Experimental investigation of flow discharge coefficient for combined system of compound weirs-orifices

Weirs and orifices (gates) are the main hydraulic structures for water level and flow discharge control, respectively. They are also extensively used for flow discharge measurement. Having a simple stage-discharge relationship is the most important reason for using these structures as measuring instruments. Also combination of these two structures causes an improvement of system performance, when the flow is heavily sediment-laden or carries considerable loads and floating objects. One of suitable solution that can be used to increase the efficiency of weir-orifice structures is compound weir-orifice. There are very limited investigations in this regard.
Material and This study has been conducted with the aim of investigating the discharge coefficient for combined compound weir (triangular-rectangular)-orifice structures. Several experiments have been carried out in a rectangular flume with length of 10m and width of 0.40m. Triangular notch weirs with angles of 45, 60 and 90 degrees and rectangular orifices have been used. The width of rectangular weir in upper section is 40cm. The results of this study showed that for compound weirs in total selected angles, with increasing of weir height ratio (relation of head water on compound weir to the weir height), discharge coefficient of compound weir increases. In spite of high variations of head water on the weir as well as triangle weir's angles, variation in discharge coefficient is very low and changes between 0.58-0.61. This fact defines the importance of precise estimation of discharge coefficient for compound weirs. Furthermore, it was found that by increasing the angle of the triangle weir, the discharge coefficient increases while for compound weir-orifice structure, the discharge coefficient decreases. The opening height of orifice is also effective on the discharge coefficient of combined compound weir-orifice structures so that with increasing of this parameter, discharge coefficient decreases. It is also found that the discharge coefficient of combined compound weir-orifice structures is slightly lower than the discharge coefficient of the compound weirs without orifice. according to the results obtained in this study, one can properly estimate the discharge coefficient of combined compound weir-orifice structures given the simple data such as geometry of compound weir and orifice and also the flow depth in upstream of the weir and hence, utilize this structure in the main irrigation networks and canals. In this case, the proper control of water surface elevation by compound weir as well as suspended sediment transport through the orifice would be achieved.