Determination of Discharge Coefficient for Combined Flow over a Cylindrical Weir and Submerged Gate and its Correlation with Separate Flow over the Weir and Under the Gate

Weirs and gates have numerous applications in open channels as flow measurement and water-level control structures; therefore، their combination provides a structure، which offers the advantage of both structures in the passage of floating objects over the crest of the weir and the discharge of the suspended sediment under the gate. In the present study، a laboratory investigation was implemented to the variations of discharge coefficient of cylindrical weir-gate according to dimensionless parameters H/D (the ratio of upstream depth to the structure’s diameter) and H/a (the ratio of upstream depth to the gate opening) using a 10m long flume whose width and height were 0. 6 and 0. 7m، flume (10×0. 6×0. 7 meters of length، width and height، respectively). Furthermore، the variations in the trend of weir and gate discharge coefficient were determined by measuring the separate velocity profile under and above the structure and compared them to those of the discharge coefficient of the combined model of cylindrical weir-gate. The results show that the discharge coefficient of the combined cylindrical model decreases initially and then rises due to an increase in the dimensionless parameters، H/a and H/D. The trend of discharge coefficient in the combined model is decreasing for gate and shows an ascending trend at first، then becoming eventually constant for the weir. Moreover، the structure’s diameter and the gate opening affect the discharge coefficients of weir، gate، and the combined weir-gate model. The cylindrical weir-gate discharge coefficient ranges between 0. 75-1. 05 under the experimental conditions. An equation with a significant correlation for the weir-gate discharge coefficients estimation was obtained.