Introduction
Measuring, recording, and monitoring water flow in the waterways and irrigation and drainage networks is essential for demand-driven volumetric water delivery. Measuring fluids in open channels is more complex than in closed channels because the uncertainty and degree of freedom of flow are greater in open channels. On the other hand, these irrigation systems require low-cost and accurate measuring instruments or structures. Due to their economic efficiency and low uncertainty, flow measurement structures based on the discharge-scale relationship, such as weirs and flumes, are used. Among them, measuring flumes are one of the basic structures in irrigation networks, which, if constructed correctly, have high accuracy. In this study, the submergence threshold and discharge under submerged conditions were tested in a porous cut-throat flume with triangular and trapezoidal throats. The main goal of this study is to obtain an empirical relationship for finding the submergence threshold and also the throughflow under submerged conditions of a cut-throat flume with a gabion structure.
Methodology
The experiments were conducted in the hydraulic laboratory of the Water Engineering Department of the Faculty of Agriculture, Razi University. For this purpose, a laboratory channel with a rectangular cross-section of 37 cm wide, 60 cm high, and 6 m long was used. To construct the gabion structures used in this study, 6 mm rebar was used, and the structures were filled with aggregates with different porosity percentages and the aggregates were divided into three different porosity percentages. The flumes were constructed with 3 different flume heights, 4 different throat openings, and 3 different porosity percentages. Finally, in this study, 36 measurement structures and 330 experiments under submergence threshold conditions and 880 experiments under submerged flow conditions were used to calibrate the mathematical relationship and examine the effect of variables on flow. To estimate the appropriate mathematical relationship, first, appropriate dimensionless groups were obtained between the variables used using Buckingham's theory, and then, using nonlinear regression and Gene Expression Programming methods, mathematical relationships were obtained with appropriate accuracy to estimate the submergence threshold and submerged flow of the flume.
Results and Discussion
The results indicate that for a constant discharge, if the opening and porosity of the materials are constant, with increasing flume height, the depth of submergence threshold increases by 5 to 28 percent. If the height and porosity of the flume materials are considered constant, with increasing the throat opening at a constant discharge, the structure is submerged faster and the depth of submergence threshold decreases, which varies from 9 to 26 percent. A flume with greater height and lower porosity and opening percentage has a higher submergence threshold, and as a result, is more resistant to submergence and submerges more slowly. On the other hand, by increasing the flume opening for a constant water depth, the value of dimensionless discharge increases between 9 and 66 percent. This is observed with an increase in the porosity percentage and a decrease in the height of the structure. In this condition, a larger flow rate passes through the flume and a larger flow rate range can be measured. Finally, by considering the dimensionless groups obtained by dimensional analysis, mathematical relations for estimating the depth of the flume submergence threshold and relations for estimating the flow discharge under submergence conditions were obtained between the dimensionless groups using two softwares: SPSS 26 and GeneXproTools 5.0. These relations were separated in order to increase the measurement accuracy for flumes with triangular and trapezoidal throats.
Conclusion
This study showed that the use of a porous flume helps to widen the range of flow measurement under submerged conditions and confirms the use of this type of flume in submerged flow conditions. On the other hand, by increasing the size of the flume, can be prevented from becoming submerged. However, it is worth noting that the use of gabion structures in alluvial waterways with high sediment concentrations may cause the flume pores to fill and the porosity of the structure to change over time.