Field Calibration of Flow over Tilting Weirs in Canals

Tilting weirs are widely used to regulate water levels in open channel irrigation systems and can also provide flow measurement capability that helps optimize irrigation efficiency and ensure equitable water distribution. This case study demonstrates adaptation of lab-based tilting weir ratings to field settings, where flow behavior is more complex. We compile and analyze an extensive data set derived from independent studies of five structures within three canal systems, featuring both lined and unlined channels with discharge ranging from 0.35 to 15 m3/s (12 to 530 ft3/s). Results indicate that lab-based rating equations can be calibrated to account for site-specific variations (channel roughness, flow separation, etc.) using a field correction factor, KF, applied to the discharge coefficient in the rating equation. The average KF value across the seven data sets was 0.92, an 8% reduction from the laboratory. The case study demonstrates that tilting weirs can serve multiple purposes in the field, offering both stage regulation and discharge measurement with mean absolute percentage errors of approximately ±6.2%. Guidelines are given for optimizing tilting weir performance within engineered canals and additional research needs are identified for addressing complex natural channels.