Investigation of hydraulic conditions and determination of the discharge coefficient of the combined structure of semi-circular-rectangular weir and sliding gate using a laboratory model

Introduction
The overflow structure is used to organize and control the water level in open canals and relaxation ponds. Composite sharp edge overflows are designed in different ways and are made up of several openings in such a way that when there is a shortage of water, The flow only passes through the main section of the spillway and more discharge is created on it, and during the flood, with the increase of the discharge, the upper section also starts working, and as a result, it prevents the return of water and the increase of the water level upstream of the spillway. In this situation, the measurement is done with more accuracy (Boss, 1988). Due to the ability to control the water level and the ease of construction and measurement of flow intensity, sharp edge spillways and sliding gates have been widely investigated. On the other hand, most of the overflows in their upstream cause the formation of a region with relatively still water, which creates favorable conditions for sediments and waste materials to settle in the water, which is considered one of the defects of this structure. With the combination of overflow and gate, the difficulties and disadvantages of using each one alone can be reduced so that materials with sedimentation properties are easily emptied from the gate part and mineral and floating materials are emptied from the overflow (Negm et al, 2002).
Methodology
The experiments of this research were carried out in the hydraulic laboratory channel of the Faculty of Agriculture of Birjand University. The channel used in this experiment has a rectangular cross section with a width of 0.3 meters, a length of 10 meters, a height of 0.5 meters and a maximum flow rate of 30 liters per second. The current research was carried out in the form of establishing flow with constant opening of the gate and different flow rates, as well as establishing flow with opening of different gates and constant flow rate in two slopes of 0.004 and 0.002 to determine the flow coefficient. In order to carry out the present experiment, a semi-circular-rectangular overflow structure and a sliding gate is made of galvanized sheet and installed in the test section.
By including all effective parameters and dimensional analysis by Buckingham’s method and considering ρ, v and y as repeated variables, the general equation of dimensionless parameters is obtained as follows after simplification.  In the present study, dimensionless parameters Hg/D, y/D and  were investigated.
Results and discussion
According to the collected data and the hydraulic and geometrical parameters of the structure, the discharge coefficient of the structure was investigated. The flow coefficient of the combined structure was calculated in two states of constant gate and different flow rates, and constant flow rate and different gates. Also, in order to control some of the experiments, the discharge coefficient of the combined structure was investigated in two slopes of 0.004 and 0.002. In all these researches, the discharge coefficient of the combined structure was between 0.6 and 0.8.
Conclusion
The test results show that the discharge coefficient depends on whether the gate is submerged or not and the height of the upstream water. Also, the flow coefficient in the conditions of water passing through the gate decreases with the increase of Y/D parameter, and the flow coefficient increases with the increase of Y/D when water passes through the gateand semi-circular-rectangular overflow. By changing the slope of the floor, the discharge coefficient has not changed significantly and with the decrease of Hg/D, the discharge coefficient is almost fixed and tends to a constant number of 0.74.