Effect Of Bed Roughness Research Articles

Conservation equations for open-channel flow: effects of bed roughness and secondary currents

AbstractTime-averaged velocity fields in uniform open-channel flows over rough beds may exhibit spatial heterogeneities due to the effects of bed roughness and secondary currents (SCs). The latter typically originate from the turbulence anisotropy and spatial heterogeneity introduced by the solid and mixed corners (i.e., between sidewalls and water surface), but may also appear due to roughness spanwise heterogeneities, e.g., associated with patchy vegetation distributions or streamwise sediment ridges on the channel bed. In this paper, we propose rigorous conservation equations for momentum, kinetic energy and fluid stresses accounting for the contributions of bed roughness and SCs, separately. Particular attention is given to the terms regulating the energy exchanges between roughness-induced and SC-related motions, which are expected to provide information on the physical mechanisms leading to the generation of roughness-induced SCs. The proposed approach is illustrated using a large-eddy simulation of a rough-bed open-channel flow.

Impacts of bed roughness and orientation on hydraulic jump: A review

Hydraulic jumps are a prevalent phenomenon in flows through spillways, chutes, and sluice gates. As hydraulic jumps exhibit substantial kinetic energy, the downstream of a hydraulic structure is prone to scour. To mitigate downstream scour and enhance energy dissipation, hydraulic jumps are often directed into stilling basins with various bed configurations, including horizontal, sloping, rough, and their combinations. This review compiles numerous analytical and experimental studies on hydraulic jumps under various bed conditions. The effect of bed roughness on sequent depth ratio, roller and jump lengths, shear stress, and energy dissipation is critically reviewed. The impacts of roughness height, flow Froude number, and bed angle on jump characteristics are discussed, substantiated by comparative analyses for distinct roughness heights. The results indicate that bed roughness intensifies shear stress, resulting in augmented energy dissipation and reductions in jump length and sequent depth. Additionally, the analytical and empirical equations proposed by researchers for different jump scenarios are discussed, and their applicability under various conditions is summarized. Finally, it suggests considering the scale effect in future research to refine the comprehension of jump stability over adverse slopes.

Large-eddy simulation of free-surface turbulent channel flow over square bars

The results of large-eddy simulations of free-surface turbulent channel flow over spanwise-aligned square bars are used to investigate the effects of bed roughness and water surface deformations on the root-mean-squared velocity fluctuations, dispersive shear stress, double-averaged Reynolds shear stress, wake kinetic energy and double-averaged turbulent kinetic energy. Two bar spacings, corresponding to transitional and k-type roughness, at similar Reynolds and Froude numbers are considered. The main peak of all statistical quantities occurs at the bar crest height. The effects of a standing wave at the water surface in flow over k-type roughness is marked by a local peak under the water surface for all statistical quantities considered here except wall-normal and spanwise velocity fluctuations. Quadrant analysis shows that sweeps and ejections are the strongest events contributing to both dispersive and double-averaged Reynolds shear stress but their contributions are different for the two bar spacings. Examining the budgets of dispersive and double-averaged Reynolds shear stress reveals that the dominant terms of these stresses are pressure–strain correlation and pressure transport and the contribution of wake production is similar for both of these stresses but with opposite sign. In addition to the main role of the bars in consuming or producing wake kinetic energy through production and transport and convection, the standing wave at the water surface in flow over k-type roughness induces large convection in the bulk flow too. The dominant terms in the double-averaged turbulent kinetic energy budget are similarly production, transport and convection. Large shear production renders large temporal fluctuations than spatial fluctuations of flow variables. The interaction of bars, bed and water surface is seen in the convection term in flow over k-type roughness.

Incipient Motion of Exposed Microplastics in an Open-Channel Flow.

