Velocity Contour Weighting Method. II: Evaluation in Trapezoidal Channels and Roughness Sensitivity

The Velocity Contour Weighting Method (VCWM) was developed in Part I to accurately estimate the cross-sectional average velocity of a prismatic channel flow using acoustic Doppler velocity meter (ADVM) measurements of centerline velocity. Here, the VCWM is validated by its successful application to 25 different concrete-lined trapezoidal channels used for irrigation water delivery. At each site, the cross-sectional distribution of velocity is measured by an acoustic Doppler velocimeter (ADV), which is moved horizontally and vertically through a sampling grid. Multiple tests at some sites led to a total of 51 sets of cross-sectional measurements. ADVM measurements are simulated by interpolating ADV measurements along a set of vertically aligned centerline coordinates typical of ADVM deployments. Subsequent application of the VCWM gives an estimate of the cross-sectional average velocity. Secondly, the velocity-area method is applied to the ADV data to directly measure the cross-sectional average velocity for comparison with the VCWM estimate. Based on this comparison, relative percent errors in the VCWM for all 51 tests were within $\pm 6.3\%$ using a probable surface roughness ($k_s$) for the finished concrete of 0.0006 m without calibration. A sensitivity analysis shows that a range of realistic roughness values for finished concrete can be used without degrading the accuracy of the cross-sectional average velocity predictions by more than an additional $\pm 1\%$. Hence, the method is relatively insensitive to poorly characterized roughness values.