Influence of Supply-Channel Velocity on Farm Delivery Meter Gate Flow Measurement

The work presented here is an extension of previous papers that updated the calibration of round (Armco-type) meter gates (three sizes: 0.30, 0.46, and 0.61 m), developed ratings for rectangular gates (two sizes: 0.46 and 0.61 m) on round pipes, and examined errors and uncertainty related to using these devices for water discharge measurement. Previous works examined gate discharge ratings under low supply-channel flow velocity perpendicular to the gate discharge pipeline. Here, additional testing was conducted to test the hypothesis that higher velocities in the supply channel would show decreased meter gate flow compared to the low-velocity ratings, but that the published gate calibration method would still yield accurate flow-rate calculations. All testing was conducted in a test facility with the gate discharge pipe set perpendicular to the supply channel, as is common in field installations. Velocities up to $0.94\mathrm{m}/\mathrm{s}$ ($3.1\mathrm{ft}/\mathrm{s}$) were tested with the smaller gate and $0.66\mathrm{m}/\mathrm{s}$ ($2.2\mathrm{ft}/\mathrm{s}$) for all other gates. These velocities are on the upper end of velocities found in common earthen irrigation canals (and in many lined channels at the farm delivery level) in California. Interestingly, results indicate that the Froude number of the supply-channel flow did not have a statistically significant (at an $\alpha$-level of 0.01) influence on gate discharge coefficients. Discharge percent error and uncertainty were examined to compare the discharge coefficients presented in the literature to the discharge measured during the testing at different supply-channel velocities. Under recommended operating conditions, the uncertainty was within $\pm 5\%$ without adjustments for supply-channel velocity. This extended work supports earlier recommendations that meter gates can be an accurate flow measurement device for farm water delivery flow measurement if installed and operated correctly.