This International Standard specifies a method for the estimation of sub-critical flow of clear water in smooth, straight, rectangular prismatic open channels with a vertical drop and discharging freely. Using the measured depth at the end, the flow in rectangular channels (horizontal or sloping) with confined nappe and unconfined nappe may be estimated. The advantages and disadvantages of this device and other types of weirs and flumes, as well as the relative accuracies of each of these devices, are given in the annex.