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Sediment suspension is vital to the geomorphological and ecological processes in rivers. However, previous research has mainly focused on the suspension of sediments with uniform grain sizes, neglecting the complexities arising from sediment mixtures. This study investigates the suspension behaviors of both uniform and mixed-size sediment grains in an oscillating grid turbulence environment. The experimental apparatus comprised a seven-panel grid system, which oscillated at varying frequencies to generate homogeneous turbulence within a water column. The results revealed that for uniform sediments, the sediment diffusion coefficient increases nonlinearly with oscillating frequency and particle size, and for sediment mixtures, the suspension of particles is influenced not only by particle inertial effect but also by the interaction between coarse and fine particles. The presence of coarse grains leads to an enhancement in the sediment diffusion coefficient for fine grains, whereas the reverse influence is minimal. Additionally, the size fraction for the suspended sediment is influenced by various factors, including the oscillating frequency, the height of suspension, and the type of the sediment bed. This study also investigates the mechanisms of sediment-carrying capacity from an energy conservation perspective. It shows that the power required to keep sediment in suspension can be adopted as a new parameter for assessing sediment-carrying capacity. These insights are expected to have broader implications for the comprehension of suspended sediment transport in rivers. [ABSTRACT FROM AUTHOR]

Zero-plane is an important reference level included in the logarithmic velocity profile. The prediction of the zero-plane displacement remains challenging, especially in open-channel flows with submerged vegetation. In this study, we first review the existing formulas for estimating the zero-plane displacement. Then, we conducted a series of laboratory experiments of open-channel flows with submerged rigid vegetation, by varying flow rate, water depth, vegetation density, and stem diameter. By utilizing particle image velocimetry (PIV) technique, the spatio-temporal averaged (double-averaged) flow velocity and Reynolds shear stress profiles were obtained. The data analysis shows that the zero-plane displacement is almost proportional to the surface layer depth for the shallow flow cases. As the flow depth increases, the zero-plane displacement tends to be constant and increases with decreasing vegetation density. This result substantiates the hypothesis that the zero-plane displacement is related to large-scale eddies. Lastly, a heuristic model is developed to explain possible variations of the large-scale eddies, which yields a formula for predicting the zero-plane displacement for flows subjected to submerged rigid vegetation. [ABSTRACT FROM AUTHOR]

Local scour processes can be divided into initial, progressing, and equilibrium stages. Due to its short duration and high scour rate, the initial stage has not been well investigated to date. By applying laser-based technique and particle image velocimetry, this study conducted a series of measurements of scour hole profile and flow field at the beginning of scour hole development for the case of plane jet–induced scour. The results show that the scouring process in the initial stage is dominated by the wall shear, and thus can be classified as shear scour, whereas that in the progressing stage largely is affected by the turbulent vortex, and therefore can be referred to as vortex scour. The two modes of scour could be characterized by different mechanisms of sediment erosion. A bedload model is proposed to analyze the sediment transport for the shear scour, and the results agreed well with the experimental data. [ABSTRACT FROM AUTHOR]

This note concerns variations of the friction factor in the two transitional regimes, one between laminar and turbulent flows and the other between fully smooth and fully rough turbulent flows. An interpolation approach is developed to derive a single explicit formula for computing the friction factor in all flow regimes. The results obtained for pipe flows give a better representation of Nikuradse's experimental data, in comparison with other implicit formulas available in the literature. Certain modifications are also made for applying the obtained friction formula to open-channel flows. DOI: 10.1061/(ASCE)0733-9429(2008) 134:9(1357) CE Database subject headings: Friction; Pipe flow; Open channel flow; Laminar flow: Turbulent flow; Reynolds number.

The critical condition for incipient sediment motion is formulated in this note based on the settling velocity. The formula obtained is simple, relating the ratio of critical shear velocity to settling velocity to the dimensionless sediment diameter. Comparisons are then made with other settling-velocity based formulas available in the literature. To facilitate the computation of the effective near-bed velocity at the threshold condition, a generalized law-of-the-wall function is proposed for predicting the velocity distribution under various boundary conditions. This study demonstrates that the settling velocity is equivalent to the critical near-bed velocity, which is experienced by a typical bed sediment particle under the threshold condition, but only for large sediment sizes such as sand and gravel. Comparison results show that Yang's formula is suitable for flows with small flow depth relative to sediment size while Le Roux's formula may overestimate the threshold condition for fine particles by up to 30%. DOI: 10.1061/(ASCE)0733-9429(2008)134:8(1136) CE Database subject headings: Velocity; Sediment transport; Comparative studies.

When investigating sediment transport in laboratory open-channel flows, it is often necessary to remove sidewall effects for computing effective bed shear stress. Previous sidewall correction methods are subject to some assumptions that have not been completely verified, and different values of the bed shear stress may be obtained depending on the approach used in making sidewall corrections. This study provides a quantitative assessment of the existing correction procedures by comparing them to a new sidewall correction model proposed in this study. The latter was derived based on the shear stress function and equivalent roughness size for both rigid and mobile bed conditions, which were obtained directly from experimental measurements. The comparisons show that the Einstein correction formula and the Vanoni and Brooks method generally predict relatively lower and higher bed shear stresses, respectively, while the Williams' empirical function leads to more scatter. This study also demonstrates that the widely used Vanoni and Brooks approach can be well approximated by a simple formula derived based on the Blasius resistance function. The sidewall effects, when removed in the different ways, would consequently affect the presentation of the bedload function. Experimental results of bedload transport, when plotted as the dimensionless transport rate against the dimensionless shear stress with the latter being corrected using the present model, exhibit less scatter than those associated with the previous procedures. CE Database subject headings: Shear stress; Open channel flow: Channel beds: Laboratory tests.

