Turbulence Characteristics in Mild and Steep Entrance Slopes of Pool-Riffle Sequences

Abstract

This study investigates the distributions of velocity and Reynolds stress in an artificial pool-riffle sequence for two entrance slopes of 5° and 20° in a laboratory and compares the results with a similar pool-riffle sequence in a gravel-bed river in central Iran. Quadrant analysis is applied to find out the contributions of different events on turbulent flow structures. At an entrance slope of 5°, the velocity distribution follows an exponential trend without flow separation, but for the entrance slope of 20°, the velocity distribution indicates a S-shaped pattern with flow separation. Variation of entrance slope does not influence the convex form of Reynolds stress (RS) distribution, but it influences the location of the maximum value of RS and the flow separation zone length. The results reveal that outward and ejection are the dominant events for both field and laboratory settings. The sweep displays a decreasing trend from near the bed toward the water surface, however, the outward contribution indicates an increasing trend from the bed toward the water surface. The agreement between laboratory and field results in velocity and Reynolds stress will help the river engineers to better manage complex fluvial processes. Bursting process events depend on the aspect ratio for the same entrance pool slope. The agreement in the results of velocity and Reynolds stress distributions and the dominant events of bursting process in the laboratory and field are encouraging for better restoration of rivers and decreasing the cost of projects.

Incipient Motion of Bed Material in a Channel with Varying Width and Vegetated Channel Walls

“…In other words, the channel sidewalls and wall vegetation had a major role in producing turbulence eddies, and increased turbulence intensities. This finding was also reported in previous works conducted in channels with the varying channel width [29,38], as well as either submerged or emergent vegetation in channels [9,21,39].…”

“…This phenomenon, which is defined as "dip", occurs in three dimensional flows and narrow channels due to the presence of secondary currents [35]. It was reported in previous works [21,36] that the vegetated sidewalls of a channel play a major role in generating secondary currents. In this study, the vegetated sidewalls and the varying width of the channel lead to the development of secondary currents; hence, the "dip" phenomenon appears in most velocity profiles.…”

“…This phenomenon, which is defined as "dip", occurs in three dimensional flows and narrow channels due to the presence of secondary currents [35]. It was reported in previous works [21,36] that the vegetated sidewalls of a In total, 9 laboratory experiments have been carried out in the current study. The setups for these experiments differ from natural channels.…”

“…In a channel with vegetated sidewalls and gravel bed, it is found that the presence of vegetation led to the change in the location of the maximum shear stress. Furthermore, by reducing the median size of bed material, less change in Reynolds shear stress was recognized [21]. To better understand the mechanisms of fluids transport through fibrous porous media, by using analytical solutions and numerical simulations, several studies were performed [22,23].…”

Comparison of Velocity and Reynolds Stress Distributions in a Straight Rectangular Channel with Submerged and Emergent Vegetation

“…Environmental aquatic currents are rarely free of vegetative effects. Moreover, vegetation over the bed or banks exercises a dominant influence on the fluvial hydraulic studies, including drag coefficient, stable channel design, river plan and bed-forms characteristics [1][2][3][4][5][6][7]. Despite considerable investigations on the influence of vegetation in hydraulic projects, there are not many comparative studies for submerged and emergent vegetation in literature.…”