Development of an Equation for Flow Measurement in Agridrain Control Structures.

Accurate flow rate measurements are essential for estimating the flow volume and the contaminant loads during drainage studies. Subsurface drainage control structures are used in drainage water management systems to convey water, to control and measure the discharge, and to raise the outlet elevation. By installing triangular weirs inside the control structure, the flow rate can be estimated from the thickness of the sheet of water flowing over the spillway (nappe). The basic principle is that discharge is directly related to the water depth, or head, above the crotch of the weir. Due to the design of triangular weirs, small changes in discharge cause large change in thickness of the nappe at low flow rates which allows more accurate flow measurement compared to rectangular weirs. We conducted a bench-scale experiment to calibrate a water level control structure from the commercial company Agridrain. Using an independent flow measurement gauge and pump, we measured the thickness of nappe for corresponding flow rates increasing incrementally from 20 l/min to 153 l/min using a pressure transducer. For the experiment, we used a control structure with an inside width of 152 mm. Based on the initial measurements, we developed the following equation: Q = 1.7406 H^1.9531, where Q is the flow rate in l/min and H is the thickness of the nappe in cm. The equation was developed using power regression analysis with an R² of 99%. The experiment was repeated with different number of boards to study the impact of elevation of the V-notch within the control structure. A statistical analysis showed there was no significant difference between experiments with different number of boards. The relationship we developed compares well with a theoretical curve from the literature.