Uniform flow rate calibration model for flat gate under free-submerged orifice flow

Abstract: Hydraulic structures, especially gates, have widely been used to monitor the flow for water measurement in most irrigation districts of China, due mainly to their low investment and easy measuring. However, the detecting accuracy is very limited in the field application in a traditional way, where the maximum error can reach 30% in some cases. It is urgent to enhance the measuring accuracy of water flow with gates in the irrigation water management. Much effort has been made in recent years on the flow rate measurement under a sluice gate, presenting various flow rating models. However, these previous methods almost focus only on one certain flow condition, and much less is known concerning the application of field calibration. In addition, there is remarkable uncertainty in the formulas of empirical coefficients, such as the flow coefficient and submergence coefficient. Most semi-empirical formulas were also derived from under the experimental conditions in a specific laboratory, which are quite different from the actual field application. All of those made it difficult to obtain the precise measurement of flow in the water measurement. The main goal of this study was to propose a feasible flow calculation model in the simple form, particularly suitable for the flow regimes of multiple gates. A 3D hydrodynamic model of actual sluice gate was established to explore the hydraulic characteristics of sluice under different flow conditions, thereby to verify that the selected grid size has no influence on the measurements. A combined 3D numerical simulation and indoor model experiment were carried out with a field prototype observation data, in order to propose a flow calculation model that applied to the multiple flow regimes. The experimental method was first verified the accuracy of calibration model using the measured data. The calibration effect of model was analyzed to apply for the gate in different flow modes, and thereby to check whether it can be used for the free and submerged orifice flow. Furthermore, a field data was used to validate the proposed model with the source of error. One portion of field data was selected to calibrate the model, whereas, another portion to verify the accuracy of model. Finally, a specific case was utilized to evaluate the accuracy of model, where a gate in the middle route of South-to-North Water Transfer project. The measured data collecting in September, 2020, were used for the calibration, and then the data in August, October, and November was used for the verification. The results showed that: 1) The model can be used to determine the flow rate with a relatively high accuracy, where 90.63% of data has less than 5% error; 2) The model can be used for both free and submerged orifice flow rate, where the proportion within 5% error dropped to 86.67%. Using a gate of the South-to-North Water Diversion Middle Line in August, October, and November, the accuracy of model can be verified to achieve 77.64% of data error within 5%, and 95% of data error within 10%. The proposed model is feasible, smooth, and continuous state in field application, thereby it can be expected for the promising way with more rating data to improve the accuracy of gate flow measurement.