The incipient motion threshold of microplastics (MPs), defined as the condition that is just sufficient to initiate MP movement, is key to assessing the transport and fate of MPs in water bodies, yet only a few studies have focused on its prediction. This study experimentally investigated the effects of bed roughness (smooth and rough beds) and MP properties (shapes, sizes, and densities) on the incipient velocity (Ui) and critical shear stress (τc) of exposed MPs in an open-channel flow. For a total of 19 types of MPs, Ui and τc were found to range from 0.06 to 0.21 m/s and 0.01 to 0.075 N/m2, respectively. The commonly used thresholds for sediment transport, for example, critical shield parameter Θc and movability number Λc, were established for MPs based on τc. Based on the experimental data of the literature and this study, it was found that predictions of Θc and Λc for sediments do not apply to MPs. A new explicit formula for Λc was proposed for predicting the incipient motion of MPs by introducing the dimensionless particle diameter (d*) and a new dimensionless parameter related to the particle size, density, and shape. The new formula has an absolute error of 12.3%, which is smaller than the existing formula for MPs (55.6%).

Effect of Bed Roughness and Negative Step on Characteristics of Hydraulic Jump in Rectangular Stilling Basin

Water flow often releases excess energy after passing through gates and spillways. This energy should be reduced to avoid the destruction of downstream structures. The hydraulic jump, a natural phenomenon, inevitably reduces incoming flow energy. The experiments were carried out in this study to investigate the effect of bed roughness and abrupt negative drops on the characteristics of the hydraulic jump. Also, to investigate the effect of geometric and hydraulic parameters on energy dissipation and location of the hydraulic jump, there was a change in the height of the abrupt drop and roughness for different discharges between 30 and 50 L/s and the Froude numbers were ranging from 4.9 to 9.5. The results showed that increasing the bed roughness causes a reduction of the sequent depth ratio and the relative length of the jump by 16.6% and 20.7%, respectively, and increases the relative energy loss and the bed shear force coefficient by 10% and 31%, respectively. In contrast, increasing the step height causes an increase in the sequent depth ratio and the relative length of the jump by 6.5% and 7%, respectively, and increases the relative energy loss and the bed shear force coefficient by 11% and 3.2%, respectively.

Effects of the bed roughness and beach slope on the non-breaking solitary wave runup height

In this study, a series of laboratory experiments were carried out to investigate the effects of bed roughness and beach slope on the non-breaking solitary wave runup height. Experimental measurements reveal that the bottom friction plays different roles in the solitary wave runup process for different beach slopes. Under the same incident wave condition, the maximum runup height on a smooth steel bed was found to be larger than that on a rough sandpaper bed over a 10° beach slope, whereas the same runup heights were observed for the smooth and rough beds over a 45° beach slope. By introducing the runup similarity parameter, quantitative relationships between the maximum runup height and the bed roughness, as well as the beach slope, were analyzed. On the basis of Synolakis (1987), a new empirical model for estimating the non-breaking solitary wave induced runup height was proposed, after incorporating a slope-related enhancement coefficient and a friction-related reduction coefficient. It is found that both the enhancement coefficient (being always larger than unity) and the reduction coefficient (being always smaller than unity) are large under the small relative incident wave height and steep beach slope condition. Influence of the bed roughness is more significant under the small runup similarity parameter condition, under which the runup height monotonically decreases with the increase of the runup similarity parameter for the small bed roughness, whereas it first increases then decreases with the increase of the runup similarity parameter for the large bed roughness. Nevertheless, influence of the bed roughness could be neglected for the large runup similarity parameter condition. These results are consistent with experimental observations. Finally, the present model was validated using a large amount of existing solitary wave runup data with different bed frictions and beach slopes. The good agreement between the model predictions and measured data indicates the reliability and accuracy of the present model.

Quantifying the Effects of Bed Roughness on Transit Time Distributions via Direct Numerical Simulations of Turbulent Hyporheic Exchange

AbstractWe report direct numerical simulation (DNS) results of hyporheic exchange for a flat river bed with two different particle roughness textures, at a surface flow friction Reynolds number of 395 and a bed permeability Reynolds number of 2.6. Transit time distributions (TTDs), subsurface flow patterns, and the interfacial volumetric fluxes are discussed. The transit time was quantified using a forward particle tracking method based on pure advection by three‐dimensional, pore‐resolved, time‐mean velocities. Results show that bed roughness induces deep subsurface flow paths that yield a TTD with a power‐law tail. Roughness obstructs the surface flow, creating interfacial pressure variations which induce subsurface flow. Next, the molecular diffusion is accounted for based on a random walk method and is shown to increase transit times regardless of roughness texture. This work demonstrates that particle roughness on a macroscopically flat sediment bed can induce significant hyporheic exchange that is fundamentally similar to that induced by bedforms.

