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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]

Experiments were conducted to systematically investigate the nearly homogeneous turbulence generated by multiple oscillating grids. Altogether 54 runs were completed by employing five different grids. For each run, four to nine layers of grids were installed, which were operated with different frequencies, strokes and grid spacings. The particle image velocimetry was used to obtain flow velocity characteristics. Instantaneous flow measurements reveal that the flow structure varies depending on the grid configuration. For regular grids, turbulent wakes of nearly uniform size were generally observed. In comparison, for double-scale grids, the size of wakes varied with the bar width. In the near-grid region, the horizontal distributions of Reynolds stresses are strongly influenced by the grid geometry, showing obvious inhomogeneity especially for wide bars. The inhomogeneity reduces as the distance from the grid increases. At the midplane between two neighboring grids, the flow characteristics appear similar for all grid configurations. The size of the region where the turbulence appears nearly homogeneous is affected by the grid spacing and stroke. Generally, the double-scale grid can produce a greater homogeneous region than the regular grid. This study shows that the decay law of turbulence intensity developed for a single oscillating grid can be successfully extended to multiple oscillating grids. In addition, a theoretical interpretation of the decay law is presented by considering the variation in the velocity and size of wakes when evolving away from the grid. [ABSTRACT FROM AUTHOR]

Viscous effects on bedload sediment transport rates have been seldom examined in previous studies. They may become significant in the low transport regime, as evidenced by experimental data. However, the problem cannot be resolved using classical bedload models that involve the shear excess, for example, Meyer-Peter–Müller formula. By applying the concept of the critical pickup probability for incipient sediment motion, this study derives a new bedload formula that takes the fluid viscosity into consideration. The results show that the relationship between the dimensionless bedload rate and the Shields number varies significantly with the viscosity-dependent parameter in the low transport regime, which partially accounts for the scattering of bedload data. In addition, the present study proposes a simple approach to the evaluation of the critical bedload transport rate that is associated with the Shields curve for incipient sediment motion. [ABSTRACT FROM AUTHOR]

Difficulties are often encountered in estimating the bed shear stress in vegetated flows over a sediment bed due to complex interactions of the flow, sediment, and vegetation. Without involving any empirical partition of the bed friction and vegetation resistance, this study presents an analytical approach to evaluate the bed shear stress in the presence of vegetation in light of the phenomenological theory of turbulence. Then, the bed-load transport rate in vegetated flows is computed, with a result that agrees well with the bed-load transport formula developed for unvegetated flows. In addition, it is also shown that the present method is applicable not only to uniformly distributed vegetation but also to vegetation patches. However, further efforts are needed to extend the present model for flexible and submerged vegetation conditions. [ABSTRACT FROM AUTHOR]

This paper presents an extensive review of the characteristics of a propeller jet-induced flow field and its impact on scour formation. As the driving force of scour, the jet flow behavior is first explored, including both the free and bounded jets under various boundary conditions. Accordingly, their resulting scour, namely, unconfined and confined scour, are expounded in terms of the temporal and spatial development of the scour depth. Hitherto, existing methods for predicting the scour depth are principally developed empirically, which exclude in-depth scrutiny into the mechanism governing the scouring process. With the detailed flow fields obtained in recent research, this paper aims to piece together a state-of-the-art understanding of the scouring mechanism governing confined propeller scour, that is, the combined effect of jet flow and junction flow on the scour hole development. By comparing the similarities and differences between the scouring mechanisms in open and closed quays, this paper provides a comprehensive insight into the scouring process associated with bounded propeller jets. Moreover, in terms of the near-bed flow variations during the scouring process, implications for sediment transport under highly turbulent flow are discussed. Finally, future research needs are identified. [ABSTRACT FROM AUTHOR]

Whether or not a sediment particle is entrained from a channel bed is associated with both average bed shear stress and shear stress fluctuation, the latter being flow-dependent and also related to bed irregularities. In the first part of this study, a preliminary analysis of possible fluctuations induced by bed roughness is presented for the case of an immobile plane bed comprised of unisized sediments. The result shows that the roughness-induced variation is generally comparable to that associated with near-bed turbulence, and both variations can be approximated as log-normal in terms of probability density distribution. The bed particle mobility is then analyzed by considering the effects of shear stress fluctuations. The relevant computations demonstrate that with increasing shear stress fluctuations, the probability of the mobility of a bed particle may be enhanced or weakened. For the case of low sediment entrainment, the probability is increased by turbulence. However, the probability may be reduced by the shear stress fluctuation if the average bed shear stress becomes relatively higher. This study shows that the critical condition for initial sediment motion could be overestimated if the Shields diagram is applied for the condition of flows with high turbulence intensities. [ABSTRACT FROM AUTHOR]