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Weirs possess an essential role in dam safety and should spill floods with high return period. The designers can enhance the width of the weirs to increase the discharge capacity. But this has sometomes topography and economic limitations. Arced weirs can be considered as an alternative. A arced weir is a arcuate of a circle in plan-view that provides an increase in crest length for a given channel width that increas the flow capacity for the same head. Also when modification and capacity increase in existing spillways are necessary, this structure is recommended. In this paper, the hydraulic performance of arced weirs located in a reservoir has been studied experimentally. Firstly, dimensionless parameters affecting the performance of arced weirs is introduced using Buckingham π theorem. Then effect of arc angle (θ) and head water ratio (H0/P) on hydraulic performance of arced weirs was experimentally investigated and hydraulic performance of the tested arced weir geometries was compared with a linear configuration. For this purpose, Arches with different radius of curvature from linear to semi-circular configurations ( ) and various head water ratio Were studied. To simulate reservoir conditions, a reservoir simulator was designed and built. Laboratory observations show that the converging of flow over a arced weir causes a locally bulge in the downstream of the weir. This phenomenon was named as flow mound. Results show that arc angle (θ) and head water ratio (H0/P) have a direct effect on the flow mound and an increase in each of them leads to mound height rise. The head-discharge relationship for arced weirs was determined by using a general form of the rectangular weir equation. Data from physical models were used to determine discharge and upstream head for the flat crested weirs installed in the reservoir. from discharge curves, it was found that with increasing angle of weir, that provide an increase in crest length for a given channel width, flow capacity increases for a given upstream head. Discharge coefficients as a function of H0/P for arced weirs are also presented and is compared with linear configuration. The results show that with increasing H0/P, discharge coefficient is declined for each tested configuration. Also with increasingθ , that leads to greater convergence of flow passing over the weirs, discharge coefficient decreases. Efficiency parameter is defined as the ratio of discharge of arced weir to that of liner weir with a same width. From efficiency curves it can be understood that the semi-circular weir can improve efficiency up to about 45%. However for all tested weirs , efficiency decreases with increasing H0/P and it gets close to 1. Finally, based on the results and limitations of this study, a methodology for the design of arced weirs located in the reservoir is presented. By using this method, the geometric parameters if an arced weir that is able to pass a certain flow rate for a given hydraulic head, will be determined.

Keywords: Dam, arced weir, head water ratio, discharge, Discharge coefficient

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