Flow Measurement with Trapezoidal Free Overfall

Abstract

The trapezoidal free overfall is analyzed by the one‐dimensional momentum equation, including an assumed pressure distribution at the brink. The pressure distribution is based on previous measurements in rectangular and triangular open channels. The predicted relationship between brink depth and discharge is compared with experimental data from a laboratory flume, and excellent agreement is evident. A calibration chart, applicable to any mild slope trapezoidal channel, is developed theoretically and checked against the data from the present study and data from previous studies. Again excellent agreement is noted. Comparison with previous predictive techniques, in which the pressure distribution at the brink was assumed to be uniformly atmospheric, illustrates the importance of including this pressure distribution in the analysis.