Bed Shear Stress Distribution Research Articles

Uncertainty quantification of bank vegetation impacts on the flood flow field in the American River, California, using large‐eddy simulations

AbstractBank vegetation plays a key role in both hydrodynamics and morphodynamics of natural rivers; however, these effects are often unaccounted for in the computational flow dynamics of natural waterways. Recent studies using the large‐eddy simulation (LES), however, have attempted to gain insights into the impacts of bank vegetation on the mean flow field of the natural rivers using a vegetation model, which applies a sink term to the momentum equations of motion. This approach accounts for the effects of the vegetation and provides a practical approach to account for the complex patches of bank vegetation in large‐scale rivers. To implement the vegetation model, a drag coefficient reflecting the overall resistance of vegetal structures to the flow is needed, but due to the lack of calibrated data and range of size, density and type of vegetation, this parameter can be a significant source of uncertainty in the model results. In this study, we use uncertainty quantification (UQ) to investigate the hydrodynamics and bed shear results when a bank vegetation is incorporated in an LES model. To this end, we used the polynomial chaos expansion and Monte Carlo sampling techniques to determine the uncertainties associated with the drag coefficient in the vegetation model and from uncertainties in the bed roughness and inflow discharge. The UQ analysis provided spatially varying confidence levels for the spanwise and vertical distribution of velocity magnitude and for the bed shear stress distributions. In addition, Sobol indices were computed to indicate the relative influence that each parameter had on the overall uncertainty. In general, it was found that uncertainty in flow discharge was the dominant source of uncertainty; however, the drag coefficient in the vegetation model and the bed roughness parameter also made significant contribution to the uncertainty near the banks and bed, respectively.

Experimental Study of Secondary Flow in Narrow and Sharp Open-Channel Bends

Secondary flow is a prominent feature of channel bends; it alters the streamwise velocity and bed shear stress distributions. Experiments were conducted to investigate the complex pattern of secondary flow in a narrow and sharp open-channel bend and the underlying mechanism of generation of multiple circulation cells. Compared with the moderate bends, the sharp bends are characteristic of multiple circulation cells from the 90° section. In addition to the curvature-induced circulation cell (S1) and turbulence-induced counter-rotation circulation cell (C1) near the outer bank, another circulation cell (S2) was observed near the inner bank and was attributed to flow separation. A term-by-term analysis of the vorticity equations indicates that the centrifugal term favours S1 and C1 while opposing S2. The turbulence-related term accounts for the formation of C1 and S2. The advective transport term redistributes vorticity and maintains the existence of S2. The dependence of secondary flow structure on Reynolds number and aspect ratio was also explored. With an increase in the Reynolds number from 23000 to 37000, both the strength and size of C1 are reduced by 50%, whereas the size of S2 increases by 20%, and its strength slightly decreases. With a decrease in the aspect ratio from 3.3 to 2, the strengths of S1, S2, and C1 are doubled, and the sizes of C1 and S2 increase by 90% and 20%, respectively.

Scour Development Around an Oblong Bridge Pier: A Numerical and Experimental Study

The complex flow structure around bridge piers is challenging for both experimental and numerical studies. Therefore, investigating the capabilities of Computational Fluid Dynamics (CFD) tools in resolving the flow structure and the mechanism of sediment entrainment into and out of the scour hole remains a challenging task. In this study, the scour depth around an oblong bridge pier and the bed shear stress distributions in time and space were numerically investigated using the Computational Fluid Dynamics (CFD) tool Sediment Simulation In Intakes with Multiblock option (SSIIM). Clear water scour conditions and sand of known granulometric composition were considered in accordance with the experimental study carried out. Laboratory data and the results of a scour characterization around a 0.11 m wide oblong bridge pier were considered to calibrate and validate the numerical model. The averaged form of the Navier–Stokes equations was considered to simulate the turbulent flow fields in anticipation of long time scales. The results show that calibrated numerical models can reproduce measured scour depths in the laboratory environment with considerable accuracy, with an average relative error of less than 3%, especially around oblong bridge piers.

