Sediment transport by Nian Sheng Cheng
This study presents a scaling analysis of the time development of clear-water scour depth at brid... more This study presents a scaling analysis of the time development of clear-water scour depth at bridge piers. It shows that the widely-used exponential formula can be theoretically derived with scaling arguments. The derivation provides connections between the physical pier-scouring process and two empirical constants used in the formula. The dependence of the two constants on sediment coarseness is calibrated using published laboratory data with scour duration ranging from 49 to 1094 hours. The analysis presented is restricted to the conditions including steady flow, clear-water scour, narrow circular cylindrical pier and uniform sediment with low coarseness.
In the investigation of sediment transport, it is necessary to differentiate various definitions ... more In the investigation of sediment transport, it is necessary to differentiate various definitions of angle of repose (AoR) available in the literature. The static AoR, composed of upper and lower angle of slope, forms just before and after slope instability, while the dynamic AoR can be observed when sediment grains are moving continuously down an inclined plane. In the present study, a series of laboratory experiments was conducted to measure static and dynamic angle of repose for uniform natural sediments with median diameter of 0.28-4.38 mm. The results show that the different slope angles have different characteristics. The upper and dynamic AoR increase slightly with increasing grain diameter, while the lower AoR is not sensitive to changes in sediment size and may assume a constant value. The average of the upper and lower AoR is equivalent to the dynamic AoR, and the difference between them increases with increasing grain diameter. The present study suggests that the different angles of repose should be treated with caution when applying in investigations of bedload transport, dune migration and local scour development.
In the present study, vortices were generated in open channel flow with a cross-flow cylinder ins... more In the present study, vortices were generated in open channel flow with a cross-flow cylinder installed horizontally near the bed. Sediment pickup rates were then measured over the channel bed downstream the cylinder using a sediment lift. The experimental data show that the pickup rate increases exponentially in the presence of vortices. Two different relationships can be clearly observed between the pickup rate and the maximum root-mean-square (rms) value of the streamwise velocity fluctuation, one for the cylinder-obstructed flow and the other for the unobstructed flow. The results imply that the vortex-induced sediment pickup cannot be explained based on the traditional boundary layer theory.
Sediment pickup rate is useful for deriving bedload function and specifying boundary conditions f... more Sediment pickup rate is useful for deriving bedload function and specifying boundary conditions for numerical modeling of sediment transport. Previous pickup functions were developed based only on the data measured under the plane bed condition, and they may not be applicable for dune-covered beds. The present study compares pickup rates for three bed conditions, i.e. plane bed, fixed dunes and mobile dunes. All pickup functions are formulated by involving the densimetric Froude number instead of the Shields number. The results show that the sediment pickup rates measured over fixed dunes can be described using the pickup function developed for the plane bed. However, the presence of mobile dunes causes a considerable reduction in the pickup rate when the densimetric Froude number is greater than 5, which is largely due to flow-particle and particle-particle interaction. This study also shows that under mobile dune conditions, the particle step length for dune bed conditions scales with the dune length rather than the particle diameter.
Experimental data collected in open channel flows show that the sediment pickup rate is better co... more Experimental data collected in open channel flows show that the sediment pickup rate is better correlated with the densimetric Froude number than the Shields number. An empirical formula is then proposed for estimating the pickup rate. It does not involve the critical condition for the incipient sediment motion, and therefore can be applied to sediment entrainment of different stages including weak sediment motion. In spite of the fact that the experiments were carried with a limited range of sediment particle diameters from 0.23 to 0.86 mm, the result obtained is also supported by several series of data reported in the literature.
The representative grain size of a sediment bed can be defined as the mean-volume diameter, which... more The representative grain size of a sediment bed can be defined as the mean-volume diameter, which varies with the geometric standard deviation of sediment size. The obtained results explain why the representative sediment size can be approximated to be D84 for typical gravel beds. The analysis is finally extended to quantify the dependence of the equivalent sand roughness on the geometric standard deviation of grain size distribution.
Laboratory experiments were conducted to measure sediment pickup rate over two-dimensional fixed ... more Laboratory experiments were conducted to measure sediment pickup rate over two-dimensional fixed dunes. Measurements were performed over both stoss and lee sides of the dune with sediments of = 0.23, 0.44 and 0.86 mm. Flow velocity and turbulence were also measured by using an Acoustic Doppler Velocimeter (ADV). By analysing the experimental data, an empirical sediment pickup function based on depth-averaged flow parameters was proposed to estimate the pickup rate over the dune.
