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The stage-discharge relationship is a type of resistance to flow evaluation that is used to determine the depth or the hydraulic radius if the flow discharge, channel cross-sectional shape and the properties of bed materials are specific. Wide channels as well as plane bed rivers, due to having higher ratios of width to depth, b/h, they have specific hydraulic characteristics than that of the normal open channels. In this research work, the hydraulic characteristics of widening channels including flow pattern, stage-discharge relationship and flow resistance, by using a physical model of a rigid boundary with 60 m length, 1.5 m wide and a bed slope of 0.001 that has b/h ratios of 12 to 56 are experimentally investigated. The construction material of channel model is concrete bed and concrete block walls were used. To insulate the channel bed and walls, a thick layer under the concrete was also used. Due to its wide width and length, the physical model of this study has low scale effects and the obtained results are closer to the normal open channels as well as the plain bed rivers. Based on the values of obtained for the Froude, Reynolds and Shear Reynolds numbers in model, it was found that the flow regime is subcritical and turbulent. According to the results of model, stage-discharge-tailgate relationship is shown plotted and it is found that as the flow rate increases, the tailgate height that required at the downstream end of the channel to produce a normal depth, increases. On the basis of normal depths obtained in the widen channel model, a reliable stage-discharge relationship is presented. The results show that this relationship has a higher coefficient than that of a natural channel stage-discharge relationship. Results, also reveals that between the calculated and measured the Manning coefficient, n, in the wide channel model, a difference (about 10%) is observed. The results of the research show that, for discharges greater than 35 lit/s, the measured Manning roughness coefficient, n is very close to the n values presented in literature. However, for the discharges less than 35 lit/s, the measured values of Manning’s n is 15% higher than the amounts provided in literature. The results of the present research show that by increasing the ratio of hydraulic radius to the roughness height of the bed (Rh/ks), the Manning roughness, n, decreases linearly. Also, based on the results, it was found that by increasing flow depth, The Manning roughness, n, decreases and by increasing Reynolds number, Re, the Manning n decreases linearly. The results also show that in the wide channel model, the Manning roughness coefficient, n, decreases from 0.022 to 0.016 by increasing Froude number, Fr, from 0.26 to 0.4, but n values increases by increasing depth ratio, b/h. According to the results of the experiments, in a range of Reynolds number, 700

Keywords: Wide Rigid Boundary Channel, Physical Model, Stage-Discharge Relationship, Flow Resistance

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