Evaluating the Effect of Downstream Channel Width Variation on Hydraulic Performance of Arched Plan Stepped Spillways

In the present study, a 3D physical model with a scale of 1:50 was developed based on the Germi-Chay dam. The model was used to assess the impact of downstream channel width variation on the efficiency of arched plan stepped spillways. Four ratios of downstream channel width to spillway crest width (Wch/W) were considered which range from 0.214 to 0.286. The hydraulic parameters such as water surface profile, static pressure, discharge coefficient, energy dissipation, and stage-discharge were assessed for each model. Results of the experiments indicated that the values of the discharge coefficient increased with increasing the upstream head values before the submergence state occurrence. However, in this state, the downstream channel width variations had no significant impact on the discharge coefficient. On the other hand, with spillway submergence, influence on discharge coefficient was observed when width variations occurred. Observations showed that an increase in the width ratio caused a decrease in static pressure. In addition, the results of the submerged state clearly showed the effect of the width ratio variations on energy dissipation. For a particular discharge, the highest width ratio led to minimal energy dissipation. Results revealed that the most efficient model was when Wch/W = 0.286. This model was the only one that successfully passed the probable maximum flood discharge in the maximum allowable height (5 m).