Investigation of the effect of internal weir's scale on hydraulicparametersof modern horseshoe spillway

Modern horseshoe spillways are combination of external and internal weirs. Determining the internal weir’s hydraulic parameters is essential for design of this type of spillways. In this study, by constructing physical model of modern horseshoe spillway including three different internal weir scales, some hydraulic parameters of modern horseshoe spillways like discharge coefficientof internal weir (CO), water surface profiles and discharge rate through the external and internal weirs were investigated, and the effect of changing internal weir’s scale on the CO mas examined. Comparison of the percentage of discharge flowing through the internal weir with scales of 1/(2.5)and 1/5 , indicated that by increasing the Lw , the discharge flowing through the internal weir decreases. By increasing the internal weir’s scale from 1⁄10to1⁄5and1⁄2.5 , the head above the modern horseshoe spillway at the same total discharge, declines to 44, 48 and 56% (on average) respectively in comparison to rectangular weir head with the same width. The results showed that by increasing the scale of internal weir, the ratio of internal weir discharge to external weir discharge Q1/Q2 decreases while the CO increases. For investigation of the effect of varying internal weir’s area on the CO, ratio of the area to multiplying of internal and external weirs length was used and it was found that in the spillway with scale of1⁄2.5 , by increasing the ratio of , the CO increases. Based on the results it can be claimed that modern horseshoe spillways with scales of 1/(2.5) has hydraulically the best performance.