This paper summarizes the results of an experimental study on the influence of an external turbul... more This paper summarizes the results of an experimental study on the influence of an external turbulence field on the bed load sediment transport in an open channel. The external turbulence was generated by (1) a horizontal pipe placed halfway through the depth h; (2) a series of grids with a clearance of about one-third of the depth from the bed, and extending over a finite length of the flume; and (3) a series of grids with a clearance in the range (0.1–1.0)h from the bed, but extending over the entire length of the flume. Two kinds of experiments were conducted: plane-bed experiments and ripple-covered-bed experiments. In the former case, the flow in the presence of the turbulence generator was adjusted so that the mean bed shear stress was the same as in the case without the turbulence generator in order to single out the effect of the external turbulence on the sediment transport. In the ripple-covered-bed case, the mean and turbulence quantities of the streamwise component of the velocity were measured, and the Shields parameter, due to skin friction, was determined. The Shields parameter, together with the RMS value of the streamwise velocity fluctuations, was correlated with the sediment transport rate. The sediment transport increases markedly with increasing turbulence level.
Classical theories developed for investigating bedload sediment transport are applicable largely ... more Classical theories developed for investigating bedload sediment transport are applicable largely for channel beds of a gentle slope. Examples are the well-known Shields diagram for the evaluation of the critical condition for initial sediment motion and bedload transport formulas derived with deterministic or probabilistic approaches. How to apply the existing bedload formulas for steep channels remains A c c e p t e d M a n u s c r i p t N o t C o p y e d i t e d 2 challenging.
International Journal of Sediment Research, 2011
The evaluation of the sidewall friction could be inconvenient in the implement of the Vanoni-Broo... more The evaluation of the sidewall friction could be inconvenient in the implement of the Vanoni-Brooks sidewall correction procedure. Using the Colebrook-White equation and Nikuradse's pipe friction data, two explicit formulae are developed in this note for finding the sidewall friction factor. They are applicable for various sidewalls that are either hydrodynamically-smooth or rough, or in the transitional regime.
JOURNAL OF HYDRAULIC ENGINEERING-ASCE, Sep 2011
By applying incomplete self-similarity argument, this study presents a structural analysis of mod... more By applying incomplete self-similarity argument, this study presents a structural analysis of models for predicting bed-material load discharge, which can be formulated consistently according to the number of independent variables considered. The coefficients involved in the proposed models are calibrated with published laboratory and field data (comprising almost 6600 records). By comparing with the six bed-material formulas that are recommended in the recently updated ASCE manual on sedimentation engineering, the proposed models show significant improvements on the prediction of bed-material load discharge. This study also implies that the model developed based on regular regression analysis could be enhanced by considering interaction terms of independent variables.
Water Resources Research, 2004
1] Many formulas available in the literature for computing sediment transport rates are often exp... more 1] Many formulas available in the literature for computing sediment transport rates are often expressed in terms of time mean variables such as time mean bed shear stress or flow velocity, while effects of turbulence intensity, e.g., bed shear stress fluctuation, on sediment transport were seldom considered. This may be due to the fact that turbulence fluctuation is relatively limited in laboratory open-channel flows, which are often used for conducting sediment transport experiments. However, turbulence intensity could be markedly enhanced in practice. This note presents an analytical method to compute bed load transport by including effects of fluctuations in the bed shear stress. The analytical results obtained show that the transport rate enhanced by turbulence can be expressed as a simple function of the relative fluctuation of the bed shear stress. The results are also verified using data that were collected recently from specifically designed laboratory experiments. The present analysis is applicable largely for the condition of a flat bed that is comprised of uniform sand particles subject to unidirectional flows. INDEX TERMS: 4558
Journal of Hydraulic Engineering-asce, 2005
When investigating sediment transport in laboratory open-channel flows, it is often necessary to ... more 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 include some assumptions that have not been completely verified, and therefore 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 obtained directly from experimental measurements. The comparisons show that the bed shear stress is generally overestimated by the Vanoni and Brooks (1957) correction approach but underestimated by the Einstein (1941) formula. In addition, low precision was obtained from the prediction by the Williams' (1970) empirical equation.
Journal of Hydraulic Engineering-ASCE, 2004
Laboratory experiments have recently confirmed that the streamwise particle velocity is largely l... more 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.
Advances in Water Resources, 2003
The thickness of the bedload layer is a crucial parameter for evaluating sediment transport rates... more The thickness of the bedload layer is a crucial parameter for evaluating sediment transport rates in open channel flow, but it is often determined empirically. Based on the concept of the hydrodynamic diffusion related to particle–particle interactions, an analytical model is proposed in this study for computing the thickness of the bedload layer. The coefficient of diffusion is assumed to be associated with the momentum transfer induced by the random particle motion and thus can be derived from the shear-induced particle stress. The analytical result shows that the ratio of the bedload thickness to the particle diameter depends on the dimensionless particle diameter and dimensionless bed shear stress. Differences are also examined between the present study and a few empirical formulas that are derived from experimental results for limited bed conditions.
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Sediment transport by Nian Sheng Cheng