Nonuniform-unsteady flow in open channels with streamwise sloping beds having uniform upward seepage is theoretically analyzed. Expressions for the Reynolds stress and bed shear stress are developed, assuming a modified logarithmic law of velocity profile due to upward seepage, and using the Reynolds and continuity equations of two-dimensional open channel flow. The computed Reynolds stress profiles are in agreement with the experimental data. DOI: 10.1061/(ASCE)0733-9399(2005)131:4(451) CE Database subject headings: Reynolds stress: Shear stress: Open channel flow: Streamflow; Nonuniform flow: Unsteady flow; Seepage; Hydraulics.

In this study, we investigated a vertical dilute sediment-laden jet both experimentally and theoretically. First, an instantaneous whole-field velocimetry tool, particle image velocimetry, was applied to measure the sediment and fluid mean and fluctuating velocities of a downward sediment-laden jet at the same time. Subsequently, an analysis was performed based on two-phase conservation equations for both downward and upward jets. The analysis shows that the mean sediment velocity can be taken as the sum of fluid velocity and the settling velocity in both cases. For the downward jets, the decay rate of the centerline sediment concentration increases with the sediment settling velocity while decreases with the initial discharge velocity. The zone of flow establishment for the sediment velocity is found to be longer than that of the fluid. For the upward jets, the maximum rise of the sediment particles and their deposition distribution on the ground were derived theoretically. The predicted results compare well to the experimental data in the literature. CE Database subject headings: Sediment transport; Submerged jets; Velocity; Two-phase flow; Pipe flow.

Laboratory experiments have recently confirmed that the streamwise particle velocity is largely less than that of the fluid in sediment-laden flows. This velocity lag is investigated analytically in the present study based on the drag force exerting on a particle in the presence of other neighbors. The normalized drag force or the hindrance coefficient is found generally dependent on the particle concentration, particle Reynolds number, and specific gravity. The velocity lag is then derived by relating the hindrance coefficient to the shear stress distribution for uniform sediment-laden open channel flows. The analysis shows that the profile of the velocity lag, when normalized by the shear velocity, is associated with the shear Reynolds number, dimensionless particle diameter, and specific gravity. For the dilute condition, the velocity lag distribution varies only with the shear Reynolds number, and the lag can be ignored if the shear Reynolds number is less than unity. The theoretical predictions are comparable to limited experimental results. CE Database subject headings: Suspended sediments; Velocity profile; Two phase flow; Open channels; Interactions.

This study is concerned with two hydrodynamic parameters that are essential for the construction of submerged dikes in rivers with sand-filled geosynthetic bags. First, laboratory study and field observation are conducted to investigate the horizontal settling distance of sandbags in open channel flows. Sandbag models used for laboratory tests are prepared according to similarity requirements. An analytical formula for evaluating the settling distance is then derived based on the experimental data. The predictions are then compared to the field measurements. The observed settling distance varies significantly and its probability density distribution is positively skewed. This study also presents a formula for computing the critical flow velocity at incipient sandbag motion for two extreme conditions. DOI: 10.1061/(ASCE)0733-950X(2004)130:2(98) CE Database subject headings: Geosynthetics; Settling velocity; Incipient motion; Open channels; Hydraulic models.

The Ergun equation that is widely used in chemical engineering is generalized for describing seepage through the sediment boundaries in this study. Comparisons with the measurements of the hydraulic gradient for flows through packed beds show that the Ergun equation may not be suitable for the transitional zone where both viscous and inertial effects are of the same order of magnitude. The generalized Ergun equation is then used to evaluate the critical shear velocity for incipient sediment motion subject to seepage. The computed results are finally compared to the experimental data and those predicted using the previously proposed approach. CE Database subject headings: Velocity; Seepage; Equation of motion; Sediment; Computation.

With the assumption that the bed shear stress fluctuates in a lognormal fashion, the probability density function (PDF) of the standardized bed shear stress is derived as a function of the relative shear stress intensity. The PDF is more skewed with larger relative intensities, but approaches a Gaussian function when the relative intensity is small. The computed PDF agrees well with the reported experimental data for flows over a smooth boundary. The higher-order moments of the bed shear stress, skewness, and kurtosis, are shown analytically to be also dependent on the relative intensity. The theoretical dependencies are then compared to a number of measurements available in the literature. The Reynolds number effect on the relative intensity is also discussed. DOI: 10.1061/(ASCE)0733-9399(2003) 129:1(126) CE Database keywords: Shear stress; Skewness; Reynolds number; Beds.

An exponential formula that does not involve the concept of the critical shear stress is derived in this study for computing bedload transport rates. The formula represents well various experimental data sets ranging from the weak transport to high shear conditions. Comparisons of the present study are also made with many previous bedload formulas commonly cited in the literature. CE Database keywords: Bed load; Sediment transport; Transport rate; Shear stress.