Investigation of Discharge Coefficient of Compound Arched Circular-Trapezoidal Sharp-Crested Weirs

The compound arched circular-trapezoidal sharp-crested weirs are combinations of a circular arch notch at the bottom part and a trapezoidal notch at the top of the cross section. In this research, the discharge coefficient (Cd) of compound arched circular-trapezoidal sharp-crested was investigated. The experiments were conducted on weirs having the crest width of 15 cm, crest height of 15, 20, 25 cm and circular arch notch height of 5, 7.5 cm with different curve radii and the side walls slope of 0.5 (0.5H:1V) in a laboratory flume. The results indicated that the amount of discharge coefficient was in the range of 0.56 to 0.81. For a given hydraulic head, the amount of Cd is reduced approximately up to 5% by increasing the height of weir crest. In addition, for a given hydraulic head, by changing the height of circular arch notch, the amount of Cd for the compound arched circular-trapezoidal sharp-crested weirs increased about 25 to 30 percent with respect to the trapezoidal weirs. According to the root mean square error (RMSE) with value of 0.0563, the amount of Cd for the compound arched circular-trapezoidal weirs was estimated with acceptable accuracy, using a linear combination of the discharge relationships of circular and trapezoidal weirs. In this research, the discharge continuity over the compound weir for the various hydraulic heads was examined using the experimental data. The results showed that the least amount of discontinuity could be seen in the wide range of discharges.