Effects of bed roughness on a horseshoe vortex in overland water flowing past a cylinder

A horseshoe vortex (HV) system is formed around a bed-mounted circular cylinder in overland water flowing past the cylinder; however, the effect of bed roughness on HV still lacks clear recognition. The present study experimentally investigated the characteristics of HV upstream of a bed-mounted circular cylinder in a flume, where sand of four grain sizes was glued to the flume bed to produce the bed roughness. A particle image velocimetry system was mounted on the flume to facilitate a high-resolution (63 pixel/mm) observation of a flow field. A total of 28 cases with flow depths varying from 0.39 to 1.09 cm and cylinder Reynolds numbers (ReD) varying from 2700 to 5800 were selected to evaluate the effects of the bed roughness on HV in overland flow conditions. Measurements were then conducted on the upstream flow of a cylinder in the symmetry plane. Bed roughness significantly affected HV, in turn destroying the three vortices. Subsequently, time-averaged primary HV features on the three beds with lower roughness levels were analyzed in terms of the separation point, location, radius, and swirling strength. With an increase in the bed roughness, the separation point moved toward the cylinder, and the primary HV moved toward a free surface; however, the effect on the streamwise location was nonsignificant. A positive relationship was observed between the radius of the primary HV and the bed roughness, whereas a negative relationship was observed between the swirling strength and bed roughness. For beds with the highest roughness levels, all vortices were destroyed, and there was no clear HV topology in the time average sense. Finally, the motion process of the HV was summarized as showing an increase in the bed roughness. The present study investigated HV features with different bed roughness levels and has the potential to enhance our theoretical understanding of the relationship between HV kinematic characteristics and the bed roughness.

Experimental study of obstacle and bed roughness effects on behavior of turbidity current

Laboratory experiments were carried out to study the behavior of turbidity current under subcritical approach flow condition. In order to study the effects of solid obstacle and bed roughness on turbidity current, 24 experiments were conducted. Two trapezoidal macro-roughness elements with 0.01 m and 0.03 m height and three triangular obstacles with 0.1 m to 0.3 m height were used. The parameters such as front velocity, velocity profiles, body height, flow discharge, suspended load transport rate and efficiency of the obstacle were determined. It was found that under subcritical flow regime, the main portion of the turbidity current over the rough bed is controlled, if the obstacle height is 2–3 times of the body height. The growth of the front height depends on the inlet sediment concentration, the roughness elements and the obstacle height. The front velocity upstream of the obstacle over the smooth bed is independent of the obstacle height and significantly influenced by roughness elements. Simultaneous use of the bed roughness and obstacle significantly reduces the front velocity downstream of the obstacle. The flow discharge per unit width significantly decreases downstream of the obstacle. Simultaneous use of an obstacle and roughness elements decreases the flow discharge per unit width by about 90%. An obstacle with height equal to 0.75 of the initial height of the turbidity current with the smooth bed blocks about 65% of the suspended sediment. While, simultaneous use of the obstacle and the roughness elements blocks about 95% of the suspended sediment.

On the runup parameterisation for reef-lined coasts

The degradation of coastal ecosystems in recent years, combined with more intense storms and greater sea levels associated with climate change, are likely to increase vulnerability to coastal flooding along reef-lined coasts. Therefore, there is a need to accurately predict extreme water levels to identify areas with high vulnerability and implement mitigation measures. Runup parameterisations allow a rapid assessment of coastal vulnerability at a regional to global scale, however these formulations are primarily developed for beaches. Hydrodynamic forcing and reef geometry are key parameters for the estimation of coastal flooding in reef environments. The present study aims to develop runup parameterisations for an idealised 2DV reef-lined coast profile using a widely validated nonlinear non-hydrostatic numerical model (SWASH). The numerical model is employed to simulate different combinations of wave conditions, water levels, and reef geometries. A machine learning (ML) approach, in the form of genetic programming, is used to identify the most suitable predictors for wave runup based on the numerical results. Analysis of runup results suggests that runup parameterisations can be improved for reef environments by incorporating the crest elevation, lagoon width, reef flat depth, and forereef slope. A dimensional and non-dimensional parameterisation that include reef geometry are presented. Further research efforts should be devoted to incorporate the effects of bed roughness and three-dimensional processes in this framework that were not taken into account in the present work.

