Cylindrical Central Baffle Flume for Flow Measurements in Open Channels

This work experimentally investigated a modified venturi flume formed by placing portable cylinders vertically upright in trapezoidal channels, referred to herein as the cylindrical central baffle flume (CCBF). This mobile device causes flow constriction that creates a critical flow condition. Experiments were conducted in a specially fabricated experimental trapezoidal channel, having the facility to change the side slopes. The side slope of the channel was varied between 0.50 H:1 V and 2 H:1 V at an interval of 0.25 H. Experimental investigation of a mobile flume for such a wide range of side slopes has never been reported. Different variables describing the flow through this flume were selected, and a number of forms of equations relating stage with discharge were developed using Buckingham’s $\mathrm{\Pi }$ theorem and self-similarity hypothesis. The various forms of the stage–discharge equation were calibrated using the measurements made during the laboratory experiments. The stage–discharge equations were evaluated for fit by statistical measures, root mean square error (RMSE), relative mean error (RME), predicted residual error sum of squares (PRESS), and predicted ${\mathrm{R}}^2$. Based on the values of the statistical measures, two forms of stage–discharge equation were selected. The two selected forms were further compared for their performance, first by using additional experimental observations recorded for the same contraction ratio and then by using experimental observations collected for the changed flume dimensions. The performance of the selected forms of equation was also assessed by comparing them with a model available in the literature for a similar flume in a trapezoidal channel having a $1:1$ side slope. Based on the results of the comparison and distribution of errors, one of the two forms of stage–discharge equation that had a maximum relative error between predicted and measured discharge of less than 10% was proposed for flow measurement in such flumes under free flow conditions. The proposed mathematical model proves useful for its versatility, as it was developed and validated for a range of side slopes and contraction ratios, and is valid for a submergence ratio up to 62%. The proposed flume facilitates flow measurement at any desired location including small laterals or turnouts in agricultural settings or water treatment plants conveying water with trapezoidal channels. The flume is mobile, inexpensive, easy to install, and does not require high maintenance.