Effect of the hydraulic conditions and structure geometry on mathematical modelling of discharge coefficient for duckbill and oblique weirs

 
    Weirs are one the most important hydraulic structures for flow control, water measurement and regulating of upstream water elevation in canals, irrigation networks and rivers. For a given of channel width, duckbill and oblique weirs have longer effective length in comparison with the usual rectangular sharp crested weirs, and this could be effective on their discharge coefficients and efficiency. Discharge coefficient is a function of weir geometry and hydraulic conditions. Therefore, in this study discharge coefficients were specified for duckbill and oblique weirs with three different wall angles and three weir heights and were compared with the corresponding measured values in usual rectangular weir. Three mathematical dimensionless equations were finally developed for these weirs, describing the relationship of discharge coefficients with the weir geometry and hydraulic conditions. It was found that for all types of weirs the discharge coefficient decreases with increasing discharge or upstream head. However, this reduction has sharp slope for the weirs with higher angles. For duckbill weirs the discharge coefficients were measured 0.5-0.7, 0.4-0.73, and 0.3-0.75 for the angles of 45, 60, and 75 degrees, respectively. For oblique weirs the measured discharge coefficients were 0.66-0.9, 0.64-0.95, and 0.54-0.9 for the angles of 15, 30, and 45 degrees, respectively. The results of this research show that the duckbill and oblique weirs have more efficiency for low head in upstream and are able to pass more discharge with lower upstream head.