Velocity distribution and shear velocity behaviour of decelerating flows over a gravel bed

Afzalimehr, Hossein; Anctil, François

doi:10.1139/cjce-26-4-468

Cited by 16 publications

(24 citation statements)

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“…Fitness of the parabolic law to the outer layer data. Afzalimehr & Anctil (1999) showed that the parabolic law method fits nicely to the outer region data (y/h > x) over a fixed-gravel-bed. Furthermore, figure 3 shows the fitness of the parabolic law method to the outer layer data (y/h > x) for the mobile bed in this research.…”

Section: Bed Shear Stress Estimationmentioning

confidence: 92%

“…Since ρ is a constant for water, therefore with estimation of shear velocity (u * ) the bed shear stress can be calculated. Several methods are available to estimate the bed shear stress of non-uniform flows, such as the Reynolds stress method (Kirnoto & Graf 1995;, the parabolic law method (Afzalimehr & Anctil 1999) and the log-law method (Afzalimehr & Anctil 2001). In general, the log-law applies only for the data near-bed where considerable uncertainty exists in the measurement, such as the determination of the reference bed level, especially for gravel-beds (Afzalimehr & Anctil 2000).…”

Section: Bed Shear Stress Estimationmentioning

confidence: 99%

“…The shear velocity is estimated using the parabolic law by the data of the outer layer and the following equation (Afzalimehr & Anctil 1999):…”

Section: Bed Shear Stress Estimationmentioning

confidence: 99%

“…where is the slope of the regression equation between u/u max and [1 − (y + 0·25d 50 /h + 0·25d 50 )] 2 ; u is the mean point velocity at the distance y measured from the reference level, u max is the maximal velocity at the water surface; and λ is a constant that depends on the deviation point of the outer layer from the inner layer (x) in each velocity profile which is defined as follows (Afzalimehr & Anctil 1999):…”

Section: Bed Shear Stress Estimationmentioning

confidence: 99%

“…Nezu & Nakagawa 1993). These methods can result in different estimate of shear stress and the Shields parameter particularly for non-uniform flow condition through a study reach (Afzalimehr & Anctil 1999). Therefore, to obtain more realistic insight for calculation of shear stress and thus the critical Shields parameter values, it is vital to analyse the shear stress distribution for nonuniform flow.…”

Section: Shields Parametermentioning

confidence: 99%

See 4 more Smart Citations

Influence of decelerating flow on incipient motion of a gravel-bed stream

Afzalimhr

Dey

Rasoulianfar

2007

Sadhana

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An experimental study on incipient motion of gravel-bed streams under steady-decelerating flow is presented. Experiments were carried out in a flume with two median grain sizes, d 50 = 16·7 mm for a fixed-bed case and d 50 = 8 mm for a mobile bed case. In addition, an effort is made to determine a simplified method for the estimation of bed shear stress in decelerating flow over fixed and mobile beds for use in field situations. From the observation of eleven fixed-bed and nine mobile-bed velocity profiles, it is revealed that the parabolic law method (PLM) and the Reynolds stress method are comparable for estimation of shear velocity in general. Also, the results show that the shear stress distribution adopts a convex form over fixed and mobile beds. Due to this form the critical Shields parameter value for decelerating flow is less than the reported values in literature.This paper supports Buffington & Montgomery (1997) statement that less emphasis should be given on choosing a universal shields parameter, and more emphasis should be given on choosing defendable values based on flow structure.

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Section: Bed Shear Stress Estimationmentioning

confidence: 92%

Section: Bed Shear Stress Estimationmentioning

confidence: 99%

“…The shear velocity is estimated using the parabolic law by the data of the outer layer and the following equation (Afzalimehr & Anctil 1999):…”

Section: Bed Shear Stress Estimationmentioning

confidence: 99%

Section: Bed Shear Stress Estimationmentioning

confidence: 99%

Section: Shields Parametermentioning

confidence: 99%

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Influence of decelerating flow on incipient motion of a gravel-bed stream

Afzalimhr

Dey

Rasoulianfar

2007

Sadhana

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Effect of non‐uniformity of flow on velocity and turbulence intensities over a cobble‐bed

Afzalimehr

2009

Hydrological Processes

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Abstract:Non-uniform flows encompassing both accelerating and decelerating flows over a cobble-bed flume have been experimentally investigated in a flume at a scale of intermediate relative submergence. Measurements of mean longitudinal flow velocity u, and determinations of turbulence intensities u 0 , v 0 , w 0 , and Reynolds shear stress u f w f have been made. The longitudinal velocity distribution was divided into the inner zone close to the bed and the outer zone far from the bed. In the inner zone of the boundary layer (near the bed) the velocity profile closely followed the 'Log Law'; however, in the outer zone the velocity distribution deviated from the Log Law consistently for both accelerating and decelerating flows and the changes in bed slopes ranging from 2% to C2% had no considerable effect on the outer zone. For a constant bed slope (S D š0Ð015), the larger the flow rate, the smaller the turbulence intensities. However, no detectable pattern has been observed for u 0 , v 0 and w 0 distributions near the bed. Likewise, for a constant flow rate (Q D 0Ð040 m 3 /s), with variation in bed slope the longitudinal turbulent intensity profile in the longitudinal direction remained concave for both accelerating and decelerating flows; whereas vertical turbulent intensity (w 0 ) profile presented no specific form. The results reveal that the positions of maximum values of turbulence intensities and the Reynolds shear stress depend not only on the flow structure (accelerating or decelerating) but also on the intermediate relative submergence scale.

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Effect of Adverse Pressure Gradient and Different Vegetated Banks on Flow

Afzalimehr

Moradian

Gallichand

et al. 2015

River Research & Apps

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This study examines velocity components in 3D and shear stress distributions in flows with adverse pressure gradient over a gravel‐bed channel using three types of vegetation on banks, rice, straw and Typha stems. Results of this experimental study are compared with those for flows in the bare bank channel. Results show that the flows with adverse pressure gradient are not self‐similar in vegetated and bare banks channels. The logarithmic law and parabolic law can be used to well describe flow in the inner and outer regions, respectively, but with different relative flow depths. Shear stress distributions depend on nature of bank roughness and the secondary flow effects because of small values of the aspect ratio and adverse pressure gradient. Quadrant analysis is performed to investigate the effect of different vegetation covers on the Reynolds stress distributions at the central axis of channel and near the vegetated banks. Vegetation covers influence the contributions of different quadrants to the shear stress distribution, showing that sweeps and ejections are dominant events for all vegetated banks for near bed region. However, near the water surface the contributions of outwards and inwards for banks with Typha stems are more important than those for rice and straw. Instead of using a single value for friction factor, for different vegetated banks, a range of friction factor has to be applied in river restoration projects. Vegetated banks result in an increase of more than 60% in friction factor in comparison with that for bare banks under adverse pressure gradient flows. Copyright © 2015 John Wiley & Sons, Ltd.

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Velocity distribution and shear velocity behaviour of decelerating flows over a gravel bed

Cited by 16 publications

References 0 publications

Influence of decelerating flow on incipient motion of a gravel-bed stream

Influence of decelerating flow on incipient motion of a gravel-bed stream

Effect of non‐uniformity of flow on velocity and turbulence intensities over a cobble‐bed

Effect of Adverse Pressure Gradient and Different Vegetated Banks on Flow

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