Open-channel flows through emergent rigid vegetation: Effects of bed roughness and shallowness on the flow structure and surface waves

Free-surface flows through a staggered cylinder array were investigated in an open-channel flume. The cylinders simulated rigid emergent vegetation. Specifically, we studied four flow cases with a two-factor design comprising flow rate (7 and 13 l s−1) and bed-surface state (hydraulically rough and smooth). We have primarily assessed the effects of bed roughness and shallowness on the time-averaged flow structure and the transverse fluctuating flow motion in the cylinder wake. Secondarily, the effects of the former on the vortex-shedding-induced surface waves were quantified. To gain further insight into the bed roughness effect on flow structure, we conducted transient flow simulations using a hybrid Reynolds-Averaged Navier–Stokes/Large Eddy Simulation turbulence model. For all cases, downstream of a cylinder, an upward flow occurs and two counterrotating secondary current cells develop. The two cells bring high-momentum fluid from the high-speed region into the cylinder wake, resulting in a near-bed streamwise velocity-bulge. The measured upward flow and velocity-bulge are greater for the rough-bed cases than for the smooth-bed cases. The simulated upward flow and velocity-bulge increase with an increasing roughness height, while secondary currents decay faster in the longitudinal direction. For the rough-bed cases, in the cylinder wake, the transverse fluctuating flow motion is hindered by the rough-bed induced turbulence over the whole water column, irrespective of the shallowness level. Coupled with the fluctuating flow motion, we have observed for three flow cases noticeable surface oscillations (termed “seiche waves”), whose amplitude decreases with decreasing flow depth. Under the combined effects of strong shallowness and a rough bed, seiche waves vanished.

Experimental and numerical study of bed roughness effect on longitudinal dispersion

Abstract The effects of bed roughness on the longitudinal dispersion coefficient (DL) were experimentally and numerically investigated in the present study. The tracer experiments were first carried out in a circular flume with a diameter of 1.6 m over both smooth and rough beds (coarse sand) with four sizes (ks = d65) of 1.04, 2.09, 3.01, and 4.24 mm. In addition, the one-dimensional advection–dispersion equation was numerically solved. The longitudinal dispersion coefficient was calculated by comparing the numerical and experimental breakthrough curves. The results showed that by increasing the bed roughness height (from zero to 4.24 mm), the longitudinal dispersion coefficient increased by 34%. In addition, the longitudinal dispersivity (λ = DL/V) increased with increasing relative roughness (ks/h), so that the range of longitudinal dispersivities in smooth bed experiments were 0.037–0.049 m and for rough bed (ks = 4.24 mm) were 0.07–0.084 m. In other words, with increasing the bed roughness height from zero (smooth bed) to 4.24 mm, the longitudinal dispersivities increased from 0.037 to 0.077 m, indicating an increase of about 108%. Furthermore, a relationship was developed using non-dimensional longitudinal dispersion (DL/(Vh)) as a function of relative roughness (ks/h). It can be concluded that taking into consideration bed roughness as the driving force of shear dispersion would improve predictive equations of the longitudinal dispersion in the rivers. As the bottom of all natural rivers has roughness elements with different sizes, the results of this study will definitely be useful in estimating the longitudinal dispersion coefficient in natural rivers and quantifying the effect of roughness in the longitudinal dispersion coefficient equations.

