We consider a horizontal porous layer of constant thickness with Darcy friction and derive the nonlinear Lagrangian equations for the mean drift of fluid due to plane acoustic waves propagating along the layer. One of the bounding planes is assumed to be impermeable to fluid motion, while the other is permeable. The porous lattice is taken to have the same acoustic impedance as the fluid in the pores. Because the Darcy friction is linear in velocity, the flow in the porous medium is irrotational. However, this friction introduces dissipation in the fluid, so that the Lagrangian drift velocity in irrotational acoustic waves can be uniquely determined. The divergent horizontal Lagrangian mean drift due to spatially damped acoustic waves induces a vertical flow through the permeable boundary which may transport accumulated material into the fluid. Of particular interest here is the possible vertical transport of microplastics from reservoirs in the deep-sea benthic sediments into the ocean water caused by seismic primary waves. Analysis in Lagrangian coordinates yields the wave-induced mass transport directly. Acoustic waves induce horizontal and vertical Lagrangian mean drift in a porous layer. Vertical drift may transport accumulated microplastics into the fluid.
In urban canyons, depending on their configuration, trees can reduce wind speeds and thus may hinder pollutant dispersion under certain conditions, although they also offer other benefits such as shading and pollutant deposition. However, the aerodynamic and shading effects of trees, coupled with thermally driven buoyance from solar-heated walls, still lack systematic research. This study employs computational fluid dynamics (CFD) simulations to investigate the combined effects of tree planting configurations and solar-induced wall heating on flow and traffic-related pollutant dispersion in an urban street canyon. Four planting densities ( P_d = 0.25–1), five trunk heights (h = 0.18H–1.06H), and four wall heating conditions including isothermal (ISO), leeward-wall heating (LWH), windward-wall heating (WWH), and all-wall heating (AWH) are systematically analyzed. Results demonstrate that (1) Regardless of the combination of planting densities and wall heating scenarios, when tree crowns stay below rooftops (h ≤ 0.62H), increasing trunk height enhances canyon flow and thus reduces pollutants, particularly at higher planting densities ( P_d ≥ 0.5), whereas crowns exceeding building height (h ≥ 0.84H) obstruct roof-level airflow and hinder dispersion; (2) AWH reduces pollutants most effectively (23.5–67.5
In this study, a second-order accurate temporal discretization scheme is developed and implemented within an incompressible Smoothed Particle Hydrodynamics (ISPH) framework to enhance numerical stability and accuracy in highly transient free-surface flows, such as mountainous river flows interacting with obstructions. The proposed formulation is first subjected to a systematic particle resolution study and validated against well-established laboratory dam-break experiments. The validation results demonstrate good agreement in terms of flow evolution, leading-edge propagation, and free-surface dynamics, confirming the reliability of the developed model. Building upon this validated framework, an extensive parametric investigation is conducted to examine the effectiveness of rectangular obstacle arrays in damping the kinetic energy generated by dam-break flows, which serve as idealized representations of natural or engineered structures in steep river corridors. The effects of obstacle number, width, and height are analyzed independently using the ratio of total instantaneous kinetic energy to the initial potential energy as a quantitative performance metric. The results reveal that increasing the number of obstacles significantly enhances energy dissipation up to a threshold, beyond which further increases yield diminishing returns. While variations in obstacle width exhibit a relatively minor influence on energy attenuation, obstacle height is identified as the dominant parameter governing kinetic energy damping in mountainous river flows, primarily due to the enhanced pressure drag induced by increased flow blockage against rectangular-shaped obstacles. Overall, the findings highlight the critical role of obstacle configuration in mitigating the destructive potential of flash floods in mountainous rivers and demonstrate the capability of the proposed high-order ISPH model as a robust tool for the design and assessment of energy-dissipating structures in hydraulic engineering applications.
