Rating Rectangular Farm Delivery Meter Gates for Flow Measurement

Traditional meter gates for farm delivery flow measurement from an open channel conveyance have traditionally incorporated round canal gates (Armco type) for control. In recent years, some irrigation water agencies (i.e., irrigation districts) have replaced deteriorating round gates with lower-cost rectangular gates that cover round holes. Similar to the situation described in a companion paper, where round gates were examined, there have been no investigations into flow measurement uncertainty using the existing rating tables for these gates. In this study, two commonly used rectangular gate sizes, 0.46 m (18-in.) and 0.61 m (24-in.), were tested under scenarios of various gate openings, upstream heads, and head differences. Coefficient of discharge ($C_d$) values were computed based on actual gate open areas. These improved $C_d$ values were used to generate new discharge rating tables for 0.46 m (18-in.) and 0.61 m (24-in.) rectangular meter gates. Limitations for these rectangular gates are discussed. If guidelines presented in this paper and in the companion paper are followed, the average instantaneous flow measurement uncertainty that could be expected is better than $\pm 5\%$. However, uncertainty is higher (up to approximately $\pm 9.5\%$) at the lower end of the recommended gate openings [0.10 m (4 in.)] for these rectangular gates.