3-D Numerical Study of a Bottom Ramp Fish Passage Using Smoothed Particle Hydrodynamics

Worldwide, the overwhelming number of man-made barriers in fluvial systems has been identified as one of the major causes of the reported staggering average declines of migratory fish. Fish passages have been shown to help mitigate such problems. Close-to-nature types of fish passages, such as bottom ramps, bypass channels, and fish ramps can be used to minimize the impact of artificial steep drops (e.g., weirs) on the migration of aquatic fauna, especially in cases of low-head barriers. This study focuses on the characterization of the flow pattern in a bottom ramp. A 3-D numerical model based on the meshless smoothed particle hydrodynamics (SPH) method was successfully validated and then employed for the simulation of turbulent free-surface flow in a straight channel with complex geometry. The effects of bed roughness, channel slope, and flow rate were quantified in terms of flow depth, velocity fields, and area‒velocity ratios. During the study, several new tools were developed, leading to new functionalities in pre-processing, solver, and post-processing which increase the applicability of DualSPHysics in the field of eco-hydraulics.

A Theoretical and Experimental Investigation of Advancing Surge Waves.(Dept.C)

This research work provides analytical solution of the positive surge waves, Propagating either upstream or downstream based on the momentum principle. The solution based on the numerical solution of the governing equations of motion (the characteristics method) is also highlighted. Experimental work was carried out to make a comparative study between the actual measurements and the corresponding theoretical relationships for a horizontal plain bed channel. The effect of bed roughness on the shape and characteristics of such waves was under consideration.

Investigating the Effects of Bed Roughness on Incipient Motion in Rigid Boundary Channels with Developed Hybrid Geno-Fuzzy versus Neuro-Fuzzy Models

In present study, the comprehension of different types of bedforms generated by different bed roughness, in rectangular and circular cross-sectional rigid boundary channels and the resistance generated by the bed roughness against the flow are evaluated. Moreover, the critical condition of sediment motion under bed roughness influence is investigated. This study presents novel contributions in solving engineering problems that suffer from the lack of knowledge on the incipient motion of sediment particles in rigid boundary channels under bed roughness effects. Furthermore, soft computing and evolutionary computation methods have been combined to develop a new novel predictive model. The Evolutionary GENOFIS also aims at improving the ANFIS tool depends on Sugeno Fuzzy Inference System. The advantages of the proposed hybrid GENOFIS approach over the ANFIS tool is its novel ability to tackle the incipient prediction problem by using less fuzzy based rules and to represent the consequent part of the model as a constant, linear or non-linear function as well as with possible simultaneous combinations of them. For validation and comparison purposes, model performances of hybrid GENOFIS, ANFIS and data-based linear regression approaches are evaluated via experimental data partitioned for testing purposes. The model results are compared through corresponding RMSE and CE values. It is found that the hybrid GENOFIS model results outperformed the ANFIS and linear regression models. Consequently, novel experimental data-driven incipient motion formula and the hybrid GENOFIS approach are proposed to modeling complex incipient motion problems that are full of uncertainty and complication.

Field measurements of shear stress and friction in the surf zone

AbstractMeasurements of near‐bed shear stress were undertaken in the shallow subtidal zone at Durras Beach, NSW, Australia using a sideways‐looking acoustic velocity meter installed within the wave boundary layer. The wave climate was swell‐dominated and wave conditions comprised shoaling and breaking waves as well as surf bores. The sediment at the field site was medium‐grained sand, and observations of bedform geometry were conducted using a pencilbeam‐sonar system. Using frequency‐filtering techniques, the measured stresses were partitioned into terms representing turbulent (Reynolds) stress, stresses due to gravity and infragravity‐scale oscillatory motions, and wave‐turbulence‐mean current cross‐terms. Gravity wave‐orbital scale motions contributed the largest fraction of the stresses, comprising 24% on average, followed by long‐wave advection of vertical orbital motion (16%). The presence of wave orbital‐scale motions near or at the water/sediment interface was likely due to the porous nature of the seabed, facilitating interfacial flow. Shear stresses did not scale with bed roughness but exhibited a linear relationship with the relative wave height. This indicates that for the experimental conditions, surf zone processes overwhelmed bed roughness effects on shear stress and friction. Calculations of the wave friction factor, fw, showed that in a natural surf zone, this was a factor 3–4 larger than conventional predictions. © 2020 John Wiley & Sons, Ltd.

