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Image velocimetry has proven to be a promising technique for monitoring river flows using remotely operated platforms such as Unmanned Aerial Systems (UAS). However, the application of various image velocimetry algorithms has not been extensively assessed. Therefore, a sensitivity analysis has been conducted on five different image velocimetry algorithms including Large Scale Particle Image Velocimetry (LSPIV), Large-Scale Particle Tracking Velocimetry (LSPTV), Kanade–Lucas Tomasi Image Velocimetry (KLT-IV or KLT), Optical Tracking Velocimetry (OTV) and Surface Structure Image Velocimetry (SSIV), during low river flow conditions (average surface velocities of 0.12–0.14 m s − 1 , Q60) on the River Kolubara, Central Serbia. A DJI Phantom 4 Pro UAS was used to collect two 30-second videos of the surface flow. Artificial seeding material was distributed homogeneously across the rivers surface, to enhance the conditions for image velocimetry techniques. The sensitivity analysis was performed on comparable parameters between the different algorithms, including the particle identification area parameters (such as Interrogation Area (LSPIV, LSPTV and SSIV), Block Size (KLT-IV) and Trajectory Length (OTV)) and the feature extraction rate. Results highlighted that KLT and SSIV were sensitive to changing the feature extraction rate; however, changing the particle identification area did not affect the surface velocity results significantly. OTV and LSPTV, on the other hand, highlighted that changing the particle identification area presented higher variability in the results, while changing the feature extraction rate did not affect the surface velocity outputs. LSPIV proved to be sensitive to changing both the feature extraction rate and the particle identification area. This analysis has led to the conclusions that for surface velocities of approximately 0.12 m s − 1 image velocimetry techniques can provide results comparable to traditional techniques such as ADCPs. However, LSPIV, LSPTV and OTV require additional effort for calibration and selecting the appropriate parameters when compared to KLT-IV and SSIV. Despite the varying levels of sensitivity of each algorithm to changing parameters, all configuration image velocimetry algorithms provided results that were within 0.05 m s − 1 of the ADCP measurements, on average.

Two-dimensional free-surface flow past a submerged rectangular disturbance in an open channel is considered. The forced Korteweg–de Vries model of Binder et al. (Theor Comput Fluid Dyn 20:125–144, 2006) is modified to examine the effect of varying obstacle length and height on the response of the free-surface. For a given obstacle height and flow rate in the subcritical flow regime an analysis of the steady solutions in the phase plane of the problem determines a countably infinite set of discrete obstacle lengths for which there are no waves downstream of the obstacle. A rich structure of nonlinear behaviour is also found as the height of the obstacle approaches critical values in the steady problem. The stability of the steady solutions is investigated numerically in the time-dependent problem with a pseudospectral method.

Well‐developed karst aquifers consist of highly conductive conduits and a relatively low permeability fractured and/or porous rock matrix and therefore behave as a dual‐hydraulic system. Groundwater flow within highly permeable strata is rapid and transient and depends on local flow conditions, i.e., pressurized or nonpressurized flow. The characterization of karst aquifers is a necessary and challenging task because information about hydraulic and spatial conduit properties is poorly defined or unknown. To investigate karst aquifers, hydraulic stresses such as large recharge events can be simulated with hybrid (coupled discrete continuum) models. Since existing hybrid models are simplifications of the system dynamics, a new karst model (ModBraC) is presented that accounts for unsteady and nonuniform discrete flow in variably saturated conduits employing the Saint‐Venant equations. Model performance tests indicate that ModBraC is able to simulate (1) unsteady and nonuniform flow in variably filled conduits, (2) draining and refilling of conduits with stable transition between free‐surface and pressurized flow and correct storage representation, (3) water exchange between matrix and variably filled conduits, and (4) discharge routing through branched and intermeshed conduit networks. Subsequently, ModBraC is applied to an idealized catchment to investigate the significance of free‐surface flow representation. A parameter study is conducted with two different initial conditions: (1) pressurized flow and (2) free‐surface flow. If free‐surface flow prevails, the systems is characterized by (1) a time lag for signal transmission, (2) a typical spring discharge pattern representing the transition from pressurized to free‐surface flow, and (3) a reduced conduit‐matrix interaction during free‐surface flow. Key Points Design of a hybrid model that considers unsteady and nonuniform flow The model considers the transition between free‐surface and pressurized flow Considering free‐surface conduit flow is significant for karst characterization