Image velocimetry has gained significant attention for flow measurement in field rivers, with recent efforts combining satellite videos with various image velocimetry techniques to enhance cost-effectiveness, resolution, and public accessibility. However, challenges remain, including lower image resolution and frame rates compared to conventional methods, as well as pixel offsets caused by continuous satellite motion. Moreover, the accuracy of satellite-based flow measurement has yet to be systematically evaluated due to the difficulty of obtaining instantaneous and precise surface flow fields in large rivers. This study applied three mainstream image velocimetry methods, namely space-time volume velocimetry (STVV), optical flow methods (OFM), and large-scale particle image velocimetry (LSPIV), to analyze satellite video footage of the Chongqing section of the Yangtze River. A hydrodynamic simulation was conducted to generate reference instantaneous flow fields corresponding to the video acquisition date. Results indicated that all three methods can derive large-scale flow fields with acceptable accuracy (the root mean square errors of velocity magnitude for STVV, OFM, and LSPIV were 0.3218 m/s, 0.8131 m/s, and 0.8003 m/s) when measurement parameters were appropriately adjusted, demonstrating the feasibility of satellite-based river velocity monitoring. Among them, STVV achieved the best performance, with regression lines closely aligned with y=x. The coefficients of determination were 0.86 for flow direction and 0.82 for velocity magnitude, with a relative error of 14.24
Flash floods in high-relief Himalayan River corridors present significant and escalating geomorphic hazards, yet the precise local mechanisms governing their destructive power remain inadequately constrained. This study presents a scenario-based analysis of a catastrophic flash flood in the Harshil-Dharali corridor of the Bhagirathi River, Uttarakhand, India, a representative high-risk mountain environment. We introduce an integrated methodology that couples daily discharge records with hydraulic modeling across 15 high-resolution, DEM-derived river transects, validated against satellite-derived (NDWI) erosion mapping. Our results reveal critical hydrodynamic amplification at specific geomorphic nodes. On August 5, 2025, Unit Stream Power (USP) abruptly increased to over 1000 W/m2, with corresponding cross-sectional velocities of 3–5 m/s, at four key transects (XS5, XS6, XS9, and XS14). These modeled high-energy zones exhibit a strong spatial correlation with observed erosion footprints. Notably, USP at transect XS14 exceeded 1700 W/m2 (and XS9 exceeded 3000 W/m2) despite only moderate discharge, a phenomenon attributed to extreme channel slope and severe morphological constriction. At XS9 and XS14, an amplifying effect reveals fundamental nonlinearities in mountain river hydraulics. At these constrictions, channel width ( w ) narrows by at least 40 Q=V· A ), these geometric narrowing forces an abrupt increase in velocity ( V ) to maintain the same discharge. As flow accelerates, the regime often shifts from subcritical to supercritical (Froude number > 1). Energy is then dissipated through hydraulic jumps and intense turbulence. It explains the exponential spike of Unit Stream Power (USP⩰ƮV), which exceeded the theoretical threshold for bedrock incision ( 1000 W/m^2 ). Thus, hazards were not solely driven by hydrological input ( Q ), but were amplified mechanically via constriction ratio—acting like a hydraulic nozzle to focus kinetic energy onto the banks. The findings confirm that localized geomorphic controls, such as channel narrowing and steepening, rather than discharge magnitude alone, were the dominant drivers of the event’s destructive potential. Concurrently, flood-driven geomorphic instability triggered episodic pulses of carbon mobilization, with peak annual losses exceeding 5000 tC. This represents a substantial climate-relevant flux, of nearly 20,000 tCO2e directly linking catastrophic erosion events to disruptions in the terrestrial carbon cycle. The proposed methodology provides a robust and rapid framework for identifying hydro-geomorphic hotspots. This approach enhances flood risk assessment and mitigation planning in structurally controlled mountain catchments worldwide, offering a transferable tool for proactive disaster management in sensitive and developing regions.