Numerical Analysis of the Flow Structure around Inclined Solid Cylinder and Its Effect on Bed Shear Stress Distribution

The flow-inclined cylinder interaction is an application area in the industry (i.e., offshore wind turbines and pile-supported near-shore structures). Findings of recent studies have revealed the significance of eco-friendly coastal structures that needs the utilization of inclined cylinder. The primary purpose of this study was to better understand the influence of inclination on flow, turbulence, and bed shear stress character. To achieve this objective, a three-dimensional numerical code (the Reynolds-averaged Navier-Stokes model) was used. The numerical model was calibrated based on eleven velocity profiles obtained by point measurements data of the wake region of the inclined cylinder. The mean flow, turbulence, and secondary flow characteristics around the bodies were extensively investigated, particularly at points where experimental measurements are inapplicable with intrusive turbulence measurement devices. The findings of the study revealed that as the inclination of the cylinder increased, the coherent structures that largely control the flow dynamics in the wake zone became stable rather than cyclical. Specifically, it was determined that although vorticity couples underpinned the flow field behind the vertical cylinder, large-scale streamwise vortices replaced the visible coherent structures when the cylinders were inclined (LSCSVs). When the cylinder inclined 42 degrees, the reduction in amplification factor (τ0 / τ∞) over the bed was roughly fifty percent in terms of quantity. This finding shows that inclination is a streamlined form for a cylinder and may reduce the collapse risk due to scour.

Numerical Investigation of Bed Shear Stress and Roughness Coefficient Distribution in a Sharp Open Channel Bend

The helical flow leads to the redistribution of the bed shear stress and roughness coefficient in open-channel bends, influencing the transport of sediment and riverbed evolution process. To explore the mechanisms underlying this redistribution, a 3D numerical simulation of a 180° sharp bend was performed by solving the Reynolds-averaged Navier–Stokes (RANS) equations using the Reynolds stress equation model (RSM) as the anisotropic turbulence closure approach. The results indicate that the high bed shear stress zone appeared in the inner bank region from the 50° to 110° sections, and the section maximum bed shear stress gradually shifted outwards, owing to the advective momentum transport by the circulation cells. The quantitative analyses of the terms in depth-averaged Navier-Stokes equations indicate that the contributions of the cross-stream circulation and cross-flow to the downstream bed shear stress were of the same order of magnitude, and the overall contribution rate of the cross-stream circulation was 20%. The contribution rates of the cross-flow, cross-stream circulation, turbulence, and pressure gradient to the transverse bed shear stress were approximately 30%, 7%, 3%, and 60%, respectively, indicating that the pressure gradient term arising from the transverse water surface slope played a dominant role. The Chezy resistance coefficient showed an overall decreasing trend along the bend. Therefore, an effective expression considering the streamwise variation along the centreline and transverse variation was successfully established to predict the uneven distribution of the Chezy resistance coefficient.

Numerical investigation on the influence of a spoiler structure for local scour protection

As a Marine structure, monopile foundations can significantly change the hydrodynamic conditions around the piles, leading to local scour. The main causes of local scour around monopile foundations are horseshoe vortex, downward flow, and wake vortex. In this study, a large-diameter monopile foundation of offshore wind power is taken as the research object. A novel spoiler structure is proposed and a three-dimensional numerical model is established. Based on numerical simulation, the influence mechanism of spoiler structure on local scour around monopile is investigated in constant current. The influence of length L and height H2 of the spoiler on the location and strength of local horseshoe vortex is discussed, and the distribution characteristics of the bed shear stress are obtained. By comparing the distribution of local horseshoe vortex and bed shear stress with and without spoiler, it is found that the spoiler ribs can significantly reduce the strength of horseshoe vortex and the regional shear stress between ribs. The local horseshoe vortex presents three flow mechanisms under different geometric spoiler structures, namely independent horseshoe vortex, disturbed horseshoe vortex, and separated horseshoe vortex. In the three horseshoe vortex states, the protected bed area has a linear relationship with the spoiler geometric length or height. Additionally, the spoiler exhibits good scour protection characteristics with L/H2=1. By analyzing the flow field characteristics around the spoiler, the influence mechanism of ribs on the horseshoe vortex and shear stress distribution around the monopile foundation is revealed. Overall, the novel spoiler structure shows promising results for local erosion protection, and it has practical importance for guiding the protection design of monopile foundations.