Recently, the authors considered a thin steady developed viscous free-surface flow passing the sharp trailing edge of a horizontally aligned flat plate under surface tension and the weak action of gravity, acting vertically, in the asymptotic slender-layer limit (J. Fluid Mech., vol. 850, 2018, pp. 924–953). We revisit the capillarity-driven short-scale viscous–inviscid interaction, on account of the inherent upstream influence, immediately downstream of the edge and scrutinise flow detachment on all smaller scales. We adhere to the assumption of a Froude number so large that choking at the plate edge is insignificant but envisage the variation of the relevant Weber number of $O(1)$. The main focus, tackled essentially analytically, is the continuation of the structure of the flow towards scales much smaller than the interactive ones and where it no longer can be treated as slender. As a remarkable phenomenon, this analysis predicts harmonic capillary ripples of Rayleigh type, prevalent on the free surface upstream of the trailing edge. They exhibit an increase of both the wavelength and amplitude as the characteristic Weber number decreases. Finally, the theory clarifies the actual detachment process, within a rational description of flow separation. At this stage, the wetting properties of the fluid and the microscopically wedge-shaped edge, viewed as infinitely thin on the larger scales, come into play. As this geometry typically models the exit of a spout, the predicted wetting of the wedge is related to what in the literature is referred to as the teapot effect.

The small-scale velocity gradient is connected to fundamental properties of turbulence at the large scales. By neglecting the viscous and non-local pressure Hessian terms, we derive a restricted Euler model for the turbulent flow along an undeformed free surface and discuss the associated stable/unstable manifolds. The model is compared with the data collected by high-resolution imaging on the free surface of a turbulent water tank with negligible surface waves. The joint probability density function (p.d.f.) of the velocity gradient invariants exhibits a distinct pattern from the one in the bulk. The restricted Euler model captures the enhanced probability along the unstable branch of the manifold and the asymmetry of the joint p.d.f. Significant deviations between the experiments and the prediction are evident, however, in particular concerning the compressibility of the surface flow. These results highlight the enhanced intermittency of the velocity gradient and the influence of the free surface on the energy cascade.

The sedimentary structure of tailings is of high significance to the engineering design and safety management of tailings dams. However, due to a lack of accurate measurement techniques for the flow field and hydrodynamic conditions of tailings reservoirs, it is challenging to study the complicated sedimentary structure of tailings dams from the perspective of fluid mechanics. This study focuses on developing a large-scale particle image velocimetry (LSPIV) system in a 20 m long and 2 m wide deposition model flume to measure the flow field characteristics during the ore-drawing process accurately. According to the surface flow field characteristics measured by LSPIV, the tailings in the flume can be divided into three zones, namely the fan-shaped zone, channel zone, and laminar flow zone. Then, a simple method for estimating the flow rate of the slurry was proposed using the surface velocities measured by LSPIV. The flow rate of iron tailings slurry in the flume displays a decreasing trend along the flow direction. The variation of the flow rate of tailings slurry can be described by an exponential function. After the deposition of tailings slurry, the sedimentary characteristics of tailings are investigated, and the distribution of iron tailings particles is discussed in combination with the flow field of the tailings slurry. The LSPIV system can be applied to further deposition model tests of different slurry concentrations, discharge flow rates, and tailings compositions to investigate the effects of these factors on the tailings flow and deposition.

River flow monitoring is essential for many hydraulic and hydrologic applications related to water resource management and flood forecasting. Currently, unmanned aerial systems (UASs) combined with image velocimetry techniques provide a significant low-cost alternative for hydraulic monitoring, allowing the estimation of river stream flows and surface flow velocities based on video acquisitions. The accuracy of these methods tends to be sensitive to several factors, such as the presence of floating materials (transiting onto the stream surface), challenging environmental conditions, and the choice of a proper experimental setting. In most real-world cases, the seeding density is not constant during the acquisition period, so it is not unusual for the patterns generated by tracers to have non-uniform distribution. As a consequence, these patterns are not easily identifiable and are thus not trackable, especially during floods. We aimed to quantify the accuracy of particle tracking velocimetry (PTV) and large-scale particle image velocimetry (LSPIV) techniques under different hydrological and seeding conditions using footage acquired by UASs. With this aim, three metrics were adopted to explore the relationship between seeding density, tracer characteristics, and their spatial distribution in image velocimetry accuracy. The results demonstrate that prior knowledge of seeding characteristics in the field can help with the use of these techniques, providing a priori evaluation of the quality of the frame sequence for post-processing.

