The Experimental Study of the Effect of Gate Position on Flow Discharge Coefficient of Rectangular-Labyrinth Weir-Gate

The combined weir-gate structure has various applications in hydraulic engineering and can eliminate some of the shortcomings of each individual application. The combined weir-gate structure, with the ability of simultaneous passage of the material to be deposited through the gate section and that of the suspended material from the weir, prevents the accumulation of the sediments and the suspended materials behind the weir and helps increase the measurement accuracy and flow passing. Some studies conducted on the labyrinth gates are related to triangular and trapezoidal labyrinth weirs. However, regarding the performance, the rectangular labyrinth gates could be built and run better in some areas. Moreover, in the studies carried out in the field of combined simultaneous flow above the weir and below the gate, the combined weir models with sliding gates have been rarely addressed. One of the significant issues in the combined rectangular labyrinth weir-gate structure is the position of the gate which has not been investigated yet. Therefore, this study aims to investigate the effect of the gate position on the discharge coefficient of the rectangular labyrinth weir-gate and that of increasing the weir height on the discharge coefficient of the rectangular labyrinth weir-gate in different gate positions. This study investigates the effect of gate position on the discharge coefficient using physical models of rectangular labyrinth weir-gate, made of Plexiglas with 15 and 20 cm heights in 15 positions of gates in different situations. All experiments in this research were conducted in a glass rectangular channel with the length of 12, width of 0.5, height of 0.7 meters, and longitudinal slope of zero degrees. The discharge varied from 10 to 50 liter per second. To measure the depth of water at the upstream weir and the triangular weir, a depth-measuring device with the precision of 0/1 millimeter was used. Considering the width of the laboratory flume and the decreasing effect of walls surface tension of the rectangular labyrinth gate-weir, two cycles were selected for the weir. The weirs with the crest of 90 ° (straight) and thickness of 10 mm were made of 1, 2, 3, and 4 sliding gates. The models were mounted on a four-meter upstream channel. In general, 390 experiments were conducted in this study. To investigate the effect of gate position on the flow discharge coefficient, experiments were done using 1, 2, 3, and 4 gates in different positions. For weir models with heights of 15 and 20 cm and a gate, when the gate is located in the forehead of the upstream weir (model 4), the discharge coefficient was more than those of two other models (2 and 3) due to the gate’s perpendicular position to the flow direction. For the two-gate mode in model 6 in both weirs with heights of 15 and 20 centimeters, the discharge coefficient was higher than other 3 models, due to both gates’ perpendicular positions to the flow direction. Models 10 and 14, with 3 and 4 gates had a higher discharge coefficient. In these cases, when the number of the gates’ perpendicular positions to the flow direction increased, they had a greater effect on the discharge coefficient than other states did. The results showed that the discharge in the combined weir-gate structure was more than that in weirs with no gates; that in fact, showed the effect of the gate in a rectangular labyrinth weir on the discharge increased. According to the results, the discharge coefficient in the rectangular labyrinth weir gate was more than that in the rectangular labyrinth weir with no gate. In other words, the gate effect on the discharge coefficient increased in a rectangular labyrinth weir. However, considering the results, the discharge coefficient increase in the low current range (less H/P) was greater than that in the high current range (higher H/P). In fact, the result revealed that the gate in the rectangular labyrinth weir-gate in the low current range (less H/P) had more impact on the discharge coefficient increase and, consequently, on the transit of discharge increase. Given that the reduction of the discharge coefficient with the increase in the H/P ratio results from the increase in water jets interference in adjacent cycles and the increase in local weir submergence, the flow below the gate also caused additional, new interference within the flow of water at the bottom of the structure and, consequently, reduced the discharge coefficient. The highest efficiency of the combined rectangular labyrinth weir-gate model in less H/P is recommended. According to the obtained results to achieve the highest efficiency, the maximum water ratio (H/P) was recommended to be less than 0.5 and 0.7, respectively for weirs with the height of 20 and 15 cm. This issue almost corresponds to Lux’s results who suggested the maximum water ratio (H/P) for a trapezoidal labyrinth of 0.45-0.5. This study investigated the effect of gate position on the discharge coefficient in the combined rectangular labyrinth weir-gate structure. According to the results in the studied states, when gates were located perpendicular to the flow direction, they had higher impact on the discharge coefficient. Moreover, the discharge and discharge coefficient in the rectangular labyrinth weir-gate were more than those in their counterpart with no gates. In fact, the gate in a rectangular labyrinth weir led to an increase in discharge and discharge coefficient.