Experimental study on hydraulic performance of water-gaging plate for U-shaped canal

Abstract: In view of lack of effective regional water measuring facilities for gentle slope canal with silt current in irrigation areas of northern China, a portable U-shaped flat water measuring device was proposed as a flow water measuring equipment in the field. This study was to investigated its hydraulic performance of flat water measuring facilities based on prototype test. The prototype test was carried out in Northwest A & F University in Yangling, Shannxi of China. In order to explore dynamic characteristics and deflection phenomenon of a draping thin plate under the impact of open channel flow, the relationship between discharge and deflection angle of the plate was determined. The flow pattern was analyzed, and the moving water flow was divided into 3 parts. The formula for calculating the outlet flow of gate was applied to the flow relative to measuring device, and the flow calculation model was established. The undetermined coefficients in the flow coefficient calculation model were estimated and a unified formula for flow rate was obtained. The flat shape was made up of a rectangle and a semicircle, arc radius were respectively 16, 12, 10 and 8 cm, and the corresponding contraction ratios were 0.715, 0.547, 0.439 and 0.337. The measuring device was installed at 5.0 m far from the inlet of upstream of U-shaped channel. The base slope of the channel was 1/2 000. The triangular weir was installed at the end of the downstream of the channel to measure current flow. A total of 10 sections were used to observe the flow characteristics. The current range of U plate measuring device was 9-44 L/s (9 work conditions for each contraction ratio). The results from the prototype experiment was used to compute model parameters and validate the simulation results. The result showed that the maximum relative error between the measured flow rate and the calculated flow rate was 6.9%, with an average relative error of 3.2%. For the contraction ratio of 0.439 and 0.547, the plate flow measurement errors were less than 5%, which met the water measured requirement of irrigation area. It indicated that the current calculation model had a high accuracy. With the same contraction ratio, the relative head loss ratio decreased with the increase of flow rate, the relative head loss ratio increased with the increase of plate contraction ratio. The ratio of relative head loss was more than 10% when the plate contraction ratio was 0.715 plate at the low flow rate (about 10 L/s in this test), and the relative head loss ratio of the remaining plate was less than 10%. Besides, the maximum relative head loss of plate contraction ratio of 0.337 and 0.439 was not higher than 6%. After comprehensive analysis, 0.547 to 0.439 were chosen as the best plate contraction ratio. In practical application, devices such as wind prevention, instrument protection should be installed externally to reduce the influence of wind load and meteorological factors on measurement accuracy. The trash gate should be installed along the upstream to intercept the floating objects such as weeds. When this device was not used for measuring discharge and before the channel froze in winter, the device should be turned up to the vertical statement, which didn't affect the channel water delivery. According to different sizes of U-shaped canals, the appropriate bottom arc radius should be selected to make the plate.