Impact of Bed Slope on Accuracy of the Horizontal Plate for Measuring Water Flow in U-Shaped Channels

Abstract

【Background】Most irrigation districts in northern China are flatten and water in their channels is muddy. Existing facilities for measuring water flow in channels could cause backward water flow in their upstream and are prone to sediment deposition. Horizontal plate is a new and simply structured facility to measure water flow and can alleviate sediment deposition. While the impact of the discharge and other factors such as plate deflection angle, water depth in both upstream and downstream, and plate profile on performance of the plate method has been well documented, how bed slope of a channel affects hydraulic characteristics of the plate remains poorly understood. 【Objective】The purpose of this paper is to plug this knowledge gap, systematically investigating the effects of bed slope on reliability and accuracy of the horizontal plate for measuring water flow. 【Method】U-shaped channel commonly used in irrigation areas in northern China was taken to test the method, and the volume of the horizontal plate with optimal contraction ratio of cross-section (0.439) was selected as the test object. With a total of 18 combinations, which included three bed slope ranging from 0.000 2 to 0.001 with each associated with 4~7 flow rates ranging from 10 to 44 L/s, variation of the water-surface line and the plate deflection angles in each combination were measured, from which we analyzed the impact of the bed slope on water-surface line, relative water head loss, energy conversion coefficient, plate deflection angle and comprehensive flow coefficient. Using the formula of sluice flooding discharge, a semi-empirical flow formula with the bed slope as independent variable was used to fit the data. 【Result】Under the same water flow rate, the hydraulic jump length behind the plate, the energy conversion coefficient and the comprehensive flow coefficient all increased as the bed slope became steeper; in contrast, the water-surface line, the relative water head loss and the plate deflection angle all decreased as the bed slope increased. For all tested flow rates, the average relative error of the fitting formula for water flow rate was 2.6%, with a maximum of 6.5%, meeting the requirement for water flow in irrigated areas. 【Conclusion】The bed slope of channel has a significant influence on accuracy of the horizontal plate for measuring water flow in U-shaped channels, and a formula considering the bed slope was proposed to calculate water flow in U-shaped channels.

Keywords

• u-shaped channel
• volume level plate
• flow formula
• bed slope