Leaky barriers are in‐stream natural flood management solutions designed for peak flow attenuation, whose effectiveness is dependent on their design. Flow around leaky barriers (LB) composed of three cylindrical logs were investigated using large‐eddy simulation. The main LB configuration considered vertically aligned logs, with other layouts inclined at 15°^{\\circ}$ , 30°^{\\circ}$ , and 45°^{\\circ}$in the upstream and downstream directions. Results reveal that the frontal projected blockage area of the LB leads to an increase in the upstream flow depth, with momentum being redirected toward the bottom gap, creating a primary wall‐jet, whose peak velocity and coherence varied depending on LB design, however, attained a similar decay downstream. The porous LBs allowed for distinct internal flow paths that generated secondary jets, either diverting momentum upwards or downwards depending on the direction of the barrier inclination, impacting main flow features and turbulent characteristics. Turbulent kinetic energy and vertical Reynolds shear stress decreased when the barrier was inclined downstream. In the upstream inclination cases, these showed no significant variation, with magnitudes similar to those in the vertical configuration. Bed shear stress decreased with increasing barrier angle, reducing the risk of local scour and sediment mobilization. The vertical LB achieves the maximum backwater rise at the expense of promoting larger sediment bed mobilization. Structural loads on the logs vary with LB inclination, with drag forces decreasing as barrier angles increase. Hydrodynamic findings, evaluated through five design criteria, show that upstream‐inclined designs, particularly with large barrier angles, exhibit improved relative performance compared to other designs. Key Points Study examines leaky barriers of cylindrical logs with inclined upstream and downstream designs for flood management Barrier layout affects flow dynamics, turbulence, and risk of sediment mobilization Upstream‐inclined designs show enhanced flood management performance

Advances in flow monitoring are crucial to increase our knowledge on basin hydrology and to understand the interactions between flow dynamics and infrastructures. In this context, image processing offers great potential for hydraulic monitoring, allowing acquisition of a wide range of measurements with high spatial resolution at relatively low costs. In particular, the particle tracking velocimetry (PTV) algorithm can be used to describe the dynamics of surface flow velocity in both space and time using fixed cameras or unmanned aerial systems (UASs). In this study, analyses allowed exploration of the optimal particle seeding density and frame rate in different configurations. Numerical results provided useful indications for two field experiments that have been carried out with a low-cost quadrocopter equipped with an optical camera to record RGB videos of floating tracers manually distributed over the water surface. Field measurements have been carried out using different natural tracers under diverse hydraulic and morphological conditions; PTV’s processed velocities have been subsequently benchmarked with current meter measurements. The numerical results allowed rapid identification of the experimental configuration (e.g., required particle seeding density, image resolution, particle size, and frame frequency) producing flow velocity fields with high resolution in time and space with good agreement with the benchmark velocity values measured with conventional instruments.

In the present study, biochar was prepared from scrap wetland macrophyte Arundo donax at different temperatures and then was evaluated for its feasibility as substrate to enhance nitrogen removal performance of surface flow constructed wetlands (SFCWs). Three groups of SFCW systems with different addition of biochar (0, 10, 20%, v / v ) were constructed to investigate the effect of dissolved organic matter (DOM) released from biochar on nitrogen transformation. Results showed that the concentration of DOM released from biochar widely ranged from 6.01 to 125.67 mg L −1 , and the DOM amount decreased with increasing pyrolysis temperature. Five humic acid-like components of DOM were identified by the parallel factor analysis (PARAFAC) model. The release capacity of DOM from biochar is observed to be closely related to microbial nitrogen removal efficiency. Enhanced removal efficiencies of NO 3 − -N (81.16%) and total nitrogen (85.62%) were achieved in SFCWs with 20% biochar, which was higher than SFCWs with 10% biochar (62.74 and 73.83%) and the control groups with no biochar (36.16 and 57.90%), respectively. Increased plant height in SFCWs with more biochar addition confirmed the positive effect of biochar on plant growth. Results from the present study suggested that the application of wetland plant-derived biochar was a promising strategy to enhance treatment performance and utilization of waste biomass resource in SFCWs.