A weir is a device that measures the flow of water from streams, basins or open channels. This device is commonly used for outflows from wastewater treatment facilities. There are several types of weirs, including Cipoletti (trapezoidal), broad-crested and V-notch. The calculation of flow over a weir can be simple if you have a means of measuring the water in front of the weir, the appropriate formula and a scientific calculator. It is important to note that the formulas for flow calculation differ for the different types of weirs..
Measure the head, or the height of the water above the weir bottom, at an upstream distance that is at least four times the depth of the water in the weir. The measurement should be taken in feet. This value will be the value for "h" in the final equation.
Measure, in feet, the length of the weir crest immediately above the weir. This value will be that for the symbol "L" in the final equation. This measurement is not needed for V-notch weirs.
Calculate the weir flow in cubic feet per second. The complete equation for calculating the flow of water over broad-crested and Cipoletti weirs is Q = k * L * h^3/2, where h is raised to the power of 3/2 and the value of k is 1.705 or 3.367, respectively. The value of h^3/2 means that the value of h is first cubed, and then the square root is taken. This calculation can be done easily on a scientific calculator. Alternatively, you can visit a website that has a Cipoletti weir calculator to complete the calculation. Some websites have the capability to convert the flow into metric units, if needed.
For V-notch weirs, the formula varies depending on the angle of the V-notch. The formula for weirs having V-notch angles of 90, 60 and 45 degrees is Q = k * h^2.5, where the value of k is 2.5, 1.443 or 1.035, respectively. For a V-notch weir having an angle of 30 degrees, the formula is Q = 0.685 h^2.45. There are also online sources for doing this calculation.
Online resources make the calculations easier, but the basic calculations are not very difficult. If you do these measurements on a regular basis, you can create a spreadsheet in which you can embed the formula. This will make record-keeping and calculations very easy. Double check your calculations if you do them by hand to make sure they are correct. Remove any debris, such as water weeds, twigs or branches, from the weir before taking measurements. Debris can effect calculations for smaller weirs more than for larger weirs. Conducting a regular check of the weir for debris is a good standard operating procedure.
If you are measuring anything other than water, you should wear safety glasses and gloves.