AbstractThe design and calibration of a simple rectangular flume with central cylindrical contraction is presented. The flume is designed based on principles of critical flow in open channels. Critical flow is created through contraction of flow cross section by installing a vertical cylindrical column at the center of a rectangular open channel. The stage–discharge relationship was calibrated using a laboratory prototype with various configurations. A previously developed model was recalibrated using the laboratory data for the proposed flume. The results of the proposed stage relationship were also compared with results from a computational fluid dynamics model (CFD). The results and analysis show that if the piezometric head is measured at midstream at the upstream side of the contracting column, then stage–discharge relationship can be developed, which is independent of the contraction ratio. The stage–discharge function developed in this paper can be used for design and calibration of the flume regardless of the flume size or contraction ratio. The stage–discharge relationship equation is also compared with another set of independently measured laboratory data.