Turbulence anisotropy and intermittency in open-channel flows on rough beds

An experimental campaign, based on Particle Image Velocimetry measurements in a laboratory flume with different median sediment sizes in the no-motion condition, has been carried out aiming at investigating the effects of bed roughness on turbulence anisotropy in two different vertical zones of the turbulent open-channel flow. An analysis of turbulence anisotropy, which relies on second-order structure functions and anisotropy angle, has been performed. The scale-dependent anisotropy level has been quantified, verifying the tendency of the system to span from large-scale anisotropy, due to the main shear of the boundary layer, to small-scale isotropy. Isotropy is well-established for the largest sediment sizes. High-order structure function analysis reveals that intermittency is more pronounced in the near-bed layers, where the flow is more populated by coherent vortices. Spectral anisotropy and intermittency strongly characterize the transport properties of turbulence and are, therefore, important phenomena for natural bed rivers.

CFD Simulation of Velocity Distribution in a River with a Bend Cross Section and a Cubic Bed Roughness Shape

The variation of bed roughness along a channel cross section especially if the cross section with bend affects the velocities in that part of the section may be used to change the flow patterns in the channel and as a mean of river training. In this research the Computational Fluid Dynamics (CFD) for modelling the effect of bed roughness on the free surface flows in open channel has been validated for a straight river, the effect of angles of attack downstream the bed roughness has been investigated. This study presents the results and analysis of numerical simulations that were carried out to compute Manning’s coefficient of artificial geometric roughness element (cubic shape). Artificial roughness elements were fixed on bed of the flume within the test section in different spacing and angles of attack. A sequence of CFD simulations using the k-ω SST model are compared with a set of laboratory data for the influence of flow at a free surface in open channel with cubic shape bed roughness. The numerical results gaining confidence in a modelling of Volume of Fluid (VOF) technique and allows for modelling of more complex hydraulic structures by including the effect of angles of attack to the simulation.

Effect of bed roughness on flow field around spur dyke in a rigid bed meandering channel

ABSTRACT Effect of bed roughness on flow field and turbulence characteristics due to spur dyke in rigid bed meandering channel is presented in this study. This is an extension of authors’ earlier work. In the experiments, cement mortar, sand particles on cement mortar and small aggregates on the cement mortar surfaces are used to generate different bed roughness. 3-D velocity measurements are obtained with Acoustic Doppler Velocimeter (ADV). In a rigid bed meandering channel without spur dyke, flow concentrates near the convex bank up to inflection point in the downstream of Apex. Inflection point moves towards the upstream with increase in the roughness of the channel. The water surface slope along the center line increases with increase in bed roughness. In presence of spur dyke, the maximum velocity in the channel decreases with increase in bed roughness. When the spur dyke is located on the convex bank of Apex, the length and width of separation zone decrease with increase in bed roughness. The magnitude of turbulence intensities decreases with increase in roughness.

Anisotropy in the Free Stream Region of Turbulent Flows through Emergent Rigid Vegetation on Rough Beds

Most of the existing works on vegetated flows are based on experimental tests in smooth channel beds with staggered-arranged rigid/flexible vegetation stems. Actually, a riverbed is characterized by other roughness elements, i.e., sediments, which have important implications on the development of the turbulence structures, especially in the near-bed flow zone. Thus, the aim of this experimental study was to explore for the first time the turbulence anisotropy of flows through emergent rigid vegetation on rough beds, using the so-called anisotropy invariant maps (AIMs). Toward this end, an experimental investigation, based on Acoustic Doppler Velocimeter (ADV) measures, was performed in a laboratory flume and consisted of three runs with different bed sediment size. In order to comprehend the mean flow conditions, the present study firstly analyzed and discussed the time-averaged velocity, the Reynolds shear stresses, the viscous stresses, and the vorticity fields in the free stream region. The analysis of the AIMs showed that the combined effect of vegetation and bed roughness causes the evolution of the turbulence from the quasi-three-dimensional isotropy to axisymmetric anisotropy approaching the bed surface. This confirms that, as the effects of the bed roughness diminish, the turbulence tends to an isotropic state. This behavior is more evident for the run with the lowest bed sediment diameter. Furthermore, it was revealed that also the topographical configuration of the bed surface has a strong impact on the turbulent characteristics of the flow.