Parameterizing V-notch Weir Equations for Flow Monitoring in a Drainage Control Structure

We developed a user-friendly online tool that can be used by drainage water management professionals to accurately estimate drainage volume across pro conditions and sizes of water control structures. As more edge of field and drainage water management conservation practices are designed and implemented, we are attempting to harmonize this data to make comparisons and track performance of these practices. Our protocol provides an easy-to-use online tool that should allow other researchers to quickly understand the flow regime and how to calculate the flow rate in their system.

Our findings will improve our ability to understand the performance of systems that require monitoring of flow. This work will also improve tracking loss of nutrients and drainage water. Our results provide a uniform method for monitoring drainage flows by researchers and agencies.

Future efforts will include different sizes and configurations of control structures to ensure accuracy of the tool across applications. To begin, connect the inlet section of the control structure to the pump assembly, which consists of a pump, a flow meter, and the necessary pipes, fittings, and valves. Then connect the outlet section of the control structure to the drainage pipe.

Measure the top width and depth of the V-notch weir using a measuring tape, followed by the distance between the tracks where the stop logs and V-notch weir fit in the drainage control structure. Now insert the stop logs between the tracks one at a time. Stack additional stop logs above the existing ones in the control structure until the desired height is reached, and place the weir plate firmly above the top stop log.

Now measure the inner height of the control structure and the distance between the top of the control structure and the top of the V-notch weir using a measuring tape. Calculate the height of the V-notch crest from the bottom of the control structure using the equation. Then firmly connect the manometer tube or stilling well to the control structure.

Affix a measuring tape to the outside wall of the control structure next to the manometer, with the zero value corresponding to the base of the interior of the control structure. Start the pump and allow water to fill the inlet chamber, ensuring that water flows through the weir. Adjust the valve and the pump assembly to select a flow rate of approximately 15 to 25 gallons per minute, monitoring the display on the flow meter.

Allow the flow to stabilize for at least three minutes and increase the flow rate if the head is less than two inches. Once the flow is stable, record the flow rate reading from the flow meter. Take 8 to 10 consecutive readings within two minutes and calculate the average flow rate.

Now measure the height of water in the stilling well or manometer tube. If the water level fluctuates, record the high and low readings over 30 seconds and determine the average height. Then calculate the head using the equation.

Increase the flow rate by approximately 15 to 20 gallons per minute, and wait for it to stabilize to obtain the flow rate and corresponding head. Increase the flow rate at least five more times until the water level reaches near the top of the V-notch. Enter the values of the head and corresponding flow rates into an Excel spreadsheet.

To plot the measured flow rates in the head, select the data range, then click Insert, followed by Charts and Scatter. To fit the power function, right-click on the data points in the chart and select Add trend line, followed by Power, Display equation on chart, and Display R-Squared value on chart. Click the weir flow equation coefficients calculator on the webpage.

Select the unit between metric SI and US customary units using the radio buttons. In the Size of control structure section, keep the default selection of six inch or 15 centimeter as the user provides their own weir equation coefficients. Click Yes in response to the prompt:Do you have your own calibration equation?

And enter the values of parameters A and B, corresponding to the equation for flows contained within the V-notch. Enter the values of the weir specifications in the Input section of the tool. Retrieve the values of coefficients for overtopping flows from the coefficients for V-notch weir equation table and the plot of flow rate against the head from the Results section of the tool.

Open the tool as described earlier, and select the unit and size of the control structure using the radio buttons. Click No in response to Do you have your own calibration equation? Enter the values of the weir specifications.

Retrieve the values of coefficients for flows with the V-notch weir and overtopping flows from the table and the plot of flow rate versus flow depth relative to the V-notch crest in the Results section of the tool. Use of weir coefficients provided by the tool for a downstream weir in a multiple chambered control structure may result in incorrect flow rates at high heads because of increased flow velocities. The calibration of a stainless steel 45-degree V-notch weir in 15, 20 and 30 centimeters Agri Drain control structures demonstrated that flow rates within the V-notch could be estimated with high accuracy using a dedicated weir equation with root means square error less than 0.20 liters per second, and percentage bias less than one.

The estimated flow rates for overtopping flows using weir equation coefficients from the weir flow equation coefficient calculator tool were similar to the measured values.