Vortex formation is one of the serious problems in dam operation. In the present work, to study the vortex and its effects on orifice spillways of Karun III dam, two physical models were studied. A partial (with only one orifice) and comprehensive model (with four orifices). Experiments showed that in orifice spillways, regarding to their special conditions (changing discharge by changing the reservoir elevation) when reservoir elevation decrease, at the first strength of vortices increase and after reach to a maximum value again decrease. This research also showed that, by increasing in vortex strength, the discharge coefficient may reduce till 10%. Measuring the critical submerged depth showed that between theoretical methods to determine this parameter, Gordon and Knauss have the better results. Comprehensive model showed that, operating together of all four Karun III orifice spillways had positive effect on reducing the strength and type of formed vortices.

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