Gravity currents flows in general and powder snow avalanches in particular are dense flows occurring on horizontal but also mild or steep slopes. They occur on smooth boundary (open slope) but also in interaction with obstacles of multiple types (trees, rocks, avalanche protection structures). In this context, an experimental study has been led. Finite size volumes of dense fluid have been released in a flume in horizontal configurations up to an inclination of 30^∘ . Front position evolution with time together with typical flow depth have been measured by video recording and image processing. Flow regimes without obstacles (acceleration, slumping, buoyancy versus inertia regimes) have been studied and compared favourably with previous existing results. Obstacles made out of long thin elements but also square blocks of different heights have been organized in aligned or staggered configurations. The implication of their presence in the flume on the dense flows behaviors (front velocity and typical flow depth) has been investigated. In the vast majority of cases, the front velocity of the flow is reduced down to 50% from the non-obstructed case. There is also an influence on the flow depth, either increasing or decreasing it, depending on the blocks height and organization. Obstacles have been shown to be more effective at reducing the flow speed on horizontal or mild slopes than on steep slopes. Long thin elements are very efficient in slowing down the flows. This is all the more noticeable as their obstacle to void volume is an order of magnitude lower than the one of the square blocks. Connections with powder snow avalanches observations or mitigating implications have been proposed in the case of interaction with trees and organized buildings.
The total dissolved gas (TDG) supersaturation caused by high dam discharges pose significant ecological risks to aquatic organisms inhabiting downstream river systems, thereby representing a critical concern in hydropower development. Aquatic vegetation zones, distinguished by their characteristic low-flow regimes and intricate vegetative structures, function as essential habitats for aquatic biota. Consequently, it is imperative to examine the dissipation dynamics of supersaturated TDG within these aquatic vegetation zones. Previous studies have conceptualized the dissipation of supersaturated TDG as a process involving liquid-gas interfacial transfer, solid wall adsorption, and the internal dissipation. While, the factors influencing the internal dissipation coefficient and its quantitative characterization remain inadequately elucidated, resulting in an incomplete understanding of the overall supersaturate TDG dissipation process. To address this gap, flume experiments and numerical simulations were conducted in the present study. The determinants affecting the internal dissipation coefficient were identified. Findings indicate that the internal dissipation coefficient exhibits a positive correlation with water depth. Under equivalent vegetation densities, scenarios characterized by elevated flow velocities and increased turbulence intensities correspond to higher internal dissipation coefficients. Moreover, for consistent flow conditions, an increase in vegetation density is associated with an augmented internal dissipation coefficient. Through multiple regression analysis, a predictive formula for the internal dissipation coefficient was derived. This research advances the quantitative understanding of supersaturated TDG dissipation in vegetated flow environments and provides a theoretical foundation for subsequent investigations in this research field.
The construction of eco-friendly waterways in mountainous rivers places higher demands on fish-friendly hydrodynamic environments, while the hydrodynamic interactions occurring during fish schooling provide an important basis for understanding suitable swimming and habitat conditions. In this study, computational fluid dynamics was employed to investigate the hydrodynamic interactions of four typical schooling configurations, namely tandem, side-by-side, triangular, and diamond formations, using a representative fish species from the upper Yangtze River as the model organism. The underlying mechanisms were further analyzed in combination with wake structures and near-body flow characteristics. The results show that, in the tandem configuration, the blockage effect of the trailing fish on the incoming flow increases the thrust and swimming efficiency of the leading fish by 2.6
This study investigates suspended sediment transport in turbulent open-surface flows with periodic coverages, represented by alternating covered and open strips with coverage ratios of 30 R_n < 1.6 ), the total suspended sediment transport rate exhibits a substantial decline under periodic coverage, signaling a shift toward deposition-dominated regimes. In contrast, for coarser sediments ( R_n > 1.6 ), transport rates demonstrate relative insensitivity to surface modifications, as the sediment mass remains predominantly concentrated in the near-bed region, away from the surface-induced turbulence. Under non-equilibrium conditions, the adaptation length required to reach equilibrium is strongly modulated by the upstream SSC, the Rouse number, and deposition rates. These findings provide a theoretical basis for predicting sediment dynamics under discontinuous surface boundaries, offering critical insights for the design and environmental impact assessment of hydraulic infrastructure such as floating solar photovoltaic (FPV) systems.