Modelling shear stress distribution in ice-covered streams

Abstract Distribution of bed shear stress is the critical factor in regulating the meandering of single-thread rivers. However, the impact of ice cover on bed shear stress is largely unknown. In this study, we develop a theoretical model of cross-stream momentum balance to examine the distribution of bed shear stresses in ice-covered meandering rivers. To validate the theoretical model, field surveys were carried out in a river reach of the Red River in Fargo, North Dakota. Data monitoring was completed using an Acoustic Doppler Current Profiler to obtain time-averaged velocity profiles. Our theoretical model indicates that an ice covering develops high-shear zones near both the inner and outer banks, which might exacerbate sediment transport and enhance bank erosion. Velocity measurements confirm the results of the proposed model and demonstrate a clear impact of meandering river banks on velocity profiles and secondary flow patterns under ice cover. Based on our results, we hypothesize that ice cover increases turbulent stresses near banks, which in turn lead to the enhancement of the bed shear stress. Our work provides new insights into the impact of ice cover on bed shear stress distribution, which could play an important role in driving sediment-transport processes and the long-term morphodynamic evolution of meandering rivers seasonally covered by ice.

Experimental and numerical study on the flow characteristics around spur dikes at different length-to-depth ratios

A spur dike is an in-stream structure, one end of which is situated on the bank of a channel, while the other extends into the current. Its primary functions include protecting stream banks and enhancing aquatic habitats. The motivation of this study stems from the hypothesis that a wake parameter, defined as the product of the length-to-depth ratio of spur dikes and the bottom friction factor, serves as the key parameter in characterizing the flow pattern around these structures. The objectives of this study are two-fold: (1) to determine the critical value of a wake parameter for the transition from three- to two-dimensional flow fields around spur dikes, and (2) to examine the bed shear stress distributions around spur dikes with varying length-to-depth ratios. Flow visualization experiments and large-eddy simulations (LESs) were conducted with length-to-depth ratios ranging from 1 to 7.5, corresponding to wake parameter values between 0.005 and 0.075. The experimental and numerical results revealed various flow structures around and in the wake of the spur dike. When the length-to-depth ratio was close to 1, the two- and three-dimensional flow patterns coexisted around the spur dike. However, as the length-to-depth ratio increased, the three-dimensional flow structures diminished and the two-dimensional flow structures became more pronounced. For an length-to-depth ratio of 7.5, the flow pattern showed fully two-dimensional structures featured by an upstream and a pair of downstream recirculation regions with negligible secondary flows, a horseshoe vortex, and a free-surface vortex. A horseshoe vortex system contributed to a significant increase in both mean and root mean square bed shear stresses near the tip of spur dikes, especially when the wake parameter values were low. The upstream recirculation zone was associated with very low bed shear stress at high values of the wake parameter. The results of this study corroborated that the flow around the spur dikes becomes fully two-dimensional when a wake parameter exceeds a critical value of approximately 0.04.

On the Morphodynamics of a Wide Class of Large‐Scale Meandering Rivers: Insights Gained by Coupling LES With Sediment‐Dynamics

AbstractIn meandering rivers, interactions between flow, sediment transport, and bed topography affect diverse processes, including bedform development and channel migration. Predicting how these interactions affect the spatial patterns and magnitudes of bed deformation in meandering rivers is essential for various river engineering and geoscience problems. Computational fluid dynamics simulations can predict river morphodynamics at fine temporal and spatial scales but have traditionally been challenged by the large scale of natural rivers. We conducted coupled large‐eddy simulation and bed morphodynamics simulations to create a unique database of hydro‐morphodynamic data sets for 42 meandering rivers with a variety of planform shapes and large‐scale geometrical features that mimic natural meanders. For each simulated river, the database includes (a) bed morphology, (b) three‐dimensional mean velocity field, and (c) bed shear stress distribution under bankfull flow conditions. The calculated morphodynamics results at dynamic equilibrium revealed the formation of scour and deposition patterns near the outer and inner banks, respectively, while the location of point bars and scour regions around the apexes of the meander bends is found to vary as a function of the radius of curvature of the bends to the width ratio. A new mechanism is proposed that explains this seemingly paradoxical finding. The high‐fidelity simulation results generated in this work provide researchers and scientists with a rich numerical database for morphodynamics and bed shear stress distributions in large‐scale meandering rivers to enable systematic investigation of the underlying phenomena and support a range of river engineering applications.

