Study on effects of ogee spillway guide walls changes on its hydraulic performance

This study aimed to analyse the design of spillway in curved plan and downward angle of convergence. In this plan the effective length of the spillway is reduced from the crest to the toe for several times. Tests were carried out in two stages. In the first stage، a physical model of the real sample was prepared for a specific site in the scale of 1:50. In this stage، experiments were performed for eight different discharges making up to 25 to 150% of the design discharge of the real sample. The spillway demonstrated a satisfactory performance up to the discharge of 1. 13Qd (Design Discharge). Gradually an increase in the discharge led to a decrease in the performance of the spillway so that with a discharge of 1. 26 Qd the spillway was fully submerged and the downstream channel took control of the discharge. Observations and experiments indicated that one of the factors contributing to the swamp of the spillway was a severe variant of the guide wall (120 degrees) as well as the considerable decrease in the ratio of channel length (Lch) to crest length (L)، i. e. Lch/L=0. 214. In the second stage، the effect of variation of convergence angle of the guide walls of the spillway was studied. To this end، a physical model with three convergence angle of the guide walls، including a 0- degree angle and ratio of Lch/L=0. 98،; a 60-degree angle with ratio of Lch/L=0. 32; and a 90-degree angle with ratio of Lch/L=0. 26 was tested. The effect of variations of angles in discharge coefficient was reported to be negligible before the swap. At the 0-degree angle with ratio of Lch/L=0. 98، the highest discharge was 1. 99 because the model was not submerged in a discharge equal to the PMF discharge of the real sample. In all angles، the static pressure on the crest was reduced while it was increased in the shoot and toe. So that، in the crest a ratio of Hp/Hd=-0. 217was recorded for a discharge of 1. 13 Qd. At the angles of 90 and 120 degree، higher discharges led to swamp of the spillway and a change of the current flowing on the spillway from supercritical to subcritical. The high pressure was reported to be Hp/Hd=4. 35 of the 120-degree angle for a discharge of 1. 5Qd. Since at the 60-degree angle (Lch/L=0. 32)، the length of the crest was smaller (45% less than the 120-degree angle with Lch/L=0. 214)، it was selected as the proper angle. Key words: Angle of convergence، Critical discharge، Discharge ratio، Physical model، Static pressure