Modelling secondary cells and sediment transport in rectangular channels

Omran, Mazen; Knight, Donald W.

doi:10.1080/00221681003726288

Cited by 8 publications

(4 citation statements)

References 15 publications

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“…Quite remarkably, little sand deposited near the flume centerline. Although we have no data to support this hypothesis, this may be the result of a nonuniform distribution of bed shear stress and the existence of secondary currents (McLean, 1981;Omran & Knight, 2010). During the first sand pulse (Figure 5d), deposition represented a volume of sand reaching 143 cm 3 (or 371 g).…”

Section: Pulse Transportmentioning

confidence: 95%

Effects of Sand Addition and Bed Flushing on Gravel Bed Surface Microtopography and Roughness

Bertin

Friedrich

2019

Water Resources Research

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Gravel riverbeds naturally present a range of sediment size that can span several orders of magnitude. However, fine sediment (i.e., size <2 mm) additions affect sediment transport and are potentially harmful to the river ecology. Limited research has been done to understand the effects of sand addition on gravel bed topography. For this study, we conducted flume experiments testing the effectiveness of topography remote sensing to measure the response of a water‐worked gravel bed to varying sand additions and bed flushing. Repeated measurements of the bed topography with through‐water photogrammetry show that sand deposits primarily in low‐lying areas of the bed, where it is sheltered. Observed changes in bed elevation standard deviation, skewness, and inclination index indicate a smoother bed, as sand fills the depressions on the surface and conceals gravel imbrication. The overall bed morphology (e.g., small bedforms and coarse grain arrangement), however, is preserved. Increasing the sand supply resulted in a gravel bed buried by sand and the formation of sand dunes. Bed flushing with a flow sufficient to move sand allowed a return of the bed roughness to the prepulse conditions, while flushing the bed with a discharge equivalent to the armoring discharge reworked the normally stable surface layer without removing infiltrated sand from the subsurface. This new data set contributes to providing missing information on the small‐scale topographic effects of sand addition upon gravel beds. It might be used with measurements of near‐bed flow hydraulics obtained in other studies to explain variations of sediment transport.

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Section: Pulse Transportmentioning

confidence: 95%

Effects of Sand Addition and Bed Flushing on Gravel Bed Surface Microtopography and Roughness

Bertin

Friedrich

2019

Water Resources Research

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“…Other studies based on the Shiono and Knight method can be found in Liao and Knight (2007), Tang and Knight (2008) and Omran and Knight (2010). Apart from SKM, some other methods for analysing the conveyance capacity of compound channels have been proposed.…”

Section: Introductionmentioning

confidence: 99%

Uniform flow formulas for irregular sections

Spada¹,

Tucciarelli²,

Sinagra³

et al. 2015

Preprint

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Abstract. Two new methods for uniform flow discharge computation are presented, validated and compared with other available formulas. The first method derives from the well-known Huthoff algorithm, which is first shown to be dependent on the way the river cross-section is discretized into several sub-sections. The second method assumes the vertically averaged longitudinal velocity to be a function only of the friction factor and of the so-called "local hydraulic radius", computed as the ratio between the integral of the elementary areas around a given vertical and the integral of the elementary solid boundaries around the same vertical. Both integrals are weighted with a linear shape function, equal to zero at a distance from the integration variable which is proportional to the water depth according to an empirical coefficient β. Both formulas are validated against (1) laboratory experimental data, (2) discharge hydrographs measured in a real site, where the friction factor is estimated from an unsteady-state analysis of water levels recorded in two different river cross sections, (3) the 3-D solution obtained using the commercial ANSYS CFX code, computing the steady state uniform flow in a short reach of a prismatic channel, with known water level in the downstream section.

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“…In the absence of detailed data about the distribution of the friction factor (f) across the cross section of the channel, an overall friction factor could be used for each panel (Omran and Knight 2010). f was therefore computed from the corresponding Manning's coefficient (n) and applied as a constant in main channel and floodplain panels.…”

Section: Calibration For Skm Modelmentioning

confidence: 99%

Application of the Shiono and Knight Method in asymmetric compound channels with different side slopes of the internal wall

2018

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The Shiono and Knight Method (SKM) is widely used to predict the lateral distribution of depth-averaged velocity and boundary shear stress for flows in compound channels. Three calibrating coefficients need to be estimated for applying the SKM, namely eddy viscosity coefficient (λ), friction factor (f) and secondary flow coefficient (k). There are several tested methods which can satisfactorily be used to estimate λ, f. However, the calibration of secondary flow coefficients k to account for secondary flow effects correctly is still problematic. In this paper, the calibration of secondary flow coefficients is established by employing two approaches to estimate correct values of k for simulating asymmetric compound channel with different side slopes of the internal wall. The first approach is based on Abril and Knight (2004) who suggest fixed values for main channel and floodplain regions. In the second approach, the equations developed by Devi and Khatua (2017) that relate the variation of the secondary flow coefficients with the relative depth (β) and width ratio (α) are used. The results indicate that the calibration method developed by Devi and Khatua (2017) is a better choice for calibrating the secondary flow coefficients than using the first approach which assumes a fixed value of k for different flow depths. The results also indicate that the boundary condition based on the shear force continuity can successfully be used for simulating rectangular compound channels, while the continuity of depth-averaged velocity and its gradient is accepted boundary condition in simulations of trapezoidal compound channels. However, the SKM performance for predicting the boundary shear stress over the shear layer region may not be improved by only imposing the suitable calibrated values of secondary flow coefficients. This is because difficulties of modelling the complex interaction that develops between the flows in the main channel and on the floodplain in this region.

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Modelling secondary cells and sediment transport in rectangular channels

Cited by 8 publications

References 15 publications

Effects of Sand Addition and Bed Flushing on Gravel Bed Surface Microtopography and Roughness

Effects of Sand Addition and Bed Flushing on Gravel Bed Surface Microtopography and Roughness

Uniform flow formulas for irregular sections

Application of the Shiono and Knight Method in asymmetric compound channels with different side slopes of the internal wall

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