Understanding the hydraulic resistance and stability of boulders is essential for river engineering, river morphology, and hydraulic structure design. This study quantifies hydrodynamic forces acting on a cubical block resting on a smooth, rigid bed under near-critical to supercritical flow conditions, with bed slopes of 0.4-−1.3 1.2 ≤ Fr ≤ 2.8 . Time-resolved force measurements were used to derive drag and lift coefficients ( C_D and C_L ), and their fluctuations. The drag coefficient decreases systematically with increasing relative submergence (h/H), while the lift coefficient follows a nonlinear asymptotic recovery, shifting from negative toward positive values as h/H increases. Smoothing the block edges reduces drag by 20–30 ^∘ ) produces only minor differences (within ± 5% ). The drag coefficient exhibits a consistent trend with Fr, and the lift coefficient becomes tightly organized when expressed as a function of h/H, from which empirical relationships are derived for partially submerged transitional flow. Standing waves, linked to upstream flow deceleration, are associated with increased unsteadiness of drag and lift forces and higher mean values of C_D and C_L . These results clarify how geometry, flow regime, and free-surface instabilities control C_D and C_L under near-critical and supercritical conditions, providing a force-based understanding of boulder–flow interactions on smooth, rigid beds.
Based on the independently developed water jet experimental platform, experiments and numerical studies were conducted on the inclined jet erosion of cohesive soil under different jet velocities (U0), inclination angles (θ), and target distances (h) to reveal the dynamic evolution characteristics of the three-dimensional flow field inside the erosion hole. A physical model for cohesive soil jet scouring was established, in which the Bingham rheological parameters were incorporated into the solver via User-Defined Function (UDF), and the Volume of Fluid (VOF) method was employed to track the evolution of the water–soil interface. Experimental results indicate that, with increasing θ, the scour hole morphology gradually transforms from “deep and narrow” to “shallow and wide,” accompanied by a significant reduction in sediment accumulation and a marked enhancement in inner-wall stability. The maximum scour depth ( ε_m ), radius ( r_m ), and volume ( ξ ) increase markedly with increasing U₀. When θ = 30°, the scour hole length and volume reach their maximum values, with r_m = 832 mm and √(ξ) = 511 mm, respectively. Whereas the maximum scour depth ε_m = 590 mm is obtained at θ = 22.5°. The erosion process induced by the inclined jet can be divided into three stages: initial impingement, unstable expansion, and dynamic equilibrium, during which the scour hole morphology evolves from an elliptical shape to a wing-like pattern and ultimately develops into a stable scour hole. On this basis, a dimensionless scouring equation for predicting the equilibrium scour depth was established by combining theoretical analysis with experimental data. The scour depth exhibits a logarithmic growth with dimensionless time. This study provides important guidance for optimizing subsea pipeline installation and improving the efficiency of dredging operations.
The micro-morphological structure of riverbeds represents the basic unit of bed surface morphology. Bedload clusters, the most common microforms in mountain rivers, constitute the main component of the surface armouring layer and significantly affect bed stability, sediment transport, and flow conditions. In this study, a fully resolved computational fluid dynamics–discrete element method (CFD-DEM) coupling model, based on the immersed boundary method (IBM) and large-eddy simulation (LES), was developed to simulate the evolution of uniform bedload clusters over rough beds under varying flow intensities. The mechanisms of cluster formation, disintegration, and their influence on flow structures were examined. Results show that cluster formation occurs in two modes: collision-dominated and wake-capture, with the latter tending to evolve into streamwise-aligned patterns. Disintegration arises from self-breakup or collision-induced breakup, with transverse and streamwise velocity fluctuations as key triggers. Larger clusters generate more extensive wake low-pressure zones with increased peak negative pressure, revealing a self-organizing trend in spatial evolution. Clusters with complex morphologies are prone to vortex breakdown, which enhances energy dissipation. Moreover, clusters significantly modify local bed shear stress distributions: a high-stress zone develops at the upstream edge and a low-stress zone downstream due to wake shielding. Importantly, the peak stress magnitude is independent of cluster planar shape. These findings clarify the dynamic interactions between bedload clusters and turbulent flow, providing insights into sediment transport processes and riverbed stability in natural rivers.
