is one of the key external factors that might affect an organism’s swimming behavior, along with other aspects such as food resources and predators. Most previous studies on collective behavior of schooling fish have been conducted in relatively simplified tanks, with few focusing on collective behavior in complex flow environments with obstacles. In this study, complex hydrodynamic environments were constructed by arranging staggered cobbles in the flume to investigate the collective behavior and upstream tactics of juvenile fish under different hydrodynamic conditions, including low- (0.4 Ucrit) and high-flow (0.8 Ucrit) conditions. The results indicate the following: (1) Under high-flow conditions, schooling fish tend to swim side-by-side with stronger cohesion when confronted with the impact of high flow velocity; (2) in low-flow conditions, schooling fish display increased polarization, evidenced by reduced bearing angles between neighboring and focal fish, signifying improved group coordination; and (3) in high-flow conditions, schooling fish use the sheltered area behind obstacles to go upstream, while in none-obstructed zone, migrating upstream is challenging. This study provides a new approach for studying fish swimming behavior in natural-like environments and offers insights and theoretical references for fish habitat restoration works.

The mixing mechanisms of density-stratified fluids in natural and engineered aquatic systems play critical roles in ecological and engineering management. This study investigated the mixing processes of the density fields induced by ship motion in a two-layer density-stratified environment through laboratory experiments. A controlled experimental setup comprising a plexiglass flume and a ship model system was employed, integrating particle image velocimetry and planar laser-induced fluorescence techniques to synchronously measure velocity and density fields and analyze the temporal evolution of mixing under varying ship speeds. Three distinct mixing stages were observed: pre-ship entry, dominated by molecular diffusion across a stable interface; ship passage, compressing the light fluid layer while preserving stratification; and post-ship wake, characterized by turbulent entrainment and mixing driven by wake disturbances. Results showed that higher ship speeds amplified the turbulence intensity and density change rate, prolonging fluid entrainment and mixing duration. Quantitative analysis demonstrated a linear increase in the peak density change rates and turbulence intensity with ship speed, with extended decay times, due to enhanced wake energy. Furthermore, the turbulence decay followed an exponential power law, reflecting energy dissipation modulated by stratification. These findings provide valuable theoretical insights for optimizing vessel operations and mitigation measures in stratified waterways.

Abstract: After the construction of the Pinglu Canal, the operation of the ship lock induces saltwater intrusion into the Qingnian Reservoir, threatening the drinking water supply of Qinzhou City. Despite its significance, studies on the dynamics of low?salinity and non?tidal saltwater intrusion in large artificial canal?reservoir systems remain limited. This study therefore establishes a three?dimensional hydrodynamic and salinity transport numerical model validated against field measurements and laboratory experiments, with discrepancies of less than 0.05 m for water levels and 4% for salinity transport, to systematically investigate the spatiotemporal evolution of saltwater intrusion in the Qingnian Reservoir.This work provides novel insights by quantifying the relationship between upstream inflow and intrusion distance, and by deriving specific operational criteria for drinking-water safety based on stratification analysis.The results show that saltwater intrusion displays a distinct salt?wedge pattern, with the bottom saline layer intruding up to 7.54?km. The maximum intrusion distance is positively correlated with the salinity boundary at the upstream approach channel and negatively correlated with upstream inflow. Specifically, a 411% increase in runoff (from 21.8 to 111.3 m³/s) reduces the intrusion distance by 70% (5.36 km), indicating that higher upstream discharge effectively suppresses the upstream advance of saltwater.Density?driven vertical salinity stratification develops, with the stratification coefficient increasing along the intrusion path. Specifically, safety?margin analysis confirms that under upstream salinity?boundary conditions of 1‰ to 3‰, the use of surface water intakes can reliably maintain output salinity within permissible limits. The research results provide a theoretical basis and technical support for the water resource safety of large?scale estuarine canal projects. Keywords: Pinglu Canal; Saltwater intrusion; Salinity stratification; Numerical simulation.