Flow Pattern and Erosion in a 90-Degrees Sharp Bend around a W−Weir

Different flow-altering methods, such as W−Weirs, have been developed to reduce erosion. For this study, we performed two experiments: (1) installing a W−Weir in various positions to determine the best angle for placement, and (2) investigating the variation of flow patterns and bed shear stress distribution in a 90-degree sharp bend by measuring the 3D components of flow velocities, with and without W−Weirs, where the greatest scour depth occurs. The results from the three installation angles indicate that less scour depth and volume of sediment removal occur when the weir is located close to the end of a bend. In addition, the value of the secondary circular power without a weir was higher than the position with a weir; however, this value significantly increased at 70 degrees due to turbulence flow near the W−Weir. This secondary flow power reduction at 45 degrees with a W−Weir increased by 65.8 percent for a Froude number value of 0.17, and by 29.8 percent for a Froude number value of 0.28, compared to values without the W−Weir, respectively.

Spatial and temporal variability of shorefaces: A morpho-hydrodynamic controlled system

Shorefaces are transitional zones between the shelf and surfzone/beach systems. They are subdivided into ‘upper’ and ‘lower’ shoreface sectors which display particular morphodynamic behaviour, with the upper shoreface morphologically active over short (annual) time scales, and the lower shoreface evolving over much longer time scales (decadal and beyond).We examine morphodynamics using three distinct examples from the high-energy coast of Ireland. Numerical wave modelling (SWAN) is used to characterize wave energy dissipation across the shoreface. Our results show that zonation patterns and the physical extent of shorefaces display distinct spatial as well as temporal variations. The shoreface’s physical limits and internal zonation patterns are driven by the combination of local wave forcing and pre-existing shoreface morphology. Shoreface behaviour therefore, falls within a spectum between gradual and intensive wave energy dissipation.We show that particular seabed morphology controls the extent of, as well as the bed shear stress distribution across the shoreface. This can have significant implications for any adjoining beach systems as waves arriving at the surf zone acquire specific characteristics depending on the shoreface configuration, even under similar offshore wave conditions. Additionally, the variation of shoreface extent impacts the volume of sediment available for transport, both to the nearshore zone as well as local beach systems. The influence of shoreface morphology is therefore an important, but largely overlooked element in the study of mesoscale coastal behaviour as well as coastal response to global climate change and secular sea-level rise.

Reynolds stress modeling of supercritical narrow channel flows using OpenFOAM: Secondary currents and turbulent flow characteristics

In this study, the full Launder, Reece and Rodi pressure-strain model, and nonlinear boundary damping functions were incorporated in OpenFOAM® to simulate the turbulence-driven secondary currents in supercritical narrow channel flows, such as in sediment bypass tunnels. Five simulations were performed under uniform flow conditions covering Froude numbers from 1.69 to 2.56 and aspect ratios (channel width to flow depth) ar from 0.9 to 1.91 to investigate the formation of secondary currents and their impacts on longitudinal velocity, turbulence characteristics, and bed shear stress distribution. The numerical results of the maximum longitudinal velocity and the average shear velocity show marginal deviations, of less than 2.6%, from two-dimensional experimental results acquired under decelerating flow conditions. However, some differences are observed for the secondary currents and for the vertical turbulence intensity and Reynolds shear stress in the outer flow region, especially for cases with higher flow nonuniformity (that can influence the surface perturbation) whose influence is missing in the numerical model. No intermediate vortex is observed for ar = 1.91. However, it develops for lower ar and detaches from the free surface vortex when ar ≤ 1.05. Such vortex bulges the longitudinal velocity contour lines inward and the zone of higher longitudinal velocity narrows and deepens with a decrease in ar. The decrement reduces the magnitude of the normalized maximum secondary velocity. It also affects the bottom vortex which alters the bed shear stress distribution.