Various scales of turbulent coherent structures (TCSs) individually and collectively contribute to pollutant dispersion in urban areas. Although it is well known that surface heating significantly modifies urban turbulent flows, its impact on TCSs with different scales and their interactions still needs further investigations. In this study, we examine the impact of surface heating on different scales of TCSs and their associated turbulent pollutant flux over an idealized building array. Two large-eddy simulations with and without surface heating are conducted using the PArallelized Large-eddy simulation Model (PALM). The spectral proper orthogonal decomposition (SPOD) and the amplitude modulation (AM) analysis are applied to extract different scales of TCSs and to examine their interactions, respectively. The surface heating enhances low-speed streaks at and above the roof level while it leads high-speed regions to be less organized into streaks. The surface heating preferentially enhances large-scale coherent structures and therefore substantially increases the relative importance of large-scale coherent structures for roof-level TKE and turbulent pollutant flux compared with mid-scale and small-scale coherent structures. The surface heating also enhances the modulation of small-scale turbulent momentum and pollutant fluxes by large-scale coherent structures above the roof level. This enhancement of the modulation mainly originates from the strengthened large-scale upward motions. This study emphasizes the necessity of applying modal decomposition and scale-interaction analysis together to better understand both scale-dependent characteristics of TCSs and interactions among TCSs with different scales.
The interaction between various greening forms and the heating conditions of building walls in street canyons significantly influences both pedestrian thermal comfort and the traffic-related pollutants distribution. In this study, six types of greening forms and three wall heating conditions were simulated. The study shows that of the various urban greening strategies, street trees and vertical vegetation most significantly alter airflow dynamics and enhance air quality. When combining multiple greening approaches, the integration of vertical wall greening and hedges yields the most favorable outcome for enhancing pollutant dispersion. The windward wall heating substantially alters the airflow structure, whereas leeward wall heating exerts the least influence. Under conditions where all three walls are heated, thermal buoyancy dominates the flow dynamics within the canyon, leading to significant changes in airflow and a notable improvement in ventilation performance, with an 8.99
To clarify the complex hydrodynamics of bend-straightened reaches of continuous bend channels, three-dimensional simulations were performed with a Reynolds Stress Model (RSM) coupled to the Volume of Fluid (VOF) method. The numerical experiments examined water-surface fluctuations, velocity structures, and turbulent kinetic energy under varying planform geometries, characterized by the junction angle α and curvature K, and under different Froude numbers Fr. Related implications for channel stability, ecological functions, and navigation management were also assessed. The results indicate that all hydraulic parameters exhibit an approximately linear dependence on Fr, while being jointly modulated by α and K. Streamwise velocity deflection and secondary flow structures generated in the upstream unit persist into the downstream unit, leading to average increases of 3.7
Understanding the three-dimensional flow turbulence generated by the combined presence of a bridge abutment and adjacent piers is essential for accurately predicting local scour, a primary cause of hydraulic-structure failure. Although the vortical fields associated with piers and abutments have been studied extensively in isolation, their coupled effect on coherent turbulence and sediment entrainment remains insufficiently clarified. This study evaluates the predictive capability of turbulence diagnostics obtained under rigid-bed conditions, prepared after prior mobile-bed experiments, by organizing all results along four longitudinal transects (Rows 1–4) to ensure direct comparability between hydrodynamic parameters and scour patterns. High-resolution Acoustic Doppler Velocimeter (ADV) measurements collected across multiple depths and cross-sections were used to quantify turbulence intensities, Reynolds shear stress (RSS), quadrant events, and sweep-to-ejection (STE) dynamics around a rectangular abutment and two circular bridge piers. The results demonstrate that the narrow interaction zone between