Hydraulics of River Flow under Arch Bridges: Report No. 5, Discussion - "Roughness Spacing in Rigid Open Channels"
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This paper is a discussion of an ASCE paper which presented the effect of longitudinal and transverse spacing of roughness on the flow in rigid open channels. The authors continue the discussion by reporting the results obtained by applying the analysis to a different type of roughness element consisting of round bars, and to consider a possible extension to field conditions. The study of the roughness effect was made in connection with the model testing of arch bridges.Keywords:
Hydraulics
Hydraulic roughness
Abstract Time-averaged velocity fields in uniform open-channel flows over rough beds may exhibit local spatial heterogeneities due to the effects of bed roughness and secondary currents (SCs). The latter typically originate from the turbulence anisotropy and spatial heterogeneity introduced by the solid and mixed corners (i.e., between sidewalls and water surface), but may also appear due to roughness spanwise heterogeneities, e.g., associated with patchy vegetation distributions or streamwise sediment ridges on the channel bed. In this paper, we propose rigorous conservation equations for momentum, kinetic energy and fluid stresses accounting for the contributions of bed roughness and SCs, separately. Particular attention is given to the terms regulating the energy exchanges between roughness-induced and SC-related motions, which are expected to provide information on the physical mechanisms leading to the generation of roughness-induced SCs.The proposed approach is illustrated using a large-eddy simulation of a rough-bed open-channel flow.
Hydraulic roughness
Momentum (technical analysis)
Large-Eddy Simulation
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The effect of bed roughness and sediment load on energy dissipation under wave action has been analysed with the aid of unidirectional flow relationships and laboratory data. An expression is proposed for estimation of the equivalent roughness height. The paper also raises questions which need further study.
Hydraulic roughness
Wave height
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Hydraulic roughness
Roughness length
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Hydraulics
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Coefficients of roughness, n are characterized as parameters representing the channel roughness and flow resistance. The Manning equation has been recognized as the most proper formulae to represent the open channel flow application. This study is about determining the mean roughness coefficients for an open channel with different bed and side walls roughness. The experiments were conducted at the Hydraulic and Hydrology Laboratory in Universiti Malaysia Pahang. The experiments were performed on a rectangular open channel with glass sides and a flat bed. The size of the rectangular open channel was lOm long, 0.3 m wide and 0.46m deep. The experiments were carried out using two different types of roughness; 5mm gravel and 2 mm gravel. There were two channel conditions, bed and sidewalls having the same roughness, and only the channel bed had the roughness while the sidewalls were smooth. For both conditions; experiments were conducted with certain three fixed slope gradients which are 1:100, 1:300, and 1:500. The fixed flow rates,
4m 31s, 8 m 3/s and 12 m 31 were also set by adjusting the water pump. The data obtained was converted into graph form before performing the analysis. The heights and velocities were recorded at the upper stream and lower stream of the open channel under different roughness conditions. The results showed that the coefficient of roughness for the open channel with wall roughness is higher than the channel without sidewall roughness and larger grain size will give higher roughness coefficient. It also can be concluded that channel slope and surface roughness were the main factors in determining the roughness coefficient.
Hydraulic roughness
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1. FUNDAMENTALS OF OPEN CHANNEL FLOW 2. ENERGY AND MOMENTUM PRINCIPLES 3. NORMAL FLOW 4. GRADUALLY VARIED FLOW 5. DESIGN OF OPEN CHANNELS 6. HYDRAULIC STRUCTURES 7. BRIDGE HYDRAULICS 8. INTRODUCTION TO UNSTEADY OPEN-CHANNEL FLOW
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Data of flume experiments with bed forms are used to analyze and compare different roughness predictors. In this study, the hydraulic roughness consists of grain roughness and form roughness. We predict the grain roughness by means of the size of the sediment. The form roughness is predicted by three approaches: Van Rijn (1984), Vanoni & Hwang (1967) and Engelund (1966). The total roughness values (friction factors) are compared with the roughness values according to the DarcyWeisbach equation. Results show that the different methods predict different friction factors. In future research uncertainties in the hydraulic roughness will be taken into account to determine their influence on the computed water levels.
Hydraulic roughness
Flume
Roughness length
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Hydraulics
Open system (computing)
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