Bulk Flow Characteristics of a Gravel Bed River with Instream Emergent Vegetation

ABSTRACT Aquatic vegetation and morphology are integral components of fluvial systems, study of which is important due to ecological and engineering projects. Often, studies in fluvial systems are carried out in the laboratory as collecting data in the field is expensive and difficult, and even risky. A field study was carried out to measure the hydraulic and geometric parameters of the bed and the banks of a gravel bed river with instream emergent vegetation in northern Iran. The aim of this study was to gain insight into the effect of bed forms and vegetation patches on flow resistance and distribution of velocity and bed shear stress. The boundary layer characteristics method was employed to calculate the shear velocity and friction factor, the resulted friction values are compared with that of Manning’s equation. The friction of bed forms was estimated using a linear superposition of friction. Results showed that the boundary layer characteristics method was suitable for calculating the shear velocity under various conditions, moreover, the distribution of velocity and bed shear stress were discussed. The findings of this study will be useful in river management and restoration.

Bed and wall shear stresses in smooth rectangular channels

Various empirical equations for mean bed and wall shear stresses have been proposed for smooth rectangular open channels. Considering that the distribution of bed shear and wall shear stresses is affected by the cross-sectional shape, the gravity force based on the flow depth used to non-dimensionalize shear stresses in previous studies is modified to a parameter that includes the width and wetted-perimeter. This yields a new set of dimensionless equations for mean bed and wall shear stresses, and the wall/bed shear force ratio. These equations compared well with all available experimental data over a wide range of aspect ratios from 0.31 to 50. These equations also yield new relationships for bed and wall friction factors.

Quantifying the relationship between structural deformation and the morphology of submarine channels on the Niger Delta continental slope

AbstractThe processes and deposits of deep‐water submarine channels are known to be influenced by a wide variety of controlling factors, both allocyclic and autocyclic. However, unlike their fluvial counterparts whose dynamics are well‐studied, the factors that control the long‐term behaviour of submarine channels, particularly on slopes undergoing active deformation, remain poorly understood. We combine seismic techniques with concepts from landscape dynamics to investigate quantitatively how the growth of gravitational‐collapse structures at or near the seabed in the Niger Delta have influenced the morphology of submarine channels along their length from the shelf edge to their deep‐water counterpart. From a three dimensional (3D), time‐migrated seismic‐reflection volume, which extends over 120 km from the shelf edge to the base of slope, we mapped the present‐day geomorphic expression of two submarine channels and active structures at the seabed, and created a Digital Elevation Model (DEM). A second geomorphic surface and DEM raster—interpreted to closer approximate the most recent active channel geometries—were created through removing the thickness of hemipelagic drape across the study area. The DEM rasters were used to extract the longitudinal profiles of channel systems with seabed expression, and we evaluate the evolution of channel widths, depths and slopes at fixed intervals downslope as the channels interact with growing structures. Results show that the channel long profiles have a relatively linear form with localized steepening associated with seabed structures. We demonstrate that channel morphologies and their constituent architectural elements are sensitive to active seafloor deformation, and we use the geomorphic data to infer a likely distribution of bed shear stresses and flow velocities from the shelf edge to deep water. Our results give new insights into the erosional dynamics of submarine channels, allow us to quantify the extent to which submarine channels can keep pace with growing structures, and help us to constrain the delivery and distribution of sediment to deep‐water settings.

Experimental investigation on bed shear stress distribution in the roughened compound channel

The roughness in the floodplains in a compound canal and its impact on hydraulic parameters such as the shear stress and their estimation are one of the problems that have attracted the attention of engineers. In this research, the aim is to investigate the effect of artificial on the floodplain of the compound channel on hydraulic parameters. In experiments, slope of channel bed was 0.0015 and three different discharges have been used. The four types of rigid roughness were used to investigate the effect of these parameters. These roughness elements were arranged with zigzag state with two distances of 4 k and 8 k (k is the height of roughness) in the floodplain. A Preston tube with an external diameter of 3 mm that equipped with dynamic pressure sensors was used to compute the shear stress. The Patel calibration curve was used in order to convert the difference between the static and dynamic pressure measured by the Preston tube to the shear stress values. The results showed that for the zigzag arrangement with the density of 4 k, the shear stress is reduced due to the high roughness density and the greater roughened area. In a rough bed, the shear stress in floodplain was significantly higher than smooth bed, and the stress distribution is such that it has descending trend from the main channel toward the wall of the floodplain. The shear stress increase for roughness with a spacing of 8 k is 22–36% higher than the similar hydraulic condition in a smooth bed and the shear stress for condition with the presence of a cylinder with D = 3 cm and roughness spacing of 8 k was 14–18% higher than the shear stress of bed without a cylinder and the same roughness density. The shear stress for condition of the presence of a cylinder with D = 6 cm and roughness spacing of 4 k is 24–30% more than the roughened plain with distances of 4 k.