The investigation of the three-dimensional morphology of large-scale structures is crucial for advancing the understanding of the physical mechanisms underlying turbulent transport. Tomographic particle image velocimetry measurements were performed to investigate the three-dimensional morphological characteristics of large-scale turbulent structures in open-channel flows. Taylor's frozen turbulence hypothesis and the two-point correlation method were employed to directly infer the three-dimensional morphology of large-scale turbulent structures. The main attention is paid to the wall-normal evolution of the structural inclination angle and spatial length scale. The Reynolds number invariance of the structural morphological characteristics was also investigated, which provided useful information for speculating on the three-dimensional shapes of large-scale structures in open-channel flows. The results indicated that the large-scale structures of open-channel flows exhibited an inclination angle ranging from 10 degrees to 30 degrees. The free surface in open-channel flows did not exert a decisive influence on the variation of the structural inclination angle; however, it influenced the distribution of the wall-normal length scale of large-scale structures in the near-surface region. A piecewise linear increase in the spanwise width scale was observed with increasing water depth, with a slope of 0.1 in the logarithmic region and a slope of 0.3 for y > 0.4H. The variation of the streamwise length scale within the logarithmic region aligned closely with the findings observed in the atmospheric boundary layer. The streamwise and wall-normal length scales decreased as the Reynolds number increased. The spanwise width scale and inclination angle of large-scale structures in open-channel flows all exhibited Reynolds number invariance. This study will facilitate settling the severe debate on the existence of large-scale structures and lay the foundation for the wall-bounded turbulence theory.

Large earthquakes trigger catastrophic geological hazards, which can impact the runoff and sediment transport not only in affected basins but also in downstream channels. This is particularly true for the Wenchuan Earthquake, which struck on May 12, 2008, in the Longmen Mountain region. In this study, data were collected from representative hydrological stations, and statistical methods along with the Sediment Budget Model were used to investigate the combined impact of the Wenchuan Earthquake and the subsequent intense precipitation on runoff and sediment load changes in rivers affected by it, as well as sedimentation amounts in the Three Gorges Reservoir (TGR). The results revealed an obvious increase in the sediment load at stations along the Fu, Tuo and Min Rivers. Compared to 1999-2007, the annual average sediment load at the stations along the Fu, Tuo and Min Rivers increased by 102.6%-421.9% from 2008 to 2020, except at Gaochang Station. The intense post-Wenchuan Earthquake precipitation events in July 2013, July 2018, and August 2020 led to substantial increases in sedimentation within the TGR, estimated to be approximately 156.9%, 103.8% and 80.1% higher than in normal years. Based on sediment yield data from 2008 to 2020, the removal of earthquake-induced fine-grained sediment is projected to take approximately 33 years. These findings facilitate the understanding of changes in runoff and sediment loads not only in the Upper Yangtze River Basin and the management of the TGR but also in other similar basins.

Navigable river channel construction increasingly emphasizes balance of ecological sustainability and waterway transportation. Dike structures are widely applied as they enhance habitat complexity and promote nutrient cycling, yet the mechanisms governing organic debris advection, dispersion, and cloud diffusion in complex hydrodynamics remain unclear. This study focuses on Chaotianmen–Fuling reach of the upper Yangtze River to investigate flow structures and the diffusion behaviors of floating leaves and their grouped clouds withinengineered habitats between dikes. Settling experiments on eight common leaf species in static water were first conducted, followed by flume tests and three-dimensional hydrodynamic simulations under different dike configurations. In addition, the leaf drifting process in a natural river was simulated using a Euler–Lagrange approach. Results indicate that recirculation zones with a length-to-width ratio of 3:1 optimize leaf retention, while increased channel constriction enhances turbulence and leaf entry into pools; submerged bars reduce retention efficiency. Moreover, numerical simulation revealed that turbulence and secondary circulation accelerate dispersion of both floating debris and granular clouds, whereas dikes create localized retention zones forming nutrient-rich microhabitats. Application to the real fluvial reach confirmed that the proposed design-channel constriction ratio of 0.33 and bar spacing three times the bar length-improves retention and aggregation of organic and granular matter. These findings provide a scientific basis for integrating habitat restoration with navigation channel construction, offering a model for ecological waterway development in large rivers.

The leakage of the buried water pipelines will not only cause waste of water resources, but also cause secondary disasters that will endanger the safety of life and property of residents along the pipeline. Therefore, leakage monitoring is crucial for buried water pipelines. In this study, based on the active heated fibre optic optical frequency domain reflectometry (AHFO-OFDR), the buried water pipelines leakage monitoring test considering the influence of leakage size, flow velocity and other factors was carried out, and the corresponding finite element numerical model was established to verify the reliability of the test results. The research results show that the AHFO-OFDR technology can realize the accurate positioning of the pipeline leakage point and the leakage quantity and leakage velocity can be roughly judged. The relevant conclusions are consistent with the numerical simulation results, so it is considered that the experimental results obtained by AHFO-OFDR have high accuracy.