Laboratory examination of Comparison of the effect of the porous screen,, continuous obstacle, and continuous porous obstacle on the edge of a stepped spillway

Step chutes as a structure are commonly used in earth dams and weighted concrete dams (Chanson, 2001). The presence of a step in the spillway acts like a roughness compared to a smooth chute, which causes the amount of air to enter and as a result, the amount of energy dissipation in the direction of the spillway step increases. In recent decades, extensive research, often laboratory-oriented, has been conducted by researchers to identify the type of flow, the effect of step dimensions, the onset of aeration, and the mechanism of energy dissipation. Further research has attempted to place continuous and discontinuous obstacles and roughness at the bottom of the steps in Floors and edges of step with a variety of shapes and arrangements, deformation of steps, creating angles along the spillway, creating angles in the floor of the steps and edge obstacle, artificial aeration in the steps investigated the hydraulic conditions. In general, in some cases, depending on the height of the obstacle used, the slope of the spillway, the outlet flow, the type of obstacles and the location of the obstacle, the depreciation effect has been positive or negative.

The flume used was direct, with a length of 10 m, a width of 1.2 m and a height of 1.2 m in the first 2 m, and 1 m in length of the flume with maximum flow rate 150 liters per second. Measurement of water depth in the tail-water and upstream was carried out using a point gage with an accuracy of ± 1 mm. The spillway have 8 steps, where the vertical length of step (h) is 10.9 cm, the horizontal length of step (L) is 31.3 cm, and the total height is 87 cm, while at a 1:2 slope, the length of step is 20.9 cm and the total height is 88 cm. The image was recorded by Sony FS5 camera with 240 frames per and second, along with 3 LED150 projectors.

In the 1: 3 slope in the transition regime, the obstacles used had a positive dissipation effect compared to the flat step. In this slope and flow regime, the porous obstacle (EP) has a greater dissipation effect than the porous screen (ES) and the full obstacle (EO), respectively, and in all three expressed arrangements (EO, ES, EP) in this regime the relative height of 0.38 had the highest dissipation rate. The superiority of the porous obstacle in this regime in energy dissipation is due to the three-dimensionality of the porous obstacle, which depletes the flow energy. Then in the procedure regime for 1: 3 slope, the results show that all three arrangements used have increased energy dissipation compared to the flat step, in this case, respectively, porous screen (ES), porous obstacle (EP) and full obstacle (EO), respectively. Had the highest energy consumption. For example, at the maximum flow rate, the flat steep energy dissipation rate is 46%, which is 55% for the porous screen (ES), 52% for the porous obstacle (EP) and 49% for the full obstacle (EO).

The present study compares the use of continuous obstacle, porous obstacle (3 dimensional) and screen obstacle (2 dimensional) with three heights at the edge of the step at two slopes of 1: 3 and 1: 2 in all three flows regime of nappe, transitional, and skimming. 1- The placement of continuous obstacle with different shape (filled continuous, porous obstacle and screen obstacle) at the edge of spillway for both slopes of this research causes the onset of the flow regime shift to the flat step and start of the inception point of free aeration (IP) was transmitted to a lower step toward the downstream relative to flat step on both slopes of the present research. 2- Based on the BIV results and a comparison of energy dissipation, it can be stated that continuous obstacles that can expand the mixing zones (including MZ and RF) increase energy dissipation. In fact, the recirculation zone has less dissipation effect than the mixing zone. 3- The creation of a porous obstacle and a porous screen on both slopes has increased energy dissipation compared to the full obstacle in all three flow regimes. This effect will increase with increasing the relative height of the obstacle until it reaches the pool (RZp) on the steps.