How does the bed surface impact low‐magnitude bedload transport rates over gravel‐bed rivers?

AbstractBedload transport is a complex phenomenon that is not well understood, especially for poorly sorted sediment and low transport rates, which is what is typically found in alpine gravel‐bed rivers. In this paper, the interaction between bedload rate, bed stability and flow is investigated using flume experiments. Significant differences in bedload rates were observed for experiments conducted on beds formed with the same gravel material but presenting diverse arrangements and bedforms. Tests were performed under regimes of low transport rate, which are mainly controlled by gravel‐bed roughness. Different scales of roughness were identified using the statistical characteristics of detailed bed elevation measurements: grain, structure and large bedform scales. The role played by these different roughness scales in bedload dynamics was examined. For quasi‐flat beds, bed stability was quantified using a combination of bed surface criteria describing grain and structure scales. It was found that bed stability affects the bedload rate directly and not only through its influence on the flow or on the incipient motion. For beds with large bedforms, the analysis of bedload dynamics also showed the importance of accounting for effective bed shear stress distributions. An empirical bedload model for low transport regimes was suggested. Compared with previous formulae developed for alpine rivers, this model accounts for bed stability and distribution of effective bed shear stress. It significantly improves the understanding of gravel dynamics over complex beds such as arranged beds or those with large bedforms. However, further tests are needed to use the model outside the range of conditions of this study. © 2019 John Wiley & Sons, Ltd.

Modeling Bed Shear Stress Distribution in Rectangular Channels Using the Entropic Parameter.

The evaluation of bed shear stress distribution is fundamental to predicting the transport of sediments and pollutants in rivers and to designing successful stable open channels. Such distribution cannot be determined easily as it depends on the velocity field, the shape of the cross section, and the bed roughness conditions. In recent years, information theory has been proven to be reliable for estimating shear stress along the wetted perimeter of open channels. The entropy models require the knowledge of the shear stress maximum and mean values to calculate the Lagrange multipliers, which are necessary to the resolution of the shear stress probability distribution function. This paper proposes a new formulation which stems from the maximization of the Tsallis entropy and simplifies the calculation of the Lagrange coefficients in order to estimate the bed shear stress distribution in open-channel flows. This formulation introduces a relationship between the dimensionless mean shear stress and the entropic parameter which is based on the ratio between the observed mean and maximum velocity of an open-channel cross section. The validity of the derived expression was tested on a large set of literature laboratory measurements in rectangular cross sections having different bed and sidewall roughness conditions as well as various water discharges and flow depths. A detailed error analysis showed good agreement with the experimental data, which allowed linking the small-scale dynamic processes to the large-scale kinematic ones.

Investigation of Shear Stress Distribution in a 90 Degree Channel Bend

Shear stress is a key parameter that plays an important role in sediment transport mechanisms; therefore, understanding shear stress distribution in rivers, and especially in river bends, is necessary to predict erosion, deposition mechanisms and lateral channel migration. The aim of this study is to analyze the shear stress distribution near a river bed at 90-degree channel bend using a depth-average method based on experimental measurement data. Bed shear stress distribution is calculated using the depth-averaged method based on velocity components data has been collected from a 3D-ADV device (three-dimensional acoustic doppler velocity) at different locations of a meandering channel. Laboratory experiments have been made at the hydraulic laboratory of the RCRFIDF (Research Center for River Flow Impingement and Debris Flow), Gangneung-Wonju National University, South Korea to provide data for simulating the incipient motion of the riverbed materials and then predicting the river morphological changes in the curved rivers. The calculated results show that the maximum value of shear stress distribution near the riverbed in the different cross sections of the surveyed channel occurs in a 70-degree cross section and occurs near the outer bank. From the beginning of a 40-degree curved channel section, the maximum value of the shear stress occurs near the outer bank at the end of the channel.

Analysis of Bed Shear Stress Distributions in Compound Open Channels Using Large Eddy Simulation.

Analysis of Bed Shear Stress Distributions in Compound Open Channels Using Large Eddy Simulation.