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In this study, the bedform dimensions of an alluvial bed in a unidirectional flow were experimentally investigated. A series of flume experiments was conducted; 700 sets of flume and field data were used in developing formulae for predicting the bedform dimensions on alluvial beds in unidirectional flow. Bedform dimensions include the length and height of bedforms generated by the lower and upper flow regimes; the resistance coefficient for the flow in different flow regimes is introduced into the proposed formulae. The momentum boundary-layer thickness was introduced as an independent variable instead of the flow depth. Based on a large amount of flume and field data, the coefficients of each parameter were determined; four typical formulae were used to compare the accuracy of the proposed formulae. The experimental results show that the momentum boundary-layer thickness, hydraulic radius, and resistance coefficient for the flow in different regimes correlate well with the bedform dimensions. The calculation results show that the dimensionless particle size should not be ignored in the calculation of bedform dimensions. The bedform dimensions have an obvious trend of rapid increase with an increase in the ratio of flow depth to sand size (H/d). The bedform dimensions obtained using the van Rijn method and the Engelund and Hansen method did not represent the variation trend of the bedform length in the upper flow regime with an increase in H/d when H/d was greater than 103. The calculations using the proposed formulae are more accurate and reasonable than those in previous studies predicting the bedform height and length on an alluvial bed in a unidirectional flow. [ABSTRACT FROM AUTHOR]

In this study, the bedform dimensions of an alluvial bed in a unidirectional flow were experimentally investigated. A series of flume experiments was conducted; 700 sets of flume and field data were used in developing formulae for predicting the bedform dimensions on alluvial beds in unidirectional flow. Bedform dimensions include the length and height of bedforms generated by the lower and upper flow regimes; the resistance coefficient for the flow in different flow regimes is introduced into the proposed formulae. The momentum boundary-layer thickness was introduced as an independent variable instead of the flow depth. Based on a large amount of flume and field data, the coefficients of each parameter were determined; four typical formulae were used to compare the accuracy of the proposed formulae. The experimental results show that the momentum boundary-layer thickness, hydraulic radius, and resistance coefficient for the flow in different regimes correlate well with the bedform dimensions. The calculation results show that the dimensionless particle size should not be ignored in the calculation of bedform dimensions. The bedform dimensions have an obvious trend of rapid increase with an increase in the ratio of flow depth to sand size (H/d). The bedform dimensions obtained using the van Rijn method and the Engelund and Hansen method did not represent the variation trend of the bedform length in the upper flow regime with an increase in H/d when H/d was greater than 103. The calculations using the proposed formulae are more accurate and reasonable than those in previous studies predicting the bedform height and length on an alluvial bed in a unidirectional flow.

Turbulent burst corroborate many significant events of bed-load transport in open channel flows. Present study aims to focus the role of turbulent bursting occurrences in an alluvial mobile and immobile sand bed stream under dynamic equilibrium flow conditions. Experiments were conducted over a sand bed (d50 = 0.65 mm) under clear water and mobile bed conditions with three Reynolds numbers for each bed configuration. Longitudinal bed-slopes, S = 0.1 and 0.075% were used. The instantaneous three-dimensional velocities were recorded by an acoustic Doppler velocimeter (ADV) in a laboratory open channel of 12 m long. The quadrant analysis revealed that the coherent structures contributed significantly to sediment mobility over the alluvial stream bed. Four bursting events were identified. The ejection and sweep burst events were dominant and considered as main energy source for bed-load transport processes. In case of rigid under clear water conditions, ejection and sweep events close to the bed contributed approximately 64 and 60% to the total Reynolds shear stress production whereas, outward and inward interaction events contributed moderately by 10 and 14%, respectively. This is attributed to arrival of low-speed fluid streaks from the near-bed zone and become no longer effective due to the arrival of high-speed fluid streaks from the upper region. In contrast, under mobile bed conditions, sweep events contributed approximately 70% of the Reynolds shear stress production and much dominant in comparison to its counterpart ejection events. Turbulent sediment fluxes were triggered by the sweep events under mobile bed conditions and governed by the arrival of high-speed fluid streaks. However, the contributions from inward and outward interaction events were found rather weak. [ABSTRACT FROM AUTHOR]

An experimental investigation of the equilibrium depth of local scour around side-weirs located on an alluvial river is presented in this paper. A sand bar in the middle of the channel and a scour hole close to the weir side is formed because of changes in shear stress field and velocity at the downstream end of the side weir. The depth of clear-water scour increases by time and approaches the equilibrium state asymptotically depending on approach flow velocity. The equilibrium depth of scour increases by the increase of the dimensionless parameters of approach flow velocity, water head ratio, side-weir length and sediment size. Although there is no scour for the dimensionless approach flow velocity less than 0.45, it increases almost linearly with the dimensionless approach flow velocity and the maximum value of scour depth occurs when it is equal to 1.0. [ABSTRACT FROM AUTHOR]