Parameterizing V-notch weir equations for flow monitoring in a drainage control structure

Authors: KATUWAL, SHEELA - Oak Ridge Institute For Science And Education (ORISE); CRAIG, ANDREW - Iowa State University; RUPIPER, ANDREW - Iowa State University; Rogovska, Natalia; JOHNSON, GABRIEL - Iowa State University; ISENHART, THOMAS - Iowa State University; Malone, Robert

Submitted to: The Journal of Visualized Experiments (JoVE)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Weirs are structures with an opening (of various shapes) used for measuring flow in a channel or a drainage control structure. Assigning correct values for the weir parameters is important for determining accurate water and nutrients losses from tile drains and evaluating the effectiveness of conservation practices which treat water from drained agricultural fields. However, a unified method to develop weir equations for flow monitoring in a drainage control structure is lacking. In this video article, we describe the method for obtaining weir equation parameter values for a V-shaped weir (V-notch weir) in a drainage control structure in the laboratory. For users who do not have access to a laboratory, the video article also describes the use of our recently developed online tool “Weir Flow Equation Coefficients Calculator” for obtaining accurate parameter values for V-notch weirs. This method and the tool allow farmers, conservationists, land managers, water quality specialists, and researcher to obtain accurate parameter values of the weirs they use to measure water flow and nutrients loads from drained agricultural fields.

Technical Abstract: Accurate estimation of drainage discharge (flow rate or water volume per unit time) is important for calculating nutrient loads delivered to surface water and assessing performance of edge-of-field conservation practices. Determination of drainage flow rates involves monitoring of flow depth (head) in a drainage control structure and the use of an appropriate equation for the weir installed in the control structure to accurately estimate flow rate based on flow depth. Prior studies that have developed calibrated equations for weirs in control structures have resulted in different equations (i.e., different values of weir flow equation coefficients) for the same type of weir and the size of the control structure. In this protocol, we describe the procedure for the experimental setup for water flow rate and depth measurements in a drainage control structure and the development of a calibration equation for flows contained within a V-notch weir. In addition, we also describe how the calibration equation developed in the laboratory for flows contained within the V-notch can be used to derive a calibration equation for overtopping flows, i.e., when the water level exceeds the top of the V-notch weir, using an online tool “Weir Flow Equation Coefficients Calculator” developed by the authors. The tool can also provide weir flow equation coefficients for flows contained within the V-notch and overtopping flows for various sizes of Agri Drain water control structures even if the user does not have their site-specific flow rate vs depth relationship, which is also described in the protocol.