the abutment and the upstream pier constitutes the dominant source of three-dimensional turbulence. Elevated near-bed RSS and strongly dominant sweep (Q4) events in this region represent the principal drivers of sediment entrainment, while the vertical concentration of turbulent energy close to the bed is shown to be more decisive for scour initiation than the magnitude of vertical velocity alone. This critical zone corresponds directly with the deepest scour measured in the mobile-bed tests. Downstream, turbulence progressively weakens, ejection (Q2) events become more prominent, and reductions in RSS and STE coincide with decreasing scour potential toward the second pier and the flow-recovery region. The unified presentation of results along four longitudinal transects confirms that RSS distributions, Q4–Q2 bursting behaviour, and STE profiles obtained under fixed-bed conditions provide robust, physically consistent predictors of scour in compound pier–abutment systems. These findings also strengthen recent hydrodynamic frameworks by confirming that pier–abutment spacing fundamentally governs vortex interaction and near-bed momentum transfer. The insights gained here offer improved guidance for scour prediction and the resilient hydraulic design of bridge foundations in fluvial and coastal environments.
The motion of solid particles floating on a water surface driven by a point-sourced bubble plume was investigated experimentally. Particle image velocimetry and particle tracking velocimetry revealed that the particles moved more slowly than the surrounding water surface flow. In the most significant condition, the particle velocity was only about 50
Einstein’s stochastic bedload transport framework often underperforms near incipient motion because of simplified representations of particle step length and sediment pick-up probability. To address this issue, a two-stage single-particle model is developed in which bedload motion is represented by a contact phase (rolling) followed by a detachment phase (saltation), while grain protrusion is incorporated through bed-surface protrusion statistics. The model clarifies how protrusion, sediment specific gravity, and near-bed velocity fluctuations influence the step length to particle size ratio. The predicted mean step length, together with a near-threshold closure for pick-up probability, is then embedded into Einstein’s stochastic framework to derive a corrected bedload transport function. The resulting formulation improves predictions for uniform sediments near incipient motion and remains applicable to fully rough flows without the need for additional fitting parameters. This framework is most relevant to gravel-bed rivers and mountain streams where near-threshold transport influences bed stability and sediment budgets.
The Tuotuo River is a typical braided river located in the Yangtze River source area. An increasingly warmer and wetter climate in recent years has noticeably modified the runoff, sediment flux, and vegetation in the Tuotuo River Basin, consequently altering its braiding morphology. To evaluate morphological changes, the braiding in the Tuotuo River was examined using remote sensing images covering the period from 1990 to 2023. During this period, water discharge and air temperature increased by 18.6 m3/s and 0.95 ℃ per decade, respectively. The width of its braided channel has increased by 24.2 BI_T3 ), which represents the average number of branches in a cross section, has increased over the past three decades by 8
The water and sand can have violent interactions in flushing in which the free surface and water–sand interface both experience fragmentation and severe deformation. In this study, the EI-MPS method (explicit incompressible version of moving particle semi-implicit/simulation) is employed to simulate water–sand interactions. This study aims to further validate the EI-MPS method in simulating the flushing flow and to investigate the flushing rate in a tank by the numerical method. To validate the numerical method, the flushing flow in a tank is simulated with different particle distances and similar free surface and water–sand interface profiles are obtained in good agreement with experimental measurements. The initial water height and depth of the sand layer in the tank are respectively examined in the flow to calculate the flushing rate. It is found that the water–sand interface has violent variation in the flow. The flushing rate of the sand layer increases sharply in the early stage of the flow and then gradually approaches a constant level. When keeping the initial water height constant, the shallower sand layer can be flushed easily with larger flushing rates.
