Introduction

Considerable accuracy in studies, correct location, correct assessment of water needs that will be met by building a dam, building a dam according to the materials available in the region, etc. are among the most important things considered in order to increase the advantages and make dams more practical.Given the nature of dams, which are artificial walls placed against the flow, challenges arise during floods with different return periods, which, if not predicted correctly, can lead to irreparable accidents. For this purpose, modeling with an appropriate scale in a laboratory environment has always been considered with the aim of evaluating the hydraulic behavior of different parts of the dam. This, in addition to the design engineers' knowledge of the dam's performance, will also reduce economic costs. Therefore, the present study was conducted with the aim of examining the results of the 1:20 scale hydraulic model of the Dasht-e Palang Dam and evaluating the performance of the relaxation basin and spillway.

Methodology

The construction site of Dasht-e Palang Dam is located in Bushehr Province, Dashti County, and Shanbeh and Tasuj Districts from the perspective of geographical divisions. The study area includes the Dasht-e Palang River watershed with a geographical range of 51 degrees and 28 minutes to 52 degrees and 5 minutes east longitude and 28 degrees and 31 minutes to 29 degrees and 13 minutes north latitude.Construction of the hydraulic model of Dasht-e Palang Dam is the refinement of the spillway design in order to ensure proper and safe flood discharge under different operating conditions. Considering the dimensions of the steps, the spillway length and other hydraulic characteristics and considering the limitations in the laboratory environment, a scale of 1:20 was determined for the construction of the model.

Results and Discussion

The highest and lowest energy dissipation efficiency of the rapid is about 72 and 40 percent, which correspond to the flow rates of 1000 and 4000 cubic meters per second, respectively. By evaluating the energy dissipation efficiency values, it is clear that with increasing flow rate, the energy dissipation efficiency has decreased. With increasing flow rate, the water depth on the rapid increases and the ratio of step height to water depth at each section decreases, and therefore the effect of the step's interference with the flow on energy dissipation decreases. In order to examine the flow pattern at the end of the rapid, inside the calm and backwater basins, important information is obtained in terms of the location and formation of the hydraulic jump. At a flow rate of 1000 cubic meters per second, a submerged hydraulic jump is formed in the basin. At a flow rate of 3000 cubic meters per second, a strong hydraulic jump is formed in the basin. The position of the jump formation is at the beginning of the pond, and the subsurface currents near the bottom of the pond move at high speed and, upon impacting the bottom water, come upstream and eddies are formed in this area. At this discharge, the hydraulic jump is also formed completely inside the pond, and the flow in the downstream is uniform with short waves. At a discharge of 3300 cubic meters per second (as the design discharge of the stilling pond), the flow conditions are largely similar to the flow conditions at a discharge of 3000 cubic meters per second, but the position of the hydraulic jump formation and also the flow fluctuations are slightly different. At the design discharge, the pond has a hydraulic jump formed at a distance of 3 to 4 meters from the beginning of the pond and continues to the end of the pond.

Conclusion

In evaluating the performance of the stepped rapid at low flow rates, it was found that at flow rates less than about 280 cubic meters per second, the thin blade of the flow separated from the surface of the step after hitting the bottom of the first step and, passing over several other steps without engaging them, hit the surface of the rapid again. Regarding the investigation of the flow pattern on the stepped rapid and also the location of the formation of the starting point of the flow aeration, experiments were conducted in a wide range of different flow rates and the results were compared with several different experimental relationships, which were observed to be in close agreement. Hydraulic parameters including water depth and piezometric pressures along the spillway and stepped rapid were measured for flow rates of 500, 1000, 2000, 3000 and 4000 cubic meters per second. The flow velocity values were not measured only at a flow rate of 500 cubic meters per second due to the shallow depth of the flow, but were measured at other flow rates, similar to other parameters. According to the measured data, the energy dissipation efficiency of the stepped rapid was calculated. Accordingly, the efficiency values for flow rates of 1000 to 4000 cubic meters per second were obtained from about 72 to 40 percent. From the results obtained from the model, it was determined that from low flow rates to a flow rate of about 3000 cubic meters per second, the hydraulic jump is completely formed inside the stilling basin and the basin has a good efficiency in energy dissipation.